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Atlantic Climate VariabilityExperiment Prospectus

SummaryEnergetic, large-scale variability is observed

in the atmosphere and ocean of the Atlantic Sec-tor on interannual and decadal time scales. It ismanifested as coherent fluctuations in tempera-ture, rainfall, surface pressure and temperaturereaching eastward to central Europe and north-ern Asia, southward to subtropical West Africaand westward to North and South America witha myriad of well documented impacts on societyand the environment. The coordinated changesin the position of mid-latitude atmospheric pres-sure centers and winter storm tracks over the At-lantic, as well as the strength of the trade winds,are commonly referred to as the North AtlanticOscillation (NAO). In the tropical Atlantic, sur-face temperature variability and associatedchanges in the inter-tropical convergence zoneand the Hadley circulation occurs on interannualto decadal time scales, phenomena we collectivelycall Tropical Atlantic Variability (TAV). The NAOand TAV are reflected in marked changes in windstress on the ocean, as well as air-sea heat andfresh water fluxes. These in turn induce substan-tial changes in the wind- and buoyancy-drivenocean circulations, water mass transformation andpossibly poleward heat transport. Indeed manyocean properties show strong links to overlyingatmospheric variability, suggesting that much ofthe observed ocean variability is driven by theatmosphere. Whilst it is known that the Atlanticocean plays a central role, through its thermoha-line circulation, in the mean climate, its role inmodulating atmospheric variability on interannualto decadal time scales is less clear.

We propose an Atlantic Climate VariabilityExperiment (ACVE) to address the overarchingquestion: In what ways does coupled atmosphere-ocean dynamics in the Atlantic sector play anactive role in climate variability? Its primary goalis to quantitatively test and improve our under-

standing of mechanisms and models of atmo-spheric and ocean processes that lead to climatevariability in the Atlantic and its global conse-quences. We must determine whether observedand simulated variability is fundamentallycoupled, whether one component drives variabil-ity in the other, whether modeled variability bearsany relation to observed variability, and whethermodes of variability once identified are predict-able. Improvement of atmospheric and oceaniccirculation models, especially boundary layerformulations, is a central part of ACVE.

The specific objectives of the Atlantic ClimateVariability Experiment are to:• Describe and model coupled atmosphere-

ocean interactions in the Atlantic sector, quan-tify their influences on the regional and largerscale climate system, and investigate theirpredictability.

• Assemble a quantitative historical and realtime data set that may be used to test, improveand initialize models of coupled Atlantic cli-mate variability.

• Investigate the sensitivity of the meridionaloverturning circulation of the ocean tochanges in surface forcing and assess the like-lihood of abrupt climate change.Some of the processes involved in climatic

modulation of the North Atlantic will requirebroad-scale study of entire gyres or basins; othertopics can be examined in focused process ex-periments. Some investigations must extend overa decade so as to observe a full cycle of the vari-ability of interest. But clever treatment of exist-ing historical observations will provide an impor-tant long-time-scale perspective. Understandingthe causes of interannual to decadal climate vari-ability demands a new type of research program.The current global network of sustained obser-vations is too thin to support detailed quantita-tive study of specific climate modes. Moreover,

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typical process studies do not last long enough toresolve decadal-time-scale variability.

What is needed is an integrated program ofsustained observations, process studies and mod-eling conducted jointly by meteorologists andoceanographers that last a decade or longer. Theproposed Atlantic Climate Variability Experimentneeds to develop time-dependent, basin-scaleanalyses of the Atlantic ocean and atmosphere,based on an economically efficient observingsystem having significantly greater coverage anddensity than that provided by routine observationsavailable today. The phenomenological focus willbe the NAO and TAV modes of climate variabil-ity. The ACVE will encompass model-data syn-thesis in which mechanisms, key processes, andglobal teleconnections will be addressed quanti-tatively inside an improving coupled climatemodel framework. A prerequisite is to developan active dialogue between observers, modelersand theoreticians.

This document provides the scientific objec-tives and framework for achieving an interna-tional Atlantic CLIVAR program. It represents aconsensus document of U.S. and European sci-entists reflecting the findings of two U.S. work-shops: in Lamont (September, 1997) and Dallas(February 1998); and a joint U.S.-European meet-ing held in Florence (May 1998). A list of scien-tist who attended these meetings is attached insection 8.

After a brief introduction in section 1, the sci-entific background is set out in section 2. Section3 discusses some of the societal impacts of At-lantic-sector climate variability. The scientificquestions are set out in section 4, and a proposedstrategy for addressing them is given in section5. Programmatic context is discussed in section6, along with an outline of the next steps neces-sary to advance the planning and begin the imple-mentation for this project.

of future climate fluctuations and the extent towhich they may be predictable, and b) understand-ing the potential impact of climate change due tochanging levels of greenhouse gases.

A substantial proportion of the climate vari-ability in the Atlantic region is associated with aphenomenon known as the North Atlantic Oscil-lation (NAO). Although the spatial pattern of theNAO appears to be a natural mode of atmosphericvariability, its phase may be influenced by sur-face, stratospheric, or even anthropogenic pro-cesses. The NAO has undergone major low fre-quency variation this century. The rising NAOstate over the past twenty years has been accom-panied by a strengthening of the Pacific-NorthAmerica (PNA) pattern of atmospheric variabil-ity, and together they have resulted in a globalpattern of cooling of the mid-latitude oceans, andwarming over northern hemisphere land masses.

Several mechanisms have been proposed toaccount for the observed low-frequency variabil-ity in the NAO including atmospheric responseto changes in sea surface temperatures, variabil-ity of deep atmospheric convection in the trop-ics, and the effects of changing external forcessuch as solar insolation, volcanic eruptions, andanthropogenic emissions of climatically impor-tant trace gases. Alternatively, the NAO may sim-ply be a natural internal mode of the atmosphere.Distinguishing between these hypothesizedmechanisms is a high priority. Low frequencyvariation of the Atlantic Ocean is also poorly un-derstood. SST anomalies have been observed topropagate slowly through the upper-ocean gyrecirculations, formed in response to, or possiblyinducing changes in the NAO. The phase of theNAO also appears to be correlated to the forma-tion of deep water in the Greenland, Iceland,Norwegian and Labrador Seas, which in turn in-fluence the strength and character of the Atlanticmeridional overturning circulation (MOC). Al-though changes in the MOC can occur naturally,there is also the possibility that they could be trig-gered by anthropogenic emissions. A breakdownof the MOC could lead to drastic changes in glo-bal climate. A program investigating Atlantic vari-ability should therefore also explore those atmo-

1. IntroductionThe climate of the Atlantic Sector has exhib-

ited considerable variability on a wide range oftime scales. Improved understanding of this vari-ability is essential for a) assessing the likely range

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spheric, oceanic and ice processes which are criti-cal to the MOC.

Substantial variability is observed in the tropi-cal Atlantic region on interannual and decadaltime scales. Sea surface temperature (SST)anomalies have been observed both north andsouth of the equator, along with regional changesin winds and precipitation. Following the successof ENSO modeling in the Pacific, study of low-latitude Atlantic variability has highlighted thetropical Atlantic’s response to variable atmo-spheric forcing, and the atmospheric response totropical SST gradients. Key questions in both theAtlantic and Pacific include the nature and con-sequences of tropical-subtropical interactionsbetween the free atmosphere and the ocean, aswell as inter-ocean interactions viateleconnections within the tropics or involvingthe higher-latitude PNA and NAO patterns.

Most important, the predictability of climatefluctuations in the Atlantic region needs to beestablished. Great progress has been made in thelast 20 years developing understanding of theevolution of ENSO in the Pacific and its influ-ence on future weather patterns. It is timely tobegin comparable studies of Atlantic modes ofclimate variability. Even if it turns out that thepredictability of such modes is limited, improvedunderstanding of climate variability in the Atlan-tic region is essential to better assess the poten-tial impact of climate change due to changing lev-els of greenhouse gases, and the possibility ofrapid climate change associated with changes inthe MOC.

2.1 — The North Atlantic OscillationEarlier this century, meteorologists noticed

that year-to-year fluctuations in winter air tem-peratures on either side of Iceland were often outof phase. When temperatures were below normalover Greenland they were above normal inScandinavia, and vice versa. Concomitant fluc-tuations in rainfall, and sea level pressure (SLP)were also documented, reaching eastward to cen-tral Europe, southward to subtropical West Af-rica and westward to North America. Walker andBliss (1932) noted a tendency for pressure to beanomalously low near Iceland in winter when itis anomalously high near the Azores and south-west Europe. This mode of climate variability hascome to be known as the North Atlantic Oscilla-tion (NAO), a name first used by Walker in 1924(see Rogers, 1990). The horizontal structure ofthe NAO is plotted in the Figure 2.1a. In con-trast to the mean flow, the NAO has a pronounced“equivalent barotropic” structure (vertical phaselines, Wallace and Gutzler, 1981, Kushnir andWallace, 1989) and increases in amplitude withheight in rough proportion to the strength of themean zonal wind. This result is consistent withthe notion that the NAO has a structure that isclose to that of a stationary external Rossby mode(Held, private communication) and maybe insepa-rable from true dynamics of the zonally symmet-ric circulation. It appears to be a “resonant struc-ture” (a “neutral mode” of the climatology;Marshall and Molteni; 1993), which can be ex-cited in a number of different ways.

The vertical structure of the NAO gives usstrong clues as to its likely mechanism. Eddy-mean-flow interaction (i.e., dynamics internal tothe atmosphere) is thought to be the primary forc-ing mechanism on short time-scales. Eddy-poten-tial vorticity flux forcing readily projects on tothe vertical structure of the NAO.

The NAO index, defined as the normalized

1 Detailed discussions of climate variability phenomena in and over the Atlantic Ocean are given in a “WhitePaper” on the Atlantic Climate Variability (http://geoid.mit.edu/accp/avehtml.html) and elsewhere (e.g., http://www.clivar.ucar.edu/hp.html). Some of that material may be also viewed at the ACVE web page (http://www.ldeo.columbia.edu/~visbeck/acve/). See also (http://www.cgd.ucar.edu/cas) and the international CLIVARdocuments (http://www.clivar.ucar.edu/hp.html) D1-D3.

2. Modes of Atlantic ClimateVariability

To motivate the science plan described below,we give here a brief review of what is known aboutAtlantic climate variability1.

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SLP difference between Iceland and the Azores,from 1864 to the present day is plotted in Figure2.1b. Positive values indicate stronger-than-av-erage middle-latitude westerly winds. Such asimple index is chosen because of the availabil-ity of data - it clearly cannot accurately capture

the seasonal variations in the NAO (Barnston andLivezey; 1987), or the possibility that the centersof the actual pattern may not overlap these loca-tions.

Paleo indicators of the NAO suggest that ithas been coherent over long periods of the 700-

a)

b)

Figure 2.1 — a) Sea Level Pressure projected on the NAO index (courtesy of J. Hurrell). b) The NAO index from1864 to 1996 defined as the difference in normalized pressure between Lisbon and Stykkisholmur, for the wintermonths, December-March (Hurrell, 1995).

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year stable isotope record from the GISP-2 icecore from central Greenland (White et al., 1996).Similar evidence emerges from the tree-ring data(Cook et al; 1998). H. Wanner (personal commu-nication, 1998), however, reconstructs the NAOfrom proxy and documentary sources and hypoth-esizes that decadal-scale cold phases during theLate Maunder Minimum (1675-1715) were re-lated to successive negative NAO events. If themain forcing mechanism of the NAO were dueto its interaction with synoptic-scale eddies, thenone might expect its frequency spectrum to bewhite. The spectrum of the wintertime NAO in-dex is slightly red (power increases with period)with an indication of enhanced power in somefrequency bands (though not large enough to beconsidered statistically significant; Wunsch,1998). However, frequency domain analyses(Mann and Park, 1996; Tourre et al., 1998), sug-gests that these relatively weak spectral peaksrelate to basin-wide coherent phenomena and soperhaps are real. Thompson and Wallace (1998)have drawn attention to the fact that the NAOmay be the regional manifestation of a hemi-spheric mode of variability centered over the Arc-

tic Ocean. This “Arctic Oscillation” - the AO - isan equivalent barotropic, zonally symmetric struc-ture which couples the troposphere with the strato-sphere (Perlwitz and Graf; 1995) and modulatesthe strength of the polar vortex, as in the zonalindex of Rossby et al (1939). Because the energydensity of the mode remains high well into thestratosphere, it has been suggested that the NAOcould be modulated through its interaction withthe polar vortex and hence be sensitive to an-thropogenic influences in the stratosphere. Cause-and-effect within the stratosphere are not yetsorted out; one extreme view is the “Charney-Drazin” picture of upward propagating energyfrom the energetic winter troposphere, with thestratosphere both conditioning the waveguide, andturning the energy into a breaking planetary wave.Another is that an increasingly strong polar strato-spheric vortex could actively excite the tropo-spheric NAO.

2.1a — Covarying patterns in the oceanThere are clear indications that the North

Atlantic Ocean varies significantly with theoverlying atmosphere. Expectation of an im-

80oW 40oW 0o 40oE 80oE 60oS

30oS

0o

30oN

60oN

Correlation Wintertemperature - NAO

0.2-0.2 0.4-0.4 0

Figure 2.2 — Regression between winter SST and land surface temperatures and the NAO index (courtesy of M.Visbeck and H. Cullen)

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print of the changing atmosphere on the ocean,and perhaps a coupling between climate changesignals in the two fluids, follows from severalrelated points:

First, changes in the NAO, are reflected inmarked changes in surface wind stress, air-seaheat exchange, and the evaporation-precipitationbalance (Cayan, 1992a) and sea-ice. Heat fluxanomalies directly drive month-to-month SSTtendencies in the North Atlantic (Cayan, 1992b).Figure 2.2 shows the regression of SST on theNAO index during winter. It reveals a tri-polarpattern - a cold subpolar region, warmth in middlelatitudes and a cold region between the equatorand 30°N. This is also the leading pattern of SSTvariability during winter. Its emergence is con-sistent with the spatial form of the anomaloussurface fluxes associated with the NAO pattern,as pointed out by Cayan (1992a, b). It appearsthat the strength of the correlation increases whenthe NAO index leads SST indicating that SST isresponding to atmospheric forcing on monthlytime scales (Frankignoul 1985; Battisti et al 1995;Delworth; 1997). Changes in sea-ice cover in boththe Labrador and Greenland Seas are well corre-lated with the NAO, with a particularly strongatmospheric feedback.

Second, the NAO has its largest amplitude inwinter. The interior ocean has a selective memoryfor winter conditions, both in the subduction ofwinter conditions in to the thermocline (Stommel,1979; Marshall et al, 1993; Williams, et al., 1995),and in the seasonal sequestering of winter condi-tions from below the seasonal thermocline. Third,the ocean acts as an integrator of the high-fre-quency forcing acting on it. If an SST anomaly is‘tied’ to a deep thermal anomaly, which is re-ex-posed each winter (Alexander and Deser, 1995),then the SST anomaly can re-emerge year afteryear and so might have a greater ability to alter,through anomalous fluxes, the overlying atmo-sphere. Sea surface temperature observationsdisplay a myriad of long-term (interannual anddecadal) responses. The pattern of SST variabil-ity is indicative of the ability of the ocean to storeheat locally and move it around (Deser andBlackmon, 1993; Hansen and Bezdek, 1996). The

spreading of SST anomalies along the path of theGulf Stream and North Atlantic Current is of par-ticular interest (Sutton and Allen, 1997). Herewinter SST anomalies born in the western sub-tropical gyre are found to propagate eastward witha transit time of a decade or so. Subsurface oceanobservations provide an even clearer depictionof long-term climate variability because the ef-fect of the annual cycle and month-to-month vari-ability in the atmospheric circulation decays rap-idly with depth. A large freshening event (dubbedthe Great Salinity Anomaly, GSA) occurred inthe sub-polar gyre in the early 1970s (Dickson etal., 1988). Observations suggest it may have beenlinked to coherent changes in the overlying at-mospheric circulation and an extreme manifesta-tion of the quasi-decadal fluctuations (Reverdinet al., 1997; Belkin et al; 1998). The GSA eventfollowed the deep reaching changes in tempera-ture and salinity in the North Atlantic, from thelate 1950s to the early 1970s (Levitus, 1989).These changes coincide with a multi-decadal fluc-tuation of SST and atmospheric conditions(Kushnir, 1994; Hansen and Bezdek, 1996). Vari-ability in the temperature of the upper subtropi-cal ocean has also been linked with fluctuationsin the NAO index (Molinari et al., 1997). Ondecadal time scales, the intensity of convectiontends to see saw between the Labrador Sea andthe Greenland-Iceland Seas (Dickson et al., 1996)orchestrated by fluctuations in the NAO. The linkbetween propagating SST anomalies and the prop-erties of water (McCartney et al., 1996) mixed atthe end of winter, shows that the end-product ofthe transformation process, Labrador Sea Water(LSW), underwent extended warming and cool-ing trends consistent with the phase of the NAO.

2.2 — Tropical Atlantic VariabilityThe dominant low-frequency climate phe-

nomenon in the tropical Atlantic is the covaryingfluctuation of tropical SST and trade winds(Nobre and Shukla, 1996, and Figure 2.3a). Thetime series of the joint SST and wind pattern (Fig-ure 2.3b) clearly contains long-term componentsand a noticeable imprint of decadal variability.These fluctuations exhibit a pattern of basin-wide

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SST anomalies straddling the mean position ofthe ITCZ, with concomitant changes in trade windintensity. The Northern Hemisphere trades, andthe associated cross equatorial flow, seem to de-pend sensitively on SST. Very little wind vari-ability is found south of the equator. Weaker thannormal trades are associated with positive localSST anomalies, and vice versa (Nobre and Shukla,1996; Enfield and Mayer, 1997). The northerntrades are also affected by SST changes south ofthe equator, such that colder than normal SST inthe South Atlantic correspond to weaker thannormal trades in the North Atlantic and vice versa.Early statistical analysis of SST data revealed adipolar structure (particularly when referenced tometeorological anomalies, Moura and Shukla,1981) and led investigators to dub the pattern the“tropical Atlantic SST dipole.” However, SSTvariability north and south of the equator are notcorrelated (Houghton and Tourre, 1992; Enfieldand Mayer, 1997, Mehta, 1998), suggesting thatit is the change in the cross-ITCZ SST contrast towhich the trades respond. While it is becomingapparent that SST variability on both sides of theAtlantic ITCZ is not well described as a tempo-rally coherent seesaw fluctuation, it does seemthat climate variability in the tropical Atlantic issensitive to the cross equatorial SST differences(Hastenrath and Heller, 1977; Servain, 1991;Ward and Folland, 1991; Folland et al., 1991).The build-up of the North Atlantic SST anomalylags behind the local change in the wind circula-tion by a month or two (Nobre and Shukla, 1996),much like the lag between the atmosphere andocean in mid-latitudes (Frankignoul, 1985). How-ever, as the SST anomaly builds up, the NorthernHemisphere trades adjust, so that on seasonal timescales the atmosphere feels the SST anomalybuildup and responds to it (Nobre and Shukla,1996). A mode of tropical Atlantic atmosphere-ocean variability has been identified that is akinto ENSO in the Pacific (Covey and Hastenrath,1978; Philander, 1986; Zebiak, 1993) and in-volves variations in SST along the equator. Theseperturbations in equatorial winds and SST are notas dominant as their Pacific counterpart, appar-ently because the Atlantic basin is much narrower

than the Pacific, and do not appear to be consis-tently sustained (Zebiak, 1993). However, theydo exert some influence over the adjacent landmasses.

The Atlantic sector clearly responds to thePacific El Niño although the relative importanceof this remote forcing in tropical Atlantic vari-ability is still unresolved. (Hastenrath et al., 1987;Hameed et al., 1993; Nobre and Shukla, 1996;Enfield and Mayer, 1997). The ENSO influenceis transferred through the atmosphere, via changesin both the Walker circulation and Northern Hemi-sphere extratropical teleconnections (i.e., the Pa-

Figure 2.3 — The dominant pattern of tropicalAtlantic variability revealed by a joint EOF analysisof surface wind stress and SST monthly anomalies,September 1963 to August 1987. Spatial structuresof the first EOF mode and the associated timecoefficients are illustrated in (a) and (b), respec-tively. Contours represent SST loading values,interval of 0.1°C. Vectors depict wind-stressloadings. The unit vector on the lower-left cornerrepresents an easterly wind-stress anomaly of 1.0dyn cm-2. The time coefficients have been normal-ized by their own standard deviations (Nobre andShukla, 1996).

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cific North American Pattern, PNA). ENSO isthought to particularly effect the western flank ofthe Northern Hemisphere subtropics, during thewinter and spring following the onset of ENSO.It appears that ENSO effects further east in theGulf of Guinea is rather small (Enfield and Mayer,1997).

Finally, there exists a link between tropicalclimate variability and the NAO (see Figure 2.2).Variability of the NAO can change the intensityof the trades in the eastern subtropical North At-lantic and thence SST. Moreover, the NAO itselfmay be sensitive to subtropical SST anomalies(S. Jewson, personal communication, 1998, showthat there is a significant model response with andwithout the tripole SST pattern of Figure 2.2, andargues that it is the subtropical SST anomalieswhich are the most important). It has even beensuggested (Tourre et al; 1998, Rajagopalan, per-sonal communication; Robertson et al, personalcommunication) that there is a link between theNAO and tropical SST in the South Atlantic. Itremains to be seen whether the induced changesin tropical SST can feed back on the extratropi-cal circulation through mechanisms akin to thoseacting in the Pacific.

2.3 — Thermohaline circulation and abruptclimate change

The mean circulation of the ocean plays a keyrole in meridionally transporting water proper-ties such as heat and freshwater, carbon and nu-trients. In concert with meridional atmosphericfluxes, ocean transports balance the earth’s glo-bal heat and hydrologic budgets. At 25°N, theAtlantic’s subtropical gyre circulation carriessome 1.2 PW of heat northward: approximately60 percent of the net poleward ocean flux and 30percent of the total flux by ocean and atmosphereat this latitude (Hall and Bryden, 1982). Thispoleward heat flux is of course intimately associ-ated with the watermass transformations that takeplace as thermocline waters move north and areultimately converted by air-sea interaction intocold North Atlantic Deep Water (NADW). Ther-mocline waters of the South Atlantic, represent-ing some mix of Indian Ocean and Pacific water

masses, move north within the S. Atlantic sub-tropical circulation and enter the tropical regimelargely at the western boundary. There, signifi-cant water mass transformation occurs within theenergetic equatorial circulations, with muchwarmer waters emerging northward. The warm-water link between tropical and northern subtropi-cal circulations is complex, involving bothstrongly varying Ekman transport in the interiorand highly time-dependent western boundarycurrent flows. Upon entering the subtropical gyre,the northward thermocline water flow is incor-porated into the Florida Current/Gulf Stream sys-tem and related interior circulation. The role ofthe Atlantic in a world of increasing CO2 emis-sions has received increased attention of late.About 60 percent of the global oceanic CO2 up-take may take place in the Atlantic sector, a con-sequence of its intense meridional overturningcirculation. However, some model projectionssuggest that in only a few decades, Atlantic cli-mate might radically shift into a different equi-librium with much reduced MOC. Evidence forrapid climate shifts in the past has been found inseveral paleo records and is attributed to changesin the strength of the MOC. A weaker MOC willresult in a reduced poleward oceanic heat trans-port and might dramatically reduce oceanic CO2uptake and more importantly, rapidly cool Eu-rope and the northeastern American continent.One might rightfully question the accuracy ofsuch projections. However, the dramatic impacton the western world of such a scenario, even ifhighly unlikely, justifies a concerted effort tomonitor the evolving climate in the Atlantic sec-tor and improve fundamental understanding ofthe coupled climate system.

3. Impacts of Atlantic ClimateVariability

3.1 — The North Atlantic Oscillation• Temperature and global warming

The NAO exerts a dominant influence onthe wintertime temperatures of the NorthernHemisphere. Surface air temperature and SST

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in wide regions across the North Atlantic ba-sin, in eastern North America, the Arctic,Eurasia and the Mediterranean, are signifi-cantly correlated with NAO variability.Changes in temperature over land (and relatedchanges in rainfall and storminess, see below)are of serious consequence to a wide range ofhuman activities.

Changes of more than 1°C in surface tem-perature can be associated with a one stan-dard deviation change in the NAO index oc-cur over the northwest Atlantic and extendfrom northern Europe across much of Eurasia(Hurrell, 1995; Hurrell and van Loon, 1997,see Figure 3.1). Temperature changes overnorthern Africa, the Middle East and thesoutheast U.S. are also notable. Of particularinterest is the contribution of the NAO vari-ability to the recent trend in Global/hemi-

spheric mean wintertime temperature. Hurrell(1996) demonstrated that both NAO andENSO are linearly related to this climate trendwith NAO dominating a larger land area thanENSO. Thompson and Wallace (1998) arguethat an index of the AO is more strongly cor-related to the temperature trend over Eurasiathan the NAO. Perlwitz and Graf (1995) sug-gest that the trend in the NAO and the associ-ated trend in wintertime temperature overEurasia (and other details of the trend pattern)are a manifestation of the anthropogenicgreenhouse effect. They hypothesize thatstratospheric increases in CO2 result in en-hanced radiative cooling of the polar strato-sphere in winter, leading to a strengtheningof the polar vortex. They then invoke strato-sphere-troposphere interaction to explain therecent positive trend in the NAO index

Figure 3.1 — Observed surface temperature change associated with a one standard deviation of the NAO index.The regression coefficient was computed for the winters of 1935-1996 (from Hurrell and van Loon, 1997). Contoursare in 0.1°C. Dark (light) shading indicate positive (negative) changes.

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through enhancement of tropospheric station-ary waves.

• Precipitation and StormsChanges in mean circulation patterns over

the North Atlantic are accompanied by pro-nounced shifts in the storm tracks and asso-ciated synoptic eddy activity. These effect thetransport and convergence of atmosphericmoisture and can be directly tied to changesin regional wintertime precipitation (Figure3.2, see also van Loon and Rodgers, 1978;Lamb and Peppler, 1987; Hurrell, 1995). Drierthan normal conditions occur during highNAO index winters over much of central andsouthern Europe, the northern Mediterraneancountries, and west North Africa. At the sametime, wetter than normal conditions occurfrom Iceland through Scandinavia. OverNorth America the effect of NAO on precipi-tation is not strong. However an out of phaserelationship between the NAO index andsnowfall over New England was found. TheNAO-related changes in precipitation patternshave directly affected different Europeaneconomies, in particular because of the longstretch of consecutive winters in which theNAO continued to intensify. Over the Alps,for instance, snow depth and duration over

the past several winters have been among thelowest recorded this century, causing eco-nomic hardship to those industries dependenton winter snowfall (Beniston, 1997). Severedrought also prevailed throughout Spain andPortugal affecting olive harvests. In contrast,increases in wintertime precipitation overScandinavia was related to recent positivemass balances in the maritime glaciers ofsouthwest Norway, one of the few regions ofthe globe where glaciers have not been re-treating.

Storminess in the midlatitudes is associ-ated with the path of synoptic disturbancesand their frontal systems. Indeed, this is thesource of the above- indicated changes inmidlatitude rainfall patterns. It is well knownthat changes in low-frequency patterns areassociated with changes in storm activity andshifts in the storm tracks (Lau, 1988; Rogers,1990). The NAO is clearly linked to system-atic changes in storm tracks over the North-ern Hemisphere from North America toEurasia and the Mediterranean (Rogers,1997).

There is a connection between the season-ally-averaged NAO and low frequency vari-ability within a season (e.g., blocking) over

Figure 3.2 — Precipitation anomalies associated with the NAO; E-P is plotted, computed as a residual of theatmospheric moisture budget using ECMWF global analyses, for high NAO index minus low index winters - seeHurrell, 1995.

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the Atlantic (see Nakamura, 1996). There isweaker intra-seasonal variability nearGreenland and the Labrador Sea and stron-ger intra-seasonal variability over Europe,when the NAO index is positive (and visa-versa).

The ocean integrates the effects of stormsin the form of surface waves, to exhibit amarked response to long lasting shifts in thestorm climate, as is the case with variabilityrelated to the NAO. The recent rising trend inthe intensity of the NAO has had a profoundeffect on the surface wave climate of the NorthAtlantic. The trend in measured wave heightshas been supported by many anecdotal reportsabout increasing wave heights encountered bymariners and North Sea oil rig operators. Re-cent studies by Bacon and Carter (1993) andKushnir et al. (1997) have tied, in a morequantitative manner, the increase in waveheights to the increase in wintertime stormi-ness and mean wind speeds in the North At-lantic during the last 30 years or so.

• Fisheries and ecosystemsChanges in the NAO have been associ-

ated with a wide range of effects on the ma-rine ecosystem too. This includes changes inthe production of zooplankton and the distri-bution of fish (e.g., Fromentin and Planque1996; Friedland et al., 1993, Drinkwater,1994). The NAO has been linked to bloomsof the “brown tide” unicellular algae in coastalwaters of Eastern Long Island, which devas-tate the local commercial scallop fishery(LaRoche et al., 1997).

Evidence has also recently emerged that,in Scandinavia, the NAO influences tempo-ral and spatial variability in timing of plantgrowth: the length of the plant growth seasonvaried by 20 days between extremes of theNAO index (Post et al., 1997). Through ef-fects on vegetation and climatic conditions,the NAO was furthermore observed to influ-ence several aspects of life history and ecol-ogy of terrestrial, large mammalian herbi-vores, including: phenotypic variation, fecun-dity, demographic trends, and population dy-

namical processes (Post et al., 1997). Influ-ences of the NAO are evident among fivespecies of ungulates in populations inGreenland, Canada, the U.S., Great Britain,Norway and Finland.

3.2 — Tropical Atlantic Variability• Rainfall and droughts

Although sea surface temperature anoma-lies in the tropical Atlantic are weaker thanthose associated with the Pacific El Niño, theycan cause disastrous climate hazards over theAmericas and Africa. In the tropical Atlantic,rainfall is governed by the annual migrationof the subtropical high pressure cells on bothsides of the equator and the changes in theirstrength, as well as the north-south swings ofthe Intertropical Convergence Zone. Many ofthe continental regions bordering the tropicalAtlantic experience a sharp seasonal contrastin rainfall where half the year is wet and theother half dry. Some of these regions areblessed with abundant rainfall, while othersare semiarid. It is in these semiarid regionsthat a delay in seasonal rainfall or a signifi-cant drop in its total amount can bring seri-ous hardship to their inhabitants. Such is thesituation in sub Saharan Africa (the Sahel,15°W-15°E,15-20°N) with its boreal summerrainfall, and the northeast corner of Brazil(Nordeste, 35 45°W, 2-10°S) where it rainsin the boreal winter. Many meteorologistshave for quite some time been concerned withthe dynamics of climate anomalies in thesetwo regions. Hastenrath and collaborators(Hastenrath and Heller, 1977; Hastenrath,1978; Hastenrath et al., 1984; Hastenrath andGreischar, 1993, Moura and Shukla, 1981;Ward and Folland, 1991; Nobre and Shukla,1996) found that tropical Atlantic SST distri-bution and the associated anomalies in sealevel pressure and wind are leading factors indetermining the anomalies in seasonalNordeste rainfall. These same climatic fac-tors were also found to be strongly linked withrainfall anomalies over the Sahel and west-ern tropical Africa (Lamb, 1978; Hastenrath,

12

1984; Folland et al., 1986; Folland et al.,1991).

Rainfall anomalies in the Nordeste dis-play a wide range of time scales with cleardecadal components (Figure 3.3a). Such longtime scales are also present in the Sahel rain-fall time series (not shown). The correlationbetween seasonal rainfall in the Nordeste andtropical Atlantic rainfall indicates an out ofphase relationship between SST anomalies inthe ocean regions underlying the Northern andSouthern Hemisphere trade wind region, re-spectively (Hastenrath and Heller, 1977;

Markham and McLain, 1977; Moura andShukla, 1981). Dry Nordeste years tend tooccur when SSTs north of the equator arewarmer than normal and SSTs south of theequator are colder than Normal. Rainfall insubtropical west Africa is more complex butalso displays considerable dependence on asimilar out of phase variation of SST in thetropical Atlantic (Lamb, 1978; Lough, 1986;Lamb and Peppler, 1987). It rains more whenNorthern Hemisphere subtropical SST arewarm, and Southern Hemisphere SST arecold. Recent attempts to understand the dy-

1840 50 60 70 80 90 1900 10 20 30 40 1950 60 70 80 90

-2

-1 0 1 2

a) rainfall index (Nordeste)

Year

anom

alie

s

30S

20

10

EQ

10

20

30N

80W 60W 40W 20W 0 20E

5 m/s

b) rainfall, SLP, and surface vector wind

c) SST and surface vector wind

Figure 3.3 — a) Seasonal rainfall anomalies over the Nordeste region of Brazil. b) Correlation of Nordeste rainfallindex with rainfall anomaly (shaded), wind vector and sea surface pressure (contours). c) Correlation of Nordesterainfall index with SST (shaded) and wind vector (courtesy of Mitchell).

13

namics of this asymmetric SST distributionare described in Carton et al., 1996, Nobreand Shukla, 1996 and Chang et al., 1997.

• HurricanesInterannual variability of the seasonal fre-

quency of Atlantic hurricanes is related to,amongst other factors, the sign and amplitudeof the SST anomaly in the subtropics (Gray,1990; Shaeffer, 1996). Moreover, Gray (1990)also suggests that tropical cyclone intensityis correlated with the multi-decadal fluctua-tion of North Atlantic SST (Kushnir, 1994).In fact, Gray anticipates a return to a stormiertropical climate, similar to that of the 1950s,when the North Atlantic finally recovers fromits recent cold phase. Such an increase wouldthreaten the densely populated coastal regionsin the Gulf of Mexico and the North Ameri-can Atlantic seaboard which have developedmarkedly during the relatively benign 1970sand 1980s.

3.3 — Abrupt Climate ChangeThe possibility of a major change in the oce-

anic meridional overturning circulation undergreenhouse forcing has been proposed byManabe, Stouffer, Broecker, and Rahmstorff, inseparate works based on a variety of coupledGCMs, and ocean-only GCMs. “Abrupt climatechange” has received a particular burst of atten-tion in the literature and the popular press throughthe suggestion that the increasing freshwatertransports and warmer atmospheric temperaturesin a greenhouse world will blanket the high-lati-tude oceans with a buoyant layer, and cause in-stability or even collapse of the MOC. Model for-mulation of the air-sea fluxes of heat and mois-ture, the sites of deep convection in simulations,and many other dynamical aspects of the modelsare in question. Yet one cannot ignore the possi-bility that increased fresh-water input and atmo-spheric high-latitude temperature could suppressthe MOC. The impact, particularly on Eurasianclimate is potentially severe. The fairly low prob-ability is thus multiplied by a large potential im-pact, and thus demands attention.

4. The scientific challengesGiven the societal, economic and ecological

impact of interannual to decadal time scale cli-mate variability, it is of paramount importance toimprove our dynamical understanding of climatefluctuations in and over the Atlantic Ocean, tostudy the predictability of these fluctuations, andto quantify their impact on regional weather pat-terns and short time-scale climate variability. Sev-eral potentially important mechanisms of basin-scale atmosphere-ocean interaction and couplinghave been proposed. These physical processesneed to be explored and dynamical hypothesesadvanced and tested within the Atlantic ClimateVariability Experiment. In particular, data analy-sis and the collection of new observations mustbe guided by theoretical and modeling work thatexplore mechanisms underlying the coupled sys-tem.

We will first review the basic hypothesis forcoupled Atlantic Climate Variability and thenidentify a set of related key issues that need to beaddressed during ACVE. We then focus on thecomponent parts of the atmosphere-ocean systemto reveal their inherent nature in isolation, andfinally proceed to a consideration of the coupledbehavior both regionally and on large scale.

4.1 — Hypotheses for Coupled Atlantic Cli-mate Variability

On interannual to decadal time scales thereare preferred modes of variability of the coupledatmosphere-ocean system. The basic hypothesiswhich needs to be explored during ACVE is: Thepreferred modes of climate variability are signifi-cantly influenced by ocean-atmosphere interac-tions local to the Atlantic and distinct from themodes of either the ocean and atmosphere in iso-lation. For this hypothesis to hold, SST anoma-lies must exert a significant influence on atmo-spheric flows. In particular, the atmospheric re-sponse to SST anomalies must significantly in-fluence the flux of heat, moisture and momen-tum at the ocean surface.

In contrast, the null hypothesis is: The atmo-sphere has internal modes of variability with asomewhat red spectrum. The ocean integrates this

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essentially stochastic forcing, responding morerobustly to the lower frequencies and thereby red-dening the response spectrum. Interdecadal vari-ability then is either a result of decade to decadechanges in atmospheric boundary conditions dueto anthropogenic, solar, or volcanic effects, or,more likely, represents the low frequency tail ofa red spectrum.

The major challenge of ACVE is to determinethe validity of these two hypotheses by means ofobservations, numerical simulations and theoreti-cal studies. Because the source of climate vari-ability can be internal (natural) and external dueto anthropogenic forcing, attempts to test the hy-potheses must take these factors into the consid-eration.

The strength of active ocean-atmosphere cou-pling is likely to vary spatially. In middle/highlatitudes, the diabatic heating is confined withinthe lower boundary of the atmosphere, making ifdifficult to thermally force the NAO directly atthe ocean surface. Therefore, weak air-sea feed-backs are anticipated. The ocean, then, is prima-

rily driven by the surface fluxes associated withinternal variability of the atmosphere that reflectsthe spatial pattern of the NAO. The NAO not onlyprovides a dominant forcing to middle/high lati-tude SST variability and meridional overturningcell (MOC) in the Atlantic ocean, but also mayplay a significant role in exciting variability inthe tropics (Figure 4.1). In contrast to heating inhigher latitudes, convective heating in the trop-ics extends much deeper into the atmosphere,making the tropical atmosphere more responsiveto SST forcing. Thus, active ocean-atmospherecoupling is more likely to occur in the lower lati-tudes.

A coupled mechanism has been proposed forthe tropical Atlantic whereby the cross-equato-rial SST gradient plays a crucial role for the posi-tion of the ITCZ. This in turn will influence thestrength of the trade winds and feedback to tropi-cal SST anomalies (Figure 4.2). Moreover, thevariability of the ITCZ may have an importantremote impact on the NAO, possibly through re-arranging the Hadley circulation (Figure 4.1). It

Figure 4.1 — Mechanisms of Atlantic Variability (courtesy of J. Marshall).

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4.2 — Modes of Variability

THE UNCOUPLED SYSTEM

is, therefore, plausible that the NAO and TAV areinterrelated. Exploring interrelationships betweenthe NAO and TAV will be one of the major focusof ACVE.

Additionally, TAV is subject to remote influ-ence of the ENSO related variability in the tropi-cal Pacific (Figure 4.2). The remote influence maytake place either through an anomalous walkercirculation due to an eastward shift of the atmo-spheric convection, or through North Hemisphericatmosphere teleconnections (i.e., PNA). Howthese two processes are related is currently notclear.

The AtmosphereIt is well known that atmospheric GCMs, with

prescribed SST, display NAO-like fluctuations.Although no single theoretical explanation for theNAO is widely accepted, it seems that its funda-mental dynamics arise from atmospheric pro-cesses. There is probably no single cause; ratherthe NAO appears to be a preferred mode of theatmosphere that can be excited in a number ofdifferent ways, its position and spatial structure

Figure 4.2 — Mechanisms of Tropical Atlantic Variability (courtesy of P. Chang).

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linked to the basic characteristics of the large-scale atmospheric circulation. However, it is im-portant to determine whether atmospheric pro-cesses alone can lead to long-term, decadal cli-mate variability in the Atlantic Basin with ob-served amplitudes, phases and scales. Distin-guishing between coupled and uncoupled modesof variability is observationally difficult. Researchspecifically designed to explore the dynamics ofNAO-type variability is required. These investi-gations should address the following questionsregarding various aspects of planetary wave dy-namics governing tropospheric quasi-stationaryanomalies over the Atlantic:• What is the relative importance of topographic

and thermal forcing of the extra-tropical plan-etary wave field over the Atlantic?

• Does the Atlantic-sector atmosphere displaysensitivity to atmospheric deep convectiveactivity within the basin, particularly in thetropics?

• What is the role of land surface processes andtopography on both sides of the basin in ex-citing and regulating NAO variability and itsvariants?

• What is the relevance of instability mecha-nisms (barotropic/baroclinic) and synoptic-scale excitation and feedback on Atlantic ex-tra-tropical low-frequency atmospheric vari-ability?

• How do the properties of horizontal and ver-tical propagation (stratospheric connections),as well as non-linear atmospheric interactions,affect NAO and other teleconnections?Could they lead to multi-year variability as

an internal atmospheric phenomenon?The response of the atmosphere to sea-sur-

face temperature anomalies has been the subjectof many observational and modeling studies go-ing back to the very first atmospheric GCMs. Abroad consensus between observations, theory,and models exists with regard to the response ofthe atmosphere to tropical SST anomalies, in par-ticular over the Pacific Basin. However, there isno clear theoretical understanding of, or consen-sus about, the response of the atmosphere to SSTanomalies in mid-latitudes. Indeed, GCM mod-

eling experiments yield perplexing results whichseem to depend on model properties, model inte-gration methodology, and the geographical andseasonal distribution of the SST anomalies. Re-solving this issue is crucial to the understandingof decadal variability in the North Atlantic (andelsewhere) and to making progress in our studiesof the coupled interaction. The atmospheric re-sponse to the evolving pattern of low-latitude SSTin different basins of the oceans remains an im-portant research topic, in particular the atmo-spheric linkages between the tropical Atlantic andother ocean basins. Key subjects include the at-mospheric response to dipole-like SST perturba-tions. Atmospheric studies should also be de-signed to understand the remote influences of thePacific ENSO on the tropical Atlantic variability.

Questions that need to be answered regard-ing midlatitude ocean-atmosphere coupling are:• What is the sensitivity of the atmosphere to

the location and amplitude of SST anomaliesand subsurface thermal anomalies?

• What are the spatial patterns of theatmosphere’s response to mid-latitude SSTanomalies. What are the atmospheric dynami-cal mechanisms responsible for setting it upthese patterns? Is the atmospheric responselinear, or do the patterns depend on the polar-ity of the SST anomaly?

• How does the response of the atmosphereevolve in time? Does the response depend onthe season, and on the time scale of the oce-anic thermal anomalies?

• Is the observed near-surface circulation pat-tern over the tropical Atlantic Ocean repro-ducible by forcing an AGCM with a dipole-like SST perturbation? If so, what are the un-derlying dynamics determining the circula-tion pattern? How does the modified circula-tion pattern affect the surface heat flux?

• How important are continental heat sourcesto the tropical Atlantic circulation?

• What are the atmospheric connections be-tween tropical and extra-tropical climate vari-ability in the Atlantic? What influence dotropical Atlantic SST anomalies have on ex-tra-tropical atmospheric variability?

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• Through what physical mechanisms does thePacific Ocean exert influence on the tropicalAtlantic?

• Why does the ENSO influence appear to bestronger in the northern tropical Atlantic thanin the southern tropical Atlantic?

The OceanIt has been shown through theory and mod-

els that because of its large thermal heat capac-ity, the ocean can respond locally to high-fre-quency stochastic atmospheric forcing on sea-sonal and inter-annual time-scales (Hasselmann,1976; Frankignoul, 1985). Decadal variability inthe ocean surely involves non-local processes aswell, such as the vertical and horizontal exchangeof water masses. It is thus important to explorethe following issues:• What are the preferred patterns of oceanic

variability and their characteristic time scales?What dynamical mechanisms govern them?

• How do anomalies propagate through theocean? What is the oceanic teleconnectivity?

• What changes in horizontal gyre transportsand meridional overturning intensity resultfrom random and sustained surface forcing,possibly orchestrated by slowly propagatinginternal Rossby waves or advection of strati-fication anomalies?

• Is the Atlantic much affected by conditionsimposed at its northern and southern bound-aries with the Arctic, and Southern Oceansrespectively.

• What are the relationships between sub-sur-face ocean thermal anomalies and SST?In most modeling studies of ocean variabil-

ity, a simplified surface interaction is assumedwhere the heat and moisture exchange at the sur-face is either prescribed or derived from an at-mosphere that adjusts to the changes in SST inprescribed ways. Such studies of the forced re-sponse must be complemented by investigationsof internal (free) modes of ocean variability. Thisavenue is particularly relevant to the hypothesisthat decadal variability originates in the ocean.Studies of both forced and free variability mustbe carried out with models of varying complex-

ity. It is important to resolve the spatial and tem-poral characteristics of such variability, their pa-rameter sensitivity, and model dependence. Com-parisons with observations are crucial in deter-mining the degree of realism in the model simu-lations. As indicated above, processes that leadto SST variability should be a primary focus, butthe model-observation comparisons should en-compass other measures of the dynamical sys-tem to ensure a complete description of the vari-ability.

Studies are required that enhance understand-ing of oceanic processes that influence SST inthe tropics, including the role of gyre exchangeprocesses. Specific questions that must be an-swered include:• What is the role of ocean dynamics in on- and

off-equatorial SST variability?• What is the relative importance of horizontal

heat transport and vertical mixing/subductionprocesses?

• How significant are cross-equatorial andtropical-subtropical gyre exchanges? What isthe relative importance of western boundarycurrent transports and interior Ekman trans-ports?

• Do oceanic waves play an important role increating or modulating SST fluctuations in thetropical and mid-latitude Atlantic Ocean?

THE COUPLED SYSTEM

In studying the combined ocean-atmosphere sys-tem we need to identify what modes of variabil-ity owe their existence to active coupling betweenthe two systems and could not otherwise be mani-fested. This is clearly a question that can only beaddressed through a combination of modeling andobservational studies: modeling/theory to offerand test plausible physical mechanisms, obser-vations to examine evidence for their authentic-ity. In the tropical Atlantic system, coupled modelexperiments must also be performed to distinguishthe relative importance of external forcing andlocal air-sea processes to variability there. Exter-nal forcing must include the deterministic forc-ing associated with climate anomalies (e.g.,

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ENSO) teleconnected into the Atlantic basin, andthe stochastic forcing associated with dynamicalinstabilities internal to either the atmosphere orocean. The following questions must be ad-dressed:• What are the coupled mechanisms that can

lead to variability that exhibits characteris-tics of the observed variability in the NorthAtlantic?

• Is the coupling inherently regional or is it re-mote and/or basin-scale? If the coupling isregional, how does it depend on local proper-ties?

• What is the role of remote atmospheric andocean behavior in shaping the spatial and tem-poral characteristics of the variability?

• What is the “minimum physics” needed tocapture realistic coupled behavior? To whatdegree is this physics adequate to describe andpredict such phenomena?

• How do these simple mechanisms manifestthemselves in more complex and more real-istic models? Can particular coupled mecha-nisms be identified in these models? How dothey interact or compete with one another?

• What new observation and analysis methodsare required to test coupled variability foundin simple or complex models?

• Is the dipole-like SST variability better de-scribed as a self-sustained oscillator due tounstable air-sea interactions, or as a stabledynamical system driven by stochastic pro-cesses?

• What is the relationship between the Atlanticdipole, NAO and equatorial modes?

• How do these patterns of climate variabilityinteract with the seasonal cycle?

4.3 — Regional aspects(a) Subpolar gyre and Northern Marginal Seas

Important interactions occur between cold-fresh and warm-salty water masses in the Labra-dor and the Greenland-Iceland-Norwegian Seas,where winter-season convection forms interme-diate and deep water. Large, quasi-decadal andmulti-decadal SST anomalies, coherent with SLPanomalies, are observed in these regions. Strong

evidence exists that SST and surface salinity(SSS) anomalies propagate cyclonically aroundthe gyre and may be important in modulating theintensity of convection and the sea-saw betweenconvective activity in the Labrador and GIN Seas.The following questions need to be addressed:• What are the feedbacks and coupling mecha-

nisms that maintain these anomalies?• How is the formation of persistent sub-polar

SLP-SST anomalies linked to local and re-mote (e.g., Arctic, subtropics) anomalies insurface properties (such as ice, heat fluxes andprecipitation) and subsurface properties (suchas mixed layer depth and intermediate/ deepwater formation rates)?

• Do significant upwelling branches of theMOC exist which are local to, rather than re-mote from, the sinking branch, and how dothey effect SST and SSS?

• What is the role of the annual cycle in quasi-decadal variability?

• What is the role of sea ice?• How do the exchanges between the Arctic

Ocean and the North Atlantic affect NorthAtlantic climate variability?

(b) Subtropical/Subpolar Gyre interactionsProminent SST anomalies tend to form along

the North Atlantic Current at the boundary be-tween the subtropical and subpolar gyres. Theyare of particular interest because they have beenlinked to changes in climate over Western Eu-rope. As reviewed above, it has been suggestedthat quasi-decadal anomalies in this region arethe result of an inherently coupled phenomenon.Coupled model studies indicate that this regionis a sensitive indicator of, and may be a driver of,multi-decadal variability of the thermohaline cir-culation. Moreover, coupled models also suggestthat on decadal time scales, upper-ocean gyredynamics play an important role. In this scenario,the life cycle of SST anomalies is determined bythe interplay between the local response of theocean to variations in surface heat flux and theadvection of heat by the gyre circulation. The lat-ter counteracts the former, but lagged in time. Theatmosphere, reacting to the change in SST, is the

19

bridge connecting the local and non-local re-sponse of the ocean. In studying this mechanism,the following questions should be addressed:• What are the essential dynamics of the ocean

and atmosphere required to capture the inter-action? Are the mechanisms that have beenproposed robust? Can they be reproduced inmodels of various types?

• What is the role of the seasonal cycle in theinteraction?

• How is the interaction affected by variabilityin the other parts of the Atlantic, and how doesit affect variability elsewhere?

(c) Subtropical GyreIn the northeastern part of the subtropical

gyres much of the upper ocean thermocline ven-tilation takes place. Variability of the sea-surfaceproperties in those subduction zones can subductbeneath the surface layer. Now shielded from thedirect air-sea interaction they can propagatelargely undisturbed within the gyre circulationthroughout the subtropics/tropics unless exposedto surface entrainment zones in upwelling and thewestern boundary current regions. Specific issuesare:• Can subducted water mass anomalies survive

many years subsurface to finally reemerge andmodify sea surface conditions?

• How important are changes in the circulationrelative to changes in the surface fluxes in thegeneration of to subducted temperatureanomalies?

(d) Tropical/Subtropical Gyre InteractionDecadal TAV may be driven remotely from

the subtropics through variation of the shallowmeridional Subtropical Cells (STCs) that carrysubtropical thermocline water into the tropics.One hypothesized process involves zonal-windanomalies about the tropical-subtropical bound-ary latitudes: the winds that set the strength ofthe STCs. STC modulation eventually alters theamount of cold water that flows into the easternequatorial Atlantic. This in turn may modify theextent/intensity of the equatorial cold tongue, theair-sea heat flux and ultimately, the tropical at-

mospheric circulation. This process was likewiseidentified as a central element for the PacificBasin Extended Climate Study.

Two possible corollaries of are worth noting.First, because there is a net poleward transport ofupper-ocean water associated with the Atlantic’sthermohaline circulation, nearly all of the ther-mocline water that feeds the equatorial undercur-rent comes most directly from the southern hemi-sphere, a conclusion that is supported both byobservations and models. Thus, it is more likelythat variability of the southern STC leads to sur-face anomalies in the equatorial cold tongue. Sec-ond, because this process leads to a change in theequatorial upwelling, it initially causes an SSTanomaly pattern that is essentially symmetricabout the equator. It is not clear whether subse-quent ocean-atmosphere interactions can gener-ate asymmetric, dipole-like anomaly patterns andcause a shift in the position of the ITCZ. The mostimportant questions are:• Can subtropical thermocline water mass

anomalies indeed transit to the tropics andinfluence surface conditions in the equatorialupwelling zones?

• Do feedback mechanisms exist between theequatorial surface ocean, atmosphere andthe STCs to sustain decadal time scaleoscillations?

(e) Tropical OceanOcean-atmosphere interactions are gener-

ally expected to be important in the tropicsbecause 1) the atmosphere is sensitive tochanges in the SSTs and 2) the adjustment timescales of the tropical oceans are relatively short.The high correlation between the tropicalAtlantic SSTs and rainfall variability in theneighboring continents reflects the atmosphere-ocean coupling in the region. Modeling studiesfurther suggest that local air-sea interactions inthe tropical Atlantic can lead to a decadal TAVand an ENSO-like interannual mode. For thedecadal TAV mode, the dominant air-sea feed-back appears to be between interhemisphericSST gradient and wind-induced latent heat fluxin the tropical North and South Atlantic, al-

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though two other related feedback mechanismsmay also contribute to air-sea interactions in thetropical Atlantic:i) weakening of the trades in the warm SST

region, which decreases entrainment intothe surface mixed layer thereby causingeven warmer SST; and

ii) weakening of the trades along the NorthAfrican coast, which reduces coastal up-welling thereby warming SST in the north-eastern tropical Atlantic.

For the ENSO-like mode a dynamic interactionbetween the equatorial trades and SST (theBjerknes mechanism) is believed to be operat-ing to a certain extent. Although the prelimi-nary coupled model studies point to the poten-tial importance of regional air-sea feedbacks intropical Atlantic, many fundamental issuesremain unsolved. For example, pertinent ques-tions are:• To what extent do local air-sea feedbacks

contribute to the decadal hemispheric SSTgradient variability and ENSO-likeinterannual variability in the tropical Atlan-tic?

• How much tropical Atlantic SST variabilityis attributable to “remote” influences ofENSO in the Pacific and NAO in the NorthAtlantic?

• What are the oceanic and atmosphericprocesses responsible for the coupling inthe tropical Atlantic? In particular, what isthe relationship between the off-equatorialSST anomaly and surface heat flux anoma-lies?

The philosophy of the Atlantic Climate Vari-ability Experiment is to take a basin-wide per-spective in the study of coupled ocean-atmo-spheric variability in the Atlantic-sector oninterannual to decadal time scales. Our proposedresearch program has been jointly developed be-tween U.S. and European scientists. Its primarygoal is to understand the mechanisms responsiblefor Atlantic Climate Variability and quantitativelytest, and thereby improve, atmosphere-oceanmodels that describe them. We must determinewhether variability simulated in models is fun-damentally coupled, whether one componentdrives variability in the other, whether modeledvariability bears any relation to observed variabil-ity, and whether modes of variability once iden-tified are predictable. The improvement of atmo-spheric and oceanic circulation models, especiallyboundary layer formulations, is a central part ofACVE.

Our specific objectives are to:• Describe and model coupled atmosphere-

ocean interactions in the Atlantic Sector andquantify their influences on the regional andglobal climate system, and investigate theirpredictability.

• Understand and model the response of theatmosphere to air-sea exchanges in the Atlan-tic (in particular the anomalous exchangesassociated with observed tropical and mid-latitude SST anomalies) as a function of hori-zontal pattern, scale, amplitude and frequency.

• Provide an improved description and under-standing of the ocean’s response and poten-tial feedback onto mid-latitude, NAO-likeatmospheric forcing as a function of ampli-tude and frequency, and the associated time-varying fluxes of heat, fresh water, carbondioxide and other gases, momentum and en-ergy.

• Assemble a data set that may be used to testand improve models of coupled Atlantic cli-mate variability.

• Investigate the sensitivity of the meridionaloverturning circulation to changes in surfaceforcing and assess the likelihood of abruptclimate change.

5. A Proposed Research ProgramThe structure of this chapter is as follows:

5.1 Elements of the basin scale ACVE5.2 Analysis of Historical and Proxy Data5.3 Sustained Observations on the Basin Scale5.4 Observing the meridional overturning cir-

culation5.5 Modeling, Theory and Ocean State Estima-

tion5.6 Regional aspects of ACVE: Process Stud-

ies

21

Some of the processes involved in climaticmodulation of the North Atlantic will require abroad-scale study of the entire gyres or basins inthe context of the global atmosphere; other top-ics can be examined in focused process experi-ments. Some investigations must extend over adecade, but clever treatment of existing histori-cal observations can also provide an importantlong-time-scale perspective.

5.1 — Elements of the basin scale ACVEKey basin-scale elements of ACVE will be:

• analysis of historical and proxy data,• basin scale observational networks,• modeling, theory and state estimation (data

assimilation),• process studies embedded in the observa-

tional networks.

5.2 — Analysis of Historical and Proxy DataContinued analysis of historical instrumental

and proxy data records is an essential component.Only through such study can the spatial and tem-poral properties of recurring interannual todecadal climate variability be characterized. Inhand with modeling approaches, data analysis willbe essential to evaluate emerging hypotheses con-cerning the mechanisms of Atlantic climate vari-ability. Data analysis must also play a key role inoptimizing the design and assessing the expectederrors of future observing networks.

Proxy records of Atlantic climate variabilityare provided by tree ring and ice core stable iso-tope chronologies and marine and lake sedimentrecords. The use of such records should continueto be actively pursued. Such data provide the onlywindow on climate fluctuations before the instru-mental period. They furthermore are particularlyattractive for studying continental climate and itsrelation to oceanic conditions. Interpretation ofproxy climate records is an area where models,suitably equipped with tracer or other specializedcapabilities, will be of substantial utility. For ex-ample, atmospheric models with isotopic water-vapor/tracer capabilities can be used to syntheti-cally generate ice core records, potentially lead-ing to a better understanding of the relationship

between the observed ice-core records and cli-mate variability.

Addressing the key scientific issues discussedin Section 4 requires an interdisciplinary researcheffort. Unfortunately such efforts are too oftenlimited or hindered by the availability of, or easyaccess to, historical data. Investigators are fre-quently unaware of existing data sets, handi-capped by data access difficulties, or unaware ofdocumented problems with certain observations.New, updated or improved data sets are also dif-ficult to identify and locate. We suggest that amajor step forward would be to establish an At-lantic Climate Variability Data Center to improvedata accessibility and documentation, and foster“data archeology”.

The primary purpose of the data center willbe to catalog and provide easy access to basicobserved historical data sets for interdisciplinarystudies of Atlantic climate variability. The datacenter could be as simple as a world wide webpage, pointing toward existing, publicaly avail-able data, but maintained by a person or group ofpeople with a basic understanding of the goals ofthe ACVE. To enhance productivity, the data cen-ter should utilize the knowledge of “expert us-ers’’, soliciting advice on the advantages and dis-advantages of individual data sets from the re-searchers who process and/or use them. Such in-formation could be included as meta data, orga-nized by data set name and linked to the data setsthemselves.Recommendations:• Creation of an Atlantic Climate Variability

Data Center to improve data accessibility anddocumentation, and foster “data archeology”

• Close collaboration with activities of thepaleoclimate community organized underPAGES

5.3 — Sustained Observations on the BasinScale

Two categories of measurements are neededto form the observational basis of ACVE: (i) sus-tained, Atlantic-wide observing systems in theatmosphere and ocean and (ii) shorter-term pilotnetwork studies and process experiments. The

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design of the pilot and sustained observing sys-tems must be guided by examinations of the his-torical record, in combination with numericalmodel studies.

The goal of the sustained observing systemsis to provide comprehensive descriptions of theoceanic and atmospheric circulations believed tobe associated with Atlantic Climate Variability.This includes in particular the upper-ocean mixedlayer, the atmospheric boundary layer, and thecoupling between them. In the off-equatorial re-gions where SST exhibits prominent decadalvariation, it is important to measure the time-vary-ing air-sea heat flux, and establish the atmosphericand oceanic processes that determine it. It is alsoimportant to measure subsurface thermal struc-tures, since ocean circulation and SST anomaliesare typically associated with large changes in ther-mocline depth. In the following we outline ex-plicitly which measurements will be the centralelements for testing hypotheses of coupled cli-mate variability in the Atlantic sector.

A. Atmospheric Observing NetworksOur general knowledge of seasonal-to-

decadal atmospheric variability in the Atlanticregion will be based primarily on analyses fromthe operational weather-prediction centers and onre-analyses of historical data. The backbone ofsuch work is the World Weather Watch (WWW/GCOS) upper-air sounding network on the pe-riphery of the Atlantic and those few island sta-tions. Surface and upper-air observations fromships and commercial airliners provide additionaldata over the open ocean, largely concentratedalong shipping lanes and air corridors.

Although since the 1970s satellite observa-tions provide a significant constraint on the analy-ses, the Atlantic sector remains a data-void area.This is demonstrated in Figure 5.1 which showsthe WMO’s global upper-air network that is be-ing proposed under the Global Climate Observ-ing Systems (GCOS). Figure 5.2 shows the ini-tial GCOS Surface Network (GSN). Regrettably,the WWW upper-air network has deteriorated

Figure 5.1 — The initial GCOS Upper-Air Network (GUAN) (courtesy of GCOS Joint Planning Office, 1998).

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since the 1960s in terms of the number of sta-tions and the frequency of observations. The At-lantic basin data-void areas will seriously impactACVE process studies in which detailed knowl-edge of the atmosphere’s vertical structure is re-quired. The lack of in situ soundings over data-sparse regions also hinders efforts at the opera-tional weather analysis and climate reanalysiscenters to identify and remove regional biases intheir data assimilation and analysis systems.

Attempts to calibrate satellite instruments inthe field and to develop improved remote sens-ing retrieval algorithms benefit fundamentallyfrom new high-quality in situ soundings. Satel-lite retrievals of the vertical temperature and hu-midity structure are unable to resolve the detailsof boundary layer structure, and the weightingfunctions of the radiative flux channels measuredby the instruments place fundamental limits onthe maximum resolution that can be expected.Wind information from space is obtained at onlya few levels in the vertical.

Efforts to increase atmospheric upper-air sam-pling in remote marine environments at a reason-

able cost are underway. Atmospheric profilers andcontainerized radiosonde launchers are maturefield instruments that can be operated from re-search and commercial ships of opportunity. Moresystematic measurements may become possiblewith autonomous aircraft and robotic balloon plat-forms, which under development and will prob-ably become operational during ACVE.

It is important to realize that there is increas-ing evidence that atmospheric radiative transferequations transmit too much energy throughclouds. In the TOGA COARE region for example,models show as much as a 30 W/m2 bias in cloudyconditions compared to observations. Globally,the average bias in insolation may be about 10percent, which, if not corrected, has serious con-sequences for ocean climate models. Technicaldevelopments are underway to acquire direct fluxmeasurements autonomously from volunteer ob-serving ships (VOS). In combination with theprofiling instruments, these measurements willprovide important new information crucial forenhancing our obviously limited understandingabout the atmospheric radiation transfer and as-

Figure 5.2 — The initial GCOS Surface Network (GSN) (courtesy of GCOS Joint Planning Office, 1998).

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sociated differential heating.In addition to VOS, well-instrumented sur-

face moorings are recommended at select sites.These will return accurate reference data to verifyand/or calibrate surface meteorological fieldsfrom models, remote sensing platforms and otherin-situ measurements. Work is in progress to up-grade the quality of the surface meteorology andair-sea fluxes on a global basis from the VOS fleetis dependent on such fixed reference sites. Thereference sites will serve as primary standards forVOS data quality control and help define regionalchoices of air-sea flux formulae. The present drift-ing buoys provide a good example of how in situobservations are used to calibrate satellite data;in their case they provide the means to calibratethe AVHRR fields of SST. The reference site sur-face moorings will additionally provide surfacewind, humidity, surface short wave and long waveradiation, and rainfall estimates. Possible sitesinclude the mid-subpolar gyre near historicalOcean Weather Station “Juliet’’ or “Lima’’ andthe area east of the Grand Banks, the New En-gland Bight region that experiences polar air out-breaks, the center of the Azores-Bermuda highand the North Atlantic trade wind region to itssouth, the Gulf of Lyons in the Mediterranean Sea,and the area southwest of Cape Town in the east-ern South Atlantic.Recommendations

The primary source of atmospheric observa-tions for ACVE will be the GCOS, which is builton the observing system that supports operationalnumerical weather prediction. This implementa-tion plan requires:• maintenance of the critically important

ground and space-based components ofGCOS,

• the existing observational network to beoptimized and modernized, in particularenhancing the technologies to recoveratmospheric profile and boundary layer datain remote ocean regions,

• continuing improvements in the perfor-mance of 4-dimensional variational dataassimilation schemes that operate in near-real-time,

• availability of upgraded reanalysis productsfrom ECMWF and NCEP, and

• the free and open exchange of observationaldata sets.None of the above can necessarily be taken

for granted but require significant attention toassure their continuation. In addition, special at-mospheric observations (localized in space andtime) are required to address specific issues re-lated to improvements of the parameterizationsof essential boundary layer processes in coupledatmosphere - ocean - land models or forintercomparison of measurements made with dif-ferent kinds of instrumentation.

B. Ocean Observing NetworksPermanent upper-ocean networks are needed

to document low-latitude anomalies and the gen-eration and subsequent evolution of higher-lati-tude features such as propagating SST and SSSanomalies. Some of the required information canbe obtained by high-quality remote sensing datasets:• Sea surface temperature — SST is one of the

fundamental climate parameter which can beobserved from space. ACVE will stronglybenefit from the operational SST network.

• Sea surface elevation — Altimetry has shownto provide crucial information about changesin the geostrophic currents and heat contentas a function of space and time. Continuousaltimeter coverage is mandatory in the con-text of ACVE, since SSH data uniquely pro-vide global information about variability inthe ocean flow-field near the surface, butstrongly reflecting interior dynamics. They arecrucial to interpolate in time and space manyof the interior measurements which are nec-essarily sparse.

• Surface winds — Scatterometer data are notroutinely available. However, when they wereavailable they have greatly improved the sur-face wind stress fields and their variability.

• Sea ice — Sea ice concentration is one of thefew global measurements that we have whichdirectly relates to upper ocean fresh water.SSMI data also allow to estimate sea ice drift

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velocities.In situ networks capable to capture climate

features and their development in time will con-sist of the following elements, some of which areoperational at the time of writing or can be an-ticipated to be so in the near future:• Tropical moored array — Continuation of the

present PIRATA moored array in the tropicalAtlantic with an additional five stations is re-quired to better cover anomaly patterns in thesoutheastern and northeastern upwelling re-gimes as well as the northern tropics. West-ern boundary observations are also needed tomeasure the equatorward flow of thermocline

water. At present, PIRATA is only structuredas a pilot project and operated jointly betweenBrazil, France and the United States withfunding ending after 2000. It is essential toaugment the PIRATA array with five extrabuoys (Figure 5.3) to measure surface fluxesand the upper-ocean thermal structure. A keyarea for improved flux estimates is under theITCZ where winds are weak.

• PALACE float network — A proposal hasbeen made to deploy a global network ofPALACE floats, and we embrace this conceptfor the Atlantic. A deployment of many hun-dreds floats, covering both the north and south

Figure 5.3 — A strawman ACVE mooring network. Shown are the present PIRATA surface mooring array in thetropics and its possible enhancement, potential buoy reference stations for improving air-sea flux estimates, currentand proposed hydrographic time-series stations to be instrumented with autonomous systems, and sites wheretransport monitoring arrays might be located.

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Atlantic, enables the evolution of T, S field tobe mapped autonomously (Figure 5.4). In thetropics, the array will observe the variabilityassociated with the southern and northern el-ements of the tropical ‘dipole,’ as well as theinterior structure of the subsurface,equatorward-flowing branch of both thenorthern and southern STCs. Floats shouldalso populate the northward-flowing branchof the MOC and the paths of the propagatingmid-latitude SST anomalies. Salinity obser-vations are particularly important in the sub-polar North Atlantic, so the floats should have

salinity sensors whenever possible. The prob-lem of floats vacating some areas and clus-tering in others could be overcome in the fu-ture by new types of glider floats that can ac-tively navigate to maintain position or carryout local surveys.

• VOS-XBT lines1) Ocean measurements — To complement

the float array, particularly in regions char-acterized by strong currents and smallspatial scales, continuation of the volun-teer observing ship XBT program at theWOCE intensity is recommended (Figure

Figure 5.4 — A random realization of a 700 profiling float network. Design studies are needed to determine thenumber and regional distribution of an efficient float network for ACVE.

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5.5). Design studies are needed to estab-lish where such high-density sampling isrequired. In the tropics, the TOGA linesshould be maintained as a complement tothe PIRATA and PALACE arrays until anyredundancy is established. A proposednew line along 60°N (see below) shouldcontribute valuable information on sub-polar exchanges with the Nordic Seas.Some technical issues must also be ad-dressed. In the subpolar North Atlantic,the depth-range of T7 XBT probes is notalways sufficient to reach the mixed-layer

base. Salinity observations are particularlyimportant in the subpolar North Atlantic,and as reference data for floats and remotesensing instruments globally; the use ofthermosalinographs on volunteer shipsshould have high priority.

2) VOS meteorological measurements —The North Atlantic is particularly wellsampled by VOS meteorological obser-vations. However, an improved accuracyof the data is required. For example thepresent VOS data may underestimate theheat fluxes during winter time cold air

Figure 5.5 — The present XBT/VOS network developed under WOCE, TOGA and ACCP/ACCE. High-densitylines are marked with bold lines, low-density with thin. Design studies are recommended for determining theoptimal mix of high- and low-density XBT lines within ACVE in light of the proposed profiling float array, remotesensing tools, and the envisaged network of Eulerian stations.

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outbreaks over the western North Atlan-tic. Improved instrumentation (which isbeing developed both in the U.S. and Eu-rope) should be implemented on a subsetof the Atlantic VOS.

• Time series stations — Continuous recordsat a relatively small set of stations have provento be very effective in documenting annual-to-inter-decadal variability in the ocean. It isvitally important to continue these importantstations. While originally conducted usingresearch vessels, time series stations in futurewill be autonomous utilizing moored or free-floating profiling vehicles.- It is essential to continue stations “Bravo”

and Panulirus/Bermuda in the centers ofthe subpolar and subtropical cells, respec-tively, which already demonstrated sub-stantial value in providing information ondecadal heat storage and circulationanomalies.

- Station “Mike” in the Norwegian Sea hasgiven insight into decadal warming of theNorwegian Sea Deep Water and propa-gation of the Great Salinity anomaly. Thisstation, which monitors the properties ofthe northward flowing Atlantic waters thatultimately feed the deep water formationsites, must be continued.

- The ESTOC station was begun in 1994north of the Canary Islands in the easternsubtropical Atlantic. Together with Ber-muda, hydrographic data from this stationyields a baroclinic circulation index acrossthe subtropical gyre. Its continuation isencouraged.

- Companion station(s) on the western mar-gin of the basin would return barocliniccirculation indices for both the GulfStream and the net MOC, and wouldmonitor the meridional spreading of wa-ter mass anomalies originating in the high-latitudes.

- Similarly, two bounding stations at thelatitude of the tropical- subtropical ex-changes (i.e., near 10-15°N) could pro-vide integral measurements of the meridi-

onal baroclinic transport variability there.One station near the eastern boundarymight be part of the PIRATA array; itswestern counterpart should be locatedclose to the boundary (e.g., near the LesserAntilles).

- Other stations are under consideration forobserving the temporal evolution of heatcontent and water mass properties in as-sociation with the PALACE array, and/orproviding baroclinic transport time seriesfor key advection elements in the subpo-lar and subtropical domains.

- Expansion of the time series network intothe South Atlantic should be considered.

• Flux monitoring systems — At key sites, di-rect measurement of ocean currents is envis-aged. For example, long term Eulerian mea-surements of the dense overflows of NordicSea waters through the Denmark Straits andFaeroe Bank Channel will help quantify thechanging climate system of the northern At-lantic. Similarly, measurements in the Straitof Gibraltar would provide integral measuresof air-sea exchange with the Mediterraneanand the supply of saline intermediate waterto the Atlantic. Monitoring of the FloridaCurrent heat and volume transport would fa-cilitate study of the meridional overturningcirculation’s warm limb at subtropical lati-tude; a companion array spanning the deepwestern boundary current could track the coldlimb of the circulation. Developments nowunderway for long-term (5-year), low-costcurrent meter moorings with data telemetrywill make such programs economically fea-sible.

• Drifters Surface — Drifters have proven tobe a valuable resource for SST ground truthto satellite-based observing systems as notedabove. Drifter data have additionally yieldedinformation on upper-ocean circulation pat-terns and Ekman transports. Sensor suites ondrifters may be enhanced to measure winds,atmospheric pressure and near-surface salin-ity. On interannual time scales, the horizon-tal transport of heat in the upper ocean be-

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comes an important process. At the presenttime, there are no systematic, basin-scalemeasurements of upper-ocean circulation inthe tropical Atlantic (unlike the Pacific andIndian Oceans) leaving this area void of anydata set by which to verify modeled oceancirculations. The meridional component ofageostrophic flows (Ekman flow) is signifi-cant in the tropical region, showing speedsthere that can be twice as large as the geo-strophic currents. Moreover, the sparse drifterdata which exists (Richardson et al., 1988),indicate a net poleward transport, whereas thesurface dynamic topography indicates a netequatorward mass flux. It is therefore recom-mended that a basin-scale array of drifters bedeployed and maintained for at least one cycleof the TAV on a resolution of 2° in latitudeand 6° in longitude and within 18° of the equa-tor.

• Tide-gauges— Although the Atlantic does notallow for an extensive tide-gauge network, asdoes the Pacific, a few stations located in theeastern, equatorial region are useful for moni-toring the Atlantic El Niño-like phenomenon.Stations in the Cape Verde and Ascension Is-lands are near the centers of action of decadaldipole variability, and thus are useful indica-tors of net steric (storage) processes associ-ated with SST changes. The CPACC stationsalong the western boundary and in the AntillesIslands are useful for monitoring changes inadvection/export from the tropics via west-ern-boundary currents on interannual timescales.

• Acoustic Tomography — The combination ofaltimetry and long range acoustic tomogra-phy has been shown to be an effective meansof tracking basin-scale changes in ocean heatcontent (ATOC Consortium, 1998). Tomog-raphy can average over the noisy mesoscaleeddy field, and thus greatly enhance the sig-nal to noise ratio of climate signals. ACVEneeds to explore how a few acoustic sourcescan be used to maximize the amount of newinformation.

Recommendations:Design studies will help to determine the most

efficient mix of the above mentioned networksfor sustained Atlantic climate observations. Thenetworks will build on existing and planned glo-bal remotely-sensed data sets, those time-seriesstations remaining in operation, existing VOS-XBT, drifter and float programs, and the capa-bilities offered by newly developed floats, moor-ings and VOS technologies.

5.4 — Observing the meridional overturningcirculation (MOC)

Coarse-resolution climate models have sug-gested scenarios for global thermohaline circula-tion collapse. While these model oceans are of-ten unrealistic, the results deserve scientific at-tention. It is important to determine how realisticmodel simulations of large thermohaline circula-tions variations are at short (decadal) time scalesunder possible anthropogenic changes. For com-parison with model simulations, essential param-eters include observations of the meridional over-turning circulation rate and the associated heatand fresh water transports. Coverage at a particu-lar latitude might involve elements of sustainedobservations (see above) in combination with:• Occasional repeated high-resolution transoce-

anic hydrographic ship-based sections includ-ing tracers and current measurements.

• Moored current-meter measurements at thewestern boundary and additional stations formonitoring integrated baroclinic transports inthe interior.

• Moored bottom devices to interpolate be-tween full-depth stations, such as barotropicflow meters and inverted echosounders.A feasibility study for monitoring the MOC

needs to be carried out, analyzing variability pat-terns in existing hydrographic sections and theoutput of high-resolution numerical models. A48°N section (the division line between the sub-polar and subtropical gyres) would be a prioritycandidate for a MOC monitoring section because(i) anomaly patterns suggest a dipole of North-South heat storage anomaly across this latitude,(ii) a number of WOCE sections have already

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been obtained and can serve for pattern analysis,and (iii) intent has been expressed by BSH (Ger-many) to re-occupy this section at WOCE qual-ity as a contribution to GOOS every three years.Other potential sections for repeat top-to-bottomhydrographic coverage, possibly combined withmoored boundary arrays, are shown in Figure 5.6and should include:• Near 25°N, close to the heat transport maxi-

mum, where considerable prior knowledge onthe variability has been built up.

• 10°S, to measure the combined effects of theshallow tropical cell (STC) and the top-to-bottom MOC

• 30°S, to measure the net heat and freshwaterexchange of the Atlantic with the global cir-culation system .

• Two meridional sections to quantify thechanges in water mass inventories of the west-ern and eastern Atlantic basins. Samplingalong 52°W longitude, which would repeatmeasurements that extend back as far are1950s, is strongly suggested. The optimaltrack line in the east has yet to be identified.In conjunction with the MOC section mea-

surements, inventory observations of water masschanges and anthropogenic CO2 uptake need tobe carried out in appropriate time intervals (to bedetermined from the WOCE/AIMS analysis).Recommendations:

Quantitative design studies for an MOC moni-toring program including measurement of theassociated meridional property transports andstorage changes are required. Such studies needsto determine the sampling frequencies of the zonallines required to quantify variability in the At-lantic MOC and associated meridional transports.

5.5 — Modeling, Theory and Ocean State Es-timation

Theory and models will play a central role inthe design, implementation and synthesis of anACVE. They will facilitate refinement of work-ing hypothesis, which can then be tested usingobservations. The complexity of the models willvary from idealized studies of particular physicalmechanisms to fully coupled ocean- atmospheric

models. Despite obvious shortcomings, oceanicgeneral circulation models have achieved a suf-ficient degree of realism that they can now beemployed to address process oriented or experi-ment design questions, and to synthesize diverseobservations into ‘best-estimates’ of the quanti-ties of interest.

An active program in data-model synthesismust be a component of ACVE to both help planit and to interpret the observations. The combi-nation of model and data fields, presuming thatboth contain elements of the same variability, al-lows for a better definition of what took place inthe Atlantic Ocean than either by itself. If pre-dictability can been established, these analyseswill serve as the initial conditions for forecastsof the coupled system. A number of assimilationefforts are underway as part of WOCE AIMS andare anticipated as part of GODAE. Although tech-nology and computer power will certainly im-prove over the next decade, it is fair to state thatthe basic “know-how” has been developed and isnow available for ACVE. Additionally, the basicinfrastructure which is being proposed byGODAE, including active management of dataand forcing fields, will be of benefit to the ACVEresearch activities.

Numerical sampling studies are needed tohelp determine effective and efficient observa-tional networks. High-resolution forward mod-els can be used to investigate sampling strategies,and the rigorous framework of data assimilationcan lead to objective criteria for efficiently as-sessing sampling alternatives.

In the following, we provide some specificrecommendations for modeling studies that arehighly relevant to Atlantic Climate variability:

Mechanisms responsible for atmospheric vari-ability in the Atlantic sector.

Many atmospheric GCMs still have majorproblems simulating synoptic and lower fre-quency variability in the North Atlantic region.Rather than curving northeastward, simulatedstorm tracks are often quite zonal. The occurrenceof Atlantic blocking events is often underesti-mated. Lower-frequency variability such as that

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of the stationary waves is often poorly repre-sented. The difficulty experienced in trying toremedy these model problems reflects poor un-derstanding of the processes involved. The biasesin atmospheric GCMs are particularly evident inthe Atlantic sector, and could lead to poor repre-sentation of interannual and interdecadal variabil-ity in such models and of the fluxes used to forceocean components in coupled models. Theseuncertainties need to be reduced before forecasts

using GCMs seasonal, decadal and anthropo-genic-induced variability can be credible.Recommendations:• Studies using a hierarchy of atmosphere

models to investigate the mechanismsresponsible for atmospheric variability ontime scales from days to decades, and theprocesses that determine the spatial coher-ence of this variability

• Studies to investigate the mechanisms

Figure 5.6 — A schematic drawing of a potential ACVE repeat-section program. Hydrographic and tracer proper-ties would be sampled on these lines on an interannual time scale to quantify net changes in water mass propertiesand heat content. Thick lines should be repeated more often.

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behind atmospheric teleconnections intoand out of the Atlantic sector. For example,how can a diabatic heating anomaly in thetropics lead to dynamical changes in theAtlantic sector? Do changes in the Atlanticsubtropical jet lead to changes in the Atlan-tic inter-tropical convergence zone and viceversa ?

Influence of the Atlantic Ocean on the atmosphereLong-range forecasting relies heavily on the

influence that SST anomalies exert on the atmo-sphere. Yet the nature of this control, especiallyoutside the tropics, is very poorly understood.Experiments in which extratropical SST anoma-lies are imposed in GCMs generally indicate weakresponse by comparison with the internal vari-ability. But these results could be misleading. Itappears that the response is unlikely to be througha simple locally enhanced heating, as is often thecase in the tropics. Rather, anomalies may im-pact the development of individual weather sys-tems which tend to grow over the western NorthAtlantic and decay over the eastern Atlantic/West-ern Europe. It is unknown to which space andtime scales the atmosphere respond most strongly,nor the extent to which the structure of the GulfStream is important to the atmosphere. Thesequestions are central to possible interactive modesof the atmosphere-ocean system and to coupledmodeling of the North Atlantic in general. An is-sue of particular importance is whether or not therecent trend in the NAO index arose in responseto changes in SST.Recommendations:

To address these issues we need:• Studies that investigate the sensitivity of

individual weather systems to SST, initiallyusing high resolution forecasting modelsand cases for which much additional obser-vational data exist (e.g., FASTEX IOPs).The structure of the boundary layer, themagnitude of the surface fluxes, and thedevelopment of the whole system should becompared with observations, and the sensi-tivity to parameterizations, to the specifiedSST, and to an oceanic mixed layer need to

be determined. Additionally the sensitivityof atmospheric models to SST anomalies asa function of spatial resolution needs to beinvestigated.

• Studies that determine the scales and pat-terns of Atlantic SST to which the atmo-sphere is most sensitive, and the atmo-spheric predictability that derives fromchanging SST. Ensemble integrations ofatmospheric models forced with bothobserved and idealized SST anomalies areneeded. Particular attention should befocused on: a) whether the observedmultidecadal fluctuation in the NAO indexcan be reliably reproduced and if so, whichregions of SST are important; b) The influ-ence of tropical and subtropical AtlanticSST anomalies on the tropical and extratro-pical atmosphere. Carefully controlledstudies in which several different modelsare forced with the same SST anomalieswould be useful to check the robustness ofresults and increase ensemble sizes.

• Studies to investigate the usefulness andlimitations of atmosphere model experi-ments with prescribed SST anomalies.Under what circumstances are the results ofsuch experiments misleading? To whatextent can the results be related to thevariability that arises when the same modelis coupled to an ocean mixed layer or to adynamical ocean model? What role doatmosphere-land surface interactions play?Further development of data sets, for example

reanalysis such as NCEP and ECMWF, to im-prove characterization of the observed atmo-sphere and ocean variability.

The mechanisms responsible for variability in theAtlantic Ocean

Recent work suggests that a large fraction ofNorth Atlantic Ocean variability can be under-stood as the ocean’s response to atmospheric fluc-tuations. These fluctuations, which are dominatedby large scale patterns such as the North AtlanticOscillation, influence the wind and buoyancydriven ocean circulations as well as the convec-

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tion and subduction processes that determine theproperties of the ocean thermocline. We are, how-ever, far from understanding the detailed relation-ships between the surface fluxes of heat, freshwater and momentum and temporal variation ofthe ocean state.

Of particular importance is an improved un-derstanding of the oceanic pathways throughwhich fluctuations in surface exchange in oneregion can influence the ocean, and especiallySST, in another region at a later time. For ex-ample, fluctuations in wind stress curl and fluxanomalies over the subtropical gyre may promotethe development of heat content anomalies thatpropagate northward to affect deep convection inthe Greenland or Labrador Seas. Changes in deepconvection may in turn impact the overturningcirculation, including its contribution to GulfStream transport. Fluctuations in wind or buoy-ancy fluxes in the subtropics may also influenceequatorial SST via meridional circulation cellsthat feed equatorial upwelling.Recommendations:• Model studies to investigate the local and

non-local relationships between temporalvariations in the surface fluxes and tempo-ral variations in the ocean state. In thedesign of models and experiments specialattention should be paid to the following:- Key processes in the ‘warm water

transformation’ pathway including themechanisms responsible for the devel-opment, propagation, and decay of heatcontent anomalies, the interplay ofprocesses that influence oceanic con-vection, the processes that modulate theoverturning circulation, and the impactMOC variations have on SST.

- The mechanisms that determine SSTvariability in the tropical Atlantic,particularly the role played by dynami-cal ocean processes. Also the mecha-nisms linking the tropical Atlantic withhigher latitudes including the modula-tion of the STC’s by subtropical windsor buoyancy fluxes and the influence ofthe tropical Atlantic variability on

higher latitudes.• Studies with ocean GCMs are needed to

investigate how well the observed evolutionof the Atlantic Ocean can be simulatedgiven observed surface fluxes or relatedquantities. (Careful thought should be givento the formulation of the surface boundarycondition). Multidecade-long model simula-tions should be rigorously evaluated againstavailable surface and subsurface observa-tions, with particular attention to poorlyunderstood phenomena such as the develop-ment, propagation and decay of heat contentanomalies, and to basic features such as thedistribution of principle water masses. Acomparison between different GCMs wouldhelp to differentiate between the effect ofsurface flux errors and model errors in thesolutions. Model experiments could bedesigned to address which features in thesurface forcing are essential to reproducekey features of the observed variability.Specific points include:- Investigating the nature and the role in

the large-scale circulation of key physi-cal processes that are poorly representedin ocean GCMs, in particular the role oftopography (especially “choke points”)and of turbulent mixing.

- Development of ocean data assimilationsystems to provide best estimates of theseasonal-to-decadal variability in theAtlantic Ocean (in close collaborationwith GODAE).

Variability and Predictability of the AtlanticOcean-Atmosphere System

Coupled ocean-atmosphere GCMs are pow-erful tools for understanding the climate system.However, due to biases in both the atmosphericand the oceanic components, such models oftenhave unrealistic mean states, annual cycles, andinterannual variability. Much work remains todocument, understand and alleviate such prob-lems.

Coupled GCMs are the central tools for sea-sonal and longer time scale forecasting. There is

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evidence of seasonal predictability over Europein spring and summer but the mechanisms thatgive rise to this predictability are not understood.High priority must be given to research directedat improving the realism of coupled forecast mod-els, improving understanding of the basis for pre-dictability, and improving forecasting techniques.Recommendations:

ACVE will need:• Studies that elucidate the mechanisms

responsible for variability in the AtlanticSector in coupled GCMs, and evaluate therealism with which observed seasonal todecadal variability is simulated. Insight intomechanisms may be gained through experi-ments in which ocean-atmosphere interac-tions are restricted to certain regions (suchas the Atlantic).

• Studies of ENSO influence on the AtlanticSector. What SST anomalies patterns aredirectly forced by ENSO force in theAtlantic? Do these anomalies in turn influ-ence the atmosphere? How predictable isthe Atlantic’s response to ENSO?

• Studies to investigate the mechanismsresponsible for variability in the tropicalAtlantic, including the ‘Atlantic ENSO’.

• Research into the use of coupled models forforecasting climate fluctuations in theAtlantic Sector on seasonal and decadaltime scales, including the problems ofinitializing and assimilating data intocoupled models, and the sensitivity offorecasts to model formulation.

• Studies to investigate how changing levelsof greenhouse gases may influence climatevariability in the Atlantic Sector, and hownatural variability influences the responseof climate system to such changes.

5.6 — Regional aspects of ACVE: ProcessStudies

Regional aspects of ACVE will encompassobservationally-based process studies and focusedmodeling activities. Both will serve to enhanceinsight into mechanisms which, although local,effect the entire Atlantic atmospheric or ocean

circulation. In the following we highlight someregional processes that might deserve special at-tention and a targeted effort. Such process experi-ments are anticipated to be of shorter duration(one to five years) and embedded in the sustainedobserving networks. Focused pilot studies mayalso be needed to help design the sustained ob-servational networks.

Tropical AtlanticAGCM experiments, in which the evolving

pattern of tropical Atlantic SST is prescribed, willbe instrumental in examining the underlying dy-namics that govern the atmospheric response.Questions to be addressed include: What are theatmospheric responses to dipole-like SST anoma-lies? Are the anomalous circulations due to aboundary-layer response to SST or to a whole-scale rearrangement of the circulation in the tropi-cal Atlantic troposphere? How does the anoma-lous circulation pattern affect the surface heatflux? What is the role of land heat sources in driv-ing the anomalies?

Several oceanic processes are believed to playroles in tropical Atlantic variability such as oce-anic advection including cross-gyre/cross-equa-torial exchanges and vertical mixing/subductionprocesses. However, the relative importance ofthese processes in decadal TAV is unknown. Ex-periments using models with varying degrees ofdynamical complexity must be carried out to de-termine which oceanic processes regulate off-equatorial SST variability. For example, oceanmodels forced only with winds can be comparedto those forced only with surface buoyancy fluxes.Emphasis should be placed on the following is-sues: How can subsurface thermal anomalies in-fluence SST changes? What is the relative im-portance of horizontal heat transport and verticalmixing/subduction processes? How important arecross-equatorial and cross-gyre exchanges? Whatis the relative importance between western-boundary-current transports and interior Ekmantransport? Do oceanic Rossby waves play an im-portant role in determine SST fluctuations in theoff-equatorial SST variability?

It has been demonstrated that tropical ocean-

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atmosphere interactions can lead to decadal TAVin relatively simple coupled models (e.g., Changet al., 1997). In these models, the atmosphere isassumed to be largely determined by SST in sucha manner that the tropical atmospheric circula-tion and associated surface heat flux adjust in-stantaneously to changes in the SST. This “slaved’atmosphere may exaggerate the atmospheric feed-back to SST changes. Additionally, there are largeuncertainties in surface heat flux estimates. Suchmodels should be regarded as being near the bot-tom of a hierarchy of coupled systems, rangingfrom intermediate coupled models to fullycoupled GCMs, that are needed in order to un-derstand air-sea coupled processes in the tropicalAtlantic. Specifically, dynamically based atmo-spheric models must be coupled to a variety ofocean models, ranging from mixed-layer modelsto oceanic GCMs, in order to diagnose the im-portant feedback loops in TAV and to identify keydelays in these loops. The coupled model experi-ments will be crucial for addressing importantquestions such as: Is the dipole-like SST variabil-ity a truly coupled phenomenon? If so, where arethe regions in which the key air-sea feedbackstake place? What are the fundamental oceanic andatmospheric processes that control these feed-backs? Much more modeling work is needed tofill in this hierarchy. It is not likely that a processstudy, focused on critical issues, can be designeduntil this modeling activity is well underway. Fi-nally, it is important to emphasize that coupledmodels will be instrumental in evaluating the pre-dictability of the climate phenomenon. Existingobservations already point to a tight couplingbetween rainfall variability in Northeast Braziland Sub-Saharan Africa, as well as Caribbean andtropical-Atlantic SST variability. Coupled modelexperiments must be conducted to examine thepredictability of tropical Atlantic SSTs, andwhether seasonal-to-interannual SST forecastscan improve seasonal rainfall forecasts.

Subtropics and tropical-subtropical connectionsWe propose to study tropical-subtropical in-

teractions using model-data combinations. Oceanmodels will be used to study the effect of

midlatitude variability on equatorial thermoclinestructure and SST. Experiments with forcing con-fined to different hemispheres will also tell uswhich hemisphere is more important. Detailedanalyses of models constrained by observationswill indicate which oceanic processes (e.g., sub-duction, wave propagation and/or changes instrength of shallow meridional cells) are likely tocontribute to equatorial SST variability. Result-ant SST anomaly patterns from such experimentswill then be fed into an atmospheric model toidentify possible feedbacks. For the atmosphere-model experiments, one could specify SST in thetropical Atlantic to examine the resulting extra-tropical variability. Particular attention should bepaid to the relationships between the Hadley celland storm-track variability in midlatitudes. Re-verse experiments can also be carried out by forc-ing the atmosphere in midlatitudes to examinepossible NAO connections to tropical and south-ern-hemisphere variability. Coupled model ex-periments can also be carried to determine therelation between NAO and TAV. For example, onecan drive a coupled model with NAO-like forc-ing to see how much NAO influence there is onTAV.

Subpolar Atlantic and Subtropical-subpolar con-nections

Among the most pressing problems to be ad-dressed during ACVE is the complex of questionsassociated with the atmospheric response tomidlatitude SST/heat flux anomalies, with theGulf Stream position, and with propagatingSSTAs. Although many of those questions willbe addressed in a modeling framework, someenhancement of measurements in the subpolarAtlantic is required. The unknown consequencesof heat content anomalies along the boundarybetween the subtropical and subpolar gyre mightjustify a targeted process experiment:An anomaly process experiment

After identification by the observing system,developing upper ocean heat content anomalieswill be sampled intensively to determine theirevolution and propagation characteristics.Anomalies in the upper layers of the ocean, cir-

36

culating in both the subtropical and subpolargyres, should be sampled intensively over sev-eral annual cycles, to determine the manner inwhich their associated SST and thermal anoma-lies are modified by air-sea interaction, and thecycle of mixing and restratification. Fluid pass-ing through the Gulf Stream system is exposedto the influence of strong NAO forcing at the startof the Atlantic storm track. Its properties, changedby convective mixing, then flood the subpolar andsubtropical gyres. We need to determine the life-time and characteristic pathways of these anoma-lies, and their potential influence on the atmo-sphere above.

To carry this out, the upper ocean observingnetwork would be augmented with additionalfloats and surveys in key areas (e.g., the regionof18 degree water under the influence of strongNAO forcing). When studied in combination withdata assimilating models, the fate of the anoma-lies, their longevity and characteristic pathwayscan be mapped out. We would also like to quan-tify, as a function of time-scale, the role of theocean in the advection and transfer of heat to andfrom the surface, through subduction, Ekman lay-ers, geostrophic eddies etc.

An anomaly process experiment to investi-gate the generation, propagation and fate of tem-perature anomalies at the boundary between thesubpolar and subtropical gyres should include: alocally augment float and drifter array andEulerian time-series stations at key sites of SSTAevolution, and an instrumented monitoring lineat the southern outflow of the subpolar gyre(roughly 47°N), extending eastward to the mid-Atlantic ridge, to sample the deep and intermedi-ate southward flows as well as the northwardNorth Atlantic Current transport and T/S proper-ties.

The Polar Basins and Subpolar-polar connectionsThe central questions here include under-

standing the variations in low-salinity surfacewater and deep water mass formation/export, theirrelationships to atmospheric and ocean processes,and the adjustment of the basin-to-global scalethermohaline circulation (in terms of physical

mechanisms and inherent time scales) to thoselocalized changes. At these latitudes, the surfaceand deep waters are related through the verticalstability which in turn is strongly influenced bysea surface salinity. The time-dependent freshwater budget appears to be the clearest diagnos-tic, if not the controlling factor for deep convec-tion and the water types that result.

Strategies need to be developed to investigatethe following topics:• The export of waters from the Greenland-

Iceland-Norwegian (GIN) Seas includingthe densest Denmark Strait Overflow Water(DSOW) and the shallower boundarycurrents (including ice) that are important tothe fresh-water balance. Close cooperationwith ACSYS (joint memberships in Imple-mentation WGs) is recommended. Monitor-ing the overflows over the Greenland-Iceland-Scotland sills is considered veryimportant as these measurements may beapplied as northern boundary conditions inrestricted-domain Atlantic Ocean modelingefforts.

• The properties of those Atlantic waters thatenter the northern seas and subsequentlyinvolved in dense water mass formation andmodifying the Arctic pycnocline. Thismight include a proposed 60N VOS pro-gram on a 3-week repeat schedule betweenDenmark to Greenland.

• The actual formation mechanism(s) of thedense overflow waters to quantify howmuch of the newly ventilated deep andintermediate water is formed in the gyrecenters versus the boundary current sys-tems. Exploratory Lagrangian programsmight follow the conversion of AtlanticWater into DSOW in or about the GINSeas. Study of the entrainment mixingprocesses active within the descendingplumes of dense overflow waters is alsoneeded.

• The shallow boundary currents along theLabrador and Greenland continental shelvesthat, despite their small mass transports,carry very low-salinity waters and hence

37

have a large fresh water transport thatappear able to cap off the deep convectionsites.

• The energetics/boundary layer physics ofocean/atmosphere coupling at high latitude.To address this, the high-resolution ETAatmospheric model needs to be extendedeastward and a reanalysis program of thestandard NCEP model organized. A generalgoal is improved E-P and heat flux predic-tions. The dipolar response of sea-ice to theNAO, in the GIN and Labrador Seas.

• The ice edge provides a possibly strongocean-atmosphere feedback through surfaceheat/moisture flux. This suggests a processstudy of upper ocean/ice measurement andatmospheric energetics.

Recommendations:Detailed planning for these imbedded process

studies within the ACVE sustained observationalnetworks must begin immediately, building on themodeling work and observational program resultsof ACCE.

6. Programmatic context

bines into a coherent effort, the elements (D1-D3) of the International CLIVAR programthat deal with the Atlantic Sector. By doingso, it is hoped that a more natural and focusedstudy will result. The Atlantic is surroundedby many developed countries which will con-tribute to ACVE scientifically and financially.Given the partial overlap of physical pro-cesses and scientific questions, ACVE willmaintain close scientific ties with our Pacificcounterpart: PBECS.

• GODAE/GCOS/GOOS (Global ObservingSystem) — The proposed Global Ocean DataAssimilation Experiment (GODAE) will pro-vide a convincing demonstration of real-timeassimilation systems, with core operationalactivities to be carried out 2003-2005 on aglobal basis. The aim is to assimilate data onspace-time scales of 100 km and a few days,and produce integrated global analyses thatcan be used to support local-scale studies andinitiate model runs. This study has been re-ceived enthusiastically by the WCRP. Planscall for a multi-node operation, with differ-ent groups focusing on different oceans, al-though all will use a common model and for-mat so as to produce consistent products. Atpresent, it appears that GODAE jointly withCLIVAR UOP will foster a global profiling-float network, ARGO, as part of its sustainedsubsurface ocean observing system. The At-lantic component of this array will contributesubstantially to ACVE.

• PIRATA PACS — The international PilotResearch Moored Array in the Tropical At-lantic (PIRATA) will maintain an array of upto 14 surface moorings in the equatorial At-lantic in the 1997-2000 period. The acquireddata from this network will help guide plan-ning for the augmented array envisioned forACVE. The Pan-American Climate Studiesprogram (PACS, the US component ofVAMOS) aims to improve the skill of opera-tional seasonal-to-interannual climate predic-tion over the Americas, with the emphasis onforecasting warm season rainfall. Focal pointsinclude the relationship between boundary

The Atlantic Climate Variability Experiment hasbeen conceived as a major contributor to theCLIVAR program. As such, ACVE will providereal time assessments of the state of the AtlanticOcean in relation to climatic variations of the at-mosphere. These will be of great utility to researchprograms which seek to understand the role ofocean biogeochemistry in climate, and the im-pacts of climate variability on fisheries. There-fore strong connections exist with many other on-going or planned programs. These are summa-rized below.• International CLIVAR Program — An

important objective of both the GOALS andDec-Cen components of CLIVAR is to iden-tify and exploit incremental elements of cli-mate predictability provided by quantifiedmodes of ocean-atmosphere variability. Bydeveloping improved understanding of theNAO and TAV, the ACVE directly addressesthese CLIVAR program goals. ACVE com-

38

forcing and climate variability; processes gov-erning SST variability in the tropical Atlanticand Pacific; and the development of the mixedlayer. In its second phase, there are plans thatPACS will shift its activities into the tropicalAtlantic which will allow close collaborationwith PIRATA-ACVE.

• WOCE AIMS — The World Ocean Circula-tion Experiment (WOCE) is now entering inits Analysis, Interpretation, Modeling andSynthesis phase (AIMS). In particular the re-sults and experience gained from the AtlanticCirculation and Climate Experiment (ACCE)will benefit the planing and final design ofACVE. The analysis phase of WOCE will alsoprovide the opportunity to inspect the histori-cal and WOCE records in the light of theACVE goals.

• ACSYS — The Arctic Climate System Study(ACSYS) aims to answer two related ques-tions:- Is the Arctic climate as sensitive to glo-

bal changes as models seem to suggest?- What is the sensitivity of global climate

to Arctic processes?Hence ACSYS will focus on the climate ofthe polar oceans and will be an important part-ner in the northern part of the Atlantic wherevariability in sea ice distribution, fresh waterexport from the Arctic and the rate of warmto cold water formation might hold the key toAtlantic variability. (http://www.npolar.no/acsys/)

• PAGES — The International Geosphere-Bio-sphere Program project charged with provid-ing a quantitative understanding of the Earth’s

past environment. We anticipate that longproxy records of climate variability in theAtlantic sector will come out of this effortwhich will contribute to the ACVE goals.

• GLOBEC — International GLOBEC (nowan official IGBP program) is engaged in plan-ning for the Small Pelagics and ClimateChange (SPACC) program. Involved coun-tries include Japan, U.S., Mexico, Peru andChile. Retrospective studies, monitoring,modeling and process studies are being con-sidered. There are clear connections betweenthe physical elements of these efforts andACVE.

TimetableA definitive time table for ACVE will be de-

veloped in the coming six months. Tentative planscall for core field activities beginning 2002. How-ever, planing and implementation activities willstart as early as FY 1999, these will include de-sign studies and error estimates based on histori-cal data and numerical studies. The ACCE dataset will be a central element in these studies.

BudgetA draft budget for ACVE will be assembled

in advance of the December internationalCLIVAR meeting. It can be anticipated that a largefraction of the experiment will be covered byEuro-CLIVAR. In the U.S., the National ScienceFoundation and National Oceanic and Atmo-spheric Administration are actively involved atthis time. NASA is also involved with remotesensing elements of the program. Contributionsfrom ONR are unclear at this point.

39

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Annex

List of Participantsa) Dallas Workshop, February 1998

David BattistiUniversity of WashingtonDept. Of Atmospheric SciencesP.O. Box 351640Seattle, WA 98195david@atmos.washington.edu

A. J. BusalacchiNASALaboratory for Hydrospheric ProcessesGSFCGreenbelt, MD 20771tonyb@neptune.gsfc.nasa.gov

Jim CartonUniversity of Maryland/JCESSDepartment of MeteorologyComputer & Space Sciences Bldg.College Park, MD 20742carton@cola.umd.edu

Ping ChangDepartment of OceanographyTexas A&M UniversityCollege Station, TX 77843-3146ping@manta.tamu.edu

43

Clara DeserClimate and Global Dynamics DivisionNational Center for Atmospheric ResearchP.O. Box 3000Boulder, CO 80307-3000cdeser@ucar.edu

Randall DoleNOAA/ERL/CDCMail Code: R/E/CD325 BroadwayBoulder, CO 80303rmd@cdc.noaa.gov

Isaac M. HeldGeophysical Fluid Dynamics LaboratoryNOAAP. O. Box 308Princeton, NJ 08542ih@gfdl.gov

Nelson HoggWHOIWoods Hole, MA 02543nhogg@whoi.edu

Jim HurrellNCARP.O. Box 3000Boulder, CO 80307-3000jhurrell@ra.cgd.ucar.edu>,

W. E. JohnsRSMAS/MPOUniversity of Miami4600 Rickenbacker CausewayMiami, FL 33149wjohns@rsmas.miami.edu

Yochanan KushnirLamont-Doherty Earth Observatoryof Columbia UniversityRoute 9WPalisades, NY 10964kushnir@ldgo.columbia.edu

Bill LargeNCAR/CUP.O. Box 3000Boulder, CO 80307-3000wily@ncar.ucar.edu

Susan LozierDivision of Earth and Ocean SciencesNicholas School of the EnvironmentDuke UniversityBox 90230Durham, NC 27708-0230susan@neptune.geo.duke.edu

John MarshallMIT (EAPS)77 Mass AveCambridge, MA 02139marshall@plume.mit.edu

Michael McCartneyWHOIWoods Hole, MA 02543mike@gaff.whoi.edu

Jay McCrearyOceanographic CenterNova Southeastern University800 N. Ocean DRDania, FL 33020jay@ocean.acast.nova.edu

Robert MolinariNOAA/AOML4301 Rickenbacker CausewayMiami, FL 33149molinari@aoml.noaa.gov

P. NiilerScripps Institution of OceanographyLa Jolla, CA 92093-0230pniiler@ucsd.edu

Brechner OwensWHOIWoods Hole, MA 02543breck@rascals.whoi.edu

44

R. W. ReynoldsCoupled Model ProjectNMCNOAACamp Springs, MD 20747rreynolds@sun1.wwb.noaa.gov

Peter RhinesBox 357940University of WashingtonSeattle, WA, 98195rhines@ocean.washington.edu

Walter RobinsonDept. of Atmospheric SciencesUniversity of Illinois105 S Gregory StreetUrbana, IL 61801robinson@atmos.uiuc.edu

Tom RossbyGraduate School of OceanographyNarragansett Bay CampusUniversity of Rhode IslandNarangansett, RI 02882tom@pogo.gso.uri.edu

R. SaravananNCARP.O. Box 3000Boulder, CO 80307-3000svn@ucar.edu

Michael SpallWHOIWoods Hole, MA 02543spall@cms.whoi.edu

Detlef StammerMassachusetts Inst. of Technology77 Mass. Av.Cambridge, MA 02139detlef@lagoon.mit.edu

John TooleWHOIWoods Hole, MA 02543jtoole@whoi.edu

Martin VisbeckLamont-Doherty Earth Observatory ofColumbia UnversityRT 9WPalisades, NY 10964visbeck@ldgo.columbia.edu

John WalshDept. of Atmospheric SciencesUniversity of Illinois105 S Gregory StreetUrbana, IL 61801walsh@atmos.uiuc.edu

Carl WunschMIT 54-1524Cambridge, MA 02139cwunsch@pond.mit.edu

Steve ZebiakLamont-Doherty Earth Observatoryof Columbia UnversityRoute 9WPalisades, NY 10964steve@ldgo.columbia.edu

b) Florence workshop (May 1998)

D. AndersonEMCWFShinfield ParkREADING RG2 9AXUnited KingdomTel: + 44 1189 949 9706

Fax: + 44 1189 986 9450E-mail: sta@ecmwf.int

C. AppenzellerUniversity of Bern Climate and Env. PhysicsInstitute

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Sidlerstrasse 5 a CH - 3012Bern SwitzerlandTel: + 41 31 631 4871Fax: + 41 31 631 4405E-mail:christof@climate.unibe.ch

A. BamzaiNational Science FoundationClimate Dynamics Program, Room 7754201 Wilson BoulevardArlington VA 22230 USATel: + 1 703 306 1527Fax: + 1 703 306 0377E-mail: abamzai@nsf.gov

D. BattistiUniversity of WashingtonDept. Of Atmospheric Sciences 351640Seattle WA 98195 USATel: + 1 206 543 2019Fax: + 1 206 543 0308E-mail: david@atmos.washington

G. BoccalettiIMGA Area dellaRicerca CNRVia Gobetti 101Bologna ItalyTel: + 39 51 639 8014Fax: + 39 51 639 8132E-mail:giulio@imga.bo.cnr.it

G. BranstatorNCARP.O. Box 3000Boulder, CO 80307 USATel: + 1 303 497 1365Fax: + 1 303 497 1700E-mail:branst@ncar.ucar.edu

A. CapotondiNCAR/CUP.O. Box 3000Boulder, CO 80307 USATel: + 1 303 497 1353Fax: + 1 303 497 1700E-mail: anton@ncar.ucar.edu

S. CortiCINECA (Bologna) ViaMagnanelli 6/s Casalecchio di RenoBologna 40033 ItalyTel: + 39 51 617 1581E-mail: corti@cineca.it

M. DaveyHadley CentreMet Office London Road BracknellReading RG12 2SY UKTel: + 44 1344 85 4648Fax: + 44 1344 85 4898E-mail: mkdavey@meto.gov.uk

P. DelecluseLODYCUniv. Pierre & Marie Curie 4,Place Jussieu 75005 Paris, Cedex 04 FranceTel: + 33 1 44 27 7079Fax: + 33 1 44 27 7159E-mail: delecluse@lodyc.jussieu.fr

T. DelworthGeophysical Fluid Dynamics Lab./NOAAP.O. Box 308 Princeton UniversityPrinceton NJ 08542 USATel: + 1 609 452 6565Fax: + 1 609 987 5063E-mail: td@gfdl.gov

M. DequeMeteo-France42 Av. Coriolis31057 TOULOUSE FranceTel: + 33 5 61 07 9382Fax: + 33 5 61 07 9610E-mail:deque@meteo.fr

L. DillingNOAA/OGP1100 Wayne Av. Suite 1210Silver Spring MD 20910 USATel: + 1 301 427 2089 (ext. 106)Fax: + 1 301 427 2073E-mail:dilling@ogp.noaa.gov

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C. K. FollandHadley Centre, UKMet Office London Road BracknellBerkshire RG12 2SY United KingdomTel: + 44 1344 85 6646Fax: + 44 1344 85 4898E-mail: ckfolland@meto.gov.uk

C. GordonUK Met Office London Road,Bracknell Berkshire RG12 2SY UKTel: + 44 1344 85 6660Fax: + 44 1344 85 4898E-mail:cgordon@meto.gov.uk

B. HoskinsUniversity of Reading Dept. of MeteorologyReadingRG6 6BB United KingdomTel: + 44 1189 31 8953Fax: + 44 1189 31 8905E-mail: b.j.hoskins@reading.ac.uk

D. R. CayanScripps Institution of OceanographyLa Jolla Calif. 92093-0224 USATel: + 1 619 534 4507Fax: + 1 619 534 8561E-mail: dcayan@ucsd.edu

J. HurrellNCARP.O. Box 3000Boulder, CO 80307-3000 USATel: + 1 303 497 1383Fax: + 1 303 497 1333E-mail: jhurrell@ucar.edu

S. JewsonCGAM, University of Reading Dept. of Meteo-rology Earley GateWhiteknights P.O. Box 243 ReadingRG12 2SY UKTel: + 44 1189 87 5123 (x 4281)Fax: + 44 1189 318 316E-mail: steve@met.reading.ac.uk

B. A. KingSouthampton Oceanography Centre EmpressDock SouthamptonSO14 3ZH United KingdomTel: + 44 1703 59 6438Fax: + 44 1703 59 6204E-mail: b.king@soc.soton.ac.uk

G. J. KomenKNMIP.O. Box 201 3730 AE De Bilt The NetherlandsTel: + 31 30 2206 676Fax: + 31 30 2202 570E-mail: komen@knmi.nl

Y. KushnirLamont-Doherty Earth Observatory of Colum-bia UnversityRoute 9W Palisades, NY 10964 USATel: + 1 914 365 8669Fax: + 1 914 365 8736E-mail:kushnir@ldeo.columbia.edu

N. G. KvamstoGeophysical InstituteUniversity of BergenAllégaten 70 N-5007Bergen NorwayTel: + 47 55 58 2898Fax: + 47 55 58 9883E-mail: nils.kvamsto@gfi.uib.no

W. LargeNCAR/CUP.O. Box 3000Boulder, CO 80307-3000 USATel: + 1 303 497 1364Fax: + 1 303 497 1700E-mail: wily@ncar.ucar.edu

M. LatifMax-Planck-Institut für MeteorologieBundesstrasse 55 20146Hamburg GermanyTel: + 49 40 411 73 248Fax: + 49 40 411 73 173E-mail: latif@dkrz.de

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H. Le TreutLMD/CNRS 24 rue Lhomond 75231Paris Cedex 5 FranceTel: + 33 0144 27 84 06Fax: + 33 0144 27 62 72E-mail: letreut@lmd.jussieu.fr

D. MarshallUniversity of ReadingP.O. Box 243 Reading RG6 6BB UKTel: + 44 118 931 8952Fax: + 44 118 931 8905E-mail: davidm@met.rdg.ac.uk

J. MarshallMIT Cambridge MA 02138 USATel: + 617 253 9615Fax: + 617 253 4464E-mail: marshall@gulf.mit.edu

S. MasinaIMGA-CNR Via Gobetti 101 40129Bologna ItalyTel: + 39 51 639 8019Fax: + 39 51 639 8132E-mail: masina@imga.bo.cnr.it

W. MayDanish Meteorological Institute Lyngbyvej 1002100Copenhagen DenmarkTel: + 45 39 15 7462Fax: + 45 39 15 7460

M. McCartneyMSZ1, WHOIWoods Hole, MA 02543 USATel: + 1 508 2892 797Fax: + 1 508 4572 181

J. P. McCrearyOceanographic Center Nova SoutheasternUniversity800 N. Ocean DRDania, FL 33020 USATel: + 1 954 920 1909Fax: + 1 954 921 7764E-mail: jay@ocean.nova.edu

V. MehtaNASA University of MarylandCode 913 NASA/Goddard Space Flight CenterGreenbelt, MD 20771 USATel: + 1 301 286 2390Fax: + 1 301 286 1759E-mail: mehta@climate.gsfc.nasa.gov

R. MolinariNOAA/AOML4301 Rickenbacker CausewayMiami, Florida 33149 USATel: + 1 305 361 4344Fax: + 1 305 361 4392E-mail: molinari@aoml.noaa.gov

F. MolteniECMWF Shinfield ParkReading RG2 9AX United KingdomTel: + 44 1189 499 601Fax: E-mail: nef@ecmwf.int

K. MooneyNational Oceanic and Atmospheric Administra-tion1100 Wayne Avenue, Suite 1225Silver Spring, MD USATel: + 1 301 427 2089Fax: + 1 301 427 2073E-mail: mooney@ogp.noaa.gov

J. MurphyHadley CentreMet. Office London RoadBracknell Berskhire RG12 2SY United King-domTel: + 44 1344 856 658Fax: + 44 1344 854 898E-mail: jmmurphy@meto.gov.uk

A. NavarraIMGA Via Gobetti 101 Bologna ItalyTel: + 39 51 639 8014Fax: + 39 51 639 8132E-mail: navarra@rigoletto.imga.bo.cnr.it

48

P. NiilerScripps Institution of OceanographyLa Jolla, CA 92093-0230 USATel: + 1 619 534 4100Fax: + 1 619 534 7931E-mail:pniiler@ucsd.edu

J. D. OpsteeghKNMIP.O. Box 201 3730 AE De Bilt The NetherlandsTel: + 31 30 2206 700Fax: + 31 30 2202 570E-mail: opsteegh@knmi.nl

T. OsbornClimatic Research Unit University of EastAngliaNorwich NR4 7TJ United KingdomTel: + 44 1603 592 089Fax: + 44 1603 507 784E-mail: t.osborn@uea.ac.uk

V. PavanCINECA via Magnelli 6/340033 Casalecchio di Reno Bologna ItalyTel: + 39 51 6171 474Fax: + 39 51 6132 198E-mail: pavan@cineca.it

G. PhilanderPrinceton UniversityDept of GeoscienceGyot Hall Princeton NJ USATel: + 1 609 258 5683Fax: + 1 609 258 2850E-mail: gphlder@splash.princeton.edu

G. PiovesanDisafri University of TusciaVia S.C. de Lellis 01100 Viterbo ItalyTel: + 39 761 357 389Fax: + 39 761 357 387E-mail: schirone@unitus.it

G. ReverdinLEGOS18 Av. E. Belin, 31401 Toulouse France

Tel: + 33 5 61 33 2926Fax: + 33 5 61 25 3205E-mail:gilles.reverdin@cnes.fr

M. RodwellHadley CentreMet Office London Road BracknellRG12 2SY United KingdomTel: + 44 344 856 751Fax: + 44 344 854 898E-mail: mjrodwell@meto.gov.uk

R. SaravananNCARP.O. Box3000 Boulder, CO 80307-3000 USATel: + 1 303 497 1329Fax: + 1 303 497 1700E-mail: svn@ucar.edu

F. SchottInsitut für Meereskunde Düsternbrookerweg 2024105 Kiel GermanyTel: + 49 431 597 3820Fax: + 49 431 597 3821E-mail: fschott@ifm.uni-kiel.de

F. SeltenKNMIP.O.Box 201 AE De Bilt The NetherlandsTel: + 31 30 2206 761Fax: + 31 30 2202 570E-mail: selten@knmi.nl

J. ServainCentre ORSTOMB.P. 70 29280 Plouzané FranceTel: + 33 2 98 22 4506Fax: + 33 2 98 22 4514E-mail:servain@orstom.fr

K. SpeerCNRS IFREMERLPO B.P. 70 29280 Plouzané FranceTel: + 33 2 9822 4566Fax: + 33 2 9822 4496E-mail: kspeer@ifremer.fr

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D. StammerMassachusetts Inst. of Technology77 Mass. Av.Cambridge, MA 02139 USATel: + 1 617 253 5259Fax: + 1 617 253 4464E-mail: detlef@lagoon.unit.edu

D. StephensonMeteo-France2 Av Coriolis 31057 Toulouse FranceTel: + 33 561 07 9698Fax: + 33 561 07 9610E-mail: stephen@meteo.fr

R. SuttonUniversity of Reading CGAMDept. of Meteorology Earley Gate,WhiteknightsP.O. Box 243, Early Gate Reading RG6 6BBUnited KingdomTel: + 44 1189 875 123 (x 7842)Fax: + 44 1189 318 316E-mail: rowan@met.reading.ac.uk

L. TerrayCERFACS 42Avenue Gustave Coriolis 31057 ToulouseCedex FranceTel: + 33 561 19 3015Fax: + 33 561 19 3000E-mail: terray@cerfacs.fr

J. T. ToddNOAA Office of Global Programs1100 Wayne AvenueSilver Spring Maryland 20910-5603 USATel: + 1 301 427 2089Fax: + 1 301 427 2073E-mail: todd@noaa.ogp.gov

U. UlbrichInstitut für Geophysik und MeteorologieUni Köln Kerpener Strasse 13 D-50923 KölnGermanyTel: + 49 221 470 3688Fax: + 49 221 470 5161E-mail: ulbrich@meteo.uni.koeln.de

M. VisbeckLDEO Columbia UniversityRT 9W Palisades, NY-10964 USATel: + 1 914 365 8531Fax: + 1 914 365 8157E-mail: visbeck@ldeo.columbia.edu

J. M. WallaceUniversity of WashingtonP.O. Box 351640Seattle Washington 98195 USATel: + 1 206 543 7390Fax: + 1 206 685 3397E-mail: wallace@atmos.washington.edu

H. WannerUniversity of Bern Institute of GeographyHallerstrasse 12 CH-3012 BERN SwitzerlandTel: + 41 31 631 88 85Fax: + 41 31 631 85 11E-mail: wanner@giub.unibe.ch

R. G. WilliamsLiverpool University Oceanography Laborato-riesP.O. Box 147 Liverpool L69 3BX UnitedKingdomTel: + 44 151 794 5136Fax: + 44 151 794 4090E-mail: ric@liv.ac.uk