The Modeled Atmospheric Response to Midlatitude SST

MAY 1997 971P E N G E T A L .

q 1997 American Meteorological Society

The Modeled Atmospheric Response to Midlatitude SST Anomalies andIts Dependence on Background Circulation States

SHILING PENG

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado

WALTER A. ROBINSON

Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois

MARTIN P. HOERLING

Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado

(Manuscript received 16 July 1996, in final form 21 October 1996)

ABSTRACT

The atmospheric response to a midlatitude SST anomaly in the North Pacific and its dependence on backgroundflow are examined in a GCM. Experiments are conducted using the same warm SST anomalies but two differentmodel states: perpetual January and perpetual February. The atmospheric responses to the SST anomalies arestatistically significant in both January and February but are completely different. The anomalous circulationin January is characterized by a trough decaying with height immediately downstream of the SST anomalies.In February, the anomalous circulation is dominated by a downstream ridge growing with height. The patternsof the anomalous heights in the two months are nearly orthogonal.

Vorticity and thermodynamic budgets are diagnosed to illustrate how the anomalous circulations are maintained.Over the SST anomalies, low-level convergence and ascent are observed in both months. In January the anomalousconvergence is balanced by a residual due primarily to the forcing by submonthly transients. In February theconvergence is balanced by the advection of planetary vorticity. Analysis of the thermodynamic budget indicatesthat the intensity of the mean meridional wind downstream of the SST anomalies plays a critical role indetermining the nature of the responses in the two months. The ‘‘warm SST-ridge’’ type of response is favoredwhen the background meridional flow is relatively weak. These results demonstrate that the atmospheric responseto a midlatitude SST anomaly is strongly dependent on the background flow.

1. Introduction

During the past decade several attempts have beenmade to evaluate the effects of midlatitude sea surfacetemperature (SST) anomalies on the atmosphere. Ex-tremely diverse and complex results have been producedby various modeling experiments with midlatitudeanomalies. General circulation model (GCM) experi-ments with midlatitude SST anomalies can be dividedinto two categories based on the horizontal resolutionof the models. Perhaps because of the strong interactionsbetween the mean flow and the transients in the mid-latitudes (Branstator 1992, 1995), studies using rela-tively high-resolution models have obtained results thatare very different from those using low-resolution mod-els.

Corresponding author address: Dr. Shiling Peng, CIRES, Univer-sity of Colorado, Campus Box 449, Boulder, CO 80309.E-mail: [email protected]

So far, the low-resolution model simulations haveshown three types of results: a) no atmospheric responseto the midlatitude SST forcing; b) a baroclinic responseto the surface heating, similar to that given by a linearmodel, with a low-level trough and an upper-level ridgedownstream; c) an equivalent barotropic response to theheating with a trough growing with height (e.g., Pitcheret al. 1988; Ting 1991; Kushnir and Lau 1992; Lau andNath 1994; Graham et al. 1994; Kushnir and Held 1996).A completely different response has repeatedly emergedfrom the high-resolution model experiments (e.g., Palm-er and Sun 1985; Ferranti et al. 1994; Latif and Barnett1994; Peng et al. 1995). Given a positive SST anomalythese studies have found a downstream anomalous ridgewith an equivalent barotropic structure. Such a positivephase relationship between the SST and the geopotentialheight anomalies resembles that observed in nature (e.g.,Palmer and Sun 1985; Wallace and Jiang 1987) but isnot captured by any of the low-resolution model sim-ulations.

Large differences, however, exist even among the re-

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972 VOLUME 10J O U R N A L O F C L I M A T E

FIG. 1. The SST anomaly pattern used in the experiments. Thecontour interval is 0.5 K.

sults from the high resolution model simulations. Fer-ranti et al. (1994) conducted experiments with an ide-alized SST anomaly (;2 K maximum) in the Pacificand in the Atlantic. The model atmosphere during thewinter responded with an anomalous ridge (trough) im-mediately downstream of the warm (cold) SST anomalyover both basins. The 500-mb height anomalies wereabout 30 m but were not statistically significant. Latifand Barnett (1994) performed a perpetual January sim-ulation with a Pacific SST anomaly pattern (;1 K max-imum) taken from a coupled atmosphere–ocean modelrun. The atmospheric model showed strong sensitivityto the midlatitude SST anomaly and produced an anom-alous ridge of about 200 m at 500 mb over the surfaceheating.

Over the Atlantic, the equivalent barotropic ridge re-sponse to the surface heating, first reported by Palmerand Sun (1985), was reproduced by Ferranti et al. (1994)and Peng et al. (1995). In particular, Peng et al. (1995)showed that such a response is strongly dependent onthe background circulation state. Given an identicalwarm SST anomaly and two perpetual model states, theridge response was found in early winter (November)but not in midwinter (January). The midwinter responsewas a trough growing with height, similar to that foundin some low-resolution model experiments (Pitcher etal. 1988; Kushnir and Lau 1992). Evidence for a similarseasonal dependence is found in a recent Atlantic ex-periment with the GFDL R30 model (Y. Kushnir 1996,personal communication) and also in the observations(Peng and Fyfe 1996). These results suggest that thedifferent sensitivities to the midlatitude SST anomaliesshown by various models may have resulted, in part,from the differences in their background circulationstates. It remains to be determined whether this factormay also be responsible for the discrepancies betweenthe results of Ferranti et al. (1994) and Latif and Barnett(1994) over the Pacific.

The objective of this study is to further investigatethe atmospheric sensitivity to the midlatitude SSTanomalies in the North Pacific and, especially, its de-pendence on the background circulation states. We ap-proach the problem by conducting GCM experimentsunder perpetual January and perpetual February con-ditions. An identical midlatitude SST anomaly patternis given in both cases. We find that by shifting the basicstate by one month the model atmosphere produces com-pletely different responses to the SST anomalies. Wehave diagnosed the vorticity and thermal budgets toshow how the two balances are maintained and howthey differ from existing theories. The diagnoses alsoshow how the two balances are related to the differencesin the background flows.

The selection of a SST anomaly, the model config-uration, and the experimental design are described insection 2. The model responses are shown in section 3and the diagnoses in section 4. Finally, the summaryand discussion are given in section 5.

2. Description of the experiments

a. SST anomaly

Deser and Blackmon (1995) performed an empiricalorthogonal function (EOF) analysis of winter SSTanomalies over the Pacific (608N–208S). The first EOFpattern is found to be related to the El Nino–SouthernOscillation (ENSO) and is characterized by two out ofphase centers over the tropical and the extratropical Pa-cific. The second EOF is independent of ENSO and isdominated by a single center over the midlatitude west-ern Pacific.

Studies have shown that the ENSO-related SSTanomalies over the extratropical eastern Pacific largelyresult from the anomalous atmospheric circulations(e.g., Alexander 1990; Lau and Nath 1996). The originand maintenance of the SST anomalies over the westernPacific are probably more complicated. These anomaliesoverlay the Kuroshio current and fluctuate on a muchlonger timescale than ENSO (Deser and Blackmon1995). Possibly, they are related to decadal oscillationsof the coupled system, involving a potential positivefeedback between the atmosphere and the ocean, as de-scribed in Latif and Barnett (1994). A similar positivefeedback was observed in the experiments of Palmerand Sun (1985) and Peng et al. (1995) over the Atlantic.The reinforcement between the SST anomalies and theanomalous atmospheric circulations was captured onlyin these few studies where the model atmosphere re-sponded to a warm SST anomaly with a strong ridgeimmediately downstream.

To determine the effects of midlatitude SST anom-alies on the atmosphere, independent of ENSO, we haveconstructed an anomaly pattern based on the second SSTEOF of Deser and Blackmon (1995) by compositing thewinters with strong positive and negative SST anoma-lies. The spatial distribution of the composite SSTanomalies is further simplified by retaining only thedominant center over the western Pacific as shown inFig. 1. The magnitudes of this idealized SST anomalypattern (;2.5 K maximum) are larger than the observedSST standard deviations over this region but are withinthe range of natural fluctuations. We have kept the SST


MAY 1997 973P E N G E T A L .

FIG. 2. The 300-mb streamfunctions (in contours) and isotaches (in shading) averaged overthe four control runs for January (a) and for February (b). The contour interval is 2 3 107

m2 s21. The shading interval is 10 m s21, beginning at 20 m s21.

anomalies fixed throughout the following GCM exper-iments.

b. Model and experimental design

The model employed in this study is a climate versionof the medium-range forecast model MRF9, developedat the National Centers for Environmental Prediction(NCEP). The MRF9 model has a T40 spherical har-monic horizontal resolution (corresponding to a physicalgrid resolution of approximately 2.88 3 2.88) and 18unequally spaced sigma levels. A more detailed de-scription of the model is given by Kumar et al. (1996).By adjusting the solar zenith angle and the surfaceboundary conditions one can integrate the MRF9 modelwith or without the seasonal cycle.

Our initial effort with this model was to reproducethe results of Ferranti et al. (1994). We performed anensemble of anomaly and control runs similar to theirswith the model atmosphere evolving through the wintermonths. The SST anomaly shown in Fig. 1 was addedto the monthly SST climatology in the anomaly runs.Averaged over the winter months, the results of theseexperiments showed no response to the SST anomalyover the North Pacific. Neither quantitatively nor qual-itatively could we reproduce the pattern of Ferranti etal. (1994) with the MRF9 model. In searching for anexplanation we examined the ensemble response of eachwinter month and found strong intermonth variability.In particular, the mean height response at 500 mb overthe Pacific was a trough in January but a ridge in Feb-ruary. Only the ridge response in February appeared tobe statistically significant. These preliminary experi-ments gave rise to questions as to the causes of thisintermonth variability in the model. Did the polarity ofthe height anomalies flip randomly from month to month

due to a complete insensitivity of the model atmosphereto the SST forcing? Or did the variability arise from astrong dependence of the model responses on the back-ground circulation states? To address these questions aseries of long integrations is thus performed under per-petual January and February conditions as describedbelow. The seasonal cycle is excluded in the followingexperiments in order to simplify and isolate the problemin question.

The perpetual January and February experiments areconducted by positioning the model atmosphere in theclimatological states representative of mid-January andmid-February. The SST field used in the control runsis the climatology of the corresponding month, and thatin the anomaly runs is the climatology with the SSTanomalies shown in Fig. 1 added. The monthly SSTclimatologies are based on the NCEP blended dataset(Reynolds 1988) for the period 1950–90. For each per-petual state we have performed 4 pairs of 96-monthintegrations (i.e., a total of 384 months of control runand 384 months of anomaly run). The ensemble isformed by using initial conditions from four differentyears of a model control run with the seasonal cycle.Only the monthly mean data are archived from theselong perpetual integrations. Note that, throughout thisstudy, the air temperature from the model refers to thevirtual temperature.

3. Model responses

Before discussing the model responses to the SSTanomalies, we show in Fig. 2 the 300-mb streamfunc-tions and isotaches averaged over the four control runsfor January and February. The Pacific jet is stronger(by about 10 m s21) and exits farther to the north (seethe shading along 1208W) in January than in February.



FIG. 3. The geopotential height responses at 500 and 850 mb for January (left panels) andfor February (right panels). The contour interval is 5 m at 500 mb and 3 m at 850 mb. Areaswith the height anomalies significant at the 95% level are shaded.

This corresponds to a deeper trough in the Januarystreamfunction. Overall, the two basic states illustratedin Fig. 2 do not appear to be drastically different. Theatmospheric responses to the SST anomalies under thetwo model states are examined below as the ensemble-averaged differences between the anomaly and the con-trol runs.

a. Geopotential height and temperature

The height responses to the SST anomalies at 500and 850 mb for both perpetual January and Februaryexperiments are presented in Fig. 3. The student’s t-testis applied to these ensemble mean differences to assesstheir statistical significance. Each of the four 96-monthruns within an ensemble is treated as independent. Adetailed description of the t-test and its numerical im-

plementation are given by Press et al. (1992, chapters6.4 and 14.2). Height responses that are significant atthe 95% level are shaded. It is evident that with thesame midlatitude SST forcing the anomalous circula-tions obtained under the two model states are completelydifferent. In January, at 850 mb, there is an anomaloustrough downstream of the SST anomaly, and two anom-alous ridges at high latitudes. The trough is shallow andvanishes at 500 mb. In contrast, the ridges grow withheight and are dominant at 500 mb. The centers of theanomalous heights in January are mostly significant atthe 95% level.

The February response (Fig. 3, right side panels) ischaracterized by one ridge over the Pacific and anotherover Europe. Both ridges have an approximately equiv-alent barotropic structure and grow with height. Theridge over the Pacific is downstream of the SST anomaly


MAY 1997 975P E N G E T A L .

FIG. 4. Same as Fig. 3, but for the temperature responses. The contour interval is 0.2 K.

and is about 20 m at 500 mb. This ‘‘warm SST-ridge’’type of response is qualitatively consistent with the re-sults of Ferranti et al. (1994) and Latif and Barnett(1994) but is in sharp contrast to the ‘‘warm SST-trough’’ response found in January. The remote re-sponse over Europe also bears a resemblance to thatshown by Ferranti et al. (1994), although the mechanismfor exciting such a pattern is unclear. In both the Januaryand February responses, a zonally symmetric compo-nent is evident. The results of a t-test indicate that thecenters of the height anomalies in February are mostlysignificant as well, except the weak ridge at 850 mbover the Pacific.

The thermal responses at 850 and 500 mb are pre-sented in Fig. 4 for January and for February. Near thesurface (at 2-m height), the temperature anomalies (;2K maximum) are centered over the SST anomalies andare almost identical in the two months (not shown). At850 mb, the warming over the Pacific is centered slightly

downstream of the SST anomalies. The patterns of theanomalous temperatures in January and February aresimilar, except that there is a northward spread of thewarming along 1508W in January. This northwardspread of the warming grows with height and causesthe January and February anomalous temperatures at500 mb to be markedly different. Note that the maxi-mum warming at 500 mb in January is located around(1508W, 608N), northeast of the warming in February.The centers of the temperature anomalies are significant,as shown by the shading in Fig. 4. Overall, the thermalanomalies display a stronger horizontal spread in Jan-uary and a stronger local vertical penetration in Feb-ruary.

The vertical structures of the anomalous heights andtemperatures over the Pacific are illustrated further inFig. 5 on a vertical cross section along 408N, throughthe center of the SST anomalies. It is evident that theanomalous trough in January (Fig. 5a) is confined to



FIG. 5. The height and temperature responses over the Pacific on a vertical cross section along 408N for January (left panels) and forFebruary (right panels). The contour interval for the height anomalies (a and b) is 5 m, and that for the temperature anomalies (c and d) is0.2 K. Areas with the responses significant at the 95% level are shaded.

the lower troposphere and that the local response atupper levels is weak. In contrast, the anomalous ridgein February (Fig. 5b) grows with height and reaches itsmaximum near 300 mb. An anomalous trough is alsofound in February near the surface but is much weakerand smaller in scale than that in January. Correspondingto the height anomalies, Fig. 5 shows that the warmingof the local column is much shallower in January thanin February. The temperature anomalies that are sig-nificant at the 95% level extend only to about 700 mbin January but up to 400 mb in February. In both monthsthe warming exhibits an eastward shift with height thatis stronger in January than in February.

b. Pattern correlation

The results displayed in Figs. 3–5 show that the at-mospheric responses to the SST anomalies in the two

months are very different. To demonstrate the effect ofthe SST forcing on the month-to-month variability underdifferent background states we calculate the pattern cor-relations between the mean height responses to the SSTanomalies and the monthly height anomalies at 850 mbfor the area north of 208N. As shown in Fig. 3 the meanheight responses differ most drastically at 850 mb be-tween January and February. The pattern correlationsare calculated for eight different combinations and aresummarized in Table 1. In Table 1, Jr represents themean height response in January and Fr the mean re-sponse in February; denotes the monthly heightJ9canomalies in the January control runs from the controlmean, the monthly anomalies in the January warmJ9wSST runs, also from the control mean; and areF9 F9c w

similar to and but for February; m denotes theJ9 J9c w

month of the integration.The mean response in January Jr is calculated as the


MAY 1997 977P E N G E T A L .

TABLE 1. Summary of the pattern correlations.

Correlation DefinitionMeanvalue

CJcJ(m) 5 [J (m), Jr]′c

(m 5 1, . . . , 384)Jan control anomalies &

Jan response20.05

CJwJ(m) 5 [J (m), Jr]′w Jan warm anomalies & Jan

response0.10

CFcF(m) 5 [F (m), Fr]′c Feb control anomalies &

Feb response0.01

CFwF(m) 5 [F (m), Fr]′w Feb warm anomalies & Feb

response0.12

CJcF(m) 5 [J (m), Fr]′c Jan control anomalies &

Feb response0.01

CJwF(m) 5 [J (m), Fr]′w Jan warm anomalies & Feb

response0.01

CFcJ(m) 5 [F (m), Jr]′c Feb control anomalies &

Jan response20.03

CFwJ(m) 5 [F (m), Jr]′w Feb warm anomalies & Jan

response20.03

ensemble averaged height differences between the warmSST runs and the control runs:

Jr 5 Zjw 2 Zjc, (1)

where Z denotes the monthly geopotential height; ‘‘jw’’stands for the January warm SST runs, ‘‘jc’’ for theJanuary control runs; the overbar represents an averageover the total integration of 384 months. The mean re-sponse in February Fr is calculated similarly. The Jr andFr patterns at 850 mb are shown in Figs. 3c and 3d.These two patterns are nearly orthogonal with a patterncorrelation of 20.05.

The monthly height anomalies in the January controland warm SST runs, and , are calculated asJ9 J9c w

(m) 5 Zjc(m) 2 Zjc; m 5 1, . . . , 384J9c (2)

(m) 5 Zjw(m) 2 Zjc; m 5 1, . . . , 384,J9w (3)

where m denotes the month of the integration and theprime represents a deviation from the control mean. The

and are calculated similarly but for February. TheF9 F9c w

distributions of the pattern correlations over the 384months of integrations for each combination defined inTable 1 are shown in Fig. 6.

The distributions of CJcJ (the correlations between themonthly height anomalies in the control runs and themean response to the SST anomalies in January) andCJwJ (the correlations between the monthly anomalies inthe warm SST runs and the mean response) are shownin Fig. 6a. Figure 6a demonstrates a clear bias towardpositive correlations in the warm SST runs. A similarbias is shown in Fig. 6b for February. Thus, in com-parison with the control atmosphere in January (Feb-ruary), the presence of a warm SST anomaly gives riseto more frequent positive occurrences of the Jr (Fr) pat-tern. The mean biases in both months are significant atthe 95% level.

To determine if the SST forcing also affects the dis-

tribution of the Fr pattern in January and vice versa, thedistributions of CJcF and CJwF are calculated and shownin Fig. 6c, and those of CFcJ and CFwJ in Fig. 6d. In bothFig. 6c and 6d, the correlation distributions from thewarm and control runs are similar. There is almost nodifference between the means of CJcF and CJwF, and be-tween those of CFcJ and CFwJ. These results show thatthe presence of the SST anomaly is ineffective in al-tering the occurrence of the Fr pattern under the Januarymodel conditions and vice versa. The effect of the SSTanomaly on the atmosphere, thus, is controlled by thebackground model states.

c. Surface heat flux and precipitation

The combined anomalous fluxes of sensible and latentheat at the surface are shown in Fig. 7a for January andin Fig. 7b for February. A negative anomaly indicatesthat the atmosphere receives more heat from the ocean.Over the SST anomaly, the heat loss from the ocean isenhanced significantly in both months, and the patternsof the anomalous heat fluxes are similar. The anomalymagnitudes are about 30% stronger in January, due pre-sumably to the stronger low-level winds.

The precipitation anomalies in the two months areshown in Figs. 7c and 7d. These anomalies also rep-resent the anomalous latent heating integrated over avertical column. Figures 7c and 7d show that there isincreased precipitation, significant at the 95% level, overthe SST anomaly in January and in February. Again,the anomaly patterns are similar in the 2 months, andthe magnitudes are stronger in January.

Overall, the anomalous surface fluxes and precipi-tation shown in Fig. 7 illustrate that the forcing inducedby the SST anomalies under the two model states isspatially similar but is stronger in January. The fluxesdo not, therefore, explain why the responses of the large-scale flow in January and February are so different.Vorticity and thermodynamic budgets for the anomalousflows provide some insight into their maintenance.These budgets are considered in the next section.

4. Vorticity and thermodynamic budgets

The vorticity and thermodynamic budgets for theanomalous flows are diagnosed based on the followingequations:

]z ]z ]vu 1 v 1 bv 5 (z 1 f ) 1 R (4)]x ]y ]p

]u ]u ]uu 1 v 1 v 5 Q, (5)]x ]y ]p

where z is the relative vorticity, u is the potential tem-perature, R is the vorticity forcing calculated as a re-sidual in Eq. (4), and Q is the thermal forcing calculatedas a residual in Eq. (5). Using the monthly mean modeldata of u, v, v, and T, the budgets are calculated each



FIG. 6. The distributions of the pattern correlations for the eight combinations defined in Table 1: (a) CJcJ (dashed line) and CJwJ; (b) CFcF

(dashed line) and CFwF; (c) CJcF (dashed line) and CJwF; (d) CFcJ (dashed line) and CFwJ.

month and then averaged over the ensemble of four runs.Thus, away from the surface, the vorticity forcing Rshould describe predominantly the convergence of tran-sient eddy vorticity flux on the submonthly timescale.The thermal forcing Q mainly represents the diabaticheating and the convergence of submonthly transienteddy heat flux. Strictly speaking, the residual R includesalso the effects of other terms not explicitly expressedin Eq. (4), such as the vertical vorticity advection, twist-ing, friction, and numerical errors due to truncation.These terms generally contribute little to the budgetaway from the surface.

The validity of using the residual of the monthly meanvorticity budget as an estimate for the vorticity transportby submonthly transient eddies is checked by compos-iting these residuals based on the leading EOFs of themonthly anomalies in the 500-mb geopotential heightsin the control runs. Consistent with the results of Tingand Lau (1993) based on the full GCM equations, thestreamfunction tendencies due to these residuals are inphase with the EOFs themselves, such that the internal

variability of the model is supported by the vorticitytransports of the submonthly transients. This result in-creases our confidence in the use of the residual basedon the simplified equation as a proxy for the transportsof vorticity by the transient eddies.

The anomalous vorticity budget based on Eq. (4) isgiven as the ensemble averaged difference between thewarm and control runs. The anomalous budget at 850mb for January is shown in Fig. 8. The anomalous stream-function shown in Fig. 8a illustrates again the troughover the Pacific. The anomalous streamfunction tendencydue to the horizontal vorticity advection [i.e., 21 3 allthree terms on the left side of Eq. (4)] is shown in Fig.8b with a negative center west of the trough and a positivecenter downstream. The horizontal advection is domi-nated by the advection of planetary vorticity (i.e., the bvterm) (not shown). The anomalous streamfunction ten-dency due to the divergence term in Eq. (4) (i.e., the firstterm on the right) gives a negative center over the Pacificas shown in Fig. 8c. This negative center indicates a low-level convergence and ascent over the SST anomaly.


MAY 1997 979P E N G E T A L .

FIG. 7. The responses in the combined surface fluxes of sensible and latent heat and in the precipitation for January (left panels) and forFebruary (right panels). The contour interval for the fluxes is 15 W m22 and that for the precipitation is 0.2 mm day21. The areas with theanomalies significant at the 95% level are shaded.

Comparison of Figs. 8b and 8c suggests that the anom-alous convergence is not balanced by the advection. Theanomalous streamfunction tendency due to the forcing Ris shown in Fig. 8d, which exhibits a strong positivecenter over the SST anomaly. Clearly, in January, thefirst-order vorticity balance at 850 mb is between thedivergence term and the residual R.

This balance is completely different from the lineartheory given in Hoskins and Karoly (1981), where theadvection of planetary vorticity associated with thetrough is balanced by low-level divergence and descentover the heating. The present balance also differs fromthat described by Palmer and Sun (1985) for a case inwhich a warm anomaly in the Atlantic generates a down-stream equivalent barotropic ridge. In their model, vor-tex stretching associated with ascent over the SST anom-aly is balanced by the negative meridional advection ofplanetary vorticity on the west side of the ridge.

The anomalous vorticity budget at 850 mb for Feb-ruary is shown in Fig. 9. The anomalous streamfunctionin Fig. 9a has a ridge downstream of the SST anomaly.

The anomalous streamfunction tendency due to the vor-ticity advection (Fig. 9b) has a positive center west ofthe ridge and a negative center downstream. The vor-ticity advection in February is also dominated by thebv term. The anomalous streamfunction tendencies dueto the divergence term and the residual R are shown inFig. 9c and 9d. Again, Fig. 9c shows that there is a low-level convergence and ascent over the SST anomaly. InFebruary, however, the anomalous convergence is bal-anced mainly by the vorticity advection, not the residualR. This balance is consistent with that described inPalmer and Sun (1985) and is completely different fromthat found in January.

In the upper troposphere (e.g., at 300 mb), the anom-alous divergence associated with the SST forcing inJanuary is balanced by both the vorticity advection andthe residual, whereas that in February is mainly by theadvection (not shown). Overall, the differences in thevorticity balances between the two months are strongestat low levels, as shown in Figs. 8 and 9. The analysisof the anomalous vorticity budgets illustrates that the



FIG. 8. The 850-mb anomalous streamfunction and vorticity budget for January. (a) The anomalous streamfunction at interval of 3 3 105

m2 s21. (b)–(d) The anomalous streamfunction tendencies due to the horizontal vorticity advection, the divergence term, and the residual Rin Eq. (4). The contour interval for the streamfunction tendency is 2 m2 s22.

anomalous circulations are maintained by differentmechanisms in January and February. Yet this analysisdoes not indicate why a given basic state selects onetype of balance over the other.

Similar to the vorticity diagnosis, the thermodynamicbudget according to Eq. (5) is calculated for Januaryand for February. The anomalous temperature and bud-get at 850 mb for January are shown in Fig. 10. Thethermal anomalies (Fig. 10a) exhibit a northward spreadeast of the dateline. The anomalous thermal forcing Q(Fig. 10d) displays a positive center over the SST anom-aly and a negative center downstream. The axis of theQ anomalies is tilted to the northeast. In January, theanomalous Q is primarily balanced by the anomaloushorizontal advection, as shown in Fig. 10b. The anom-alous vertical advection contributes little to the balance(Fig. 10c). The anomalous temperature and budget at850 mb for February are shown in Fig. 11. In general,the temperature anomalies shown in Fig. 11a are similarto those in January, except the anomaly pattern in Feb-ruary is more confined to the Pacific. The anomalousforcing Q depicted in Fig. 11d also bears a strong re-semblance to that given in Fig. 10d but is weaker and

less northeast-southwest tilted. The anomalous budgetshown in Fig. 11 demonstrates that the dominant balancein February again is between the horizontal advectionand Q. Despite the largely different anomalous circu-lations in the two months, the anomalous thermal bud-gets shown in Fig. 10 and 11 are similar.

The anomalous thermal budget is further illustratedin a vertical cross section along 408N. The anomalousdistributions of each term in Eq. (5) are given in Fig.12 for January and in Fig. 13 for February. The anom-alous forcings Q shown in Fig. 12d and 13d are largelysimilar, except that the forcing is stronger and morespread out to the east in January than in February. Theanomalous zonal advection below 300 mb is character-ized by cold advection west of the dateline and warmadvection to the east (Fig. 12a and Fig. 13a). The anom-alous advection is stronger in February due primarilyto the stronger temperature gradients, as shown in Fig.5. In most of the troposphere, the anomalous meridionaladvection in the two months is dominated by warmadvection west of the dateline and cold advection down-stream (Fig. 12b and Fig. 13b). In January, the anom-alous meridional advection below 850 mb is reversed


MAY 1997 981P E N G E T A L .

FIG. 9. Same as Fig. 8, but for February.

due to the presence of an anomalous trough. In bothJanuary and February, however, there is a strong can-cellation between the anomalous zonal and meridionaladvection. The residual of the horizontal advection ap-proximately balances the anomalous forcing Q. Theanomalous vertical advection contributes to the balanceonly in the upper troposphere (Fig. 12c and Fig. 13c).

The vertical distributions of the anomalous budgetagain demonstrate a similar balance in January and Feb-ruary. The most intriguing and puzzling resemblancelies in the anomalous meridional advection. The anom-alous warm and cold advection displayed in Fig. 13bcan easily be explained as resulting from the anomalousridge developed in February. As shown in Fig. 5, thereis no anomalous ridge in January, and, yet, a similaranomalous meridional advection is produced (Fig. 13b).The anomalous meridional advection can be decom-posed as follows:

¯ ¯]u ]u ]u ]u9w2c c2 v 5 2v 2 v 2 v9 , (6)c w2c1 2 1 2]y ]y ]y ]yw2c w2c

where the overbar represents an average over the totalintegration of 384 months; the prime denotes a monthlydeviation from the control mean; and ‘‘w’’ stands for

the warm SST runs, ‘‘c’’ for the control runs, and ‘‘w-c’’ for the difference between the warm and the controlruns. The anomalous meridional advection thus is di-vided into three terms as expressed in Eq. (6): the ad-vection of anomalous temperature by the control meanwind (the first term), the advection of control meantemperature by the anomalous wind (the second term),and the advection due to the intermonth transients (thethird term).

The first and second terms in Eq. (6) calculated forJanuary and for February are shown in Fig. 14. Thetransient eddy term is small and contributes little to theanomalous meridional advection. Figure 14 demon-strates that the apparently similar anomalous advectionsshown in Fig. 12b and 13b are in fact generated bydifferent mechanisms. In January, the anomalous me-ridional advection comes from both the first and thesecond terms in Eq. (6): the first term gives the anom-alous cold advection east of the dateline and the secondthe warm advection to the west (Figs. 14a and 14c). Incontrast, the anomalous cold and warm advection inFebruary results primarily from the second term. Thus,east of the dateline, the anomalous cold advection isprovided by different processes in the two months: in



FIG. 10. The 850-mb anomalous temperature and thermodynamic budget for January. (a) The anomalous temperature at interval of 0.2K. (b)–(d) The anomalous horizontal temperature advection, the vertical advection, and the anomalous forcing Q according to Eq. (5). Thecontour interval in (b)–(d) is 0.2 K day21.

January by the advection of anomalous temperature bythe mean wind and in February by the advection of meantemperature by the anomalous wind. Figures 14a and14b show that the advection of anomalous temperatureby the mean wind in January is twice as strong as thatin February. This is caused primarily by the differencesin the climatological meridional wind and not the dif-ferences in the anomalous temperature gradients. Asshown in Fig. 15 the control mean meridional wind eastof the dateline is twice as large in January as in Feb-ruary.

The analysis of the anomalous meridional advectionindicates that the intensity of the climatological merid-ional wind downstream of the SST forcing may play acritical role in determining the nature of the responsesin January and in February. The maintenance of the tworesponses can be described as follows: near the surface,a similar warming develops over the SST anomalies inJanuary and in February. The anomalous Q is balancedby both the anomalous zonal and meridional advection.Away from the surface, the warming is shifted eastward,and the zonal wind produces anomalous cold advectionwest of the dateline and warm advection downstream.

The cold advection by the zonal wind acts primarily tooffset the anomalous Q, and the warm advection eastof the dateline is balanced by the anomalous meridionaladvection. In January, the climatological meridionalwind east of the dateline is strong and generates suffi-cient cold advection for the balance. In February, theanomalous cold advection produced by the climatolog-ical meridional wind is too weak to achieve a balance,and an anomalous ridge provides the required cold ad-vection.

The above mechanisms also provide an explanationfor the different anomalous temperature pattern obtainedin the two months. Because the mean meridional windin January works predominantly to give the cold ad-vection east of the dateline, the warming is hence pushednorthward with height as found in Figs. 4a and 4c. InFebruary, the mean meridional wind is weak and thecold advection east of the dateline is produced largelyby the anomalous ridge. Thus, the warming is moreconfined to the Pacific and penetrates deeper into thetroposphere (Figs. 4b and 4d). This diagnosis of thethermodynamic budget provides some insight into howthe background circulation states may affect the devel-


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opment and maintenance of the anomalous circulationsin the two months. Nevertheless, such a diagnosis alonecan never fully establish causality. Further experimentswith an idealized model are required to determine betterthe effects of the climatological meridional wind on theatmospheric responses to a midlatitude SST anomaly.

5. Summary and discussion

A modeling study is conducted to examine the at-mospheric response to a midlatitude SST anomaly overthe North Pacific. To determine the sensitivity of theresponse to background circulation states, experimentsare performed under two perpetual model states, namelyJanuary and February. For each state, four pairs of96-month long integrations are made with and withoutthe positive SST anomaly. The results show that byshifting the basic state by one month completely dif-ferent responses to the SST anomaly are generated. InJanuary, the anomalous circulation is characterized bya trough immediately downstream of the SST anomalies.The trough is shallow and weakens with height. In Feb-ruary, the anomalous circulation is dominated by adownstream ridge growing with height. The patterns of

the anomalous heights in the two months are nearlyorthogonal. Furthermore, the anomalous temperature inJanuary spreads northward east of the dateline andgrows with height, whereas that in February is moreconfined to the Pacific.

Vorticity and thermodynamic budgets are diagnosedto determine the mechanisms sustaining the differentanomalous circulations. Analysis of the anomalous vor-ticity budget reveals that there is low-level convergenceand ascent over the SST anomaly in January and inFebruary. The anomalous convergence in January is bal-anced by a residual, R, presumably due primarily to theforcing by submonthly transients. This balance is com-pletely different from the linear theory given by Hoskinsand Karoly (1981) and the GCM results of Palmer andSun (1985). In February, the low-level convergence isbalanced mainly by the advection of planetary vorticity,consistent with that of Palmer and Sun.

Analysis of the anomalous thermodynamic budgetdemonstrates that the thermal balances are also main-tained by different mechanisms in the two months. Inparticular, this analysis suggests that the nature of thebalance depends on the climatological meridional flow.In January the climatological meridional wind down-



FIG. 12. The anomalous thermodynamic budget on a vertical cross section along 408N for January. (a)–(d) The anomalous zonal temperatureadvection, the meridional advection, the vertical advection, and the anomalous forcing Q according to Eq. (5). The contour interval is 0.2K day21.

stream of the SST anomaly (east of the dateline) is twiceas strong as that in February. Thus, it produces sufficientcold advection to balance the warm advection by thezonal wind. In February, the cold advection given bythe mean meridional wind east of the dateline is tooweak to offset the warm zonal advection. Therefore, ananomalous ridge develops.

The results of this study support the findings of Penget al. (1995) and demonstrate once again that the at-mospheric response to midlatitude SST anomalies isstrongly dependent on the background climatologicalflow. Moreover, the intensity of the meridional winddownstream of the SST forcing is recognized as beingperhaps one of the most critical factors in determiningthe nature of the anomalous circulation. The ‘‘warmSST-ridge’’ type of response is favored when the down-stream meridional wind is relatively weak. Apparently,this mechanism also applies to the Atlantic, where the‘‘warm SST-ridge’’ type of relationship is more pre-

ferred in early winter than in midwinter (Peng et al.1995; Peng and Fyfe 1996). The meridional wind overthe midlatitude Atlantic is much weaker in Novemberthan in January.

A question remains, however, as to the mechanismresponsible for generating the anomalous ridge in thepresence of weak meridional flow (February case). Fig-ure 2 shows that the weaker meridional flow in Februaryresults in a more zonal jet exit than in January. Tingand Peng (1995) performed linear model experimentsusing different GCM background flows and an idealizedanomalous heat source over the Atlantic. They showedthat the upper-level ridge response to the heating tendsto weaken the zonal winds in the jet exit, given a back-ground flow in which the jet exit is relatively zonal (theirNovember case). It was suggested that such a changein the jet exit, where transient-eddy vorticity fluxes areimportant, would modify those eddy fluxes so as toreinforce the ridge north of the jet. The diffluence in


MAY 1997 985P E N G E T A L .


the jet exit may also provide a barotropic source ofenergy for the zonally elongated anomalous ridge (e.g.,Hoskins et al. 1983).

We plan to use idealized models to determine whetherthe above mechanisms are indeed responsible for thedifference between the January and February responsesto anomalous heating. Experiments with a linear model,such as that used by Ting and Peng (1995), will allowus to explore the direct effects of the background flow,while a storm track model, such as that described byBranstator (1995), will allow us to address the questionof transient eddy feedback. Other factors that may in-fluence the response to the SST anomaly are the strengthof the zonal flow, the position of the SST anomaly rel-ative to the jet, and the direct influence of the SSTanomaly on the transient eddies.

Given the same model but a different adjacent month,the resulting differences in the basic states are largeenough to affect substantially the responses to the SSTanomaly. No doubt the differences among the clima-tological flows simulated by various GCMs can be much

larger even for the same resolution and the same cal-endar month. Therefore, the diverse results given byvarious midlatitude SST studies can perhaps be attrib-uted largely to the differences in the model’s backgroundflows. A strong sensitivity of the responses to the meanflow has a two-fold implication for the understandingof low-frequency variability in midlatitudes. On the onehand, the sensitivity exhibited in various models maybe indicative of a largely unpredictable system in nature.On the other hand, this sensitivity may be much am-plified at times in the models due to the unrealisticallysimulated background flows. The latter seems to be thecase for the MRF9 model over the Pacific. We find thatthe differences in the mean meridional flow betweenJanuary and February are enlarged in the MRF9 modelcompared to those in the observations. By comparison,the simulated meridional flow is more realistic in Feb-ruary than in January. This suggests that if the meanflow were better simulated in January the responses tothe SST anomaly in the Pacific might not be so dras-tically different in the two months. Thus, it is possible



FIG. 14. The decomposed anomalous meridional advection according to Eq. (6) for January (left panels) and for February (right panels).(a)–(b) The advection of anomalous temperature by the mean meridional wind. (c)–(d) The advection of mean temperature by the anomalousmeridional wind. The contour interval is 0.2 K day21.

FIG. 15. The climatological meridional wind alone 408N averaged over the four control runs for January (a) and for February (b). Thecontour interval is 2 m s21.


MAY 1997 987P E N G E T A L .

that as the mean circulations simulated by various mod-els converge to the observed climatology, their re-sponses to midlatitude SST anomalies will also con-verge and offer a more realistic view of the sensitivityin nature.

Acknowledgments. We gratefully acknowledge Dr.Arun Kumar for his valuable assistance with the MRF9model. His generosity in sharing the model scripts andhis patience in answering many questions have madethis study possible. He also provided us with the dataof monthly SST climatology. Helpful discussions withDrs. M. A. Alexander, J. Barsugli, G. Branstator, C.Deser, N. Hall, Y. Kushnir, M. Newman, and C. Penlandduring the course of this investigation are appreciated.Valuable comments made by the reviewers have im-proved the rigor and clarity of the presentation. Thiswork is supported by the NOAA Climate and GlobalChange Program. W. Robinson gratefully acknowledgesthe support of NSF Grant ATM-922578 and the hos-pitality of the Climate Diagnostics Center during hissabbatical.

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The Modeled Atmospheric Response to Midlatitude SST