Monitoring the transport through Monitoring the transport through Drake PassageDrake Passage

Mike MeredithMike Meredith

Chris HughesChris Hughes

Phil WoodworthPhil Woodworth

South Atlantic Workshop, May 2007

Overview:-Overview:-

• Why? Why?

• Drake Passage transport variability on a range of timescales:-Drake Passage transport variability on a range of timescales:-– 1) subseasonal1) subseasonal– 2) seasonal2) seasonal– 3) interannual3) interannual– 4) secular (briefly)4) secular (briefly)

• What is forcing and limiting the observed transport variability What is forcing and limiting the observed transport variability

• The present and possible future state of monitoring - The present and possible future state of monitoring - minimum requirements for a Drake Passage monitoring minimum requirements for a Drake Passage monitoring system?system?

Why? :-Why? :-

• Drake Passage is a key Drake Passage is a key chokepoint for the world’s chokepoint for the world’s largest current, the Antarctic largest current, the Antarctic Circumpolar Current (ACC).Circumpolar Current (ACC).

• Heat, salt, mass, freshwater, Heat, salt, mass, freshwater, nutrients etc. are moved nutrients etc. are moved between the Atlantic, Pacific between the Atlantic, Pacific and Indian Oceans, with and Indian Oceans, with consequences for global consequences for global climate, ecosystems etc.climate, ecosystems etc. => Therefore important to know => Therefore important to know

what variability the oceanic what variability the oceanic circumpolar transport exhibits on a circumpolar transport exhibits on a range of timescales, and how it range of timescales, and how it interacts with lower latitudes.interacts with lower latitudes.

Why?Why?

Large quantities of mass, Large quantities of mass, heat, salt etc moved around heat, salt etc moved around Southern Ocean, with links Southern Ocean, with links to lower latitudes.to lower latitudes.

Divergence between Divergence between chokepoints requires chokepoints requires knowledge of the knowledge of the chokepoint transports.chokepoint transports.

How do these change?How do these change?

(Ganachaud, Wunsch)(Ganachaud, Wunsch)

How do we monitor the transport?How do we monitor the transport?

Lots of methods have been tried…Lots of methods have been tried…

• Repeat hydrographic sections (CTD, ADCP, LADCP)Repeat hydrographic sections (CTD, ADCP, LADCP)• Repeat expendable bathythermograph (XBT) sectionsRepeat expendable bathythermograph (XBT) sections• Current meter arraysCurrent meter arrays• Tide gauge dataTide gauge data• Bottom pressure recorders (BPRs) Bottom pressure recorders (BPRs) • Inverted echo sounders (IESs)Inverted echo sounders (IESs)• Satellite altimetrySatellite altimetry• Satellite gravitySatellite gravity• etcetc

• Lots of different ways…Lots of different ways…

• But one of the most useful But one of the most useful (for reasons that will become (for reasons that will become clear) is to use bottom clear) is to use bottom pressure recorders (BPRs) pressure recorders (BPRs) and tide gaugesand tide gauges

• Antarctic network is not Antarctic network is not massive, but good coverage massive, but good coverage around >½ continentaround >½ continent

• ND2 and SD2 first deployed ND2 and SD2 first deployed during WOCE; now nearly during WOCE; now nearly 20 years of data.20 years of data.

How do we monitor the transport variability?How do we monitor the transport variability?

ND2

• Models suggest that sea level/bottom pressure adjacent to Antarctica Models suggest that sea level/bottom pressure adjacent to Antarctica should be a good index of circumpolar transport should be a good index of circumpolar transport

• Also that transport changes are genuinely circumpolar on these timescales, Also that transport changes are genuinely circumpolar on these timescales, and strongly steered by bathymetryand strongly steered by bathymetry

(Hughes, Meredith & (Hughes, Meredith & Heywood, JPO, 1999)Heywood, JPO, 1999)

How do we monitor transport variability?How do we monitor transport variability?

Deployments Deployments typically of 1-2 years typically of 1-2 years duration – main use duration – main use is for subseasonal is for subseasonal and seasonal and seasonal variability.variability.

BPRs deployed from BPRs deployed from ship; depths typically ship; depths typically 1000-3000m1000-3000m

Subseasonal variabilitySubseasonal variability

Subseasonal timescalesSubseasonal timescales

(Hughes et al., GRL, 2003;(Hughes et al., GRL, 2003;

see also Aoki, GRL, 2002)see also Aoki, GRL, 2002)• Highly coherent.Highly coherent.

• No discernable lag between forcing and response.No discernable lag between forcing and response.

• No discernable lag around Antarctica.No discernable lag around Antarctica.

• Strongly related to (predicted) Drake Passage transport.Strongly related to (predicted) Drake Passage transport.

OCCAM ¼º

Forcing for circumpolar transport Forcing for circumpolar transport variability is the Southern Annular Modevariability is the Southern Annular Mode

• See-saw in barometric pressure See-saw in barometric pressure between Antarctica and lower-between Antarctica and lower-latitudes.latitudes.

• Dominant mode of extra-tropical Dominant mode of extra-tropical atmospheric variability in Southern atmospheric variability in Southern Hemisphere.Hemisphere.

• Dominant timescales are ~10 days Dominant timescales are ~10 days and longer.and longer.

• Increasing, due to likely Increasing, due to likely anthropogenic causes (e.g. anthropogenic causes (e.g. Thompson and Solomon, Thompson and Solomon, ScienceScience; ; 2002; Marshall et al. 2002; Marshall et al. GRL, GRL, 2004).2004).850-hPa height regressed on SAM index850-hPa height regressed on SAM index

• Correlation of south Drake Passage BPR data with Correlation of south Drake Passage BPR data with eastward wind from NCEP reanalysiseastward wind from NCEP reanalysis

• Shows genuine circumpolarity of forcingShows genuine circumpolarity of forcing

What is forcing the variability?What is forcing the variability?

Subseasonal timescalesSubseasonal timescales

So what is the forcing for the northern Drake BPRs …?So what is the forcing for the northern Drake BPRs …?

Equatorial/tropical Pacific winds, rather than circumpolar SAM.Equatorial/tropical Pacific winds, rather than circumpolar SAM.

Subseasonal timescalesSubseasonal timescales

(Hughes and Meredith, 2006)(Hughes and Meredith, 2006)

Related to waves Related to waves propagating along shelf propagating along shelf and slope of South and slope of South America.America.

Evidence of baroclinicity Evidence of baroclinicity in wave structure in wave structure

=> choice of => choice of deployment depth is deployment depth is critical (unlike south critical (unlike south Drake)Drake)

North Drake BPR data North Drake BPR data also shows strong also shows strong correlation with correlation with transport of Malvinas transport of Malvinas Current at ~40˚S.Current at ~40˚S.

Lag is approximately 2 Lag is approximately 2 weeks – a different weeks – a different mode?mode?

(Vivier, Provost and Meredith, JPO, (Vivier, Provost and Meredith, JPO, 2001).2001).

Seasonal variabilitySeasonal variability

Seasonal timescalesSeasonal timescales

• Circumpolar winds have strengthened in recent decades, with Circumpolar winds have strengthened in recent decades, with change strongly seasonally modulated.change strongly seasonally modulated.

• Trends in month-by-month BPR data agree with those of the SAM. Trends in month-by-month BPR data agree with those of the SAM.

• => changes in the seasonality of the circumpolar winds are inducing => changes in the seasonality of the circumpolar winds are inducing changes in the oceanic circumpolar transport. (Anthropogenic?)changes in the oceanic circumpolar transport. (Anthropogenic?)

(Meredith et al., GRL, 2004)(Meredith et al., GRL, 2004)

Interannual variabilityInterannual variability

Interannual timescalesInterannual timescales

• Faraday tide gauge is best dataset for interannual variabilityFaraday tide gauge is best dataset for interannual variability• Annual means show significant correlation with SAM, and also OCCAM Annual means show significant correlation with SAM, and also OCCAM

transport, despite baroclinic variabilitytransport, despite baroclinic variability• Range of transport is quite small (~7 Sv, c.f. ~20 Sv from CTDs)Range of transport is quite small (~7 Sv, c.f. ~20 Sv from CTDs)• Aliassing is clearly an issue with CTD sectionsAliassing is clearly an issue with CTD sections

(Meredith et al., 2004)(Meredith et al., 2004)

So what sampling interval do we need to So what sampling interval do we need to get reliable annual means?get reliable annual means?

• Answer is: < 7 days, for 95% level!Answer is: < 7 days, for 95% level!• And this presumes zero measurement error…And this presumes zero measurement error…• In practice, need continuous data from In practice, need continuous data from in situ in situ instrumentation (BPRs etc)instrumentation (BPRs etc)

(Meredith and (Meredith and Hughes, GRL, 2005)Hughes, GRL, 2005)

• Sampling requirement alone demands Sampling requirement alone demands measurement interval of shorter than 1 week.measurement interval of shorter than 1 week.

• Significant measurement errors mean that even Significant measurement errors mean that even more rapid sampling is required.more rapid sampling is required.

• Which means that repeat CTD/XBT/ADCP sections Which means that repeat CTD/XBT/ADCP sections will not capture true seasonal or interannual will not capture true seasonal or interannual variability.variability.

• Neither will altimetry along a single groundtrack.Neither will altimetry along a single groundtrack.

• In practice, need continuous data from In practice, need continuous data from in situ in situ sources.sources.

• Some combination of BPRs/tide gauges/moorings Some combination of BPRs/tide gauges/moorings etc is requiredetc is required

Interannual timescalesInterannual timescales

• ACC accelerates as ACC accelerates as eastward wind stress eastward wind stress increasesincreases

• Circumpolar Eddy Circumpolar Eddy Kinetic Energy then Kinetic Energy then begins increasing, begins increasing, limiting the increasing limiting the increasing transport of the ACC.transport of the ACC.

(Meredith & Hogg, GRL, 2006)

Why does the ACC vary so little on interannual timescales?

““Secular” changesSecular” changes

Secular changesSecular changes

(Fyfe & (Fyfe & Saenko, Saenko, 2005)2005)

• Winds over Southern Ocean have increased dramatically in Winds over Southern Ocean have increased dramatically in recent decades…recent decades…

• Has ACC accelerated in response?Has ACC accelerated in response?• And/or moved southward?And/or moved southward?• Coarse-resolution models suggest there should be a change…Coarse-resolution models suggest there should be a change…

But how would we know…?But how would we know…?

• On interannual timescales, a change in SAM index of 1 gives a On interannual timescales, a change in SAM index of 1 gives a change in ACC transport of around 6 Sv…change in ACC transport of around 6 Sv…

• If same relationship holds true for longer timescales (?), acceleration If same relationship holds true for longer timescales (?), acceleration in ACC due to trend in SAM would be small compared to aliassing in ACC due to trend in SAM would be small compared to aliassing and measurement error.and measurement error.

• And tide gauges have trends all their own.And tide gauges have trends all their own.

The Present: monitoring is rather The Present: monitoring is rather disparate, both spatially and scientificallydisparate, both spatially and scientifically

UK CTD + BPR UK CTD + BPR (WOCE SR1b)(WOCE SR1b)

US XBT/ADCPUS XBT/ADCP

France CMFrance CM

WOCE SR1WOCE SR1Russia CTDRussia CTD Spain CTDSpain CTD

Argentina CTDArgentina CTD

WOCE A21WOCE A21

US PIESUS PIES

The Future:The Future:

• More strategic thought about what the separate More strategic thought about what the separate elements are telling us, and how they are connected.elements are telling us, and how they are connected.

• Commit to having instrumentation Commit to having instrumentation in situ in situ long-term. long-term.

• Special attention to northern boundary of Drake Special attention to northern boundary of Drake Passage, and signals passing to western boundary of Passage, and signals passing to western boundary of South Atlantic.South Atlantic.

• Trends (how?).Trends (how?).

• Smarter use of remote sensing (inc. GRACE?).Smarter use of remote sensing (inc. GRACE?).

• Monitor temperature and salinity concurrently with Monitor temperature and salinity concurrently with velocity if want variability in heat & freshwater fluxes.velocity if want variability in heat & freshwater fluxes.