Stanford GES 205 - Changes in ocean transport through Drake Passage

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Changes in the ocean transport through Drake Passage during the1980s and 1990s, forced by changes in the Southern Annular ModeMichael P. Meredith, Philip L. Woodworth, Chris W. Hughes, and Vladimir StepanovProudman Oceanographic Laboratory, Bidston Observatory, Wirral, UKReceived 1 August 2004; revised 18 September 2004; accepted 14 October 2004; published 6 November 2004.[1] We present the first direct evidence that interannualchanges in ocean transport through Drake Passage are forcedby variability in the Southern Annular Mode (SAM). Thisevidence is derived from two decades (1980s and 1990s) ofsubsurface pressure measurements from the tide gauge atFaraday station (western Antarctic Peninsula), combinedwith the output of an ocean general circulation model. Inrecent decades, the SAM has moved toward a higher-indexstate (stronger circumpolar winds); this trend is not simplymonotonic, but is the product of a long-term change in theseasonality of the SAM. Whilst we cannot address directlythe effect of the long-term trend on circumpolar transport,bottom pressure data from Drake Passage during the 1990sdemonstrate that ocean transport showed the same changesin seasonality as did the SAM. This offers a mechanism foratmospheric climate change to influence directly the large-scale ocean circulation.INDEX TERMS: 4207 Oceanography:General: Arctic and Antarctic oceanography; 4215 Oceanography:General: Climate and interannual variability (3309); 4532Oceanography: Physical: General circulation; 1635 GlobalChange: Oceans (4203); 4556 Oceanography: Physical: Sea levelvariations. Citation: Meredith, M. P., P. L. Woodworth, C. W.Hughes, and V. Stepanov (2004), Changes in the ocean transportthrough Drake Passage during the 1980s and 1990s, forced bychanges in the Southern Annular Mode, Geophys. Res. Lett., 31,L21305, doi:10.1029/2004GL021169.1. Introduction[2] The Southern Ocean is unique in being zonallyunbounded, with direct connections to all the other majoroceans. Consequently, the region is key to the globalclimate system, with large quantities of heat, salt andfreshwater being distributed by the Antarctic CircumpolarCurrent (ACC), the worl d’s largest current in terms ofvolume and mass transport [Macdonald and Wunsch,1996; Ganachaud and Wunsch, 2000]. Many studies havebeen devoted to trying to quantify the transport and/orvariability of the A CC at different longitud es. DrakePassage has been the most well-studied location, since itis where the ACC is constricted to its narrowest extent(Figure 1); consequently logistics are easiest, and compli-cations with possible additional flows from the subpolar orsubtropical gyres are avoided [Whitworth, 1983; Whitworthand Peterson, 1985; Meredith et al., 1996; Cunningham etal., 2003]. Typical values for transport through DrakePassage from hydrographic sections are of order 130 Sv(1 Sv = 106m3/s), however there remain significant ques-tions relating to the interannual and longer-period variabilityin transport, and its forcing.[3] At subseasonal timescales, the circumpolar transportvariability is predominantly barotropic (independent ofdepth), and hence is well-measured by bottom pressureor sea level (corrected for the inverse barometer effect)[Meredith et al., 1996; Hughes et al., 2003]. Strong cir-cumpolar coherence in transport has been demonstratedusing data from various Antarctic tide gauges and bottompressure recorder (BPR) deployments [Aoki, 2002; Hugheset al., 2003]. It h as also been demonstrated that thissubseasonal transport variability is forced (with undetect-able lag) by the varying circumpolar eastward windsassociated with the Southern Annular Mode (SAM;[Thompson and Wallace, 2000; Thompson et al., 2000]).This is the dominant mode of climate variability in theSouthern Hemisphere outside the tropics, and is character-ised by an oscillation in barometric pressure between anode over Antarctica, and a ring over the lower-latitudeSouthern Ocean at approximately 40–50°S. The eastwardwinds over the Southern Ocean change zonally with theSAM, time series of which resemble a red-noise processwith an e-folding timescale of 10 days and increasedpower at longer periods. Given the range of timescales overwhich the SAM operates, it can have teleconnections withdiverse other phenomena: links with ENSO and the semi-annual oscillation (SAO) have been proposed, for example.[4] Of more climatic importance than the subseasonalvariability are the lower frequency (interannual period andlonger) changes in circumpolar transport. The SAM hasbeen moving toward to a higher-index state over the pastthree decades [Thompson and Solomon, 2002], with asso-ciated stronger circumpolar winds; there is also significantinterannual variability in the SAM (Figure 2). Althoughmodelling studies have hypothesised a link between theSAM and circumpolar ocean transport on these longertimescales [Hall and Visbeck, 2002], observational evidenceof this has been crucially lacking. For example, previousinvestigations of Southern Ocean transport variability havenot demonstrated a link between interannual changes in theSAM and circumpolar transport changes in the ocean[Rintoul and Sokolov,2001;Cunningha m et al., 2003;Sprintall, 2003; Sokolov et al., 2004]. As noted by Visbeckand Hall [2004], there remains a need to understand thechanges in high-latitude ocean circulation and propertiesthat occur as consequences of variability in the SAM.[5] The trend of the SAM over the past three decades isnot simply monotonic, but has been strongly modulated byseason, being significant only for certain times of year(summer and autumn). It has been argued that this changein seasonality, and the trend to a higher-index state, areGEOPHYSICAL RESEARCH LETTERS, VOL. 31, L21305, doi:10.1029/2004GL021169, 2004Copyright 2004 by the American Geophysical Union.0094-8276/04/2004GL021169$05.00L21305 1of5consequences of anthropogenic influences, such as green-house warming or ozone depletion in the stratosphere[Kushner et al., 2001; Thompson and Solomon, 2002 ;Gillett an d Thompson, 2003]. It is of great interest tounderstand what effect this has had on ocean circulation.2. Methods[6] We have investigated variability in transport throughDrake Passage using sea level data from Faraday Station onthe west side of the Antarctic Peninsula (Figure 1); this is byfar the longest series of sea level data from Antarctica. Theinstallation at Faraday (now operated by the Ukraine, andrenamed Vernadsky) consists of a


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