Surface Currents and Upwelling in Kerguelen Plateau Regions M

Home , Kerguelen Plateau

Open Access , 3 usivity for es- ff Discussions 5 Biogeosciences ects on balances between , E. Kestenare ff 2 , and S. Blain 4 , and the upwelling fluxes in the sur- 1 − , I. Durand 2 , F. Carlotti 7.4 m d 4 6846 6845 ± , Y.-H. Park 2 usivity and upwelling on the Kerguelen Plateau and the ff , B. Queguiner 3 the plateau east of the Kerguelen Islands, and upwelling driven ff , F. d’Ovidio the Plateau in the upper 50 m based on surface drifters and below 1 ff , Y. Zhu , P. Van-Beek 3 1,4 This discussion paper is/has beenPlease under refer review to for the the corresponding journal final Biogeosciences paper (BG). in BG if available. University of Massachusetts Boston, Boston,LOCEAN-IPSL MA (CNRS/UPMC/MNHN/IRD), 02125 Paris, France LEGOS (CNRS/UPS/CNES/IRD), Toulouse, France MIO (AMU/STVU/CNRS/IRD), Marseille, France LOMIC (CNRS/UPMC), Banyuls-sur-Mer, France massive phytoplankton blooms areprocesses, such associated as with the Kerguelen enrichments Plateau region of (Blain DFe et al., by 2007; physical Park et al., 2008), These physical transport processes canbiogeochemical elements have and significant their e recycling processes. 1 Introduction The Southern Ocean is theworld largest High ocean, Nutrient and Low the Chlorophyll(DFe) primary (HNLC) within region production surface of is waters the (Martin limited et by al., the 1990; availability Coale of et dissolved al., iron 1996). In a few of places, ume transports o the mixed layer basedand on standard wind deviation stress of curl verticalplateau and are velocities deep driven 0.5 basin by regions and up windveyed 1.7 to stress Sv, plateau 3.2 respectively, curl and the averaged deep in mean basin the regions are approximately 0.7 and 1.1 Sv, respectively. II) cruise. These drifterscales, least-squares data fitted were streamfunctionswind analyzed for stress based estimating fields mesoscale on mean and autocovariancestimating currents, Ekman horizontal for pumping, dispersion temporal and ofbreak Taylor’s surface current single waters. on particle the The southern di results andport eastern have of shelf revealed surface slopes the waters of shelf- theinto from Kerguelen the the Islands, deep trans- Kerguelen–Heard basin shelf o by basin crossing wind the stress shelf curl slope in both the plateau and deep basin regions. The estimated vol- Mean currents, horizontal di deep basin east of thetion Kerguelen Experiment Islands were (WOCE) studied Standard usingduring Surface 48 the Velocity World 2011 Program Ocean austral (SVP) Circula- spring drifters KEOPS deployed II (KErguelen Ocean Plateau compared Study Abstract V. Sanial 1 2 3 4 5 Received: 7 April 2014 – Accepted: 14Correspondence April to: 2014 M. – Zhou Published: ([email protected]) 12 MayPublished 2014 by Copernicus Publications on behalf of the European Geosciences Union. Surface currents and upwelling in Kerguelen Plateau regions M. Zhou Biogeosciences Discuss., 11, 6845–6876, 2014 www.biogeosciences-discuss.net/11/6845/2014/ doi:10.5194/bgd-11-6845-2014 © Author(s) 2014. CC Attribution 3.0 License. 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ff er- ff shelf iron ff 0.034 nM) on the ± the Plateau. Of crit- ff the Kerguelen Plateau ciency and potential to- ff ffi but surrounded by strong 1 − 6848 6847 for part of the photosynthetic DFe uptake de- 1 − d 2 − leading to an open unbalanced iron budget. It is even 1 − d 2 − 77 nmolm ± and vertical mixing of DFe from deep convected waters for fertilizing the surface ff The KEOPS II project was aimed at the iron related biogeochemical and biological The ACC surface water west of the Kerguelen Islands is of the typical HNLC wa- The Kerguelen Plateau is a major barrier to the Antarctic Circumpolar Current (ACC) Dominant physical processes delivering DFe to the surface waters vary in di ter (DAC), a total of 48 WOCE standard SVP drifters were used in the KEOPS II study. and how is thethe DFe 2004 added KEOPS intofrom I the World study, surface Ocean the waters Circulationdrifters current in Experiment providing field the (WOCE) a in East SurfaceTrough conceptual Kerguelen the Current Velocity (Park Basin? circulation Program upper et al., In (SVP) pattern mixed 2008).SVP A consisting layer number drifters was of of can studies estimated the have play demonstratedthe a ACC, that Southern primary WOCE and Ocean role Fawn (Park inUS et National revealing al., Oceanic meso- and 2008; and Atmospheric Zhou Adiministrationand large-scale (NOAA) et Meteorological circulation Atlantic Laboratory al., Oceanographic in (AOML) 2006, Surface 2010, Velociy Program 2013). Data Working Assemble with Cen- of the Kerguelen Plateau(referred to as the its East south Kerguelen Basin and hereafter). west andprocesses by on the the Kerguelen Gallieni Plateau Spurical and East questions, to Kerguelen where its Basin are o north DFe rich shelf waters transported o enhanced phytoplankton blooms (BlainKerguelen et and al., Heard Islands, 2007).ered known This as as plateau the a area Kerguelen–Heard culcurrents between shelf de (Park the basin, sac et is with consid- al.,DFe sluggish 2008). at currents It a of was ratemand 3–5 estimated of of cms that 208 only 31 the nmolmless vertical clear mixing what can physicalthe contribute plateau processes east contribute of Kerguelen to Islands semi-surrounded DFe by supplies the northeastern in shelf the slope deep basin o 14 Sv. It meets theslope northward (Gille, 2003; western Davis, boundary 2005). currentjoins As the this along western ACC the forming boundary Kerguelen the current Plateau Crozet–Kerguelen moves Confluence northward, flowing it eastward. ter with low DFe while the ACC surface water on the plateau is DFe-enriched with ACC from the Weddell–Enderby Basin forms the Fawn Trough Current of approximately ward (McCartney and Donohue, 2007; Park et al., 2008). In the south, a branch of the a horizontal scale ofsures 100–1000 et s al., km 2013), (Dulaiovafertilization and et were the al., considered physical 2009; as mechanismspumping, the Hatta horizontal for results mixing et such between of al., water a combined masses,(Zhou 2013; large and topographic et Mea- potential scale al., steering, vorticity 2010, o Ekman conservation 2013;amount Frants et of al., 2013). iron Thesupplies from horizontal transport rely shelf on can deliver vertical areas large physical into processes or HNLC recycling regions, ofin iron. but in the downstream Southern areas, Indian DFe Ocean (Fig. 1). The ACC is deflected by this barrier north- the Kerguelen Islands, the resultsperiment from (CROZEX) the showed Crozet the Naturalruno importance Iron of Bloom both and Export horizontalwaters (Charette Ex- transport et of al., 2007). iron Inderived from the from southern Drake the Passage, the shelf DFe-rich north shelf water of Elephant Island fertilizes the southern Scotia Sea at ing iron additions delivereda fundamental by task physical in processes estimatingtal iron-carbon carbon into sequestration export the e from surface nature iron waters fertilization becomes processes inent the nature Southern iron Ocean. fertilization sites.pared Study During I the (KEOPS 2004 I), KerguelenKerguelen the Plateau Plateau low and DFe was surface Ocean enriched water com- 500 (less by m than relatively through 0.09 high vertical DFe mixing waters process (0.19–0.51 (Blain nM) et below al., 2007; Park et al., 2008a). West of (Zhou et al., 2010; Measures, 2013),et and al., Ross 2012). sea (Orsi As andwere both Wiederwohl, depleted, 2009; dissolved the Smith and mechanisms particulatetermine of iron the iron concentrations supplies scale in for anda surface horizontal pelagic timing waters and marine of ecosystem vertical a and fluxes export bloom de- of which particulate in organic turn carbon (POC). determine Estimat- the productivity of Crozet Basin (Park et al., 2002; Pollard et al., 2009), Drake Passage and Scotia Sea 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 ff = is − 0 i τ u ) is the value 0 t , x | t ( 0 i u usivity can be conducted di- ff . Furthermore, we approximate eddy 0 ) defined as ), (2) t τ y ( , ii components of horizontal velocities, are departures from the Lagrangian mean x R 0 j v = d 6850 6849 j (Sybrandy and Niiler, 1991; Niiler et al., 1987, at time , i 1 ( and x and ), (1) − 0 i y u ), (3) , i u ) ∞ x 0 ), τ t = , y i x , → | ( t x t , − t x 0 ( t )d , commensurate with the uncertainty. κ ( 1 τ 0 j − ≈ d + is the length of the time series. Time-averaged velocities usually ) ) t 0 ( wind stress curl data product from the NASA Quick Scatterometer t T 0 i ) represents the , ◦ u y x ) , | t → ∞ 0 x ( ) as t 0 i t ( , = 0 i ∞ u 0.25 x i T u κ Z 0 ( ( h × is the perturbation velocity, angle brackets are the ensemble average, is the coordinates ( κ 1 i T ◦ = i u x ) = i = t ) u , ) x ) of the particle passing through τ x ( usivity ( ( t ( − h Statistical analysis of mean Taylor’s single particle di Following the statistical definitions of drifter motion (Davis, 1991; Zhou et al., 2000), The temperature and salinity measurements were collected during the KEOPS II ( ff ii ∞ 0 i i ij the statistical decorrelation time scale ofof drifter the motion can Lagrangian be autocovariance estimated function from the roll-o R the 0.25 (QuikSCAT) between 1999 and 2007 were used (Risien and Chelton, 2008). to be less than 1 cms cruise using adepth Seabird (CTD) 911-plus sensors which rosette weretronics system calibrated Inc., Bellevue, before with WA, and USA). dual after Standardby the conductivity-temperature- data Seabird cruise processing Electronics procedures (Seabird Inc. recommended Elec- werecies applied of during temperature, salinity post and data depthspatial processing, and data and temporal meet coverage the the from accura- standards. the Because underway of wind data the collected limited during the cruise, 1995). Drifters were trackedposition using of a the drifter ARGOS wasinterpolation further onboard developed interpolated polar-orbiting by to satellites. US positions NOAA/AOML The everywe SVP 1 assume h DAC (Hansen the based on and maximum the Poulain, positioning800 optimal 1996). m, error If twice between the two daily standard fixes deviation, is the not error greater of than a daily averaged current is expected 2 Data and methods A total of 48 WOCEand SVP drifters November were 2011 deployed onboard duringcation the of numbers KEOPS the II (IDs) cruise RV of in Marionused these October in Dufresne. drifters this The are study locations shown werein in and all which Table mixed-layer identifi- 1 drifters drogue and that elements conformed Fig.40. centered to 2. These at the These drifters 15 WOCE m drifters can standards error provide follow for in water the wind motion drag conditions at area up the ratio to center great 10 of ms than the drogue within 1 cms and mesoscale fields influxes the of early DFe spring through season,vertical upwelling and quantitative in to estimates the study East of horizontal Kerguelen horizontal Basin. and transport vertical and mixing and The objectives using these SVP drifters were to map the large scale circulation pattern u di κ κ where velocity and displacement, respectively. The notation is such that pass-filter with a filterbe window determined of by 20 trials.than days The 70 (Fig. % mesoscale of 3). variations kinetic The less energy.one The than width order first 20 of zero-crossing of days of the magnitude autocovariances contributethe filter is less more filter window at window than the can selected. the 2 days, filter window applied, andrectly is through independent analyzing from variationsDavis of (1991), Lagrangian properties of drifters as outlined by where u the time lag, and substitute the estimates of ensemble-averaged velocitiesstudy (Poulain the and variations Niiler, 1989). of To drifter motion, mean motions are removed by applying a high- 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and x τ ) deter- L τ is satisfied by is the drag co- ∗ τ D ), we have E C v , E u , where ∗ τ 2 < approximately equal to 2 days L ∗ is the mixed layer depth. For the τ τ D is the sea surface elevation, ( ζ 6852 6851 is the gravity, . (11) g ! x . (8) ∂y ∂τ ) and Ekman wind-driven currents ( ) can be estimated by combining Eqs. (4) and (5), is the water density, and G , (4) y − v ) τ ) ρ y , G G G v u ∂x x y ∂τ u τ τ ρD ρD and − −

x , (5) v + + u 0. (10) 0. (7) ( ( τ , respectively. The Lagrangian integral timescale of 4 days is chosen f f = . (9) 1 ∂ζ ∂ζ ∂y ∂x . (6) = ) are the east and north components of wind, and − yy f ρD + g g W ∂ζ ∂x ! t t R u v v − − ∂ζ ∂y d y = d G d d g W can be approximated using the Lagrangian integral timescale ( x W , τ g = = ρD − u v y E ∂y ∂y W ∂τ 0, the zero crossings of the autocovariance functions. The Lagrangian auto- ∂v τ ρD − is the Coriolis constant, D D ∞ and u = ∂y = τ ∂v f v f u = + ρD C ρD C + f = = G E ) x − + ∗ xx + G = = = = G E τ f v f v t t u v R x y x y E ( ∂x cient. If we assume the wind ﬁeld is at the mesoscale so that it is geostrophically d d ∂x ∂τ d d After computing the geostrophic currents, we combine Eqs. (8) and (10) to compute From the geostrophic current relationship (Eq. 5), we have the non-divergent condi- Considering that a drifter moves with the water parcel surrounding it, the equation of ) are the wind stress, τ τ τ τ − f u − f u ∂u ij ﬃ y ∂x

( ∂u ( The wind stress ( for throughout this paper. motion for the water parcel can be written as (Zhou et al., 2000), where mined by the integration of1971). Eq. Inoue (1), (1950) and the Ogura autocovarianceR (1952) functions indicated (Monin that andcovariance Yaglom, functions of the 20 day filtered data show ponents from current measurements and provide geostrophic current estimates. the non-divergent mesoscaleestimates wind (Eq. stress 8). The by Ekman fitting divergence can a be computed streamfunction by to wind stress balanced, we have streamfunction associated withstreamfunction to Ekman filtered currents wind-driven during currents, interpolation will fitting remove ageostrophic a com- geostrophic where ( e To obtain geostrophic currents,(Bretherton we et fit al., the 1976; filtered Dorland current and data Zhou, with 2007). a Because streamfunction there is no geostrophic The wind stress is proportional( to wind field, i.e., where τ geostrophic current ( ( tion, ( and 5 5 20 15 10 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | yy κ and xx κ the plateau, ff ) at the bottom of . Both D yx w κ and xy κ ), the upwelling ( D and between yy 6854 6853 κ and xx κ , (13) , (12) z d E. These currents surround the East Kerguelen Basin with strong E E become negative in the shelfbreak current region and positive in the ◦ ∂y ∂v ∂y ∂v yx + κ + E E ∂x ∂u usivity tensor is computed based on Eqs. (2) and (3) shown in Fig. 7. There and ∂x the plateau and moved northward into the East Kerguelen Basin, and drifter ∂u ff xy ff − 0 κ Z D = = The di The mean current field is computed using 4 day ensemble averages as the La- Drifters on the Kerguelen Plateau moved northward (Fig. 4), converged into the Using the technique fitting a streamfunction based on Eqs. (7) and (10) warrants the D ∂z ∂w rent around the eastern shelfalong slope of the the Gallieni Kerguelen Islands, Spur ameandering strong associated at eastward with current 75 theeddy polar activities front, (Fig. 6). and a large scale southward are certain similarities between are significantly higher on the Kerguelenwhile Islands shelf region than thoseEast o Kerguelen Basin. at the Gallieni SpurBasin elucidating and the a strong mean mesoscale northward drifting activities of in the the surface Eastgrangian waters. integral Kerguelen scale is equal to(Inoue, 4 1950; days Ogura, estimated by 1952). 2 The times current of the field first consists zero of crossing a northward shelfbreak cur- 41 419 merged into theIslands, shelfbreak joined current along the the confluencealong shelf between the slope the east Gallieni of shelfbreak Spurward the current departing o Kerguelen and from Kerguelen ACC, Plateau,41 and 551 drifter flowed moved 39 slowly 969 in exitedinto the east- the Kerguelen-Heard Australian shelf Antarctic basin,deployed Basin. exited in In and an the flowed area East eastward lesscated Kerguelen (Fig. than Basin, 5). 10 an Regardless km circulating array moved northward of or southwestward southward, 5 for all drifters 5 these drifters days rejoined and then bifur- contribute more than 70 % ofis kinetic at energy. The 2 first days, zeroremove most crossing these of motions, of autocovariances a which 2 day are filter contributed is applied. by tidalsoutheastern currents slope and of inertial the motions. Kerguelen To Islands, and exited through 3 pathways: drifter variations, a 20 day filter is applied. The high frequency variations less than 20 days Using the continuity equation, i.e., The 2 day averagednorth trajectories of of the these Kerguelen driftersdetermined Islands by indicate the representing an autocovariances the eastwardrandomness computed ACC mean to in (Fig. understand current drifter 2). the motions nature The (Fig. of 2 mesoscale 3). day To average separate is the large trends and mesoscale variations for a view over the large scale pattern. 3 Results nondivergent conditions of geostrophic currentsthe and best mesoscale estimates wind of fields,filter these and is current gives applied and first windthe to fields. To remove streamfunctions have motions better are with estimates,fitting tidal computed a (Bretherton and 2 using et inertial day al., the motion 1976;interpolations periods, technique Dorland and could and based then be Zhou, on 2007). biasedour These the by study streamfunction-based least-squares a area. lack Butby of drifters, if spatial the there and streamfunction is temporal fitting a coverage can of consistent help data us large to in scale remove pattern random small-mesoscale observed repeatedly w where the mixed layer depth is assumed equal to 50 m in the computation. the mixed layer is equal to and integrating it over the mixed layer depth ( 5 5 25 15 20 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . E S 1 ◦ ◦ − and 1 − 7.4 md joining the E, a dome ◦ ± 1 S and 73 − ◦ . The southeastern 1 − E. These domes and depressions are ◦ 6856 6855 E, a depression around 72.75–73 ◦ E, and a convergent center at 48.5 ◦ joining the Crozet–Kerguelen Confluence. The western 1 in the longitude, and then the mean currents and sums of − E. Along the eastern shelf slope of the Kerguelen Islands, ◦ S and 72 ◦ 4 ◦ / . The shelf break current continued onto the Gallieni Spur and 1 − E, a dome around 72 ◦ the Kerguelen–Heard shelf basin (McCartney and Donohue, 2007). If ff E, and a depression around 74–75 ◦ E and then northward into the East Kerguelen Basin or northeastward into the , respectively. Overall, the mean and standard deviation of vertical velocities ◦ 1 the plateau, respectively. − ff in the latitude and 1 ◦ 8 The meteorological forcing on deep waters can be examined from the wind stress The upwelling and downwelling regions can be seen from the domes and depres- The nondivergent mean currents computed based on Eq. (7) and the streamfunction The acceleration and Coriolis force of a drifter are balanced by the surface gradients / the waters on thecross-slope Kerguelen currents Plateau carrying are the enriched DFe enriched with waters DFe, can both fertilize the downstream. shelfbreak and len Islands equivalent to ashelf slope northward region translation of the speed Kerguelenthe of Islands Kerguelen–Heard is 2 shelf cms a basin convergent where area the ofshelfbreak drifters the current surface accelerated along waters to in the 24 cms slope,at or a crossed speed the slope of into 12accelerated cms the up to East 40–50 Kerguelen cms Basin boundary current originated from the Fawnslope Trough Current region was o not clear in this eastern The drifter trajectories revealed thethe northeastward Kerguelen movement Plateau of (Figs. the 2, surfacelated 8 around waters and the on 9). station Drifter nearly 41 20approximately 551 days 60 before deployed it at days moved traveling Station slowly 100 A2 northward km recircu- (Fig. to 4). the It took southeastern shelf slope of the Kergue- tober and November period fromPlateau QuikSCAT data are show primarily that positive thenegative curls and region on the (Fig. the 11), curls Kerguelen that inand corresponds the o to an East Ekman Kerguelen downwelling Basin and upwelling form on a large 4 Discussions 4.1 Mean currents and residence times remarkably consistent with those estimatedAt upwelling the and eastern downwelling end areas of inmight this Fig. be transect, 9. associated the with warm the water waters mass north was found of in the thecurl, polar depth and front. which in turn the upwelling and downwelling. The mean wind stress curl in the Oc- around 73.5 slope around 71 circled by the red and gray3.9 enclosed md curves are computed approximatelydriven 3.3 md by wind stress curlThe waters averaged in in the Kerguelen–Heard these shelf twoand basin 72.5 exit circled eastward regions in the up area toAustralian around 49.5 Antarctic 3.2 Basin. Thealong mean their currents paths. are weak but persistent with upwelling sions of isopycnals along the west-east transect (Fig. 10): a depression at the shelf associated with thethen Crozet–Kerguelen turns Confluence southward exits at 75 fromthe the current converges Gallieni while Spur,a in and divergent the center East at(Fig. Kerguelen 48.5 Basin, 9). the The current areal forms mean a upwelling pair of velocities on the plateau and in the deep basin acceleration and Coriolis force are computedwere (Fig. in 8). the The eastern strongest pressure slopewith gradients of the the strong Kerguelen shelfbreak Plateau current and andthe Gallieni Crozet–Kerguelen Confluence. Spur apparent The areas sea currents associated surface and gradientsgeostrophic are nature of mostly these in currents. perpendicular indicating the primary fitting are shown inthe Fig. southern 9. and The eastern results shelf confirm slopes that of a the shelfbreak Kerguelen current Islands. flows The along strong current and wind stress in Eq.surface (4), and gradients can and be directly windstress computed. stress Because is we in simply cannot Eq. separate calledcompute (4), as the the accelerations the and apparent sum Coriolis sea force, of1 the surface drifter surface gradients data gradients for were binned and the in wind convenience. to To boxes of 5 5 20 25 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | in 3 − and the are very Nm 4 1 usivity and − − ∞ yx with strong ff s κ 2 10 1 − × are found in the cm and are found on the 7 ∞ . (16) yx on the plateau con- ∞ xy 10 ∞ κ 2 yy κ 1 κ × − i . Only part of northward v 1 h and ∂y − and ∂ ∞ xy , (15) κ ∞ xx represent the mesoscale fluc- 0 ∞ yy κ are found where the Reynolds p i κ v ∞ yx h + ∂y κ 2 ∂ and 0 0 v i 0 v v v and h are found in the East Kerguelen Basin , ∂x 0 ∂ − u ∞ xy ∞ yy i κ κ v h 6858 6857 ∞ yx ∂x ∂ κ 0 + and u 0 2 . Negative values of v is the seawater density in the surface layer. On the ∞ xx ∞ yy s i + κ κ u ρ i h . Assuming the longitudinal width of the plateau equal to u ∂y ) in the deep water can be estimated from the wind stress h 1 ∂ ∂y − V ∂ and and s 0 2 v ∞ y xy 0 ∞ xx . (14) κ d u κ + + ! f / -plateau transport. 2 x i ff u ) of the East Kerguelen Basin is approximately 100 km. The mixing represent the mean, and h ∂y i ∂τ L ∂x i u ∂ is on the order of magnitude approximately 5 h 0 u − ∂x h ∂ u 20days. (17) y 0 ∞ yx u κ ≈ ∂x ∂τ and ∞ xx

is equal to d i κ

2 xy u s and L β h ∼ − ∼ 1 κ ρ

∞ xy = t t ∼ erent from those of κ The Kerguelen Plateau lies in the westerly wind region which produces a northward The meridional transport ( Some shelf waters on the Kerguelen Plateau exited into the East Kerguelen Basin d d ff M dEKE dEKE The shelfbreak current is typicallyering hydrodynamically stable that (Zhou the et al., changework 2006). of Consid- done by mesoscale Reynolds eddy stress, kinetic i.e., energy (EKE) is proportional to the 4.2 Stability of currents and horizontal mixing shelf basin and the deepcontribution East to Kerguelen the Basin. o This transport is probably a significant where Kerguelen Plateau the wind stressOctober curl and November is (O’Neill positive et withtransport al., a is 2005; mean equal Risien of to and 5.6 0.84 Chelton, m 300 2008), km the wide, meridional the northwarddepth Sverdrup of transport Sverdrup is transportwaters, approximately is this 1.7 Sv. limited transport Because by may the the not buoyancy be of limited these by downwelled deep surface depth and the slope between the others crossed the slope into the East Kerguelen Basin. curl based on the Sverdrup theory, that is, βV those in the shelfbreak current have the shortest residencemovement time. in the upper mixed layerPlateau in are general. consistent The with drifter the trajectories northwardthe on wind Kerguelen the Plateau driven Kerguelen current is (Fig. approximately 300 8).only km Considering 20 wide that km and the wide, shelf ifverges slope into all approximately a the narrow northward slope,sluggish it sluggish currents will current produce on of a the 2 current cms Plateau of 30 converges cm into the strong current on the slope, and and slowly movedmesoscale northward eddies and meanders. and TheseBasin surface eastward waters approximately resided at 20–30 in days the approximatelythe before East Kerguelen–Heard their 6 Kerguelen Shelf cms basin departures. have the Thus,in longest the residence the surface time East of waters Kerguelen 30–70 in Basin days, those have the intermediate residence time of 20–30 days, and time scale crossing the East Kerguelen Basin can be estimatedT as energy from mesoscale eddies for increasingKerguelen the Basin, strength of high mean currents. positivestress In values the transfers East of kineticof energy from meanhorizontal current scale shears ( to EKE. The mean value In general, Reynolds stressto transfers random kinetic energy smaller fromshelf scale slope larger fluctuations. presuming scale High that mean strongin values currents shelfbreak EKE. of currents Relative fluctuate leading lowwhere to values there an of is increase adi lack of strong meanshelf slope currents. region The implying that patterns stable of shelfbreak currents are capable to absorb kinetic where tuations. As the Reynoldsmean stress current can shear, be we written have as the product of eddy di 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the Kergue- , respectively, ff 1 , respectively. − 1 − over a section of 1 − . The transports from 1 − 7.4 md ± and 3.9 md 1 − , probably due to the smoothened wind E. This large scale meander around the 1 ◦ − in October and November. The upwelling 3 − 6860 6859 Nm 4 − the plateau in the upper 50 m specified based on sur- 10 ff × plateau deep basins. the plateau into the East Kerguelen Basin. The shelfbreak ff ff 0.82 − the Kerguelen Plateau. Most of those exiting drifters continued to move northward ecting their balances in o The upwelling estimates from drifter data are significant higher than that of over- The surface waters in the Kerguelen–Heard shelf basin were transported northward This value is equivalent to the mean advection time scale of 20–30 days in the East ff ff waters were transported o current turned eastward along the Gallieni Spur, and then recirculated southward after 5 Conclusions Drifters have revealed avicinity. detailed Approximate circulation 0.5 Sv pattern of inplateau the drifted the northeastward surface Kerguelen driven water Plateau bythe and and the positive 1.7 wind westerly its Sv stress wind of curl, and respectively.at the Both Ekman the the deep pumping southeastern surface due shelf water and slope to deep inwaters of waters the the converged joined Kerguelen Islands the during shelfbreak our study current, period. Some flowing along the slope northward while some Thus, the horizontalKerguelen and Basin vertical are volume equivalent,volume 0.5 fluxes fluxes and may into 1.1 play Sv, the aa respectively, significant surface a role total in waters transporting of in biogeochemical 1.6 Sv. elements the and These East 100 km, the volume transportface o drifters will bea approximately northward 0.5 Sv. Sverdrup In transport thelen of depth Plateau 1.7 Sv into below taking the the East DFetransported mixed Kerguelen rich layer, into Basin. deep there the A waters East is total o Kerguelen of Basin. 2.2 Sv of waterssmoothened on wind the stress plateau curl. The is upwellingPlateau fluxes in and the circled in areas on East the Kerguelen Kerguelen Basin are approximately 0.7 and 1.1 Sv, respectively. of vertical velocities averaged from drifter data are up too 3.2 while a fewface of current them crossing directly moved the eastward slope (Figs. approximately 2 equal and to 9). 10 cms Considering the sur- and in the combined plateau and deep basin region, the mean and standard deviation East Kerguelen Basin regions are approximately 3.3 md been observed from QuikSCAT (Fig. 11) (O’NeillCambra et et al., al., 2005; 2014). Risien This andin negative Chelton, the wind 2008; East stress Kerguelen curl Basin, will anddoming produce in isopycnals an turn shown Ekman produce in divergence an Fig. upwellingBasin 10. which is is The implied mean approximately from wind the stressdriven curl by this in wind this stress Eastdata curl Kerguelen is with only a 6.4 cmd 70high km spatial filter. variability The and upwelling the areal fluxes means computed of from upwelling drifter in the data Kerguelen have Plateau much and lieni Spur, and circulates southward aroundEast 75 Kerguelen Basin has been observedand and Donohue, discussed in 2007; several Park studies et (McCartney ically al., balanced 2008b). meander, To the accommodateguelen this sea Basin. large surface scale This height geostroph- sea mustor be surface a depressed gradient negative wind in can stress the only curl East be region Ker- maintained centered at by the a East cyclonic Kerguelen wind, Basin which has 4.3 Upwelling and cross-slope transport The currents areface primarily slope geostrophically and balanced Coriolis forcenumbers between (Fig. are the 8). generally The apparent small. accelerationsder sea These are around 48 sur- small, the drifters East that Kerguelen elucidate is,slope Basin. the a The of Rossby persistent meander the starts cyclonic from Kerguelen mean- eastward the along Islands southeastern the as shelf shelf slope, the exits shelfbreak into current, the Australian flows Antarctic northward Basin along and the then Gal- Kerguelen Basin from the trajectories ofward drifters or 39 eastward 969 and with 41 athe 551 mean which mean moved current north- current of approximatelymagnitude and in 6 cms horizontal the East mesoscale Kerguelen eddy Basin. mixing are on the same order of 5 5 25 20 10 15 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 2007. 10.1038/nature05700 ect of natural iron fertilization on ff 6862 6861 , 2009. ths, F. B., Guigue, C., Guillerm, C., Jacquet, S., Jeandel, the plateau into the East Kerguelen Basin by either ffi ff the plateau. The surface and deep waters on the Kergue- ff This project was supported by the French Agency of National Research 10.1029/2008GB003406 data, J. Atmos. Ocean. Technol., 13, 900–909, 1996. GB4014, doi: Analysis of horizontal and verticallayer in processes southern contributing Drake to Passage, natural Deep-Sea iron Res. supply Pt. in II, the 90,velocities, mixed and 69–76, topographic 2013. steering, J. Phys. Oceanogr., 33, 1167–1181, 2003. drive biological productivity in the southern Drake Passage. Global Biogeochem. Cy., 23, 1991. by autonomous floats, J. Phys. Oceanogr., 35, 683–707, 2005. Hiscock, M. R., Demarest,Hunter, M., C. Hiscock, N., W. T., Elrod,A. Sullivan, V. E., K. A., Herndon, F., Tanner, Fitzwater, S. J.,S. S. J., Brewster, L., E., J., Gordon, Selph, Jones, Ladizinsky, R. K. N., J. M., enrichment E., Smith, L., Sheridan, experiment: G., C. carbon Tozzi, Cooper, C., S., cycling2004. Twining, D., in Koblizek, B. Timothy, M., high- S., D., and and Brown, Roberts, Johnson, low-Si Z. waters, I.: Science, Southern 304, ocean 408–414, iron len Plateau Region, Biogeosciences, in preparation, 2014. and Garabato, A. C. N.: Radiumphytoplankton isotopes bloom, as Deep-Sea tracers of Res. iron Pt. sources II, fueling 54, a Southern 1989–1998, Ocean 2007. Cochlan, W. P., Millero, F.M., J., Altabet, Falkowski, P. M. G.,Chase, A., Bauer, J. Z., Hales, E., Strutton, B. Wanninkhof, P. E., R. G., Takahashi, H., Friederich, T., Kudela, G. Landry, R. M. E., R., Gorbunov, M. Bidigare, Y., R. Lance, R., V. P., Wang, Hilting, X., A. K., J. W., Vincent, D., Viollier, E.,carbon Vong, sequestration L., in and the Wagener, Southern T.: E Ocean, Nature, 446, doi: Brussaard, K., Carlotti, F., Christaki,Garcia, U., N., Corbière, Gerringa, A., L. Durand,C., J. I., Laan, A., Ebersbach, P., Gri Lefèvre, F., D., Fuda, Lomonaco,Picheral, J. C., M., L., Malits, Pondaven, A., P., Remenyi, Mosseri,L., T., J., Souhault, Sandroni, M., Obernosterer, V., Thuillers, I., Sarthou, D., Timmermans, Park, G., K. Y. R., H., Savoye, Trull, N., T., Uitz, Scouarnec, J., Van-Beek, P.,Veldhuis, M. tica, J. Geophys. Res., 106, 22401–22423, 2001. a mean crossing slopedeep current water or on mesoscale thethe plateau horizontal west is mixing. of originated Considering the from thatand Kerguelen the contacting Plateau, the with DFe it sediments deficient was on ACCBasin the enriched deep plateau, by with and water DFe either transported from through into shelfbreakpersistent the remineralization current, upwelling East in Kerguelen crossing the slope Easting current Kerguelen these Basin or deep may DFe mesoscale play rich the mixing. waters critical The into role the in surface transportwaters. aries of an Ekman divergentapproximately zone 1.1 within Sv. the This East cyclonic Kerguelenkinetic Basin meander energy with is from an mesoscale stable upwelling horizontal and of shears.the strengthened Strong East horizontal by Kerguelen mixing absorbing Basin was found o in len Plateau were transported o the current exited from the ridge. This large scale cyclonic meander marked the bound- Hansen, D. V. and Poulain, P. M.: Quality control and interpolations of WOCE-TOGA drifter Gille, S. T.: Float observations of the Southern Ocean, Part I: Estimating mean fields, bottom Frants, M., Gille, S. T., Hatta, M., Hiscock, W. T., Kahru, M., Measures, C. I., and Zhou, M.: Dulaiova, H., Ardelan, M. V., Henderson, P. B., and Charette, M. A.: Shelf-derived iron inputs Davis, R. E.: Intermediate-depth circulation of the Indian and South Pacific Oceans measured Davis, R. E.: Observing the general circulation with floats, Deep-Sea Res., 38, S531–S571, Charette, M. A., Gonneea, M. E., Morris, P. J., Statham, P., Fones, G., Planquette, H., Salter,Coale, I., K. H., Johnson, K. S., Chavez, F. P., Buesseler, K. O., Barber, R. T., Brzezinski, M. A., Cambra, R., Carranza, M., Gille, S. T., and Morrow, R.: Wind-induced Upwelling in the Kergue- Blain, S., Quéguiner, B., Armand, L., Belviso, S., Bombled, B., Bopp, L., Bowie, A., Brunet, C., Amos, A. F.: A decade of oceanographic variability in summertime near Elephant Island, Antarc- References grant (ANR-09-CEXC-006-01) to M.dation Zhou (ANT-0948 and 378). F. MZ Carlotti, wouldfor and like their to the kind acknowledge US support the by National support providingthe from Science 48 US cruise. Foun- SVP NOAA-AOML-DAC drifters This and manuscript sendingand daily is interdisciplinary drifter a marine data sciences tribute during conducted and to after in P. this P. Niiler Southern Indian for Ocean his project. hearty support to unconventional Acknowledgements. 5 5 30 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , 2009. 10.1038/nature07716 6864 6863 usion in the atmosphere, J. Meteor. Soc. Jpn., 30, 23–28, ff southern Drake Passage, Deep-Sea Res. Pt. I, 57, 1039–1048, 2010. field Strait, Antarctica, Deep-Sea Res. Pt. I, 53, 1244–1252, 2006. Reference 91/6, Scripps Inst. Oceanogr., Univ. Cali., San Diego, 1991. observations, J. Geophys. Res., 105, 21893–21910, 2000. Northern Kerguelen Plateau, Deep-Sea Res. Pt. II, 55, 566–581, 2008b. tica: seasonal iron limitation in Antarctic11336, shelf 2000. waters, J. Geophys. Res.-Oceans, 105, 11321– a sea of change, Oceanogr., 25, 90–103, 2012. 778–795, 2009. circulation in the Crozet1842, Basin 2002. during the Antares4 cruise, Deep-Sea Res. Pt.mixing II, over the 49, Kerguelen 1823– Plateau, Deep-Sea Res. Pt. II, 55, 582–593, 2008a. system using satellite-tracked drifters, J. Phys. Oceanogr., 19, 1588–1603, 1989. from eight years of QuikSCAT scatterometer data, J. Phys. Oceanogr., 38, 2379–2413, 2008. surements of the atmospheric boundaryReturn layer Current, response J. to Climate, SST 18, variations 2706–2723, along 2005. the Agulhas J., Allen, J. T.,French, Baker, M., A. Hickman, R.,Morris, A. Bakker, D. P. E., J., C. Holland, Nedelec, E.,Read, R. F. Charette, J. H., F., J., M. Seeyave, S., Nielsdottir, Hughes, Smith, A., M., T.,J., Stinchcombe, Fielding, J. Williamson, M., Planquette, S., A., R., Taylor, S., H., and Thomalla, Fones, Jickells, S. Popova, Zubkov,by G. M. J., T. E. natural Venables, R., V.: D., H. iron Southern E., fertilization, Lampitt, Ocean Poulton, Nature, deep-water R. 457, A. carbon 577–580, S., export J., doi: enhanced Deep-Sea Res., 34, 1867–1881, 1987. water-following capability of Holey-sock1964, and 1995. TRISTAR drifters, Deep-Sea Res., 42, 1951– 1952. Press, Cambridge, Massachusetts, and London, England, 769 pp., 1971. 158, 1990. Prog. Oceanogr., 75, 675–750, 2007. Processes in Delivering Dissolved Iron toDeep-Sea the Res. HNLC Pt. waters of II, the 90, Drake 77–88, Passage, Antarctica, 2013. from the shelfaustral-winter regions 2006, near Deep-Sea Res. the Pt. South II, 90, Shetland 89–101, Islands 2013. in the Drake Passage during the Zhou, M., Zhu, Y., Dorland, R. D., and Measures, C. I.: Dynamics of the current system in the Zhou, M., Niiler, P. P., Zhu, Y., and Dorland, R. D.: The western boundary current in the Brans- Zhou, M., Paduan, J., and Niiler, P. P.: The surface currents in the Canary Basin from drifter Pedlosky, J.: Geophysical Fluid Dynamics, Springer-Verlag, New York, 624 pp., 1987. Sybrandy, A. L. and Niiler, P. P.: WOCE/TOGA Lagrangian drifter construction manual, SIO Park, Y.-H., Roquet, F., Durand, I., and Fuda, J.-L.: Large-scale circulation over and around the Smith Jr., W. O., Sedwick, P. N., Arrigo, K. R., Ainley, D. G., and Orsi, A. H.: The Ross Sea in Sedwick, P. N., DiTullio, G. R., and Mackey, D. J.: Iron and manganese in the Ross Sea, Antarc- Park, Y.-H., Fuda, J.-L., Durand, I., and Naveira Garabato, A. C.: Internal tides and vertical Risien, C. M. and Chelton, D. B.: A global climatology of surface wind and wind stress fields Park, Y.-H., Pollard, R. T., Read, J. F., and Leboucher, V.: A quasi-synoptic view of the frontal O’Neill, L. W., Chelton, D. B., Esbensen, S. K., and Wentz, F. J.: High-resolutionOrsi, satellite A. mea- H. and Wiederwohl, C. L.: A recount of Ross Sea waters, Deep-Sea Res. Pt. II 56, Pollard, R. T., Salter, I., Sanders, R. J., Lucas, M. I., Moore, C. M., Mills, R. A., Statham, P. Poulain, P.and Niiler, P.P.:Statistical analysis of the surface circulation in the California Current Ogura, Y.: The theory of turbulent di Niiler, P. P., Sybrandy, A. S., Bi, K., Poulain, P. M., and Bitterman, D. M.: Measurements of the Niiler, P.P.,Davis, R. E., and White, H. J.: Water-following characteristics of a mixed layer drifter, Monin, A. S. and Yaglom, A. M.: Statistical Fluid Mechanics: Mechanics of Turbulence, MIT McCartney, M. S. and Donohue, K. A.: A deepMeasures, cyclonic C. gyre I., in Brown, the Australian–Antarctic M. Basin, T., Selph, K. E., Apprill, A., and Zhou, M.: The Influence of Shelf Martin, J. H., Gordon, R. M., and Fitzwater, S. E.: Iron in Antarctic waters, Nature, 345, 156– Hatta, M., Measures, C. I., Selph, K. E., Zhou, M., Yang, J. J., and Hiscock, W. T.: Iron fluxes 5 5 20 30 15 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | m

2000 6000 0 4000

Basin Kerguelen Basin

East Australian Antarctic

4650 4.19 7.4250 5150 24.68 65 56.82 50 41.71 65 0.37 50 40.68 66 51.09 50 39.39 68 41.41 37.8850 70 27.91 50 11.07 72 12.64 49 0.23 2.64 7149 48.15 22.25 7150 38.28 70 5.19 49 11.96 70 46.22 48.2349 72 30.74 49 36.97 72 8.90 49 28.92 12.89 7249 16.98 72 13.28 48 9.97 72 15.46 48 57.32 15.78 72 43.6348 72 17.02 48 40.32 72 17.50 48 35.22 16.25 7248 28.95 72 15.64 47 10.21 72 14.16 47 58.56 72 12.59 19.9046 71 4.18 48 50.89 71 58.85 48 48.09 42.07 7148 48.76 72 30.79 48 52.94 71 10.27 25.6348 71 56.4 48 25.83 72 51.67 48 29.92 8.06 7248 30.50 72 16.24 48 27.9 72 15.83 48 29.18 8.2 72 28.0348 72 12.32 48 27.99 72 32.99 48 27.987 47.43 7348 28.32 73 6.74 48 36.48 22.87 7448 36.64 73 59.93 57.0149 73 50.45 49 9.77 73 50.2 48 25.60 19.85 7348 45.90 72 0.73 50 45.90 71 36.95 50 18.57 71 25.49 38.51 72 25.49 72 0.82 3.781 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − Heard ‐ 6866 6865 A2 Basin E1 Island Kerguelen

ID41 33841 Date/time 323 15 (UTC) Oct 2011 06:55 41 17 346 Oct 2011 12:01 41 418 17 Latitude Oct (S) 2011 15:02 41 419 17 Oct 2011 17:35 41 417 18 Longtitude Oct (E) 2011 02:33 41 551 18 Oct 2011 07:29 41 18 549 Oct 2011 12:35 41 562 18 Oct 2011 19:46 41 559 18 Oct 2011 21:51 41 561 19 Oct 2011 00:05 39 969 19 Oct 2011 01:29 41 560 21 Oct 2011 05:40 40 21 676 Oct 2011 10:06 41 232 21 Oct 2011 12:12 40 686 21 Oct 2011 16:38 41 235 21 Oct 2011 18:41 41 109 21 Oct 2011 20:15 92 995 22 Oct 2011 00:55 92 22 996 Oct 2011 09:37 93 022 22 Oct 2011 10:02 93 021 22 Oct 2011 10:46 93 020 22 Oct 2011 13:39 81 840 22 Oct 2011 19:22 93 019 23 Oct 2011 00:24 81 23 839 Oct 2011 09:04 81 838 23 Oct 2011 15:50 81 836 24 Oct 2011 05:28 81 835 24 Oct 2011 06:16 81 837 24 Oct 2011 06:40 37 28 656 Oct 2011 11:21 37 640 28 Oct 2011 11:51 37 660 28 Oct 2011 12:15 37 651 28 Oct 2011 12:50 41 797 28 Oct 2011 13:40 41 802 1 Nov 2011 12:40 41 1 796 Nov 2011 13:10 37 543 1 Nov 2011 20:41 41 800 2 Nov 2011 03:11 41 798 2 Nov 2011 18:40 41 795 8 Nov 2011 05:50 41 784 8 Nov 2011 05:51 41 8 783 Nov 2011 09:14 41 794 8 Nov 2011 11:15 41 786 8 Nov 2011 13:24 41 785 12 Nov 2011 10:10 41 787 12 Nov 2011 10:15 37 649 15 Nov 2011 13:49 17 Nov 2011 11:54 Islands

Heard Kerguelen IDs, times and locations of 48 WOCE SVP drifters deployed. Bathymetry in the Kerguelen Islands vicinity. False gray colors represent bottom depths; Fig. 1. depth contours are 100,of 500, 1000, SVP 2000, drifters. 4000Kerguelen The and Basin inserted 6000 is m; made ﬁgure and only black for shows the dots the discussion. are enlarged deployments study area. The name of the East Table 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and u m

2000 6000 0 4000 6868 6867 Time lag (days) autocovariances, respectively. Black and purple lines v and Island

Heard Island Kerguelen

autocovariances after a 20 day low-pass ﬁlter is applied, respectively.

) s Autocorrelation (cm

‐2 2 and u Autocovariance functions and standard deviations. Red and blue solid lines are Trajectories of SVP drifters deployed during the KEOPS II cruise. Black dots are deploy- autocorrelations after a 20 day high-pass ﬁlter is applied, respectively, and yellow and green Fig. 3. bands are standard deviations of v are the Fig. 2. ment locations, and black lines are drifter trajectories. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | m

0 2000 4000 6000 shelf transport and elapse ﬀ shelf transport from the Kerguelen ﬀ 39969 41551 E2 6870 6869 41419 A2

IDs

39969 41419 41551 Drifter Kerguelen Islands Drifter 37640 37651 37656 37660 81837 Kerguelen Islands Trajectories of 5 drifters (37 640, 37 651, 37 656, 37 660 and 81 837) which were de- Trajectories of 3 drifters (39 969, 41 419, 41 551) showing o ployed in a grida less classic than bifurcation demonstrating 10 kming the waters within stability and of a current the time divergence shelfbreak near window current the less entrapping deployment location. than surround- 2.5 h. The trajectories show Fig. 5. Fig. 4. plateau. The black dots indicateare the elapse time location (days) of after deployments, deployments. and numbers next to trajectories time along their trajectories. Drifterdrifters 41 419 39 969 represents and the pathway 41 551 of a represent shelfbreak pathways current, of and direct o Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and i ) 0 t , 1 0 1 1 2 0 x ‐ | t m

− 6000 0 2000 4000 0 t ( 0 j d ) 0 t , x | 0 t ( 0 i ν h ‐1 ‐1 s =

2 ) 2 x ( cm cm/s cm yy

κ xy 8  8   50  ×10 Kerguelen Islands Kerguelen Islands ×10 1 1 2 0 0 1 ‐ 6872 6871 ). y , x = j , i )(

∞ τ ‐1 ‐1 ∞ s s usivity tensor deﬁned by

t ﬀ 2 2 , x ( cm cm xx yx κ  8  8   ≈ ) ×10 Kerguelen Islands ×10 Kerguelen Islands Kerguelen Islands → ∞ t Horizontal di , x ( 4 day mean currents in the study area from SVP drifters deployed during the cruise. κ ) x ( ∞ κ Fig. 7. Fig. 6. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 0 40 -20 20 -40 m

0 4000 2000 m/d 50 cm/s 2 ‐ 1 s ‐

50 0.01 . 1 − 7.4 md ± 6874 6873

) (red arrows). f u + , respectively, and the upwelling volume ﬂuxes in the circled plateau and t 1 70° 72° 74° 76° 78° 80° d − Kerguelen Islands v/ ,d Islands Kerguelen f v − 68° t and 3.9 md d 47° 48° 49° 50° 51° 1 The geostrophic currents estimated from drifter data (arrows) and the upwelling veloc- Binned mean currents from SVP drifters (black arrows) and apparent sea surface gradi- u/ − ities estimated (false colors).CTD Depth stations contours for are the 500,and in transect 1000, the 1500 in deep and basin Fig. circled3.3 2000 10. md m. by the The Red red areal dots and gray are mean enclosed upwelling curves are velocities computed on approximately the plateau Fig. 9. basin areas are 0.7 andin 1.1 the Sv, combined respectively. plateau The and mean basin and area std are of 3.2 vertical velocities averaged ents (d Fig. 8. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4 3 2 34.6 34.2 33.8 27.6 27.2 26.8 4 0 8 ‐4 ‐8 ‐3 ), and potential density S N m ‐4 ), salinity ( θ Up‐ Down‐welling Down‐ 6876 6875 Up‐ Islands

Down‐ Oct‐Nov mean wind stress curl ×10  (C) S  Kerguelen  70°E 71°E 72°E 73°E 74°E 75°E 0 0 0 500 500 500 Mean wind stress curl in the Kerguelen Plateau and East Kerguelen Basin from West-East transects of potential temperature ( 1000 1000 1000 ). θ σ Fig. 11. QuikSCAT data products (O’Neill et al., 2005; Risien and Chelton, 2008). Fig. 10. (