Dynamics of the current system in the southern Drake Passage

Deep-Sea Research I 57 (2010) 1039–1048

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Dynamics of the current system in the southern Drake Passage Meng Zhou a,n, Yiwu Zhu a, Ryan D. Dorland a, Christopher I. Measures b a b

University of Massachusetts Boston, Boston, MA 02125, USA University of Hawaii at Manoa, Honolulu, HI 96822, USA

a r t i c l e in fo

abstract

Article history: Received 24 March 2009 Received in revised form 1 April 2010 Accepted 27 May 2010 Available online 2 June 2010

It has long been seen from satellite ocean color data that strong zonal gradients of phytoplankton biomass persistently occur in the southern Drake Passage during austral summer and fall, where the low productivity Antarctic Surface Water (ASW) within the Antarctic Circumpolar Current (ACC) region transforms to the high productivity water. An interdisciplinary cruise was conducted in February and March 2004 to investigate potential physical and biogeochemical processes, which are responsible for transporting nutrients and metals and for enhancing primary production. To explore physical processes at both the meso- and large-scales, surface drifters, a shipboard Acoustic Doppler Current Profiler and conductivity–temperature–depth sensors were used. Analyzing meso- and large-scale hydrography, circulation and eddy activities, it is shown that the topographic rise of the Shackleton Transverse Ridge plays the key role in steering an ACC branch southward west of the ridge, forming an eastward ACC jet through the gap between the ridge and Elephant Island and causing the offshelf transport of shelf waters approximately 1.2 Sv from the shelf near Elephant Island. High mesoscale eddy activities associated with this ACC southern branch and shelf waters transported off the shelf were found. The mixing between the iron-poor warmer ASW of the ACC and iron-rich waters on the shelf through horizontal transport and vertical upwelling processes provides a physical process which could be responsible for the enhanced primary productivity in this region and the southern Scotia Sea. Published by Elsevier Ltd.

Keywords: Southern Ocean Drake Passage Elephant Island Antarctic Circumpolar Current Shelf water Mesoscale Jets Eddies Mixing

1. Introduction The Southern Ocean section in the southern Drake Passage exhibits complex topographic features of the Shackleton Transverse Ridge, shallow banks, South Shetland Islands, Elephant Island and Bransfield Strait (Fig. 1). The large scale circulation and water types have been well studied (Deacon, 1933, 1937; Hoffmann et al., 1996; Nowlin and Klinck, 1986; Orsi et al., 1995), and the relationships between deep waters and their potential paths have been extensively discussed (Brandon et al., 2004; Garabato et al., 2002; Heywood et al., 2004). Recently, the detailed mesoscale interactions between the Antarctic Circumpolar Current (ACC), Bransfield Current, outflows of the Weddell Sea waters and local waters have been addressed (Heywood et al., 2004; Hoffmann et al., 1996; Zhou et al., 2006). Associated with these waters in this area, the strong chemical and biological gradients such as nutrients, chlorophyll and zooplankton have also been studied (Barbeau et al., 2006; Dulaiova et al., 2009; Hewes et al., 2008, 2009; Holm-Hansen et al., 2006; Hopkinson et al., 2007; Huntley et al., 1991; Kahru et al., 2007;

n Correspondence to: Department of Environmental, Earth and Ocean Sciences, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, MA 02125, USA. Tel.: + 1 617 287 7419; fax: + 1 617 287 7474. E-mail address: [email protected] (M. Zhou).

0967-0637/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.dsr.2010.05.012

Reiss et al., 2009; Selph et al., 2006). The phytoplankton biomass is typically low in the ACC region while the inorganic macronutrients are typically high (El-Sayed, 1987; Hart, 1942; Holm-Hansen, 1985). The paradox of the vast areas with the high inorganic macronutrients and low phytoplankton chlorophyll (HNLC) in the Southern Ocean has stimulated many studies of the iron limitation on primary production (Buesseler et al., 2004; Chisholm and Morel, 1991; Coale et al., 2004; Martin et al., 1990a, 1990b). As the ACC flows through the southern Drake Passage, the primary productivity in the surface water is significantly enhanced extending from the Shackleton Transverse Ridge to South Georgia (Hewes et al., 2008; Holm-Hansen et al., 1997, 2004). What causes the surface water of the ACC to become so productive suddenly after the Shackleton Transverse Ridge and what kinds of physical mechanisms are driving the biogeochemical processes? In the southern Drake Passage, the main bathymetric features are the Shackleton Transverse Ridge of 20 km wide, 200 km long and less than 800 m deep, two basins over 4000 m deep to the west and east of the ridge, and the continental shelf of the South Shetland and Elephant Islands shallower than 500 m in the south. Between the Shackleton Transverse Ridge and Elephant Island there is a passage over 3000 m deep and 30 km wide (Fig. 1; hereinafter referred to as the Shackleton Gap). Such large variations in the topography can dramatically alter the current field through topographic steering (Pedlosky, 1987).

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M. Zhou et al. / Deep-Sea Research I 57 (2010) 1039–1048

59°00’

Ona Basin SB 52

Latitude

60°00’ 13

61°00’

SG EI

56

KGI

62°00’

93

LI

229

63°00’ 64°00’ 62°00’

AP 60°00’

58°00’ 56°00’ Longitude

54°00’

52°00’

Fig. 1. The Bathymetry in the southern Drake Passage and stations. The depth is in meters. Stars indicate stations used to represent water types in Fig. 2. The abbreviations are Antarctic Peninsula (AP), Elephant Island (EI), Shackleton Gap (SG), Shackleton Bank (SB), King George Island (KGI) and Livingston Island (LI). The black crosses represent stations occupied during the cruise.

The waters associated with the ACC are the warm Antarctic Surface Water (ASW), the cold Winter Water (WW) below the ASW and the warm Circumpolar Deep Water (CDW) (Hoffmann et al., 1996; Nowlin and Klinck, 1986; Orsi et al., 1995; Schodlok et al., 2002; Sprintall, 2003). The CDW can be further separated into the Upper CDW (UCDW) and Lower CDW (LCDW) based on their origins from the Indian-Pacific Oceans and Atlantic Ocean, respectively. The water in the Bransfield Strait is the coldest originating from the Weddell Sea with the local cooling influence. On the shelves of the South Shetland Islands and Elephant Island, the waters are strongly influenced by intrusions of the ACC, local runoff and cooling referred to as Antarctic Peninsula shelf waters (Deacon and Foster, 1977; Hofmann et al., 1996; Patterson and Sievers, 1980). In the southern Drake Passage, the northward and southward excursions of the ACC in this area were reported in previous studies (Amos, 2001; Hofmann et al., 1996; Orsi et al., 1993, 1995; Schodlok et al., 2002). It was hypothesized that the ACC is steered by bathymetry creating meanders over a horizontal scale of 100 km and intensive interactions between different water masses and currents lead to strong mixing and modification of waters in this region (Heywood et al., 2004; Locarnini et al., 1993). Though these results imply that the southward or northward excursion of the ACC is statistically meaningful and related to topographic features, the dynamics of this southward or northward excursion is less well understood from large scale non-synoptic survey data (Brandon et al., 2004; Orsi et al., 1995). In particular there has been no study of how the ACC interacts with waters on shelves of the South Shetland and Elephant Islands and the effects of mixing between waters on biogeochemical processes. In a recent study, the horizontal and vertical scales of the front between the waters on the shelf of the South Shetland Islands and the waters in the Bransfield Strait Basin were found to be less than 2 km and deeper than 800 m, respectively (Zhou et al., 2006). This narrow and deep front leads to a frontal jet, the Bransfield Current, with velocities of up to 50 cm s  1 and with water interleaving. Such small-mesoscale processes have previously been underestimated or missed by surveys with stations spaced over 20 km (Niiler et al., 1991). The Shackleton Transverse Ridge is located in the southern Drake Passage. To the west of the Ridge is the vast low productivity ASW within the ACC, to the south are the high productivity surface waters on shelves around the South Shetland Islands and Elephant Island, and to the east is the high productivity ASW in the southern Scotia Sea (Hewes et al.,

2008; Holm-Hansen et al., 1997, 2004). We refer to the surface waters on shelves as SSW being differentiated from the ASW within the ACC. In this region, not only is the ACC detoured southward, but also the waters of the Antarctic Peninsula shelf, Bransfield Strait and Weddell Sea exit northward to form an intense mixing region (Heywood et al., 2004; Hoffmann et al., 1996; Schodlok et al., 2002; von Gyldenfeldt et al., 2002). An eddy has been found persistently at 591310 in this region (Brandon et al., 2004; Cunningham et al., 2003). The biogeochemical and biological responses to the physical processes in the southern Drake Passage have been extensively reported (Dulaiova et al., 2009; Hewes et al., 2008, 2009; Hopkinson et al., 2007; Kahru et al., 2007; Reiss et al., 2009). There is a great interest for both physical and biogeochemical oceanographers to understand the interactions between currents, water masses and topographic features, and the coupling between physical and biogeochemical processes that lead to the transformation of a low productivity ASW to a high productivity ASW in this region, especially, the mechanisms which lead to transport and mixing of additional biogeochemically necessary elements into surface waters (Hewes et al., 2008; Hopkinson et al., 2007).

2. Data and methods The cruise was conducted from February 12 to March 24, 2004 around the Shackleton Transverse Ridge onboard the A.S.R.V. Laurence M. Gould (Fig. 1). The study area was bounded between 591S and 621S in latitude and 591W and 531W in longitude covering the deep basin, Shackleton Transverse Ridge, the shelf area of the South Shetland Islands and Elephant Island, the Shackleton Gap, and a relatively shallow (1500 m) bank northeast of Elephant Island. The stations for the large scale survey were spaced approximately 25 km apart dictated by available ship time. In the area near the shelf break, additional stations were inserted into the center of every 4 original stations that leads to a resolution of approximately 14 km. Three high resolution transects with the horizontal resolution of 2–3 km were conducted to resolve frontal structures, one of which was in the Bransfield Strait (Zhou et al., 2006). Two rosette systems were used, a regular rosette system equipped with a SeaBird 911 plus conductivity–temperature– depth (CTD) system including dual temperature and conductivity sensors (Sea-Bird Electronics, Inc., Bellevue, WA, USA) and a trace metal clean rosette with a SeaBird 911 plus CTD system including one pair of temperature and conductivity sensors. All temperature and conductivity sensors were calibrated prior to the cruise within the standard initial accuracies of 0.001 1C and 0.003 mmho cm  1, respectively. Salts were taken to ensure the stability of these sensors. Direct comparisons between these three pairs of CTD sensors in 1 m vertical bins along a vertical cast from the surface to 1000 m were made to ensure the stabilities of these three CTD systems at the beginning and end of the cruise. The standard deviations of temperature and salinity differences between these three pairs of CTD sensors in 1 m bins were all within 0.01 1C and 0.01, respectively, reflecting the small scale structures in temperature and salinity. The 95% confidence intervals for the mean temperature and salinity in a 1 m bin are less than 0.003 1C and 0.003, respectively. All CTD casts were deployed down to 1000 or 10 m above the bottom when the depth was less than 1000 m. All CTD data were processed by applying filters and corrections suggested by Sea-Bird Electronics, Inc., and the resulting data were binned to 1 m depth intervals. To support our water type analysis, we used the CTD data at a station in the Weddell Sea from a winter cruise conducted between July 3 and August 15 2006 onboard the R.V.I.B. Nathaniel

M. Zhou et al. / Deep-Sea Research I 57 (2010) 1039–1048

B. Palmer. Similar to the CTD systems on the A.S.R.V. Laurence M. Gould, two rosette systems and three pairs of CTD sensors were used with the same accuracies of 0.001 1C and 0.003 mmho cm  1 for temperature and conductivity sensors, respectively and the same standard deviations of temperature and salinity differences between these three pairs of CTD sensors in 1 m bins less than 0.01 1C and 0.01, respectively. The CTD data at Station 229 will be used as a reference for comparing temperature and salinity (T–S) characteristics between different water types (Fig. 1). A vessel-mounted (VM) 153 kHz Narrow Band (NB) Acoustic Doppler Current Profiler (ADCP) (RD Instruments, San Diego, CA, USA) was used for the direct current measurements. The ADCP was set with a bin length of 8 m, a pulse length of 8 m, and a blank after transmission of 4 m. These settings lead to a standard deviation of 13 cm s  1 for single ping velocity measurements (RDI, 1989). A 15 min ensemble average is made for the velocity measurements which reduces the corresponding error to approximately 0.6 cm s  1. A total of 60 mixed layer drifters were deployed in the study region, of which 20 drifters were deployed in January and February 2003 prior to the cruise for understanding the general surface circulation in the region and 40 drifters were deployed during the cruise in 2004 for measuring the surface circulation. These drifters consist of a spherical surface float, a coated wire tether and a Holey-sock drogue centered at 15 m (Niiler et al., 1987; Niiler et al., 1995; Sybrandy and Niiler, 1991; Zhou et al., 2002). These drifters are known to follow the water motion at 15 m within a 1 cm s  1 error in wind conditions up to 10 m s  1. The drifters were tracked using the ARGOS onboard polar-orbiting satellites. On average, 8 position fixes of each drifter were received per day. The raw ARGOS positions from each drifter were initially quality controlled and interpolated to a uniform 6-hourly time series according to the method of Hansen and Poulain (1996) by the Drifter Data Assembly Center at NOAA/AOML in Miami. The velocity was calculated by simply taking the ratio of position displacement to the time interval. A 1-day low-pass filter was applied to these data to eliminate inertial and tidal motions. The drifter data were further binned into boxes of 0.251  0.51 in latitude and longitude. The mean velocity was calculated by

3. Results 3.1. Water types and horizontal distribution The potential temperature and salinity (y–S) diagram indicates the characteristics of the ASW and CDW associated with the ACC and the surface and deep waters on the shelves of the South Shetland and Elephant Islands (Fig. 2). For studying the relationships between water masses, the y–S diagrams at selected stations are highlighted in Fig. 2 including Station 13 in the ACC region, Station 52 representing waters on the shelf, Station 56 in the Bransfield Current, Station 93 in the basin of the Bransfield Strait and Station 229 in the western Weddell Sea basin which was occupied during the austral winter cruise in 2006. The y–S data in our survey area over the Shackleton Transverse Ridge region were bounded between the y–S curves at Station 13 representing waters found in the ACC and Station 52 representing waters found on the shelf. Within the ACC, the ASW was the warmest and freshest in the study area produced by seasonal surface heating and precipitation with typical potential temperatures of up to 2–3 1C and salinity of less than 33.8. At 80–100 m, there was a layer of the cold WW around 0 1C produced during previous winter seasons. Below this temperature minimum the potential temperature quickly increased to 2 1C typical for the CDW forming the warmest deep water in the Southern Ocean with a steadily increasing salinity up to 34.5 at 400 dbars and 34.7 at 1000 dbars. The potential temperatures of the surface and deep waters in the Bransfield Strait were the coldest around 0.5 and  1 1C, respectively, much lower than those of waters on the shelves and in the ACC except the surface water found in the Weddell Sea basin during austral winter. The temperature of the SSW on shelves fell between those of the surface waters in the ACC and Bransfield Strait. Along the water column on shelves, the SSW was the warmest varying between 1 and 2 1C depending on locations; and the deep water (referred to as SDW hereafter) varied from 0 1C between 150 and 630 dbars to 0.8 1C between 630 and 1000 dbars. The y–S characteristics of water columns in the ACC, shelf regions and Bransfield Strait are significantly different and can be easily

ð1Þ

where u is the drifter velocity, hUi is the ensemble average and u is the mean current in a 0.251  0.51 box. The eddy kinetic energy (EKE) in the box is then calculated E 1D 2 9uu9 ð2Þ EKE ¼ 2 Because there were only a limited number of drifters, the statistics and the biases in estimating mean currents due to deployments cannot be computed (Zhou et al., 2000). To study the similarity of a water to the waters in the ACC or on the shelf and its association with mean currents and EKE, we define a water index (WI) as    )12 Z Z ( 1 TTA 2 SSA 2 WI ¼ þ dz ð3Þ 2Z 0 TS TA SS SA where T is the temperature, S is the salinity, and Z is the vertical depth range of the water column to which it applies. The subscripts A and S in Eq. (3) represent two reference stations A and S, respectively. In this study, we choose Station A in the ACC and Station S on the shelf. The value of WI is an index indicating the similarity (or difference) of a given water to the waters at stations A and S. When the value is close to 0, the water T–S characteristics is close to that of Station A in the ACC; and when the value is close to 1, the water T–S characteristics is close to that of Station S on the shelf.

4.0 3.0 Potential temperature

u ¼ hui

1041

ASW CDW

2.0 SSW 1.0 SDW 0.0

WW

-1.0 -2.0 33.5

BDW

BSW 34.0

WSDW

34.5

35.0

Salinity Fig. 2. The potential temperature (y) and salinity (S) diagrams for the typical water types in the study area. The small black dots are all y–S pairs in our survey area, large black dots are the y–S diagrams at Stations 13, 52, 56, 93 and 229 representing waters in the ACC shelf, Bransfield Current, Bransfield Strait basin and Weddell Sea basin regions, respectively. The dash lines are the surface st contours. The abbreviations are the Antarctic Surface Water (ASW), Circumpolar Deep water (CDW), Winter Water (WW), Shelf Surface Water (SSW), Shelf Deep Water (SDW), Bransfield Strait surface water (BSW), Bransfield Strait deep water (BDW) and Weddell Deep Water (WSDW).

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loop at 581S and 541W. In our southern survey area, the waters were transported off the shelf near the shelf break northeast of Elephant Island (Fig. 6).

identified by their y–S diagrams (Fig. 2). In the Shackleton Transverse Ridge region, the waters within upper 600 dbars were enveloped by two y–S curves of the waters in the ACC and on the shelf (referred to as the ACC and shelf type waters). The influence of Weddell Sea waters might be found below 600 dbars in the southeast corner of our survey area. Thus, for the water masses in the upper 600 dbars, the ACC and shelf types are the two primary end members. Warm waters associated with the ACC intruded southeastward from the northwest into the the Shackleton Transverse Ridge and Shackleton Gap area and exited to the northeast after passing the Shackleton Transverse Ridge while the cold shelf type waters occupied the south and southeast (Figs. 3 and 4). Two high salinity and temperature bands at 200 dbars extended northeastward north of Elephant Island and can be identified as the higher salinity CDW found at depths deeper than 500 dbars. Surrounding these bands were the shelf type waters.

3.3. Eddy kinetic energy and mixing between waters The EKE field represents the kinetic energy of mesoscale eddy fields from the instability of currents and jets. Extremely high EKE values were associated with the main ACC current north of 591S (Fig. 6). In our survey area, EKE maxima formed a northeastward band starting at the Shackleton Gap and following the east branch of the Shackleton Jet and the two high salinity and temperature bands (Figs. 4 and 6). The coupling between strong mean currents and EKE implies the instability of mean currents and mixing between water masses. We choose that Ta and Sa are the temperature and salinity of the ASW represented by Station 13, TS and SS are the temperature and salinity of the SSW represented by Station 52, and Z is the vertical depth range of upper 100 m. The values of WI for the upper 100 m of the survey area are shown in Fig. 7. The reason to choose 100 m is to cover the euphotic zone and depth of the maximum surface mixed layer. The value of WI should be interpreted as the similarity of a given water with the ASW (WI¼ 0) or SSW (WI¼1) for examining the spatial distributions of these waters. The areas occupied by the ASW and SSW are clearly shown in Fig. 7. In the broad areas, values of WI are close to 0 and 1 implying that the waters at Stations 13 and 52 well represent the T–S characteristics of the ASW and SSW. Especially, the SSW plume off the shelf north of Elephant Island is clearly indicated by the value of WI close to 1. Any water with the value of WI within (0, 1) could be formed as a percentage mixture between the ASW and SSW or through non-conservative processes such as heat fluxes and precipitation.

3.2. Horizontal circulation The combination of ADCP current measurements, dynamic height anomalies from CTD data and the mean currents obtained from drifters provide a clear view of the intruding ACC and its pathway (Figs. 4–6). In the broad southern Drake Passage region, the main ACC, over 100 cm s  1, remained north of 591S while south of 591S the ACC moved sluggishly eastward at less than 20 cm s  1 elucidated by the mean currents from drifter data (Fig. 6). A southern branch of the ACC turned and accelerated southward west of the Shackleton Transverse Ridge up to 40 cm s  1. As this southward detoured ACC branch was concentrated into the Shackleton Gap between the Shackleton Transverse Ridge and Elephant Island, it accelerated further to greater than 70 cm s  1 now referred to as the Shackleton Jet. This jet then formed a leeward anticyclonic eddy centered at Station 59 behind the Shackleton Transverse Ridge. The jet bifurcated into a western branch flowing northwestward parallel to the Shackleton Transverse Ridge that rejoined the main ACC and an eastern branch flowing northeastward with meanders and eddies, consistent with the two high salinity and temperature bands seen at 200 m (Fig. 4). The main ACC made a southward

3.4. Transects Data from the three transects depicted in Fig. 3 are shown in Figs. 8–10. The NS transect crossed the intruding ACC prior to the Shackleton Gap. The intruding water occupied the Shackleton Transverse Ridge and Gap between 601S and 611S (Stations 14 and

Temperature (C) 59°30’ NS

60°00’ 60°30’ EW

Latitude

61°00’

SG

Salinity 4

34.2

3

34.1

2

34.0

1

39.9

0

39.8

-1

39.7

σθ

Dynamic height anomalies (cm) 27.4

59°30’

27.3

60°00’

27.2

52 50 48 46 44 42 40 38

27.1

60°30’

27.0 61°00’

26.9 58°00’

56°00’

58°00’

54°00’

56°00’

54°00’

Longitude Fig. 3. The horizontal distributions of potential temperature (upper-left), salinity (upper-right), sy (lower-left), and dynamic height anomalies relative to 1000 dbars (lower right) at 20 m. The black lines and labels indicate the names and locations of three transects shown in Figs. 8 and 10.

M. Zhou et al. / Deep-Sea Research I 57 (2010) 1039–1048

Temperature (C)

1043

Salinity 4

34.55

59°30’

3

60°00’

2

34.45

1

34.40

0

39.35

-1

39.30

60°30’

Latitude

61°00’

34.50

σθ

Dynamic height anomalies (cm) 38

59°30’ 60°00’

27.60

36

27.55

34 32

60°30’ 61°00’ 58°00’

56°00’

27.50

30

27.45

28

54°00’

58°00’

56°00’

54°00’

Longitude Fig. 4. The horizontal distributions of potential temperature (upper-left), salinity (upper-right), sy (lower-left), and dynamic height anomalies relative to 1000 dbars (lower right) at 200 m.

59°00’

cm2 s-2 2000

56°00’ 50 cm s-1 57°00’

59°30’

1500 Latitude

Latitude

58°00’ 60°00’

60°00’ 60°30’

62°00’ 62°00’

57°00’

55°00’ Longitude

53°00’

500

16

61°00’

61°00’

61°30’ 59°00’

1000

59°00’

0

EI 100 cm s-1

58°00’

54°00’ Longitude

50°00’

Fig. 6. The mean currents (arrows) in cm s  1 and mesoscale eddy kinetic energy field (colors) in cm2 s  2 derived from drifter data. Data were binned into boxes of 1/41  1/41 in latitude and longitude. The abbreviations are Elephant Island (EI) and Shackleton Transverse Ridge (STR).

Fig. 5. ADCP current measurements at 55 m. Data were binned into boxes of 1/61  1/41 in the latitude and longitude.

18), while the jet of approximately 20 cm s  1 was centered between 601450 S and 611000 S (Stations 17 and 18) near the shelf break within the Shackleton Gap (Fig. 8). The eastward volume transport in the upper 1000 m was approximately 3.4 Sv. The westward flow was found north of 601300 S (Station 16) on the Shackleton Transverse Ridge as the return flow (Fig. 6). The front between the waters associated with the ACC and the shelf of Elephant Island was located at 601450 S between stations 78 and 84 within the Shackleton Gap on the SG transect (Fig. 9). Because the station spacing was approximately 2–4 km and the casts were only down to 1000 m, the scales of this front were less than 2–4 km horizontally and deeper than 1000 m. South of this front were the shelf type waters forming a small-mesoscale eddy and interleaving layers between the ACC and shelf type waters. One jet of 30 cm s  1 was found on the shelf break and was not associated with the main front between the ACC and shelf type waters contributing an eastward transport of 1.4 Sv; another jet of

20 cm s  1 around 601400 S off the Shackleton Transverse Ridge was the major intrusion of the ACC current contributing an eastward transport of 3.6 Sv. The total eastward volume transport in the upper 1000 m was approximately 5 Sv. This value is much larger than the transport estimated on the NS transect because the NS transect missed a portion of the eastward jet near the shelf break. Furthermore, the geostrophic current estimates through the Shackleton Gap were much less than the direct current measurements of the ADCP and drifters which recorded values of more than 70 cm s  1. The EW transect shows the southward intruding ACC was located west of 571150 W between Stations 11 and 17 (Fig. 10), it turned northward along the Shackleton Transverse Ridge extending out to 551 near Station 49 where a front separated the waters associated with the ACC from the shelf waters. The shelf waters were transported off the shelf occupying the offshelf region and forming a counterclockwise circulation. The northward returning

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ACC was approximately 3.6 Sv. The northward flow of the shelf type waters occurred primarily between stations 50 and 35 and was approximately 1.2 Sv.

The potential vorticity can be expressed as

4. Discussion

where z is the relative vorticity, f is the Coriolis constant and H is the layer thickness. Considering a small change in the layer thickness (DH) over a constant layer (H0), Eq. (4) can be simplified and the absolute vorticity equation can be written as

4.1. Topographic steering The topographic feature of the Shackleton Transverse Ridge plays an important role in the ACC path and local circulation around the Ridge because the ACC occupies a depth range from the surface to 2000–4000 m (Nowlin and Klinck, 1986). The Shackleton Transverse Ridge forms a topographic blockage more than 200 km long in the north–south direction and as shallow as 800 m. As the main broad ACC flows eastward at speeds of less than 20 cm s  1, the blockage of the Shackleton Transverse Ridge steers the ACC maintaining its potential vorticity (Holton, 1992).

1.2

Latitude

59°30’

1.0 0.8

60°00’

0.6 60°30’

13

55 16

0.4 0.2

61°00’

0.0 56°00’ Longitude

58°00’

54°00’

Fig. 7. The horizontal distribution of WI, the mixture index in the upper 100 m. The reference stations are Station 13 (the ACC Water) and Station 52 (the Shelf Water), marked by red stars. The indices of the ACC and shelf waters are equal to 0 and 1, respectively. The black crosses and white dots represent CTD and TMC stations, respectively.

Stns 18 0

17

  d zþf ¼0 dt H

ð4Þ

    DH d z þ f  ¼0 dt H0

ð5Þ

where the second term is the anomaly of vorticity produced by

DH. The blockage of the ACC by the Shackleton Transverse Ridge leads to an accumulation of water associated with the ACC and to an increase in the layer thickness that leads to a positive vorticity anomaly. The thickening of the ACC layer can be seen from the temperature contours along the EW transect prior to the Shackleton Transverse Ridge (Fig. 10). To compensate for such an anomaly and to maintain zero net change in the absolute vorticity, there must be an increase in negative relative vorticity. This negative relative vorticity turns the ACC southward as it approaches the Shackleton Transverse Ridge from the west, which was observed (Figs. 5 and 6). When the ACC flows over the shallow depth of the Shackleton Transverse Ridge, the change in water depth leads to a large negative DH and negative topographically induced vorticity. To compensate for this negative vorticity and for absolute vorticity conservation (Eq. (5)), the ACC must acquire a positive relative vorticity at the Ridge that turns the ACC northward. These layer thickness changes are clearly seen from the temperature core of the UCDW in Fig. 10. Such a Rossby adjustment is typical for a quasigeostrophic current over a topographic feature and has been well studied mathematically (Pedlosky, 1987). If the ACC diversion is year round, then the topographically induced circulation over the Shackleton Transverse Ridge will also be year round. This permanent feature sets the current field and the subsequent biogeochemical processes downstream.

16

15

14 3 2

500 1 0

1000 Depth (m)

0

34.6

500

34.2 33.8

1000 E

0

20 10

500

0 -10

1000 61°00’

60°30’ Latitude

W

Fig. 8. The potential temperature (upper), salinity (middle) and geostrophic currents (lower) along the NS transect indicated in Fig. 3. In the lower panel, ‘‘E’’ and ‘‘W’’ indicate the eastward and westward currents, respectively. The black triangles and station numbers mark the locations of CTD stations used.

M. Zhou et al. / Deep-Sea Research I 57 (2010) 1039–1048

Shackleton Gap 78 84 77 70

Stns 83 0

1045

15

8 3 2

500

1 0

Depth (m)

1000 0

34.6

500

34.2 33.8

1000

E

0

20 10

500

0 1000 60°40’

W

60°20’

-10

Latitude Fig. 9. The potential temperature (upper), salinity (middle) and geostrophic currents (lower) along the SG transect indicated in Fig. 3. In the lower panel, ‘‘E’’ indicates the eastward currents. The black triangles and station numbers mark the locations of CTD stations used.

Stns 4

11

0

17

SG

77

74

49

50

35

36 3 2

500

1 0

1000 0 Depth (m)

34.6 34.2

500

33.8 1000 S 0

500

1.8 Sv

1.2 Sv

3.6 Sv

10 0 -10

1000

58ο00’

56ο00’ Longitude

54ο00’

N

Fig. 10. The potential temperature (upper), salinity (middle) and geostrophic currents (lower) along the EW transect indicated in Fig. 3. In the lower panel, ‘‘N’’ and ‘‘S’’ indicate the northward and southward currents, respectively. The black triangles and station numbers mark the locations of CTD stations used. The abbreviation ‘‘SG’’ indicates the location of the Shackleton Gap.

4.2. Forming the Shackleton Jet As the ACC flows eastward and turns south due to geostrophic adjustment for the topographic blockage, the scale of deformation is determined by either the external or internal Rossby Radius (Pedlosky (1987)). Taking the depth of 4000 m, layer thickness of 2  103 m, gravity of 10 m s  2, and sy difference of 1 between the ACC and shelf type waters, the external and internal Rossby Radii are equal to approximately 2.0  103 and 45 km, respectively. Examining Figs. 3, 4 and 10, the horizontal frontal distortion by the Shackleton Transverse Ridge was about 1 degree in longitude

between Stations 74 and 49, which is approximately 56 km on the same order as the internal Rossby Radius. This implies that the layer thickness change plays the dominant role in determining the scale of currents. The southward detoured ACC met the shelf break and was forced to turn eastward. Eventually this detoured ACC was forced into the Shackleton Gap, which is approximately 30 km wide at 2000 m, and was accelerated up to 70 cm s  1. If we take the eastward velocity of approximately 10 cm s  1 (Fig. 6) for currents south of the main ACC, as it was forced southward by the 200 km Shackleton Transverse Ridge into a narrow band of the internal

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Rossby Radius of 45 km, the current would be increased by the ratio of 200–45 km i.e. from 10 to 45 cm s  1. As the current continued into the Shackleton Gap of 30 km width, its speed would be further increased by the ratio of 45–30 km i.e. from 45 to 66 cm s  1. Though such a volume conservation argument may oversimplify the dynamics of the Shackleton Jet, it provides a simple explanation of both the formation and magnitude of the jet. Considering the core of the Shackleton Jet of 66 cm s  1 in a width of 10 km (Fig. 10), the Rossby number is approximately 1. Thus, the Shackleton Jet is strongly nonlinear and ageostrophic, which may explain the discrepancy between current velocities measured by ADCP and drifters and those estimated from geostrophic currents relative to 1000 dbars. The Shackleton Jet flowed through the Shacklton Gap and then turned northward with the volume transport estimates of 5 and 3.6 Sv, respectively, based on the geostrophic currents relative to 1000 dbars. The difference between the eastward ACC transport of 5 Sv estimated through the Shackleton Gap and the northward ACC transport of 3.6 Sv along the EW transect may imply the potential existence of an eastward ACC transport along the shelf break. 4.3. Offshelf eddies

4.4. Horizontal and vertical mixing

The band of active mesoscale eddies shown by high EKE values estimated from drifter data that extended from the Shackleton Gap northeastward to 581S and 521W (Fig. 6), is consistent with large gradients of WI (Fig. 7). As the Shackleton Jet regained its geostrophic balance after the Shackleton Gap, the hydrodynamic instability of the jet can be analyzed by the horizontal shear (Holton, 1992; Zhou et al. 2006), i.e., 

@ug  9f 9 4 1 @y

after the Shackleton Transverse Ridge was lifted upward approximately 200–500 dbars comparing to the CDW prior to the Ridge by following the dome-shaped salinity contours. The uplifted CDW caused the reversal of the horizontal density gradient, i.e., after Shackleton Transverse Ridge, at a given depth, the CDW became denser than the SDW and this led to the counterclockwise circulation around the water bodies transported off shelves. Another consequence of the reversal of the horizontal density gradient is that at the density front, the denser CDW will tend to slide downward while the less dense SDW will tend to upwell on top of the denser CDW based on the quasigeostrophic dynamics (Holton, 1992; Rudnick, 1996). This can be one of the important upwelling mechanisms at fronts. The uplifting of the CDW after it traverses the Shackleton Gap requires further study. We can hypothesize that as the nonlinear ageostrophic Shackleton Jet of 70 cm s  1 rushes across the Shackleton Gap, its inertial motion or kinetic energy must then be turned into potential energy by uplifting the CDW as it slows down meeting with slow moving surrounding waters and readjusts into geostrophic balance. A more in-depth treatment of the regional physical oceanography falls beyond the scope of this paper.

ð6Þ

where ug is the geostrophic current along the jet. To set the horizontal coordinates, x is in the geostrophic current direction towards the downstream, and y is perpendicular to the jet following the right hand rule. This ratio is simply a signed Rossby number. The jet is always stable on the right hand side facing downstream while the jet becomes unstable on the left hand side when the horizontal velocity shear exceeds the Coriolis constant. Near the Shackleton Gap, the Rossby number of the Shackleton Jet is nearly 1, which provides the necessary condition for the instability of the jet. In the rest of the survey area, the magnitude of Rossby numbers is on the order of 0.1. Thus, most currents were hydrodynamically stable. If this is true, except for meanders associated with topographic features, offshelf mesoscale eddies in our survey area are primarily generated by the Shackleton Jet at the Shackleton Gap, or in other words, the Shackleton Jet is a primary source for generating offshelf mesoscale eddies in the study region. As these eddies were generated at the Shackleton Gap, they moved northeastward (Fig. 6). The interactions between the waters associated with the ACC and shelves and between currents and topographic features determine the complexity of the jet–eddy system at the Shackleton Transverse Ridge. At a given depth the waters of the ACC are typically lighter than the waters from shelves so that the density gradient crossing the front between these waters would enhance the frontal current flowing eastward or northeastward because the ACC primarily occupies the northwestern survey area while the waters from shelves occupy the southeastern survey area (Figs. 3 and 4). However, the current direction along the EW transect was reversed toward the south at Station 49 (Fig. 10). The cause of this reversal can be analyzed from the density field along the EW transect. Assuming salinity was relatively conservative, the CDW

The vertical mixing in the ACC region was limited by the stable water column. The ASW had a thickness typically 50–70 m and was at its thinnest of 20 m at Station 16 (Fig. 8). Beneath the ASW is the WW, which was found at all stations in the ACC. The existence of the WW strongly inhibits any direct connection between the ASW and CDW. The waters on the shelf consist of both local and remote contributions. The y–S property below 630 m at Station 52 is similar to that of the Weddell Sea Deep Water (WSDW) found in the 2006 austral winter cruise (Figs. 1 and 2), that confirms the similar findings by Garabato et al. (2002) and Heywood et al. (2004) that a branch of Weddell Sea outflow turned westward into the southern Drake Passage region. Above this WSDW is the SDW with very different temperature and salinity of around 0 1C and less than 34.5, respectively. Thus, the WSDW may not directly contribute to the water properties in the upper 100 m. The SDW can also be influenced by the waters in the Bransfield Strait. It has been long recognized that the deep water in the Bransfield Strait (BDW) originates from the WSDW and is significantly modified by local cooling. The shelf waters around the South Shetland Islands are considered as a mixture between the waters from the Bransfield Strait and ACC with local influence of runoff and cooling (Hofmann et al., 1996; Zhou et al., 2006). Their results support our finding that the shelf waters in the upper 600 m around Elephant Island were influenced by the waters from the Bransfield Strait more than by the CDW or WSDW (Fig. 2). Consequentially, iron and macronutrients found in the SSW and SDW will be closely related to the waters from the Bransfield Strait and biogeochemical processes on shelves (Dulaiova et al., 2009; Hopkinson et al., 2007). The ACC jet turns northward after traversing the Shackleton Gap and the area east of the jet was filled by the waters from the shelf region northeast of Elephant Island (Figs. 3, 4, 7 and 10). This offshore transport occurred around 54–551W as indicated by the EW CTD transect and the horizontal distribution of WI (Figs. 7 and 10). The water plume north of Elephant Island has the values of WI close to 1 implying the similarity to the SSW, which is consistent to the independent finding in the horizontal circulation based on ADCP and drifter data that the offshelf transport of the waters on shelves occurred north of Elephant Island (Figs. 5 and 6).

M. Zhou et al. / Deep-Sea Research I 57 (2010) 1039–1048

The detailed mechanism of how, and the locations of where, the waters on the shelf crossed the shelf break into offshelf areas is still unknown. Based on the conservation of potential vorticity, it is very difficult for the waters on shelves to cross a steep shelf break. We speculate that the nonlinear ageostrophic Shackleton Jet formed jets and eddies within which the waters on the shelf region northeast of Elephant Island were entrained and transported offshelf around 601400 S and 53.5–54.51W between Stations 50 and 35 at the shelf break.

5. Summary The Shackleton Transverse Ridge, Shackleton Gap and the shelf break play important roles in causing the southward detour of an ACC branch and forming the Shackleton Jet of 70 cm s  1 through the Shackleton Gap. The estimated volume transport of the ACC crossing the Shackleton Gap was greater than 5 Sv while the estimate of the northward ACC after the Shackleton Transverse Ridge was approximately 3.6 Sv. This implies that a significant portion of the ACC moved eastward along the shelf break. The northward offshelf transport of shelf type waters was approximately 1.2 Sv within which the large amount of iron fertilized the downstream southern Scotia Sea. The CDW is typically less dense than the SDW. As the ACC jet passes through the gap and slows down, its kinetic energy turns into potential energy by lifting its density contours up to 200–500 dbars, which results in the reversal of density gradients between the CDW and SDW. A band of mesoscale eddies were produced translating from the Shackleton Gap northeastward towards the main ACC stream. The reversal of density gradients at the front results in the less dense SDW upwelling onto the denser CDW. Both upwelling and horizontal transport can significantly mix nutrients and metals between different waters, while at the same time forming stable surface layers that prevent from deep mixing. The enrichment of nutrients and metals in these stable surface layers then leads to enhanced primary production and high phytoplankton biomass.

Acknowledgement This project was supported by the National Science Foundation grant numbers OPP-0229966 and ANT-0444040 to M. Zhou. We would like to acknowledge Raytheon Polar Service Co. for assisting with networking, data transferring and CTD casts and the crew of the A.S.R.V. L.M. Gould and the crew of the R.V.I.B. N.B. Palmer for tirelessly fighting the rough sea. References Amos, A.F., 2001. A decade of oceanographic variability in summertime near Elephant Island, Antarctica. Journal of Geophysical Research 106, 22401–22423. Barbeau, K., Hopkinson, B.M., Reynolds, R., Wang, H., Selph, K., Measures, C., Hewes, C., Malfatti, F., Manganelli, M., Azam, F., Holm-Hansen, O., Mitchell, B.G.2006. Phytoplankton iron stress across chlorophyll and dissolved-iron gradients in the southern Drake Passage. Eos, Transactions, American Geophysical Union, 87, Ocean Science Meeting Supplement, Abstract, OS33F-05. Brandon, M.A., Naganobu, M., Demer, D.A., Chernyshkov, P., Trathan, P.N., Thorpe, S.E., Kameda, T., Berezhinskiy, O.A., Hawker, E.J., Grant, S., 2004. Physical oceanography in the Scotia Sea during the CCAMLR 2000 survey, austral summer 2000. Deep-Sea Research II 51, 1301–1321. Buesseler, K.O., Andrews, J.E., Pike, S.M., Charette, M.A., 2004. The effects of iron fertilization on carbon sequestration in the Southern Ocean. Science 304, 414–417. Chisholm, S.W., Morel, F.M.M., 1991. What controls phytoplankton production in nutrient rich areas of the open sea? Limnology and Oceanography 36 1507–1511. Coale, K.H., et al., 2004. Southern ocean iron enrichment experiment: carbon cycling in high- and low-Si waters. Science 304, 408–414.

1047

Cunningham, S.A., Alderson, S.G., King, B.A., Brandon, M.A., 2003. Transport and variability of the Antarctic Circumpolar Current in Drake Passage. Journal of Geophysical Research 108, 8084. Deacon, G.E.R., 1933. A general account of the hydrology of the Southern Ocean. Discovery Reports 7, 171–238. Deacon, G.E.R., 1937. The hydrology of the Southern Ocean. Discovery Reports 15, 3–122. Deacon, G.E.R., Foster, T.D., 1977. The boundary region between the Weddell Sea and Drake Passage currents. Deep-Sea Research 24, 505–510. Dulaiova, H., Ardelan, M.V., Henderson, P.B., Charette, M.A., 2009. Shelf-derived iron inputs drive biological productivity in the southern Drake Passage. Global Biogeochemical Cycles 23, GB4014. doi:10.1029/2008GB003406. El-Sayed, S.Z., 1987. Biological production of Antarctic waters: present paradoxes and emerging paradigms. In: SCAR (Ed.), Antarctic Aquatic Biology. BIOMASS Scientific Series, Volume 7. Scott Polar Research Institute, Cambridge, England, pp. 1–21. Garabato, A.C.N., Heywood, K.J., Stevens, D.P., 2002. Modification and pathways of Southern Ocean Deep Waters in the Scotia Sea. Deep-Sea Research I 49, 681–705. Hansen, D.V., Poulain, P.M., 1996. Quality control and interpolations of WOCE– TOGA drifter data. Journal of Atmospheric and Oceanic Technology 13, 900–909. Hewes, C., Reiss, C., Kahru, M., Mitchell, B., Holm-Hansen, O., 2008. Control of phytoplankton biomass by dilution and mixed layer depth in the western Weddell-Scotia confluence. Marine Ecology Progress Series 366, 15–29. Hewes, C.D., Reiss, C.S., Holm-Hansen, O., 2009. A quantitative analysis of sources for summertime phytoplankton variability over 18 years in the South Shetland Islands (Antarctica) region. Deep-Sea Research I 56, 1230–1241. Heywood, K.J., Garabato, A.C.N., Stevens, D.P., Muench, R.D., 2004. On the fate of the Antarctic slope front and the origin of the weddell front. Journal of Geophysical Research–Oceans, 109. doi:10.1029/2003JC002053. Hart, T.J., 1942. Phytoplankton periodicity in Antarctic surface waters. Discovery Reports 21, 261–365. Hofmann, E.E., Klinck, J.M., Lascara, C.M., Smith, D.A., 1996. Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait. In: Ross, R., Hofmann, E., Quetin, L. (Eds.), Antarctic Research Series, vol. 70. American Geophysical Union, Washington, D.C, pp. 61–80. Holm-Hansen, O., 1985. Nutrient cycles in Antarctic marine ecosystems, In: Siegfried,W.R., Condy, P.R., Laws, R.M. (Eds.), Antarctic Nutrient Cycles and Food Webs, Proceedings of the Fourth Symposium on Antarctic Biology (SCOR/ SCAR), Wilderness, South Africa, 1983. Springer Verlag, Berlin, pp. 6–10. Holm-Hansen, O., Azam, F., Barbeau, K., Gille, S., Hewes, C., Kahru, M., Mitchell, B.G., Measures, C., Zhou, M.2006. Plankton community structure and iron distribution in the southern Drake Passage and Scotia Sea. Eos, Transactions, American Geophysical Union, 87, Ocean Science Meeting Supplement, Abstract, OS35M-16. ˜ e, V.E., Helbling, E.W., Silva, N., Amos, T., Holm-Hansen, O., Hewes, C.D., Villafan 1997. Distribution of phytoplankton and nutrients in relation to different water masses in the area around Elephant Island, Antarctica. Polar Biology 18, 145–153. Holm-Hansen, O., Naganobu, M., Kawaguchi, S., Kameda, T., Sushin, V.A., Krasovski, I., Priddle, J., Korb, R., Brandon, M., Demer, D., Hewitt, R.P., Hewes, C.D., 2004. Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters. Deep-Sea Research II 51, 1333–1350. Hopkinson, B.M., Mitchell, B.G., Reynolds, R., Wang, H., Hewes, C., Selph, K., Measures, C.I., Holm-Hansen, O., Barbeau, K., 2007. Iron limitation across chlorophyll gradients in the southern Drake Passage: phytoplankton responses to iron addition and indicators of iron stress. Journal of Limnology and Oceanography 52, 2540–2554. Holton, J.R., 1992. An Introduction to Dynamic Meteorology Third ed. Academic Press, San Diego, CA 511pp. Huntley, M.E., Karl, D.M., Niiler, P.P., Holm-Hansen, O., 1991. Research on Antarctic Coastal Ecosystem Rates (RACER): an interdisciplinary field experiment. DeepSea Research II 38, 911–942. Kahru, M., Mitchell, B.G., Gille, S.T., Hewes, C.D., Holm-Hansen, O., 2007. Eddies enhance biological production in the Weddell-Scotia confluence of the southern ocean. Geophysical Research Letters 34, L14603. doi:10.1029/ 2007GL030430. Locarnini, R.A., Whitworth III, T., Nowlin, W.D., 1993. The importance of the Scotia Sea on the outflow of Weddell Sea Deep Water. Journal of Marine Research 51, 135–153. Martin, J.H., Gordon, R.M., Fitzwater, S.E., 1990a. Iron in Antarctic waters. Nature 345, 156–158. Martin, J.H., Fitzwater, S.E., Gordon, R.M., 1990b. Iron deficiency limits phytoplankton growth in Antarctic waters. Global Biogeochemical Cycles 4, 5–12. Niiler, P.P., Amos, A.F., Hu, J.-H., 1991. Water masses and 200 m relative geostrophic circulation in the western Bransfield Strait region. Deep-Sea Research II 38, 943–959. Niiler, P.P., Davis, R.E., White, H.J., 1987. Water-following characteristics of a mixed layer drifter. Deep-Sea Research I 34, 1867–1881. Niiler, P.P., Sybrandy, A.S., Bi, K., Poulain, P.M., Bitterman, D.M., 1995. Measurements of the water-following capability of Holey-sock and TRISTAR drifters. Deep-Sea Research I 42, 1951–1964. Nowlin, W.D., Klinck, J.M., 1986. The physics of the Antarctic Circumpolar Current. Reviews of Geophysics 24, 469–491.

1048

M. Zhou et al. / Deep-Sea Research I 57 (2010) 1039–1048

Orsi, A.H., Nowlin Jr, W.D., Whitworth III, T., 1993. On the circulation and stratification of the Weddell Gyre. Deep-Sea Research I 40, 169–203. Orsi, A.H., Whitworth, T., Nowlin, W.D., 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research I 42, 641–673. Patterson, S.L., Sievers, H.A., 1980. The Weddell–Scotia Confluence. Journal of Physical Oceanography 10, 1584–1610. Pedlosky, J., 1987. Geophysical Fluid Dynamics. Springer-Verlag, New York 624pp. RDI, 1989pp. Acoustic Doppler current profilers, Principles of Operation: A Practical Primer. RD Instruments, San Diego 36pp. Reiss, C.S., Hewes, C.D., Holm-Hansen, O., 2009. Trends and relationships between atmospheric tele-connections and Upper Circumpolar Deep Water (UCDW) influence on phytoplankton biomass around Elephant Island, Antarctica. Marine Ecology Progress Series 377, 51–62. Rudnick, D.L., 1996. Intensive surveys of the Azores Front 2. Inferring the geostrophic and vertical velocity fields. Journal of Geophysical Research 101 (C7), 16291–16303. Schodlok, M.P., Hellmer, H.H., Beckmann, A., 2002. On the transport, variability and origin of dense water masses crossing the South Scotia Ridge. Deep Sea Research II 49, 4807–4825.

Selph, A., Measures, C.I., Brown, M.T., Apprill, A., Zhou, M., 2006. The influence of shelf-derived iron on phytoplankton growth dynamics in the Drake Passage, Antarctica. Eos, Transactions, American Geophysical Union, 87, Fall Meeting Supplement, Abstract OS32B-01. Sprintall, J., 2003. Seasonal to interannual upper-ocean variability in the Drake Passage. Journal of Marine Research 61, 27–57. Sybrandy, A.L., Niiler, P.P., 1991. WOCE/TOGA Lagrangian drifter construction manual. SIO Reference 91/6, Scripps Institution of Oceanography. University of California, San Diego. von Gyldenfeldt, A.B., Fahrbach, E., Garcia, M.A., Schroder, M., 2002. Flow variability at the tip of the Antarctic Peninsula. Deep-Sea Research II 49, 4743–4766. Zhou, M., Niiler, P.P., Hu, J.-H., 2002. Surface current in the Bransfield and Gerlache Straits measured by surface Lagrangian drifters. Deep-Sea Research I 46, 267–280. Zhou, M., Niiler, P.P., Zhu, Y., Dorland, R.D., 2006. The western boundary current in the Bransfield Strait, Antarctica. Deep-Sea Research I 53, 1244–1252. Zhou, M., Paduan, J., Niiler, P.P., 2000. The surface currents in the Canary Basin from drifter observations. Journal of Geophysical Research 105 (21), 893–21910.