Flow variability at the tip of the Antarctic Peninsula

Deep-Sea Research II 49 (2002) 4743–4766

Flow variability at the tip of the Antarctic Peninsula Anna-B. von Gyldenfeldta,*, Eberhard Fahrbacha, Marc A. Garc!ıab,c, . a Michael Schroder b

a Alfred-Wegener-Institute for Polar and Marine Research, P.O. Box 120161, D-27515 Bremerhaven, Germany Laboratori d’Enginyeria Mar!ıtima, ETS d’Enginyers de Camins, Canals i Ports, Universitat Polit"ecnica de Catalunya, Barcelona, Spain c Direccio! General de Ports i Transports, Generalitat de Catalunya, Av. Josep Tarradellas 2-6, 08029 Barcelona, Spain

Received 9 July 2001; accepted 16 March 2002

Abstract Recently ventilated water leaves the landlocked northwestern Weddell Sea near the Antarctic Peninsula and possibly spreads out into the basins of the world oceans at shallow to intermediate depths. To determine the pathways of the water through the complex topography and the flow variability, water-mass circulation and properties are described in the northwestern Weddell Sea and along the boundary of the Powell Basin by means of data from current-meter moorings and hydrographic sections. The mean flow is strongly controlled by the topography. Meso-scale, seasonal and interannual fluctuations are superimposed. The mean northward volume transport of shelf water, which represents the potential source water for intermediate layer ventilation, is estimated for the time interval between May 1996 and March 1998 to be 2.471.0 Sv. Water-mass properties suggest that much of this water leaves the Weddell Sea to Bransfield Strait and therefore does not reach the Weddell Scotia Confluence. The water masses are able to serve as the only source of Bransfield Strait deep water since the shelf water properties in the northwestern Weddell Sea vary over time within a range that corresponds to the required source waters. The Scotia Sea is supplied by water from the Powell Basin, which has varied significantly over the past two decades. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The cold, dense water that replenishes the deep world oceans from the polar regions is a vital element of the global thermohaline circulation. Heat exchange between the ocean and the atmosphere leads to deep and bottom flows of cold waters towards the lower latitudes and vice versa. In the southern hemisphere, dense water from *Corresponding author. Tel.: +49-471-4831-1884; fax: +49471-4831-1797. E-mail address: [email protected] (A.-B. von Gyldenfeldt).

several source areas around the Antarctic continent spreads towards the north into the World Ocean. Bottom water of Antarctic origin can be traced as far as 501N in the Atlantic (Schmitz and McCartney, 1993) and Pacific (Mantyla and Reid, 1983), while the fringing continental landmasses limit the spreading in the Indian Ocean. It represents the densest water mass in the global oceans and consequently underlays the southward flowing North Atlantic Deep Water (NADW). Thus, NADW is forced towards shallower depths near the Antarctic Continent and is partly incorporated into the Weddell Sea gyre where it participates in the formation of deep or bottom water.

0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 5 7 - 1

4744

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

One of the important source areas of dense water in the Southern Ocean is the Weddell Sea. The comparatively large shelves in the western and southwestern Weddell Sea are source areas of a variety of bottom waters (Gill, 1973; Foster and Carmack, 1976a; Foldvik et al., 1985; Fahrbach et al., 1994; Gammelsrd et al., 1994; Gordon, 1998). The circulation in the Weddell Sea is determined by a cyclonic gyre that extends from the Antarctic Peninsula in the west to B20–301E. The Antarctic Continent forms the southern boundary, while the mid-ocean ridges and the Antarctic Circumpolar Current (ACC) limit the gyre to the north. Below the Antarctic Surface Water with a large seasonal variation in its properties lies a layer of cold winter water (WW), which is the remnant of convection in winter. Underneath, a core of warm and salty water, the Warm Deep Water (WDW, sometimes referred to as CDW, Circumpolar Deep Water) extends to depths of about 1500 m (Foster and Carmack, 1976b; Orsi et al., 1993). It originates from NADW, influenced by the ACC and can be traced throughout the entire Weddell Gyre. It is the only water mass that is advected from outside the gyre (Orsi et al., 1993). Modification of WDW through sea ice–ocean–atmosphere interaction results in dense and newly ventilated water that can form deep and bottom water. This includes the Weddell Sea Deep Water (WSDW) with a temperature range between
0:7pyp01C and Weddell Sea Bottom Water (WSBW) with temperatures below
0.71C (Reid et al., 1977). The core of the WDW off the shelf lies in the depth of the shelf edge that rims the Weddell Sea. Mixtures from waters found over the slope and from the shelf may result in water that is dense enough to form bottom water (Gill, 1973; Foster et al., 1988). If sufficient dense water is formed on the shelves, it spills over the shelf edge and descends down the continental slope as a plume, concurrently entraining ambient water. Due to the Coriolis force, the plume is deflected to the left and thus is merged into the gyre circulation (Killworth, 1973; Fahrbach et al., 1995; Baines and Condie, 1998). WSDW can be formed directly by the mixing of WDW and sinking shelf waters (Orsi et al., 1993), but also by ascending WSBW that is

gradually mixed with WDW (Reid et al., 1977). The role of shelf water sinking in the Powell Basin to form a low saline WSDW modification is discussed by Gordon et al. (2001). Estimates of the WSBW production vary between 1.3 and 5 Sv, with the large range due to different assumptions for their determination (Carmack and Foster, 1975; Foster and Carmack, 1976b; Gordon et al., 1993, 2001; Fahrbach et al., 1994, 1995, 2001; Muench and Gordon, 1995; Gordon, 1998). The outflow of Antarctic deep and bottom water into the global ocean is affected by the pathways in the Weddell Sea, the volume transport, and their variability. The South Scotia Ridge restricts the flow of the densest portions of newly formed waters from the Weddell Sea proper while other, less dense water masses ventilated on the northwestern shelf, can leave the Weddell Sea in the vicinity of the Antarctic Peninsula. In this study we focus on the latter. The area of investigation (Fig. 1) thus comprises the Antarctic Peninsula with its adjacent shelf area, the central and eastern Bransfield Strait, Powell Basin and extends beyond the South Orkney Plateau. Due to the convergence of water masses from the Weddell Sea and the Scotia Sea, this area is referred to as WeddellScotia Confluence (WSC) (Gordon, 1967). Within the WSC, water-mass properties differ significantly from the ones of the adjacent waters. This indicates that the waters in the WSC are not only a lateral mixture of the advected water masses, but that local water-mass formation plays a significant role. Convection in winter (Deacon, 1937), lateral and vertical boundary mixing, injection of meltwater (Patterson and Sievers, 1980) or advection of shelf waters (Gordon and Nowlin, 1978), e.g. northwestern Weddell Sea shelf water (Whitworth et al., 1994) may influence the water-mass properties in the WSC. Due to the rugged and diverse bottom topography with shelves, basins and troughs alternating, many possible pathways of water from the Weddell Sea exist through the various gaps towards the north. The present study was conducted under the auspices of the Deep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) project (Muench, 1997), a component of the iAnzone

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

50°W

30°W

W 70°

60°W

56˚W

10 48˚W °W

52˚W

60 °S

S 70 °

t dS

el

South Orkney Plateau 1000 3000

Joinville I.

00

e

dg

20

i eR

60°S

uc

g e

Br

Jane Basin

e

idg

R nce

62°S

a dur

En

Joinville Ridge 500

Antarctic Sound

2000

4000

00

fi ans

South Orkney I.

Bruce Bank

64°S

30

a al B

Philip Passage

Powell Basin

t rai

3000

O c o t rki ney Trough a R i d

0

E

S

50

B ast

sin

Weddell Sea

1000

150

0

64°S

58°S

S

50 °S

° 50

62°S

n

asi

King George I.

Br

Scotia Sea

°S 60

h u t S o ge Rid p i l Phi

Elephant I.

ntr

36°W

40°W

Pirie Bank

°S 70

60°S

Ce

44°W 0°

°W 90

58°S

4745

66°S

66°S 60°W

56°W

52°W

48°W

44°W

40°W

36°W

Fig. 1. Area of investigation during DOVETAIL. The displayed isobaths from Smith and Sandwell (1997) are 500, 1000, 1500, 2000, 3000, and 4000 m.

programme (International Antarctic Zone). It focuses on the spatial and temporal features of this vital gateway of recently ventilated water into the global ocean. The fieldwork started in 1996 and comprises several international expeditions with hydrographic surveys and the deployment of current-meter moorings. Modelling efforts (e.g. Schodlok et al., 2002) complete the programme.

2. The data The data set that is used in this study comprises hydrographic sections as well as data from moored

instruments (Figs. 2, 10 and 15). In May 1996, during the ANT XIII/4 expedition, five moorings were deployed in the area of investigation and were recovered approximately 2 years later during ANT XV/4. Both cruises were carried out with R.V. Polarstern. The moorings were equipped with current meters, self-contained conductivity-temperature-depth (CTDs), and one acoustic doppler current profiler (ADCP). The upward looking ADCP, with a system frequency of 153 kHz, had been positioned near bottom, northeast off Joinville Island on the shelf. The data set comprises 22 layers (i.e. 8-m bins) above the instrument, covering most of the water column. Four moorings were

4746

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

ALBATROSS cruise (Antarctic Large Scale Box Analysis and The Role Of the Scotia Sea, Heywood and Stevens, 2000) were incorporated into the study. Older data from expeditions with R.V. Polarstern and Meteor between 1983 and 1996 are used to determine the variability and long-term changes of the hydrographic properties. For ANT XV/4 and later measurements, calibration of the probes for temperature and pressure were done by the Scripps Institution of Oceanography before and after the cruises. During the cruises the performance of the instruments was monitored at sea by simultaneous measurements with digital thermometers and pressure meters. The conductivity readings of the CTD were calibrated with water samples that were analysed for salinity with a Guildline Autosal 8400B. After the application of the corrections the accuracy of the CTD data amounts to 0.002 in salinity, 2 mK in temperature, and 2 db in pressure.

located on a transect on the southern flank of the Joinville Ridge across the continental slope southeast of the ADCP. One additional mooring equipped with an acoustic current meter was deployed during ANT XV/4 in April 1998 and was recovered by R.V. Hesperides during the cruise HE052 in January 1999 (for details on the different moorings refer to Table 1). The moored instruments had been serviced by the manufacturer before the deployment. Several deployed instruments had severe or complete malfunctions and cannot be used for this study. Full-depth vertical profiles of temperature and conductivity were measured with CTD probes. During ANT XV/4, 70 CTD stations were carried out in the target area. In January 2000, 60 CTD stations were realised with the Brazilian research vessel NApOc Ary Rongel in the framework of the AR . XVIII campaign (Schroder et al., 2002). CTD stations from Dovetail 1997 and the British

Table 1 Listing of moored instruments used in this study Mooring, position

WD

Type

SN

ID

Start date

DUR (d)

VC (cm/s)

Temp (1C)

234-1 62151.40 S 53140.30 W 215-3 63119.60 S 52146.920 W

284

ADCP SC AVTP AVT SC AVTP SC AVTPC ACM-CT ATV ATV SC AVTP AVT AVT SC AVTPC

378 1975 11892 9402 1974 11890 1977 9207 1402 9767 9206 1979 11926 11885 11886 631 12454

275 280 259 459 460 246 907 270 762 2187 2454 2455 262 2549 3474 3475 1593

15 15 14 14 15 14 14 13 13 13 13 13 12 12 12 12 25

682 390 683 683 452 684 684 686 454 686 686 686 688 688 688 346 249

6.677.2 — 0.971.5 0.471.9 — 1.873.7 — 2.272.3 1.872.4 1.373.5 6.973.4 — 2.272.7 1.674.7 5.275.5 — 9.475.3


1.01
1.07
1.19
0.84
0.84
1.17
0.87
0.56 0.52 —
1.00
1.02 0.50
0.19
0.83
0.82
0.29

206-4 63129.60 S 5216.60 W 207-4 63143.30 S 50149.200 W

465

952 2510

216-2 63157.60 S 4918.80 W

3520

Mir 2 60134.50 S 49130.80 W

1637

May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 May 1996 April 1998

Abbreviations and general comments: WD, water depth in meters; SN, serial number; ID, instrument depth in meters; DUR, duration of recording; VC, mean current velocity and standard deviation along the principal axis. Instruments that failed are not listed. Abbreviation for instrument types: ACM-CT, Falmouth Scientific three-dimension acoustic current meter with CTD sensor head; ADCP, RDI Inc. acoustic Doppler current profiler; AVT(P,C), Aanderaa current meter with temperature sensor (pressure, conductivity sensors); SC, SeaBird Electronics self-contained CTD (SeaCat).

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

record by the application of a low-pass filter with a 40-h 1/2-power point and are not discussed further. Meso-scale fluctuations cover the time scale of days and low period variations include seasonal and longer time scales. The vertically averaged (if there was more than one instrument in a mooring) mean current vectors indicate a strong topographic steering of the flow (Fig. 3). Velocities are strongest on the shelf of the Powell Basin northeast of Joinville Island (234-1, 6.5 cm s
1) and over Philip Ridge (MIR 2, 9.5 cm s
1). On the shelf southeast of Joinville Island moderate to low speeds are measured (215-3, 1.0 and 206-4, 1.8 cm s
1). Farther downslope, velocities increase again due to the influence of the bottom water plume. The flow in the upper layers is somewhat weaker than in the bottom-near layers (Table 1). In contrast to earlier deployments, mooring 206-4 recorded only in shallow levels. The earlier measurements (Fahrbach et al., 2001) show a velocity maximum in greater depths at this location. In 1989, 1990, and 1993 mean currents between 5.7 and 11.8 cm s
1 were recorded at a depth of about 900 m. Due to instrument failure

3. The flow pattern and the fluctuations Four current-meter moorings were deployed on a transect across the continental slope southeast off Joinville Island. They were part of a repeated section between the Antarctic Peninsula and Kapp Norvegia. These moorings were located in the northwestern flank of the Weddell Gyre, which transports newly ventilated water towards the northern boundary of the Weddell Sea (Fahrbach et al., 2001). A mooring with an upward looking ADCP was located on the shelf farther to the northwest (Fig. 2). The five moorings allow the measurement of the flow of the major water masses in the northwestern Weddell Sea and its variability. In the gateway between the Weddell Sea and the Scotia Sea, mooring MIR 2 measured the flow at the northern rim of the Powell Basin that restricts the free exchange of water but allows the outflow of waters in intermediate levels. The current-meter data show strong temporal variability that can be separated in three groups. Short-period variations, such as tides or inertial oscillations, which were eliminated from the data

60°W

56°W

4747

52°W

48°W

44°W

Station 123

60°S

2

60°S

MIR 2

62°S

1 3

4

62°S

216-2

64°S

234-1 (ADCP) 215-3 206-4

64°S

60°W

56°W

52°W

207-4

48°W

44°W

Fig. 2. Location of hydrographic sections and moorings. The asterisks represent the locations of the moorings used in this study. They were deployed by R.V. Polarstern during ANT XIII/4 (1998) and recovered 2 years later, except for MIR 2, which was deployed in spring 1998 and recovered in January 1999. The black triangle north of the South Orkney Islands marks the location of the reference station 123 (ANT ME 11/5) in connection with the outflow through Philip Passage. The black dots indicate the location of hydrographic stations.

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4748 56°W

52°W

48°W

9.5

60°S

60°S

MIR 2

6.5

62°S

234

62°S

1.0

215

1.8

206 64°S

56°W

52°W

4.0 3.7

207

216

64°S

48°W

Fig. 3. Mean current vectors at the mooring locations. The data are averaged over the recording interval and in the vertical if the mooring was equipped with more than one current meter. The italics indicate the mean speed in cm s
1 for each mooring site.

no data from deeper levels are available for the interval of this study, 1996–1998. Towards the open Weddell Sea the mean speed is about 4.0 cm s
1 at mooring 207-4 and 3.7 cm s
1 at mooring 216-2. In accordance with the long-term measurements (e.g., Fahrbach et al., 2001) for mooring 207, the data show a seasonal variation with mostly a maximum in the early austral winter and a minimum around December (Fig. 4) superimposed on the mean. However, phase and intensity of the seasonal cycle differ interannually. At the same site the temporal variation correlates well in the vertical, which points to a barotropic component in the flow of the Weddell Gyre. Meso-scale variability is displayed by means of current-meter vector plots in Fig. 5. For mooring 234-1 an intermediate level was chosen as representative for the whole water column. At mooring location 234-1 the flow is highly variable with high speeds of up to 50 cm s
1 at this depth level. Reversals of the flow, occur predominantly during austral spring and summer. At mooring location 215-3, low mean velocities correlate with weak

fluctuations at both depths. In 459 m depth the current velocities do not exceed 9.5 cm s
1. In the last months of 1997, the velocities temporarily reach the threshold speed of 1.5 cm s
1. The frequent current reversals suggest a comparatively high number of meso-scale eddy-like features. At the two deep moorings 207-4 and 216-2, the velocities increase towards greater depths and are more stable in direction which is the influence of the cold bottom water plume. At moorings 215-3 and 206-4 meso-scale variability dominates the seasonal cycles which are barely detectable. At mooring 206-4 it seems in times to be out of phase with mooring 215-3 (Fig. 4). The record of the shelf mooring 234-1 (Fig. 6) displays sequences of events in current speed, temperature, and salinity. The speed events, which occur at time intervals of 11–13 days, reach values of up to 50 cm s
1. The temperature events were observed in the winter period of both years as marked, short-lasting drops below a temperature of
1.51C, with a time interval of 9 and 15 days. The freezing point is reached only occasionally. The salinity record, which covers only the first year, indicates a sudden increase of salinity during the ‘cold events’. Whereas temperature and salinity events are correlated, the velocity events seem to be independent. The behaviour of the flow signal could either reflect the advection of meso-scale eddies or it could be induced by interaction with the atmosphere. To investigate the origin of the events, atmospheric forcing data from the ECMWF is used. The meridional wind component from the ECMWF reanalysis data for the time interval of 1996–1998 in the DOVETAIL area shows a peak in the power spectrum at a period between 11 and 12.5 days. Since this is the direction almost tangential to the north–south orientated topography, it is most likely that continental shelf waves are generated by wind stress along the shelf. The induced currents are of 0(10–20 cm s
1). According to observations in other regions (Huthnance, 1981), shelf waves consist primarily of the first mode, which is a long-shelf current, fairly uniform over the shelf and confined to it. Hence, it is concluded that the velocity events in record of

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4749

12

234-1 207-4 206-4 215-3

Velocity along principal axis [cm s -1]

10

8

6

4

2

0

5

6

7

8

9

10

11

12

1

2

1996

3

4

5

6

7 1997

8

9

10

11

12

1

2

3

1998

Fig. 4. Monthly mean current velocities along the principle axis at selected locations. 234-1, the tenth layer in 196 m, 215-3, the upper current meter data in 259 m, 206-4 (246 m) and 207-4 (2187 m).

234-1 are due to wind forcing that might not be local and generates rather uniform currents over the shelf. The sudden drops in temperature are most likely local convection events with associated ice formation and salt release.

4. Temperature and salinity fluctuations The temporal variability of water-mass properties on longer time scales can be demonstrated by displaying the time series of temperature and salinity in y=S diagrams (Figs. 7 and 8). The time series normally do not last longer than a year due to malfunction of the conductivity sensors. Except

for 207-4a all sensors were in the near-bottom layer (Fig. 7). The close relation of the shelf water masses at sites 234-1 and 215-3 is evident in Fig. 7. The shelf waters are clearly distinct from the slope water masses (206-4, 207-4 and 216-2) mainly due to their lower salinity. At the outer shelf, the water at 215-3 is always slightly warmer and saltier than at the shallower mooring 234-1. The envelopes of the y=S clusters of both overlap, which indicates the transition from ‘pure’ shelf water to slope water. The time variability is largest on the shelf, which is consistent with water-mass modification processes occurring on the shelf and subsequent mixing with rather constant source water masses on the slope.

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4750

10 cm/s

Jun

Oct

Aug 1996

Dec

Feb

Apr

Jun

Aug

234-1

180 m

215-3

259 m

215-3

459 m

206-4

246 m

207-4

270 m

207-4

2454 m

216-2

262 m

216-2

2549 m

216-2

3474 m

Oct

Feb

Dec 1998

1997

Mir II

1593 m

N May

Jul

Sep 1998

Nov

Fig. 5. Current-meter vector plots, 6-h averages. MIR 2 was deployed subsequent to the deployment interval of the other current meters.

The near-bottom water-mass characteristics on the slope at 206-4, 207-4 and 216-2 cluster and overlap partly. Although the data sets at 206-4 and 216-2 do not cover the whole mooring interval, a

similar time development appears at all locations (Fig. 8c, d and f). From May 1996 to April 1998 the bottom water masses trended toward cooler and fresher values. The trend is most obvious in

0.0

34.60

-0.5

34.55

-1.0

34.50

-1.5

34.45

-2.0 50

34.40 50

40

40

30

30

20

20

10

10

0

Speed (cm s-1)

Speed (cm s-1)

4751

Salinity

Pot. Temperature (°C)

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

0

5

6

7

8

9 1996

10

11 12

1

2

3

4

5

6

7 1997

8

9

10

11 12

1

2 1998

3

Fig. 6. Time series of potential temperature (black) and salinity (grey) from mooring 234-1 (upper panel). The data consist of one data point every 6 h. The lower panel shows the speed record of the tenth layer, centered around 196 m (as representative for the whole water column). The vertical lines indicate the cold events, when the temperature drops below
1.51C.

the two deeper moorings (216-2: 3475 m, 207-4b: . 2455 m, Fig. 8d and f). Schroder et al. (2002) state an overall trend in the measuring interval for 2074b to be Dy ¼
0:121C and DS ¼
0:03: At 207-4 a seasonal cycle can be identified (Fig. 8f). The temperatures are cooler during the austral summer and warmer in winter. The phase shift of approximately half a year relative to the atmospheric forcing indicates the time elapsed between the formation of the dense water and its arrival at the mooring location in the vicinity of the Antarctic Peninsula. In comparison to the other y=S time series, the data recorded in the depth level of the WDW at mooring site 207-4 shows the smallest variability. The overall trend here is towards higher temperature and lower salinity (Fig. 8e).

5. Transports on the shelf The flow of shelf water was measured for almost two years from May 1996 to March 1998 with an upward-looking ADCP on the ‘shelf mooring’

234-1. The data set consists of 22 layers in the vertical, each covering 8 m of the water column. Estimates of the transports across section perpendicular to the shelf edge, about 100 km wide and intersecting mooring 234-1 were obtained. The long-shelf component was calculated by rotating the vectors of the time series by 301. The velocity component perpendicular to the transect was used for the transport calculation. Since the cross-shelf component is small, the along-shelf component corresponds essentially to the current speed. Along the section, the sea-floor topography was estimated from the Smith and Sandwell (1997) data set and approximated with a cubic spline function. According to model results (Schodlok et al., 2002) the current velocity decreases by B0.5 cm per 20 km from the shelf edge to the coast. The section was divided into five boxes of 20 km length and the mooring data were extrapolated toward the coast with the decrease derived from the model. Since the ADCP data do not cover the whole water column to the surface, the uppermost reliable layer at 100 m depth was extrapolated to the surface by adding 3 cm s
1 to take into account the expected

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4752

1.0

207-4 a

0.5

206-4 27

.82

5

27

.80

0

5 27

.77

.75 0 27

5 .72

0 .70

27

-0.5

27

.67

5

215-3 27

.85

0

216-2 27

875

-1.0

27.

pot. Temperature (°C)

0.0

207-4 b

27.9

25

-1.5

27.

900

234-1

-2.0 34.4

34.45

34.5

34.55

34.6

34.65

34.7

Salinity Fig. 7. Time series of water-mass properties displayed as y=S diagrams for all moorings. The inset indicates the instrument and water depths of the current meters (in m) and the horizontal distances between the mooring sites (in km).

shear. At locations of the transect where the bottom depth exceeds the one of the mooring (e.g., 25 km off the coast and east of the mooring) the lowermost measured value was extrapolated as a constant to the bottom. The area of boxes reaching the bottom was calculated by the use of the spline function describing the bottom profile. The mean transport calculated for the whole measuring interval is 2.471.0 Sv (=106 m3 s
1) northward with a range between minimum and maximum of 0.4–4.7 Sv. During most of the year the transport contribution by depth level decreases by half or less towards the bottom (Fig. 9). In austral spring, the transports are generally lower and the vertical profile of transports is more uniform than during the rest of the year. This feature occurs in both years suggesting that it

recurs seasonally. Maxima of transport occur in early winter.

6. Water flow into the Bransfield Strait Waters on the northwestern Weddell Sea shelf may leave the Weddell Sea into Bransfield Strait or the Scotia Sea. The flow into Bransfield Strait was the subject of various studies in recent years (e.g., Gordon and Nowlin, 1978; Whitworth et al., 1994; Wilson et al., 1999; Gordon et al., 2000). Here, we focus on the potential effect of the observed variability of the source water masses in the Weddell Sea on the water-mass properties in Bransfield Strait. Clowes (1934) had already described composition of the waters filling the

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

(a) -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8 -1.9 -2.0

(c)

4753

(b) Apr ‘98

-0.4

Jan ‘98

-0.5

Apr ‘98 Jan ‘98

-0.6 -0.7 -0.8 -0.9 Jan ‘97

Jan ‘97 -1.0 -1.1

Jun ‘96 34.48

34.50

-1.2 34.52

34.54

34.56

34.58

(d)

Jun ‘96 34.54

-0.5

Apr ‘98

-0.5

Apr ‘98

-0.6

Jan ‘98

-0.6

Jan ‘98

34.56

34.58

34.60

34.62

-0.7 -0.7 -0.8 -0.8 -0.9 -1.0

-0.9

Jan ‘97

-1.0

-1.1 -1.2

Jan ‘97

Jun ‘96 34.60

(e)

-1.1 34.61

34.62

34.63

Jun ‘96

34.64

34.63

34.64

34.65

(f) -0.8

Apr ‘98 Jan ‘98

Apr ‘98 Jan ‘98

0.6

-0.9

-1.0

0.5

-1.1 Jan ‘97

Jan ‘97 -1.2

0.4

Jun ‘96 34.67

34.68

34.69

-1.3 34.70

Jun ‘96 34.59

34.61

34.63

34.65

Fig. 8. Blow-up of each y=S time series displayed in Fig. 7, (a) 234-1, (b) 215-3, (c) 206-4, (d) 216-2, (e) 207-4a, (f) 207-4b. The change of colour shading indicates the time. The data record has a temporal resolution of 6 h.

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4754

2

0.05

0.05

0.06

0.07

0.03

0.03

0.02 0.01

0.01

0.05

0.01

0.03

Bin depth [m]

0.02 0.01

0.02

2

0.05

0.0

0.04

3

4 0.0

0.0

0.03

0.03

4

0.0

0.06

0.08

204

0.06

0.06 0.05

0.02

172

0.05

0.04

156

0.07

4

0.06

0.0

140

0.07

0.05

0.03

0.03

0.05

124

0.04

0

108

188

[Sv]

4

0.05

0.06

0.02

0.06

236

0.03

0.04

220

252 268 5

6

7

8

9 10 11 12 1

2

3

1996

4

5

6 1997

7

8

9 10 11 12 1

2 1998

3

6

7 8 [cm s-1]

Fig. 9. Contour plot of the monthly mean volume transport over the shelf for each 8-m-layer in Sv (106 m3 s
1). The right panel shows the mean current profile in cm s
1. In the upper graph, the thick line indicates the total monthly mean transport across the shelf section, the surface and bottom layer included and the thin line the contribution from the 21 8-m bins. The areas in km2 over which the current is multiplied are: 9.76 (top layer), 0.77 (layers 21-19 each), 0.76 (layers 18-13), 0.75 (layers 12-9), 0.59 (layers 8 and 7), 0.58 (layers 6-3), 0.57 (layers 2 and 1), 11.08 (bottom layer).

basins in Bransfield Strait. Bransfield Strait consists of three basins (the west, central, and east basin) deepening towards the east. We will concentrate on the two larger basins, the central, and the east basin, which are depicted in Fig. 10. The depths in the eastern basin extend to more than 2700 m. The central and the eastern basins are separated by a sill that is B1000 m deep. The channel connecting the east basin and the Powell Basin (Figs. 1 and 10) is roughly 800 m deep. Other connections are shallower. Surface temperatures correspond closely in both basins and the temperature increases slightly with depth (Fig. 13a). However, at greater depths the temperatures decrease again to colder bottom temperatures in the shallower central basin than in the east basin. Bottom temperatures reach values of
1.71C and less in the central basin. The salinities are slightly higher in the central basin than in the east basin. Consequently, the shallower central basin contains

the denser waters and the stratification is stronger. CTD data from ANT II/2 (1983), ANT III/3 (1985) or ANT VI/2 (1987) show a difference in temperature of B0.41C and in salinity of 0.04 between the bottom waters of the two basins. Wilson et al. (1999) and Gordon et al. (2000) used typical temperature and salinity values for the end members to explain the formation of the deep waters. Wilson et al. (1999) identify WDW, WSBW and Central Bransfield Strait waters and Gordon et al. (2000) WDW, high and low salinity shelf waters, as well as Pacific Pycnocline water. However, data from the two time series from moorings 234-1 and 215-3 and the Dovetail-1997 data from Bransfield Strait (Gordon et al., 2000) show that the seasonal and interannual variability of the water masses on the shelf covers a range, which corresponds to the required source waters in relation to the end products (Fig. 11). This suggests that the shelf water could be the only

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766 60°W

59°W

58°W

57°W

61°S

56°W

55°W

54°W

4755 53°W 61°S

108

69 61° 30´S

61° 30´S

M

62°S

62°S

9 117

62° 30´S

62° 30´S 1

30

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0

1 118

270

-5

00

1000

63°S

63°S

74 144 145

63° 30´S

63° 30´S 60°W

59°W

58°W

57°W

56°W

55°W

54°W

53°W

Fig. 10. Topographic map (Smith and Sandwell, 1997) of central and eastern Bransfield Strait, the shelf off Joinville Island and parts of Philip Ridge. Locations of the hydrographic data that were used are indicated.

source of deep water in the eastern basin for depths from roughly 600 m to the bottom and in the central basin above 900 m, if it would be accumulated over sufficient time so that the variations occurring at different times can mix. The pronounced inflection points in the y=S curves between 700 and 800 m indicate an inflow of warmer (modified) WDW from the Powell Basin, which enters through a channel with the corresponding sill depth (Figs. 10 and 12a). The large variability of the shelf waters suggests corresponding variations of the water properties in the channel between the Powell Basin and the Bransfield Strait, as displayed in CTD stations that were measured in the channel (Fig. 12b). While the upper part of the profiles is dominated by seasonal variations, two different regimes can be distinguished in the lower part. A group of almost linear

y=S diagrams (stations 12 and 50) represents the dominant influence of shelf water. The remaining stations (79, 268, 269) carry the signature of warmer, and saltier water such as modified WDW originating in the Powell Basin. The data do not allow to distinguish between seasonal or longer-term variations. Farther to the northwest, on the Philip Ridge, the water column is strongly influenced by water from the Powell Basin proper. Shelf water that reaches Philip Ridge seems to be mixed rapidly so that east of 521W the typical Weddell Sea y=S profile found in the Powell Basin, dominates the water column above the ridge. Water masses in the central Bransfield Strait between depths of 900 and 1000 m can originate from end members supplied by the inflow from the Powell Basin and shelf water, as recorded by

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4756

(a)

0.0 600 0

5

27.82

5

27.80

27.77

5

0

27.72

0

5

27.70

27.67

5

27.65

27.62

5

0 27.60

0

27.57

27.55

27.52

27.50

0

93 90

800 27.87 5

-1.0

27.900

pot. Temperature (°C)

-0.5

5

215-3

92 -1.5

234-1 27.925

91

-2.0 34.2

34.3

34.4

34.5

34.6

34.7

Salinity (b)

0.0

0 5

27.80

5 27.77

5

0

27.72

27.75

0

5

27.70

27.67

0

5

27.65

27.62

5

0 27.60

27.57

27.55

0

5 27.52

215-3

27.82

234-1

900

95 -1.0

1000 27.900

-1.5

94 27.925

pot. Temperature (°C)

-0.5

27.50

0

600

96 -2.0 34.2

34.3

34.4

34.5

34.6

34.7

Salinity Fig. 11. y=S diagram of data from moorings 234-1, 215-3 and Dovetail 1997 data. The upper panel (a) shows four CTD stations from the Dovetail 1997 section through the eastern Bransfield Strait basin in combination with the greyshaded y=S time series from the shelf moorings. The same time series are displayed in the lower panel (b), this time with three stations from the central Bransfield Strait basin. The depths of y=S values indicated by the small white dots are given by encircled numbers.

moorings 234-1 and/or 215-3. To form water at greater depths, cooler and saltier end members are needed (Fig. 11b). The high-salinity form of the shelf water observed at mooring 234-1 serves the purpose. Around Joinville Island the very cold and saline water can be found within B60–70 km from

the coastline. This water then would be either locally formed or advected from sources further south as suggested by three sections across the Bransfield Strait (Fig. 13). Gordon et al. (2000) state that high-salinity shelf water might take a shortcut via the Antarctic

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

27.82 5

27.77 5

27.75 0

27.70 0

27.72 5

27.65 0

27.67 5

27.60 0

27.62 5

27.57 5

27.55 0

27.80 0

79

27.85 0

95

27.87

5

-1.0

27.900

pot. Temperature (°C)

-0.5

27.50 0

0.0

27.52 5

(a)

4757

92

27.925

-1.5

-2.0 34.2

34.3

34.4

34.5

34.6

34.7

Salinity (b)

0.0

50

0

27.77 5

27.75 0

27.72 5

27.70 0

27.65 0

27.67 5

27.60 0

27.62 5

27.57 5

27.55 0

27.50 0

27.80 0

79

27.85

-1.0

27.87 5

268

27.900

pot. Temperature (°C)

-0.5

27.52 5

12

269

27.925

-1.5

-2.0 34.2

34.3

34.4

34.5

34.6

34.7

Salinity Fig. 12. y=S diagrams of stations in Bransfield Strait and in the area of the deepest connection between the Powell Basin and the Bransfield Strait. (a) The greyshaded time series from moorings on the shelf are displayed with a ‘representative’ curve from each of the two major Bransfield Strait basins. Additionally the y=S curve from station 79 (Dovetail 1997) is displayed to show the probable source of the modified WDW. (b) Variability of the y=S curves measured in the years 1983 (stations 268/9), 1989/90 (12), 1997 (79) and 1999 (50).

Sound and thereby contribute to the formation of deep water in the central basin of the Bransfield Strait. This seems unlikely during the austral summer since according to CTD data from ANT VI/2 (November 1987) and ME 90/1 (December 1989) the temperature and the low salinity of this

shelf water excludes it from deep-water formation in the central Bransfield Strait (Fig. 14). However, during austral winter, an appropriate water mass is found on the shelf north of Joinville Island at mooring 234-1, which could flow from there into Bransfield Strait.

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

11601

11201

0

10801

(a)

10601

Between 601S and 611S, the South Scotia Ridge consists of two parallel crests that enclose an elongated depression with troughs of over 5000 m depth (Fig. 15). Near 491W, a seamount protrudes southward into the Powell Basin. At station 66 from ANTXV/4 located just behind the seamount (Fig. 13b, lower panel, Section 4), the WDW layer

-1.

-0.8

11801

is interrupted. The isotherms as well as the isohalines show strong excursions compared to the neighbouring stations (Fahrbach, 1999). This effect of the seamount on the stratification suggests an impact on the current at the northern rim of the Powell Basin, which is expected to be eastward. Meandering and shedding of eddies will occur downstream of the seamount and could be the origin of structures as the one observed at

7. Outflow through the Philip Passage

11701

4758

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200 -1.6

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S

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ANT VI/2, Section 1

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.4

.3

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13001

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12701

Distance [km]

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Depth [m]

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S

N

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ANT VI/2, Section 2

1800 2000 0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

Distance [km]

Fig. 13. Temperature sections, as indicated in Fig. 2, from the Bransfield Strait region (Sections 1–3) and through the Powell Basin (Section 4).

-1.

0

. -1

200

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10501

-1.2

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A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

0.41.0

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1.4

600 -1

0.8

.2

Depth [m]

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-1 .4

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E

6 -1.

W

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ANT VI/2, Section 3

1800 2000 50

0

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901

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0.8 0.2

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Depth [m]

7401 7501

7201 7301

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6601

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5501

5301 0

5401

Distance [km]

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-0.2

2000

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-0.3 -0.4

2500

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-0.3

3000

-0.8

3500

S

N

-0.4

4000

ANT XV/4, Section 4

4500 5000 0

50

100

150

200

250

300

350

400

450

Distance [km]

Fig. 13 (continued).

station 66. The eastward current is most likely deflected in front of the seamount to a northwestward current corresponding to the record at mooring MIR 2. Another possibility to explain the northwestward flow at this site might be a counterclockwise return flow near the crest, similar to the observations by Nowlin and Zenk (1988) near the Philip Passage. Philip Passage is the deepest (about 2000 m) of the various gaps in the rugged South Scotia Ridge surrounding the Powell Basin. Although the sill in

the Philip Passage is not deep enough for WSBW to escape into the Scotia Sea, WSDW can cross the northern rim. Schodlok et al. (2002) report the modelled mean annual export of water through Philip Passage to be 2.2 Sv. Several hydrographic surveys have been carried out along or across the ridge system. In this study we take into account CTD data from six cruises (Figs. 15 and 16) to determine the variability of the outflow on different time scales. All y=S curves from ANT II/3 in 1983 (Fig. 16a) show the

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4760

0.0

5

0

0

27.77

5

27.82

5

27.75

0

27.72

0

5

27.70

27.67

5

27.65

27.62

5

0

27.57

27.60

5

0 27.55

27.52

27.80

0

79

27.85

95

27.87 5

-1.0

132 27.900

-1.5

94

76 234-1 75

74

27.925

pot. Temperature (°C)

-0.5

27.50

0

31

-2.0 34.2

34.3

34.4

34.5

34.6

34.7

Salinity Fig. 14. y=S diagram for various shelf stations, the central Bransfield Strait and the inflow from the Powell Basin. Stations 74, 75, 76, 79 and 95 are from the Dovetail 1997 data set, station 31 and 132 were obtained during ME 90/1 and ANTVI/2, respectively. The latter two stations lie at the northern end of the Antarctic Sound. In both cases temperature and low salinity of the water excludes a contribution of this shelf water to the formation of deep water in the central Bransfield Strait.

51°W

50°W

49°W

II (1983)

48°W

4

60°S

T

47°W

60°S

54

(1992)

3000

60° 30´S

60° 30´S 65 2455

6 61°S

61° 51°W

50°W

49°W

48°W

47°W

Fig. 15. Detailed topography (Smith and Sandwell, 1997) of the South Scotia Ridge and Philip Passage with the stations used.

characteristic Weddell Sea signature, the seasonally influenced surface layer followed by a temperature minimum, then a maximum, before the temperatures decrease again linearly. The

temperatures in the WDW-influenced layer have a mean value of 0.081C and the bottom temperatures scatter around a mean value of
0.351C. The deep water shows a slight trend of warming

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

(a)

4761

(b)

ANT X/4 (1992)

ANT II/3 (1983)

0.0

-0.5

27. 800

27. 775

27. 750

27. 725

27. 800

27. 775

27. 750

27. 725

pot. Temperature (°C)

0.5

237 239 240 241 238 243 244 245

(c)

644 643 642 641 640 639 (d)

AR XVIII (2000)

ALBATROSS (1999)

(e)

750

27.8 25

27.

25

800

0.0

27.

27.

61 62 63 64 65 66 67 68 18

-0.5

210 211 212 217 213 215

(f )

ANT XV/4 (1998) 775

775

750

27.

27.

25 27. 7

27.

700

27.

27.

27.

725

0.5

750

Dovetail 97

0.0

-0.5

34.5

34.6 Salinity

34.7

00

58 57 56 55 54 123

27. 8

27.

800

pot. Temperature (°C)

27. 7

27. 700

775

750 27.

725 27.

pot. Temperature (°C)

0.5

34.5

34.6 Salinity

63 62 61 56 58 59 60 66 65 57 55 54 34.7

Fig. 16. y=S diagrams for stations displayed in Fig. 15. The four upper panels (a–d) show the data from the sections across Philip Passage. The two lower panels (e and f) include data from meridional sections through the passage. In the ALBATROSS diagram (16c), station 18 from E-Dovetail (Garc!ıa, 1998) is displayed, station 123 (ANT ME 11/5), at 60.201S and 45.651W in the one with Dovetail 97 (16e). The latter station is not included in Fig. 15 as it is outside of the mapped area. It is indicated by a black triangle in Fig. 2.

4762

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

towards the west. Fifteen years later, during ANT XV/4 (1998, Fig. 16f), the water found in the Philip Passage was profoundly different. The mean bottom temperatures are 0.121C warmer. The warming of 0.351C in the mean temperature maximum of the WDW layer is even more pronounced. The density gradient is weakened in the deeper layers. In the successive year, the data from the ALBATROSS section (Fig. 16c) corresponds closely to the data from 1998. Because the station occupied is at a greater depth, the bottom water is colder. The increase of the temperature maximum towards the west is further enhanced in 1999. The little scatter of the y=S relations that was observed in 1983 in the warm layer has changed to a rather dispersed structure. The layer thickness of water warmer than 01C has tripled between 1983 and the end of the century (Fig. 17). The depth of the upper 01C isotherm has remained constant while the lower boundary of the warm water mass has deepened. The Brazilian AR XVIII survey reported a consistent increase of temperature in 2000 and bottom-water temperatures above 01C at the deepest station of Philip Passage (Figs. 16d and 17). The temperature in the WDW layer varies widely, and the y=S curves have lost their distinct peak. Interleaving in the western part of the section indicates mixing processes in the depth level of the warm layer. It appears to increase with time and was especially pronounced in the Brazilian observations from 2000. A weakened stratification would facilitate interleaving and mixing which again would support a further decrease of gradients. Naveira Garabato et al. (2002) suggest that interaction with ‘Bransfield Strait waters’ are responsible for the high variability of the WDW west of the South Orkney Islands. In 1992 (ANT X/4), another hydrographic section was measured that was closer to the Scotia Sea (Figs. 15 and 16c). The comparison of two other sections from Dovetail 97 and ANT XV/4 with this northern section reveals deep water found at stations 58 (Dovetail 97, Fig. 16e) or 66 (ANT XV/4, Fig. 16f) is not able to reach the topographic depression that extends far to the west behind the southern sill of Philip Passage. Within this trough even at very deep stations (55, ANT

XV/4), the water is not colder than
0.371C, which displays clearly the limit given by the sill of Philip Passage. The deep water within the depression (Station 18, E-Dovetail, Fig. 16c) shows a very weak density gradient and is most likely supplied by the inflow of water from the Powell Basin via Philip Passage. The exchange with Bransfield Strait is limited to the upper layers, and the Powell Basin-derived y=S signature is dominant east of 531300 W. Water does spill over the Philip Ridge into the depression, which is documented by the current-meter record of MIR 2 (near 491300 W). But since the water over Philip Ridge is influenced by the injection of shelf water, it is too fresh and thus reaches only the upper and intermediate layers of the depression. The opening of Philip Passage into the Scotia Sea was surveyed by the ANT X/4 section in 1992 (Figs. 15 and 16b). At a depth of about 1000 m, the y=S curves display a sudden shift, almost step-like towards saltier water. Stations 54 (Dovetail 97 and ANT XV/4, Fig. 16e and f) from two meridional sections across the South Scotia Ridge also show a diversion towards saltier values for depths below B2800 m. Measurements carried out in the trough just north of South Orkney (Station 123, ANT ME11/5, 1990, Fig. 16e) display the appropriate source water for the overflow region. Water of Weddell Sea origin with potential densities higher than 27.85 is advected around the South Orkney Plateau (Nowlin and Zenk, 1988; Locarnini et al., 1993; Orsi et al., 1999) and is found beneath the Powell Basin overflow due to its higher density. In 1997, the transition is smooth but 1 year later the y=S relation shows an abrupt change from one water mass signature to the other. Water exiting the Powell Basin may thus contribute to the deep water in the Scotia Sea, but it is not dense enough to form bottom water.

8. Summary and conclusions In the vicinity of the Antarctic Peninsula and the Powell Basin, different water masses flow from the Weddell Sea proper into the Scotia Sea. The transition zone between these basins is influenced by the complex topography which determines the

-0.6 -0.8

-0.6 -1.0

-1.0

200 400

4763

- 24501

- 24401

- 23801 - 24301

- 24101

0

- 24001

- 23701

- 23901

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

-0.8

-0.8 -0.6

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-0.4 -0.2

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Depth [m]

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-0.2

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E

W

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80

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-1.0

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-1.2

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- 5801

- 5601 - 5701

- 6101

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0

- 6201

Distance [km]

0.4

Depth [m]

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0.4

800 0.2

1000 0.0

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-0.2

1600 1800

E

W

2000

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

Distance [km] Fig. 17. Two temperature sections across Philip Passage. The upper panel shows data that were obtained during ANT II/3 (1983), the lower panel AR XVIII (2000).

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

4764 60°W

56°W

52°W

48°W

44°W

40°W

36°W

58°S

58°S

60°S

60°S

62°S

62°S

64°S

64°S

66°S

66°S 60°W

56°W

52°W

48°W

44°W

40°W

36°W

Fig. 18. Suggested current pathways in the transition zone between the Powell Basin and the Scotia Sea with additional background information from Gordon et al. (2001). The light grey arrow indicates a surface-near level of the flow. The dashed arrows indicate the alternating dominance of shelf derived water or modified WDW in the channel between Powell Basin and the Bransfield Strait. Mixing is indicated on the Philip Ridge.

pathways of the water masses. The flow patterns and their variability are described by means of moored current meters, temperature and conductivity recorders. CTD surveys spanning the time interval from 1983 to 2000 are used to discuss the long-term variability in this area; however, the small number of stations restricts conclusions. The current-meter records showed the dominating effect of topography on the mean currents. Currents on the shelf were, compared to the currents on the continental slope, relatively weak southeast of Joinville Island but relatively strong northeast of the island. Indication for seasonal variability is found in most records, but in areas of weak currents, meso-scale fluctuations dominate. In general, strong currents prevail in winter,

weaker ones in summer. Temperature and salinity records at most locations reveal variations of the water-mass properties in the same time scales. The transport of shelf water to the north, which represents a potential path for feeding recently ventilated water in the Weddell Scotia Confluence, is estimated by means of an upward-looking ADCP. A northward transport of shelf water of 2.471.0 Sv is obtained as a mean value for the entire deployment interval. The monthly estimates show considerable fluctuations. However, the water from the shelf seems to feed almost exclusively into the Bransfield Strait and is part of the deep-water formation there. Bransfield Strait water exits the eastern basin into the

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neighbouring northeastern trough where it rapidly looses its signature through mixing with the water dominated by the Powell Basin outflow. Shelf water that spreads along Philip Ridge is subject to strong mixing with the adjacent water mass (Fig. 18). There is no evidence of a direct contribution of shelf water from the vicinity of the Antarctic Peninsula to the ventilation of the Scotia Sea. It either supplies the deepwater formation in the Bransfield Strait or is entrained into other water masses from the Weddell Sea. Consequently it contributes only indirectly to the ventilation of the global deep water. The observed variations of the characteristics of shelf water suggest that it could be the only source of deep water in the eastern Bransfield Strait basin, if it would be accumulated over sufficient time so that its different varieties can mix. Water derived from the Powell Basin between 700 and 800 m depth supplies a saline component that is also highly variable. The water properties of the deep water in the central basin require additionally the contribution of high-salinity shelf water which is advected around Joinville Island. Advection of high-salinity shelf water through the Antarctic Sound can only occur in winter according to the available data. During the last two decades the water properties of the flow through Philip Passage have changed profoundly. There has been a trend of warming throughout the water column, but especially clear in the WDW layer. The interleaving in the WDW layer is also found to have intensified in the recent past.

Acknowledgements We thank Karen J. Heywood and Alberto C. Naveira Garabato from the University of East Anglia for the kind provision of the ALBATROSS data as well as Bruce A. Huber of the LamontDoherty Earth Observatory for the access to the Dovetail 97 data. We express our gratitude to the three anonymous reviewers for their helpful comments.

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References Baines, P.G., Condie, S., 1998. Observations and modelling of Antarctic downslope flows: a review. In: Jacobs, S.S., Weiss, R. (Eds.), Ocean, Ice, and Atmosphere: Interactions at the Antarctic Continental Margin, Antarctic Research Series, AGU 75, pp. 29–49. Carmack, E.C., Foster, T.D., 1975. On the flow of water out of the Weddell Sea. Deep-Sea Research 22, 711–724. Clowes, A.J., 1934. Hydrology of the Bransfield Strait. Discovery Reports 9, 1–64. Deacon, G.E.R., 1937. The hydrology of the Southern Ocean. Discovery Reports 15, 3–122. Fahrbach, E., 1999. The Expedition ANTARKTIS XV/4 of the Research Vessel ‘Polarstern’ in 1998. Reports on Polar Research 314, 109pp. . Fahrbach, E., Rohardt, G., Schroder, M., Strass, V., 1994. Transport and structure of the Weddell Gyre. Annales Geophysicae 12, 840–855. . Fahrbach, E., Rohardt, G., Scheele, N., Schroder, M., Strass, V., Wisotzki, A., 1995. Formation and discharge of deep and bottom water in the northwestern Weddell Sea. Journal of Marine Research 53, 515–538. . Fahrbach, E., Harms, S., Rohardt, G., Schroder, M., Woodgate, R., 2001. Flow of bottom water in the northwestern Weddell Sea. Journal of Geophysical Research 106 (C2), 2761–2778. Foldvik, A., Gammelsrd, T., Trresen, T., 1985. Circulation and water masses on the southern Weddell Sea shelf. In: Jacobs, S.S. (Ed.), Oceanology of the Antarctic Continental Shelf. Antarctic Research Series, AGU 53, pp. 5–20. Foster, T.D., Carmack, E.C., 1976a. Frontal zone mixing and Antarctic bottom water formation in the southern Weddell Sea. Deep-Sea Research I 23, 301–317. Foster, T.D., Carmack, E.C., 1976b. Temperature and salinity structure in the Weddell Sea. Journal of Physical Oceanography 6, 36–44. Foster, T.D., Foldvik, A., Middleton, J.H., 1988. Mixing and bottom water formation in the shelf break region of the southern Weddell Sea. Deep-Sea Research 34, 1771–1794. Gammelsrd, T., Foldvik, A., Nst, O.A., Skagseth, Ø., Anderson, L.G., Fogelqvist, E., Olsson, K., Tanhua, T., Jones, E.P., Østerhus, S., 1994. Distribution of water masses on the continental shelf in the southern Weddell Sea. In: Johannessen, O.M. (Ed.) The Polar Oceans and Their Role in Shaping the Global Environment: the Nansen centennial volume. AGU Geophysical Monograph 84, pp. 159–176. Garc!ıa, M.A., 1998. Oceanografia fisica y productividad en la * confluencia Weddell-Scotia: contribucion Espanola al * proyecto internacional Dovetail. Informe de la campana E-DOVETAIL BIO Hesp!erides, Laboratori d’Enginyeria Mar!ıtima, Universitat Polit"ecnica de Catalunya. Gill, A.E., 1973. Circulation and bottom water production in the Weddell Sea. Deep-Sea Research I 20, 111–140. Gordon, A.L., 1967. In: V. Bushnell (Ed.), Structure of Antarctic Waters Between 201W and 1701W. Antarctic Map Folio Series 6, American Geographical Society, 10pp.

4766

A.-B. von Gyldenfeldt et al. / Deep-Sea Research II 49 (2002) 4743–4766

Gordon, A.L., 1998. Western Weddell Sea thermohaline stratification. In: Jacobs, S.S., Weiss, R. (Eds.), Ocean, Ice, and Atmosphere: Interactions at the Antarctic Continental Margin, Antarctic Research Series, AGU 75, pp. 215–240. Gordon, A.L., Nowlin Jr., W.D., 1978. The basin waters of the Bransfield Strait. Journal of Physical Oceanography 8, 258–264. Gordon, A.L., Huber, B.A., Hellmer, H.H., Ffield, A., 1993. Deep and bottom water of the Weddell Sea’s western rim. Science 262, 95–97. Gordon, A.L., Mensch, M., Dong, Z., Smethie Jr., W.M., de Bettencourt, J., 2000. Deep and bottom waters of the Bransfield Strait eastern and central basins. Journal of Geophysical Research 105 (C5), 11337–11346. Gordon, A.L., Visbeck, M., Huber, B.A., 2001. Export of Weddell Sea deep and bottom water. Journal of Geophysical Research 106 (C5), 9005–9018. Heywood, K.J., Stevens, D.P., 2000. ALBATROSS Cruise Report, UEA Cruise Report No. 6. Huthnance, J.M., 1981. Waves and currents near the continental shelf edge. Progress in Oceanography 10, 193–226. Killworth, P.D., 1973. Mixing on the Weddell Sea continental slope. Deep-Sea Research I 24, 427–448. Locarnini, R.A., Whitworth III, T., Nowlin Jr., W.D., 1993. The importance of the Scotia Sea on the outflow of Weddell Sea deep water. Journal of Marine Research 51, 135–153. Mantyla, A.W., Reid, J.L., 1983. Abyssal characteristics of the World Ocean waters. Deep-Sea Research I 30, 805–833. Muench, R.D., 1997. Deep ocean ventilation through Antarctic intermediate layers: the DOVETAIL program. Antarctic Journal of the United States 32 (5), 65–67. Muench, R.D., Gordon, A.L., 1995. Circulation and transport of water along the western Weddell Sea margin. Journal of Geophysical Research 100 (C9), 18503–18515. Naveira Garabato, A.C., 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. Nowlin Jr., W.D., Zenk, W., 1988. Westward bottom currents along the margin of the South Shetland Island Arc. DeepSea Research I 35 (2), 269–301. 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 (1), 169–203. Orsi, A.H., Johnson, G.C., Bullister, J.L., 1999. Circulation, mixing, and production of Antarctic bottom water. Progress in Oceanography 43, 55–109. Patterson, S.L., Sievers, H.A., 1980. The WeddellScotia confluence. Journal of Physical Oceanography 10, 1584–1610. Reid, J.L., Nowlin Jr., W.D., Patzert, W.C., 1977. On the characteristics and circulation of the Southwestern Atlantic ocean. Journal of Physical Oceanography 7, 62–91. Schmitz, W.J., McCartney, M.S., 1993. On the north Atlantic circulation. Reviews of Geophysics 31, 29–49. Schodlok, M.P., Hellmer, H.H., Beckmann, A., 2002. On the origin, rates and variability of dense water masses crossing the South Scotia Ridge. Deep-Sea Research II 49 (21), 4807–4825. . Schroder, M., Hellmer, H.H., Absy, M.J., 2002. Near-bottom variability in the northwestern Weddell Sea. Deep-Sea Research II 49 (21), 4767–4790. Smith, W.H.F., Sandwell, D.T., 1997. Sea floor topography from satellite altimetry and ship depth soundings. Science 277, 1957–1962. Whitworth III, T., Nowlin Jr., W.D., Orsi, A.H., Locarnini, R.A., Smith, S.G., 1994. Weddell sea shelf water in the Bransfield strait and Weddell-Scotia Confluence. Deep-Sea Research I 41 (4), 629–641. Wilson, C., Klinkhammer, G.P., Chin, C.S., 1999. Hydrography within the central and east basins of the Bransfield Strait, Antarctica. Journal of Physical Oceanography 29, 465–479.