Sources and distribution of fresh water in the East Greenland Current

Fresh water flowing from the Arctic Ocean via the East Greenland Current influences deep water formation in the Nordic Seas as well as the salinity of the surface and deep waters flowing from there. This fresh water has three sources: Pacific water (relatively fresh cf. Atlantic water), river runoff, and sea ice meltwater. To determine the relative amounts of the three sources of fresh water, in May 2002 we collected water samples across the East Greenland Current in sections from 81.5°N to the Irminger Sea south of Denmark Strait. We used nitrate-phosphate relationships to distinguish Pacific waters from Atlantic waters, salinity to obtain the sum of sea ice melt water and river runoff water, and total alkalinity to distinguish the latter. River runoff contributed the largest part of the total fresh water component, in some regions with some inventories exceeding 12 m. Pacific fresh water (Pacific source water S ∼ 32 cf. Atlantic source water S ∼ 34.9) typically provided about 1/3 of the river runoff contribution. Sea ice meltwater was very nearly non-existent in the surface waters of all sections, likely at least in part as a result of the samples being collected before the onset of the melt season. The fresh water from the Arctic Ocean was strongly confined to near the Greenland coast. We thus conjecture that the main source of fresh water from the Arctic Ocean most strongly impacting deep convection in the Nordic Seas would be sea ice as opposed to fresh water in the liquid phase, i.e., river runoff, Pacific fresh water, and sea ice meltwater.     

Transports of Nordic Seas water masses and excess SF6 through Fram Strait to the Arctic Ocean

To determine the exchanges between the Nordic Seas and the Arctic Ocean through Fram Strait is one of the most important aspects, and one of the major challenges, in describing the circulation in the Arctic Mediterranean Sea. Especially the northward transport of Arctic Intermediate Water (AIW) from the Nordic Seas into the Arctic Ocean is little known. In the two-ship study of the circulation in the Nordic Seas, Arctic Ocean - 2002, the Swedish icebreaker Oden operated in the ice-covered areas in and north of Fram Strait and in the western margins of Greenland and Iceland seas, while RV Knorr of Woods Hole worked in the ice free part of the Nordic Seas. Here two hydrographic sections obtained by Oden, augmented by tracer and velocity measurements with Lowered Acoustic Doppler Current Profiler (LADCP), are examined. The first section, reaching from the Svalbard shelf across the Yermak Plateau, covers the region north of Svalbard where inflow to the Arctic Ocean takes place. The second, western, section spans the outflow area extending from west of the Yermak Plateau onto the Greenland shelf. Geostrophic and LADCP derived velocities are both used to estimate the exchanges of water masses between the Nordic Seas and the Arctic Ocean. The geostrophic computations indicate a total flow of 3.6 Sv entering the Arctic on the eastern section. The southward flow on the western section is found to be 5.1 Sv. The total inflow to the Arctic Ocean obtained using the LADCP derived velocities is much larger, 13.6 Sv, and the southward transport on the western section is 13.7 Sv, equal to the northward transport north of Svalbard. Sulphur hexafluoride (SF₆) originating from a tracer release experiment in the Greenland Sea in 1996 has become a marker for the circulation of AIW. From the geostrophic velocities we obtain 0.5 Sv and from the LADCP derived velocities 2.8 Sv of AIW flowing into the Arctic. The annual transport of SF₆ into the Arctic Ocean derived from geostrophy is 5 kg/year, which is of the same magnitude as the observed total annual transport into the North Atlantic, while the LADCP measurements (19 kg/year) imply that it is substantially larger. Little SF₆ was found on the western section, confirming the dominance of the Arctic Ocean water masses and indicating that the major recirculation in Fram Strait takes place farther to the south.     

北冰洋水体对格陵兰海混合增密对流的可能影响分析

格陵兰海内发生的等密度混合后产生的增密对流是重要的对流现象之一。北冰洋正在发生快速变化,其内水团变性以及环流系统的改变都将使格陵兰海等密度混合对流发生明显变化,继而对全球气候变化产生影响。以往关于等密度混合对流的研究很少,大都集中在对流发生海域。由于等密度混合的主体是大西洋回流水与北冰洋流出水体,本文目的是探讨北极内部不同海域的水体会对混合增密对流造成的可能影响。文中定义了有效对流速度,强调水平温度梯度和垂向层化强度是影响有效对流速度的决定性因素;水平温度差越大,垂向层化越弱,产生的对流越强。发生在东格陵兰极锋处的有效对流都是大西洋的水体,一部分是在格陵兰海回流的大西洋回流水;一部分是在北冰洋潜沉并回流的北极大西洋水,该水体在北冰洋循环的时间越长,温度差越大,产生的有效对流越强。而横越北冰洋的太平洋水因密度过低而不能参与等密度混合对流,加拿大海盆主盐跃层之上的水体也都不能参与对流。北冰洋几个海盆深层水的温度差异明显,有可能与格陵兰海深层水形成有效对流;但是,由于深层水流速低、湍流混合弱、水平温度梯度小,是否可以产生有效对流尚不清楚。     

Sources to the East Greenland Current and its contribution to the Denmark Strait Overflow

Data from the East Greenland Current in 2002 are evaluated using optimum multiparameter analysis. The current is followed from north of Fram Strait to the Denmark Strait Sill and the contributions of different source waters, in mass fractions, are deduced. From the results it can be concluded that, at least in spring 2002, the East Greenland Current was the main source for the waters found at the Denmark Strait Sill, contributing to the overflow into the North Atlantic. The East Greenland Current carried water masses from different source regions in the Arctic Ocean, the West Spitsbergen Current and the Greenland Sea. The results agree well with the known circulation of the western Nordic Seas but also add knowledge both to the quantification and to the mixing processes, showing the importance of the locally formed Greenland Sea Arctic Intermediate Water for the East Greenland Current and the Denmark Strait.     

The Greenland Sea in Water 1993 and 1994: preconditioning for deep convection

We present here a detailed analysis of the time-dependent large-scale and mesoscale features observed in the Greenland Sea during winter and spring 1993 and 1994. Based on a comprehensive data set, this study gives particular emphasis to the data from hydrology and RAFOS-VCM floats. In both years, the basin underwent an overturning to 1000 and 800 m depth, respectively, with extensive horizontal variability. These inhomogeneities are shown to be closely linked with mesoscale eddies identified at a late stage during the preconditioning phase to deep convection. It is suggested that these eddies are remnants of previous convective events. One of them, found at 500 m depth in 1994, was thoroughly studied. The dynamical coupling between the eddy and the modified Atlantic Water (mAW), advected over it from the northwest, led to the deepest convected water column found within the basin during this particular year. Sea ice, absent in the gyre in winter 1994, but partly present in the colder 1993 winter, does not appear to be a strong prerequisite for deep convection. Heat loss at the surface, combined with salt input by the mAW, and remnant subsurface mesoscale eddies of past convective events, are the most likely agents that determine the depth reached by convection and the characteristics of the new Greenland Arctic Intermediate Water formed in the Greenland Gyre in 1993 and 1994.     

Sensitivity of the Atlantic Ocean circulation to a hydraulic overflow parameterisation in a coarse resolution model: Response of the subpolar gyre

We investigate the sensitivity of a coarse resolution coupled climate model to the representation of the overflows over the Greenland–Scotland ridge. This class of models suffers from a poor representation of the water mass exchange between the Nordic Seas and the North Atlantic, a crucial part of the large-scale oceanic circulation. We revisit the explicit representation of the overflows using a parameterisation by hydraulic constraints and compare it with the enhancement of the overflow transport by artificially deepened passages over the Greenland–Scotland ridge, a common practice in coarse resolution models. Both configurations increase deep water formation in the Nordic Seas and represent the large-scale dynamics of the Atlantic realistically in contrast to a third model version with realistic sill depths but without the explicit overflow transport. The comparison of the hydrography suggests that for the unperturbed equilibrium the Nordic Seas are better represented with the parameterised overflows. As in previous studies, we do not find a stabilising effect of the overflow parameterisation on the Atlantic meridional overturning circulation but merely on the overflow transport. As a consequence the surface air temperature in the Nordic Seas is less sensitive to anomalous surface fresh water forcing.Special attention is paid to changes in the subpolar gyre circulation. We find it sensitive to the overflow transport and the density of these water masses through baroclinic adjustments. The analysis of the governing equations confirms the presence of positive feedbacks inherent to the subpolar gyre and allows us to isolate the influence of the overflows on its dynamics.     

Anthropogenic carbon in the East Greenland Current

Sections of dissolved inorganic anthropogenic carbon () based on 2002 data in the East Greenland Current (EGC) are presented. The has been estimated using a model based on optimum multiparameter analysis with predefined source water types. Values of have been assigned to the source water types through age estimations based on the transit time distribution (TTD) technique. The validity of this approach is discussed and compared to other methods. The results indicated that the EGC had rather high levels of in the whole water column, and the anthropogenic signal of the different source areas were detected along the southward transit. We estimated an annual transport of with the Denmark Strait overflow (σ_θ > 27.8 kg m⁻³) of ∼0.036 ± 0.005 Gt C y⁻¹. The mean concentration in this density range was ∼30 μmol kg⁻¹. The main contribution was from Atlantic derived waters, the Polar Intermediate Water and the Greenland Sea Arctic Intermediate Water.     

Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific

Fifty years of measurements at Ocean Station Papa (OSP, 50°N, 145°W) show trends in the interior waters of the subarctic Pacific that are both impacted by short term (few years to bi-decadal) atmospheric or ocean circulation oscillations and by persistent climate trends. Between 1956 and 2006, waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen. On density surfaces found in the depth range 100-400 m (σ_θ = 26.3-27.0), the ocean is warming at 0.005-0.012 °C y⁻¹, whereas oxygen is declining at 0.39-0.70 μmol kg⁻¹ y⁻¹ or at an integrated rate of 123 mmol m⁻² y⁻¹ (decrease of 22% over 50 years). During this time, the hypoxic boundary (defined as 60 μmol O₂ kg⁻¹) has shoaled from ∼400 to 300 m. In the Alaska Gyre, the 26.2 isopycnal occasionally ventilates, whereas at OSP 26.0σ_θ has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify. To interpret the 50 year record at OSP, the isopycnal transport of oxygenated waters within the interior of the subarctic Pacific is assessed by using a slightly modified “NO” parameter [Broecker, W., 1974. “NO” a conservative water-mass tracer. Earth and Planetary Science Letters 23, 100-107]. The highest nitrate-oxygen signature in interior waters of the North Pacific is found in the Bering Sea Gyre, Western Subarctic Gyre and East Kamchatka Current region as a consequence of winter mixing to the ∼26.6 isopycnal. By mixing with low NO waters found in the subtropics and Okhotsk Sea, this signature is diluted as waters flow eastward across the Pacific. Evidence of low NO waters flowing north from California is seen along the coasts of British Columbia and SE Alaska. Oxygen in the subsurface waters of the Alaskan Gyre was supplied ∼60% by subarctic and 40% by subtropical waters during WOCE surveys, whereas such estimates are shown to periodically vary by 20% at OSP. Other features discernable in the OSP data include periods of increased ventilation of deeper isopycnals on an ∼18 year cycle and strong, short term (few month) variability caused by passing mesoscale eddies. The potential impacts of declining oxygen on coastal ecosystems are discussed.     

The East Greenland Coastal Current: Structure, variability, and forcing

The subtidal circulation of the southeast Greenland shelf is described using a set of high-resolution hydrographic and velocity transects occupied in summer 2004. The main feature is the East Greenland Coastal Current (EGCC), a low-salinity, high-velocity jet with a wedge-shaped hydrographic structure characteristic of other surface buoyancy-driven currents. The EGCC was observed along the entire Greenland shelf south of Denmark Strait, while the transect north of the strait showed only a weak shelf flow. This observation, in conjunction with water mass considerations and other supporting evidence, suggests that the EGCC is an inner branch of the East Greenland Current (EGC) that forms south of Denmark Strait. It is argued that bathymetric steering is the most likely reason why the EGC apparently bifurcates at this location. Repeat sections occupied at Cape Farewell between 1997 and 2004 show that the alongshelf wind stress can have an influence on the structure and strength of the EGCC and EGC on timescales of 2-3 days. Accounting for the wind-induced effects, the volume transport of the combined EGCC/EGC system is roughly constant (∼2 Sv) over the study domain, from 68°N to Cape Farewell near 60°N. The corresponding freshwater transport increases by roughly 60% over this distance (59-96 mSv, referenced to a salinity of 34.8). This trend is consistent with a simple freshwater budget of the EGCC/EGC system that accounts for meltwater runoff, melting sea-ice and icebergs, and net precipitation minus evaporation.     

Thermohaline variability and Antarctic bottom water formation at the Ross Sea shelf break

We use hydrological and current meter data collected in the Ross Sea, Antarctica between 1995 and 2006 to describe the spatial and temporal variability of water masses involved in the production of Antarctic Bottom Water (AABW). Data were collected in two regions of known outflows of dense shelf water in this region; the Drygalski Trough (DT) and the Glomar-Challenger Trough (GCT). Dense shelf water just inshore of the shelf break is dominated by High Salinity Shelf Water (HSSW) in the DT and Ice Shelf Water (ISW) in the GCT. The HSSW in the northern DT freshened by ∼0.06 in 11 y, while the ISW in the northern GCT freshened by ∼0.04 in 8 y and warmed by ∼0.04 °C in 11 y, dominated by a rapid warming during austral summer 2001/02. The Antarctic Slope Front separating the warm Circumpolar Deep Water (CDW) from the shelf waters is more stable near GCT than near DT, with CDW and mixing products being found on the outer DT shelf but not on the outer GCT shelf. The different source waters and mixing processes at the two sites lead to production of AABW with different thermohaline characteristics in the central and western Ross Sea. Multi-year time series of hydrography and currents at long-term moorings within 100 km of the shelf break in both troughs confirm the interannual signals in the dense shelf water and reveal the seasonal cycle of water mass properties. Near the DT the HSSW salinities experienced maxima in March/April and minima in September/October. The ISW in the GCT is warmest in March/April and coolest between August and October. Mooring data also demonstrate significant high-frequency variability associated with tides and other processes. Wavelet analysis of near-bottom moored sensors sampling the dense water cascade over the continental slope west of the GCT shows intermittent energetic pulses of cold, dense water with periods from ∼32 h to ∼5 days.     

Predictive permeability variations in a Hermod sand reservoir,Stine Segments,Siri Field,Danish North Sea

The Hermod sand of the Stine Segments, Siri Field, Danish North Sea, displays large permeability variations (1–600 mD). These permeability variations represent horizontal layering in the reservoir, largely formed by diagenetic processes. They have a significant impact on reservoir performance, because horizontal producers in fairly thin reservoirs (∼30 m) are protected against bottom-water influx and can produce water-free for long periods of time.     

The oxygen isotope composition of water masses within the North Sea

Based on measurements of the ¹⁸O isotope composition of 247 samples collected over a 3-year period we have assessed the oxygen isotope composition of water masses in the North Sea. This is the first δ¹⁸O data set that covers the entire North Sea basin. The waters lie on a mixing line: δ¹⁸O (‰_VSMOW) = −9.300 + 0.274(S) with North Atlantic sub-polar mode water (SPMW) and surface waters, and Baltic Sea water representing the saline and freshwater end members respectively. Patterns exhibited in surface and bottom water δ¹⁸O distributions are representative of the general circulation of the North Sea. Oxygen-18 enriched waters from the North Atlantic enter the North Sea between Scotland and Norway and to a lesser extent through the English Channel. In contrast, oxygen-18 depleted waters mainly inflow from the Baltic Sea, the rivers Rhine and Elbe, and to a lesser degree, the Norwegian Fjords and other river sources. Locally the δ¹⁸O–salinity relationship will be controlled by the isotopic composition of the freshwater inputs. However, the range of local freshwater compositions around the North Sea basin is too narrow to characterise the relative contributions of individual sources to the overall seawater composition. This dataset provides important information for a number of related disciplines including biogeochemical research and oceanographic studies.     

A numerical study of the South China Sea deep circulation and its relation to the Luzon Strait transport

A fine-resolution MOM code is used to study the South China Sea basin-scale circulationand its relation to the mass transport through the Luzon Strait. The model domain includes the South China Sea, part of the East China Sea, and part of the Philippine Sea so that the currents in the vicinity of the Luzon Strait are free to evolve. In addition, all channels between the South China Sea and the Indonesian seas are closed so that the focus is on the Luzon Strait transport. The model is driven by specified Philippine Sea currents and by surface heat and salt flux conditions. For simplicity, no wind-stress is applied at the surface.The simulated Luzon Strait transport and the South China Sea circulation feature a sandwich vertical structure from the surface to the bottom. The Philippine Sea water is simulated to enter the South China Sea at the surface and in the deep ocean and is carried to the southern basin by western boundary currents. At the intermediate depth, the net Luzon Strait transport is out of t     

An isopycnal view of the Nordic Seas hydrography with focus on properties of the Lofoten Basin

Few basins in the world exhibit such a wide range of water properties as those of the Nordic Seas with cold freshwaters from the Arctic in the western basins and warm saline waters from the Atlantic in the eastern basins. In this study we present a 50-year hydrographic climatology of the Nordic Seas in terms of depth and temperature patterns on four upper ocean specific volume anomaly surfaces. This approach allows us to better distinguish between change due to variations along such surfaces and change due to depth variations of the stratified water column. Depth variations indicate changes in the mass field while property variations along isopycnals give insight into isopycnal advection and mixing, as well as diapycnal processes. We find that the warmest waters on each surface are found in the north, close to where the isopycnal outcrops, a clear indication of downward mixing of the warmer, more saline waters on shallower isopycnals due to convective cooling at the surface. These saline waters come from the Norwegian Atlantic Slope Current by means of a very high level of eddy activity in the Lofoten Basin.The isopycnal analyses further show that the principal water mass boundary between the waters of Arctic origin in the west and Atlantic waters in the east aligns quite tightly with the Jan Mayen, Mohn, Knipovich Ridge system suggesting little cross-ridge exchange. Instead, the main routes of exchange between the eastern and western basins appear to be limited to the northern and southern ends of ridge system: Atlantic waters into the Greenland Sea in the Fram St and Artic waters into the southern Norwegian Sea just north of the Iceland-Faroe Ridge.Analysis of a representative isopycnal in the main pycnocline shows it to be stable over time with only small variations with season (except where it outcrops in winter in the Greenland and Iceland Seas). However, two very cold winters, 1968–1969, led to greater than average heat losses across the entire Lofoten Basin that eroded away much of the Lofoten eddy and induced the greatest temperature anomaly in the entire 50-year record. Interannual variations in isopycnal layer temperature correlate with the NAO index such that waters in the Iceland Sea become warmer than average with warming air temperatures and conversely in the Lofoten Basin.     

Export of Greenland Sea water across the Mohn Ridge induced by summer storms

The Mohn Ridge separates the Greenland Sea and the Lofoten Basin. Previous studies identified the export across the Mohn Ridge(EMR) from the Greenland Basin into the Lofoten Basin using water mass analysis and the tracer diffusion method, but there is still lack of direct current measurements. A surface mooring with four current meters was deployed on the Mohn Ridge from June 5 to June 18 in 2015, when three cyclones passed in the adjacent area. In the absence of cyclones, the flow on the Mohn R...     

Past,present and future nutrient loads of the North Sea: Causes and consequences

Comprehensive, aggregate nutrient budgets were established for two compartments of the North Sea, the shallow coastal and deeper open regions, and for three different periods, representing pre-eutrophication (∼1950), eutrophication (∼1990) and contemporary (∼2000) phases. The aim was to quantify the major budget components, to identify their sources of variability, to specify the anthropogenic components, and to draw implications for past and future policy. For all three periods, open North Sea budgets were dominated (75%) by fluxes from and to the North-East Atlantic; sediment exchange was of secondary importance (18%). For the coastal North Sea, fluxes during the eutrophication period were dominated by sediment exchange (49% of all inputs), followed by exchange with the open sea (21%), and anthropogenic inputs (19%). Between 1950 and 1990, N-loading of coastal waters increased by a factor of 1.62 and P-loading by 1.45. These loads declined after 1990. Interannual variability in Atlantic inflow was found to correspond to a variability of 11% in nutrient load to the open North Sea. Area-specific external loads of both the open and coastal North Sea were below Vollenweider-type critical loads when expressed relative to depth and flushing. External area-specific load of the coastal North Sea has declined since 1990 from 1.8 to about 1.4 g P m⁻² y⁻¹ in 2000, which is close to the estimate of 1.3 for 1950. N-load declined less, leading to an increase in N/P ratio.     

渤海夏季环流的高分辨率海浪-潮汐-环流耦合模式研究

The Bohai Sea is a shallow semi-enclosed inner sea with an average depth of 18 m and is located at the west of the northern Yellow Sea. The climatological circulation pattern in summer of the Bohai Sea is studied by using a wave-tide-circulation coupled model. The simulated temperature and the circulation agree with the observation well. The result shows that the circulation pattern of the Bohai Sea is jointly influenced by the tidal residual current, wind and baroclinic current. There exists an obvious density current along the temperature front from the west part of the Liaodong Bay to the offshore area of the Huanghe Estuary. In the Liaodong Bay there exists a clockwise gyre in the area north to the 40°N. While in the area south to the 40°N the circulation shows a two-gyre structure, the flow from the offshore area of the Huanghe Estuary to the Liaodong Bay splits into two branches in the area between 39°N and 40°N. The west branch turns into north-west and forms an anti-clockwise gyre with the south-westward density current off the west of the Liaodong Bay. The east branch turns to the east and forms a clockwise gyre with the flow along the east coast of the Liaodong Bay. The forming mechanism of the circulation is also discussed in this paper.     

Temporal changes of accretion rates on an estuarine salt marsh during the late Holocene — Reflection of local sea level changes? The Wadden Sea, Denmark

The temporal variability of estuarine sedimentation has been investigated in the northernmost part of the Wadden Sea (Denmark), using an estuarine sedimentary sequence at Ho Havn. The sedimentary sequence appears to have been deposited within the last ∼ 2000 yr based on detailed luminescence dating of the estuarine mud, whose ages range between 225 ± 40 and 2050 ± 300 yr. The age-depth profile reveals that the sedimentation rate has varied considerably in the past. Estuarine sedimentation was very rapid ∼ 1400 yr ago; the ages over almost 1 m of sediment are indistinguishable. After this accretion rate of ∼ 9 mm a^− 1, the rate dropped abruptly to ∼ 0.3 mm a^− 1 some time between 1340 and 970 yr ago. This slow rate of accretion continued until ∼ 350 yr ago, when it accelerated to ∼ 1.3 mm a^− 1. These abrupt changes in the accretion rate are possibly related to local sea level fluctuations, thus the period with low accretion rate most probably reflects a situation with a stable or decreasing relative sea level. The rapid deposition of ∼ 0.9 m of sediment within about one century some 1400 yr ago shows that large amounts of fine-grained sediment were available for deposition in the region at that time, and an increasing relative sea level was most probably responsible for the creation of the accommodation space for sedimentation. Recent studies on mudflats and salt marshes in the region also tend to show high accretion rates, indicating that the coastal lagoons could be less vulnerable and threatened by a future sea level rise than generally believed.     

Circulation and transport of water masses in the Lazarev Sea, Antarctica, during summer and winter 2006

The distribution and circulation of water masses in the region between 6°W and 3°E and between the Antarctic continental shelf and 60°S are analyzed using hydrographic and shipboard acoustic Doppler current profiler (ADCP) data taken during austral summer 2005/2006 and austral winter 2006. In both seasons two gateways are apparent where Warm Deep Water (WDW) and other water masses enter the Weddell Gyre through the Lazarev Sea: (a) a probably topographically trapped westward, then southwestward circulation around the northwestern edge of Maud Rise with maximum velocities of about 20 cm s⁻¹ and (b) the Antarctic Coastal Current (AntCC), which is confined to the Antarctic continental shelf slope and is associated with maximum velocities of about 25 cm s⁻¹.Along two meridional sections that run close to the top of Maud Rise along 3°E, geostrophic velocity shears were calculated from CTD measurements and referenced to velocity profiles recorded by an ADCP in the upper 300 m. The mean accuracy of the absolute geostrophic velocity is estimated at ±2 cm s⁻¹. The net baroclinic transport across the 3°E section amounts to 20 and 17 Sv westward for the summer and winter season, respectively. The majority of the baroclinic transport, which accounts for ∼60% of the total baroclinic transport during both surveys, occurs north of Maud Rise between 65° and 60°S.However, the comparison between geostrophic estimates and direct velocity measurements shows that the circulation within the study area has a strong barotropic component, so that calculations based on the dynamic method underestimate the transport considerably. Estimation of the net absolute volume transports across 3°E suggests a westward flow of 23.9±19.9 Sv in austral summer and 93.6±20.1 Sv in austral winter. Part of this large seasonal transport variation can be explained by differences in the gyre-scale forcing through wind stress curl.