Baroclinic Transport Research Articles

Comparison of two time-variant forced eddy-permitting global ocean circulation models with hydrography of the Scotia Sea

A comparison between hydrographic observations and output from two realistically forced z-level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12–0.40) and too warm in POCM_4C (by >2 °C) suggesting problems with the air-sea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.

Upper ocean temperature and the baroclinic transport stream function relationship in Drake Passage

Repeat hydrographic sections across the Antarctic Circumpolar Current (ACC) in Drake Passage are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function. Cross validation shows this relationship can be used to infer baroclinic transport (above and relative to 2500 m) from expendable bathythermograph (XBT) temperature measurements with an error of a few per cent. Transport errors of less than 2 Sv are obtained if temperature at depths between 600 and 1600 m is used to define the relationship. Temperature at depths above 300 m provides an unreliable index of transport because of variability in temperature‐salinity (T‐S) properties produced by air‐sea interaction. The scatter in the relationship between temperature and stream function from repeat observations along a single line is similar in magnitude to the scatter observed when data from the broader Drake Passage area are considered. In both cases, variability about the mean temperature‐stream function relationship reflects advection of water with anomalous T‐S properties. The tight relationship between temperature and stream function in Drake Passage and south of Australia suggests baroclinic transports can be inferred from XBT temperatures with high accuracy in the Southern Ocean, providing a cost‐effective means of monitoring ACC variability. However, care must be taken at the end points, particularly in the Drake Passage where the strong flow of the Subantarctic Front sometimes lies over the continental slope.

Comparison of two time-variant forced eddy-permitting global ocean circulation models with hydrography of the Scotia Sea

A comparison between hydrographic observations and output from two realistically forced z -level global ocean circulation models (OCCAM and POCM_4C) in the Scotia Sea, South Atlantic, is described. The study region includes the southern part of the Antarctic Circumpolar Current (ACC) and the northern Weddell Gyre. Despite similar formulations, the models have different strengths and weaknesses. OCCAM simulates well the horizontal circulation around South Georgia but loss of Antarctic Bottom Water distorts the mean circulation in the central Scotia Sea. A poorer bathymetric dataset in POCM_4C means that the circulation is not adequately topographically steered leading to greater zonal flow and a southward shift of the fronts of the southern ACC. In a comparison with sea surface height variability data, OCCAM overestimates and POCM_4C underestimates the maximum values. Both models have higher background variability than the satellite data. Mean monthly model output is compared with a meridional hydrographic section from the study region. The regional water masses at the time of the hydrographic section (April 1995) are recognisably reproduced in both models despite some discrepancies. The surface waters are too saline in OCCAM (by 0.12–0.40) and too warm in POCM_4C (by >2 °C) suggesting problems with the air-sea surface heat and freshwater fluxes used to force both models and the models' vertical mixing parameterisations. Anomalous mixed layer properties in winter lead to inaccurate Winter Water characteristics in both models. Slumping of Circumpolar Deep Water occurs in OCCAM, associated with the loss of the bottom water. Subsurface restoration to climatology at buffer zones prevents this slumping in POCM_4C although the densest waters are not reproduced. The models overestimate the baroclinic transport of the section by up to a factor of two and simulate a significant barotropic component of transport. Overall, both models can be used in this region in ways that utilise their strengths. Further improvements are likely to come from better bathymetric representations, surface fluxes, and bottom water formation processes, elimination of spurious diapycnal mixing, improvement of vertical mixing parameterisations, and higher resolution.

A Method for Obtaining the Mean Transports of Ocean Currents by Combining Isopycnal Float Data with Historical Hydrography

This article presents a method for obtaining the mean structure of the temperature, specific volume anomaly, and velocity of an ocean current, using isopycnal float data combined with gravest empirical mode (GEM) fields calculated from historical hydrography. A GEM field is a projection on a geostrophic streamfunction space of hydrographic data, which captures most of the vertical structure associated with frontal regions. This study focuses on the North Atlantic Current–subpolar front (NAC–SPF) current system, but the float–GEM method has broad applicability to baroclinic ocean currents in general. The NAC–SPF current system is of climatic interest, being an important conduit of warm salty waters into the northern North Atlantic. It constitutes the upper limb of the thermohaline circulation of the Atlantic Ocean and plays a crucial role in the moderation of European climate, but uncertainties regarding its transport and corresponding heat fluxes remain, mainly because the structure of the system is not well known. This paper shows how isopycnal floats can be used to obtain such estimates. The performance of the float–GEM method is tested in two ways. First, two synoptic hydrographic sections (one across the NAC and the other across the SPF) are reconstructed from simulated isopycnal float pressure measurements. The baroclinic transports of volume and temperature (relative to 1000 dbar) across the sections are well reproduced by the method: the float–GEM transport estimates have an accuracy of ±20% and a precision of ±15% or less, which result in deviations of less than ±10% from the “real” values. In the second test, horizontal maps of pressure and temperature on the δ = −12.7 × 10−8 m3 kg−1 specific volume anomaly surface (σθ ≈ 27.5 kg m−3) are produced, using RAFOS float data from two experiments that sampled the region from 1993 to 2000. These maps compare well with similar maps constructed in previous studies and establish the consistency of the method. The good performance of the float–GEM method gives confidence in this novel way of using isopycnal floats to obtain information on the structure of the ocean. Combined with the velocity measured by the floats, it has the potential to estimate absolute transports and heat fluxes along the NAC–SPF system.

Balance of Volume Transports between Horizontal Circulation and Meridional Overturn in the North Pacific Subarctic Region

Horizontal and meridional volume transports on timescales from intra-seasonal to interannual in the North Pacific subarctic region were investigated using a reanalysis dataset for 1993–2001 that was constructed from an assimilation of the TOPEX altimeter and in situ data into an eddy-permitting North Pacific ocean general circulation model. The barotropic flow is excited along east of the Emperor Seamounts by the western intensification dynamics. The volume transport of this flow compensates for that across the interior region east of the Seamounts below the summit depth of the Seamounts. The Oyashio, which is also considered as a compensation flow for the transport in the whole interior region, includes baroclinic as well as barotropic components. Baroclinic transports in the whole interior region exceed those in the western boundary region in the upper (200–1000 m) and lower (2000–5000 m) layers, and the total transport is northward (southward) in the upper (lower) layer. These excesses of the baroclinic transport are balanced by a vertical transport of the meridional overturn. The meridional overturn has a complementary relation to the basin-scale baroclinic circulation in the North Pacific subactic region.

Hydrographic and transport variability on the Halifax section

Archived data and geostrophic computations are used to examine variability in hydrographic properties and along‐shelf transport on the Scotian Shelf, with focus on the Halifax section and a decadal‐scale hydrographic anomaly during the late 1950s and early/mid 1960s. The long‐term annual cycle shows strong seasonal variations in baroclinic transport on the inner shelf and at the shelf edge, and an associated steric change in adjusted sea level (ASL) at Halifax. Regional wind‐forcing and barotropic currents make smaller contributions to the annual cycle in ASL. In contrast, regional wind‐forcing contributes about 40% of the ASL variability for periods of 6–30 days. Hydrographic sections indicate that the 1950s/1960s anomaly arose from episodic extensions of Labrador Slope Water along the shelf edge followed by on‐shelf intrusions, primarily during the fall‐spring periods of 1958–1959 and 1963–1964. It ended with excursions of Warm Slope Water into the region in 1967–1968. An analysis of monthly temperature anomalies indicates that slope temperatures can account for 50% of the variance of deep shelf temperatures 3–6 months later. Correlations between the hydrographic and baroclinic transport variations indicate increased southwestward flow associated with lower temperatures and salinities, but no evidence was found for large local current events at the onset and termination times of the 1950s/1960s anomaly. Positive correlations between wind‐adjusted ASL and steric anomalies point to the potential for sea level being helpful in monitoring baroclinic transport variability.

Mechanisms of heat, freshwater, oxygen and nutrient transports and budgets at 24.5°N in the subtropical North Atlantic

Hydrographic data from a cruise at 24.5°N in the subtropical North Atlantic are used to calculate and examine the fluxes across the section. The components of the fluxes of heat, freshwater, oxygen and nutrients through the section are analysed. After the variables are separated into section average, baroclinic profile and anomalies, the fluxes are separated into an Ekman component and its barotropic compensation, baroclinic and horizontal components. The baroclinic contribution due to the meridional overturning circulation (MOC) is responsible for the largest amount of heat transport with warmer waters flowing poleward and cooler deep waters flowing equatorward. The Ekman component and its barotropic compensation is the second-most important component of heat transport. The MOC transports about 25% of the combined meridional heat transport by atmosphere and ocean at 24°N. The Ekman and baroclinic components are responsible for the northward salt transport. Salt conservation for the Atlantic north of 24.5°N suggests that there is a net precipitation between the Bering Strait and the 24.5°N section of about 0.5 Sv. Horizontal transport is the major contribution to the southward oxygen flux, as oxygen-poor water flows northward in the Florida Straits and oxygen-rich water flows southward in mid-ocean. Baroclinic transport is the main factor in the southward transport of all nutrients: low concentration upper water flows northward and higher concentration deep water flows southward. The zonally integrated meridional cell carries a poleward heat flux of 1.51±0.39 PW, 0.14 PW of which is due to seasonal sampling, and equatorward transports of −2621±705 kmol s −1 of oxygen, −254±176 kmol s −1 of silicate, −130±95 kmol s −1 of nitrate and −12.6±6.3 kmol s −1 of phosphate. Finally we have explored the changes in ocean circulation implied by changes in silicate transport through the section. Strict silicate conservation through the section leads to a great increase in the northward deep circulation in the eastern basin that is not in accord with the present understanding of the circulation. Taking an intermediate silicate transport through the section, according to estimates of silicate sources, produces a reasonable increase in the southward Deep Western Boundary Current (DWBC) flow and an increase in the deep northward transport in the eastern deep basin, as well as a decrease in silicate and nitrate transports through the section. Fluxes of heat, salt and oxygen are not significantly affected by changes in the conditions of silicate conservation.

Transport and variability of the Antarctic Circumpolar Current in Drake Passage

The baroclinic transport of the Antarctic Circumpolar Current (ACC) above 3000 m through Drake Passage is 107.3 ± 10.4 Sv and has been steady between 1975 and 2000. For six hydrographic sections (1993–2000) along the World Ocean Circulation Experiment (WOCE) line SR1b, the baroclinic transport relative to the deepest common level is 136.7 ± 7.8 Sv. The ACC transport is carried in two jets, the Subantarctic Front 53 ± 10 Sv and the Polar Front (PF) 57.5 ± 5.7 Sv. Southward of the ACC the Southern Antarctic Circumpolar Current transports 9.3 ± 2.4 Sv. We observe the PF at two latitudes separated by 90 km. This bimodal distribution is related to changes in the circulation and properties of Antarctic Bottom Water. Three realizations of the instantaneous velocity field were obtained with lowered ADCPs. From these observations we obtain near‐bottom reference velocities for transport calculations. Net transport due to these reference velocities ranges from −28 to 43 Sv, consistent with previous estimates of variability. The transport in density layers shows systematic variations due to seasonal heating in near‐surface layers. Volume transport‐weighted mean temperatures vary by 0.40°C from spring to summer; a seasonal variation in heat flux of about 0.22 PW. Finally, we review a series of papers from the International Southern Ocean Studies Program. The average yearlong absolute transport is 134 Sv, and the standard deviation of the average is 11.2 Sv; the error of the average transport is 15 to 27 Sv. We emphasize that baroclinic variability is an important contribution to net variability in the ACC.

Modeling the variations of salinity and temperature in the large Gulfs of the Baltic Sea

The modeling of salinity and temperature in Gulf of Bothnia, Gulf of Finland, and Gulf of Riga is investigated by using a coupled sea ice-ocean Baltic Sea model. 18 years, from late 1980 to the end of 1998, have been investigated. The forcing data extracted taken from a gridded meteorological data base, sea level data from the Kattegat, and river runoff data to the different subbasins of the Baltic Sea from a hydrological data base. To improve the gridded meteorological data base a statistical model for the reduction of geostrophic winds to surface winds was developed. In the analysis it was shown that the calculated long-term salinity and temperature structures were stable and in good agreement with observations. This was made possible by using three different strait-flow models connecting the subbasins of the Baltic Sea. The seasonal and interannual variations of temperature and salinity were also well simulated by the model, implying that the coupling between the atmosphere and the Baltic Sea as well as the diapycnal mixing are reasonably well understood. The water cycle and the surface heat balance were calculated using the 18-year simulation. In the water-balance calculations it was shown that the volume flows from the large gulfs of the Baltic Sea were mainly due to baroclinic transports and that net precipitation added freshwater during the studied period, particularly to the large gulfs. From the heat-balance calculation it is concluded that the Baltic Sea is almost in local balance with the atmosphere. The Bothnian Bay, Gulf of Finland and Gulf of Riga loose heat, whereas the Bothnian Sea gains heat, calculated as long-term means.

A 6 year record of baroclinic transport variability of the Antarctic Circumpolar Current at 140°E derived from expendable bathythermograph and altimeter measurements

Repeat hydrographic sections across the Antarctic Circumpolar Current are used to derive an empirical relationship between upper ocean temperature and the baroclinic transport stream function. Cross validation shows this relationship can be used to infer baroclinic transport (above and relative to 2500 dbar) from temperature measurements with an error of a few per cent. The mean transport distribution derived from 31 austral summer expendable bathythermograph (XBT) sections over a 6 year period consists of westward flow immediately south of Tasmania, a broad band of strong eastward flow between 50° and 55°S, and three cores of eastward flow south of 55°S. By defining a second empirical relationship between surface dynamic height and cumulative transport a continuous time series of baroclinic transport is derived from altimeter measurements of sea surface height. Transports derived from altimetry in this way agree well with simultaneous in situ estimates (root mean square error in net transport is 4 × 106 m3 s−1), suggesting sea level anomalies largely reflect baroclinic changes above 2500 dbar. The 10 day sampling of the altimeter transport time series shows the irregular XBT sampling aliases variability at unresolved timescales. The standard deviation of net transport above and relative to 2500 m is 4.3 × 106 m3 s−1. The variability in net transport is largest (2.7 × 106 m3 s−1) in the quasi‐annual band (periods of 4 months to 1.5 years), slightly smaller (2.3 × 106 m3 s−1) in the mesoscale band (<4 months), and smallest in the interannual band (>1.5 years, 1.5 × 106 m3 s−1). Changes in transport are correlated with local changes in both wind stress and wind stress curl in the quasi‐annual and interannual bands, but the transport time series is too short to draw significant conclusions.

Water masses and baroclinic transports in the South Atlantic and Southern oceans

We describe the World Ocean Circulation Experiment (WOCE) A23 hydrographic section from Antarctica to Brazil, nominally along 35W. The section crossed the center of the Weddell Gyre, the Antarctic Circumpolar Current (ACC) and the Subtropical Gyre in the Argentine Basin. We precisely define the locations of fronts, the changes in water mass properties across them, and their transports. The Antarctic Slope Front was crossed above the continental slope of Antarctica, with a baroclinic transport of 4 Sv, part of the cyclonic Weddell Gyre circulation of 19 Sv. We repeated a section in the Weddell Sea occupied in 1973, and saw a marked warming of the inflowing Warm Deep Water layer by some 0.2°C, but no discernible change in the outflowing northern limb of the gyre. An inflow of recently ventilated water with the same characteristics as Weddell Sea Deep Water (WSDW) was observed flowing into the Weddell Sea from the east. The Weddell Front was crossed at 61°7'S and the Southern Boundary (SB) of the ACC (often referred to as the Scotia Front) at 58°38'S. Between these lay the Weddell-Scotia Confluence, contributing 16 Sv of eastward transport. The first crossing of the Southern ACC Front (SACCF) lay south of South Georgia at ~55°30'S. It then wrapped anticyclonically around South Georgia and was encountered at 53°40'S before retroflecting and returning eastward at ~53°30'S. The baroclinic transport was ~15 Sv at each crossing. In this region the SACCF is most clearly identified by a decrease in the salinity of the temperature minimum layer. The core of the Polar Front (PF) lay at ~49S where the isotherms plunged down sharply to the north, and transported 67 Sv. The PF and Subantarctic Front (SAF) were barely distinguishable with only one station clearly in the Polar Frontal Zone. The SAF, transporting 57 Sv, was encountered at ~48°45'S where the subsurface salinity minimum of Antarctic Intermediate Water (AAIW) began to descend. The Subtropical Front (STF) marks the boundary between the waters of the subtropical gyre and the colder, fresher subantarctic waters to the south; its southernmost crossing was at 44-45S transporting ~25 Sv. This is several hundred kilometers farther south than historical locations of the STF at this longitude. The Brazil Current Front (BCF) was encountered at ~38S transporting 43 Sv. Whereas previous observations found the STF to be the primary means for eastward flow of the waters of the Brazil Current after it has separated from the coast, during A23 we find that the BCF carries the majority of this transport. A further Deep Front, possibly marking the center of the subtropical gyre, was crossed at 34°22'S associated with a transport of 2.5 Sv. In the western Vema Channel we encountered a recirculation of the Brazil Current, flowing to the northeast with a transport of 23 Sv. The section ended prematurely without crossing the Brazil Current. To close the subtropical gyre transport would require a Brazil Current significantly in excess of the historical estimates based on shallow reference levels. Both the SACCF and STF exhibited meanders probably caused by bathymetry. The STF meander may be caused by the eastern end of the circulation around the Zapiola Rise. Both the SB and PF had associated eddies. The SB had shed a cyclonic eddy to the north, which had subsequently been capped by local water. It transported 8 Sv azimuthally. The PF had shed an anticyclonic eddy to the south, which was also capped and circulated at least 3 Sv. A small anticyclonic subsurface lens of AAIW was observed in the Vema Channel with a transport of the order of I Sv. In the Vema Channel, a level of no motion between the North Atlantic Deep Water and the Lower Circumpolar Deep Water (LCDW) gives a net northward flow of 1.2 Sv WSDW and 4 Sv LCDW from the Argentine to the Brazil Basin.

The Role of Eddy Transfer in Setting the Stratification and Transport of a Circumpolar Current

Abstract High resolution numerical experiments of a circumpolar current are diagnosed to study how lateral and vertical transfer of buoyancy by geostrophic eddies balances advection by a meridional circulation driven by surface wind stresses and buoyancy fluxes. A theory is developed in the framework of the “residual circulation” to relate the vertical and horizontal stratification set up to the transfer properties of eddies and the patterns of imposed wind and buoyancy forcing. Simple expressions are found for the depth of penetration, stratification, baroclinic transport, and residual circulation of the current. Finally, the ideas are applied to the Antarctic Circumpolar Current (ACC) and yield predictions for how its properties depend on wind and buoyancy forcing.

Seasonal Variation of Baroclinic Geostrophic Heat Transport: Is It Resolved by Existing Climatologies of the Atlantic Ocean?

Abstract Seasonal cycles of baroclinic geostrophic heat transport are systematically computed between 30°S and 40°N in the Atlantic from several hydrographic climatologies. The intercomparison of the cycles reveals very significant differences in both amplitude and phase between climatologies. The signal is better organized with latitude south of the equator and shows some robustness between 7°S and 7°N, as confirmed by the study, in parallel, of a realistic numerical simulation. North of 7°N, however, the climatological seasonal cycles exhibit no meridional coherence and very large month to month excursions. The seasonal cycle is basically considered unresolved by the climatologies at these latitudes. Part of the noise turns out to be due to the presence of erroneous data in at least two of the climatologies examined. The authors believe, however, that most of the noise results from sampling problems caused by the nonsynopticity of the databases, especially in western boundary regions of strong interannu...

Interannual sea-level variations in the Scotia-Maine region in the 1990s

The TOPEX/Poseidon altimeter data over the period from 1992 to 1999 have been analyzed to examine interannual sea-level variability in the Scotia‐Maine region (defined as the outer Laurentian Channel, the Scotian Shelf, the Gulf of Maine, and the northern Middle Atlantic Bight). A modified orthogonal response analysis is used to simultaneously remove the annual cycle and residual tides (including the semi-annual cycle). Altimetric data reveal significant interannual sea-level variability of magnitude 5‐10 cm over the shelves in the 1990s, falling to the lowest level in 1994, rising to the highest level in 1997‐1998, and falling again. The second sea-level decrease shows an overall equatorward propagation. These results are generally consistent with those detected from detrended coastal tide-gauge data at Halifax, Nova Scotia. The interannual sea-level variability is thought to be forced by fluctuations of the Gulf Stream position, which seems to be related to the North Atlantic Oscillation, and the baroclinic Labrador Current transport.

Investigation of the Brazil Current front variability from altimeter data

The southwestern Atlantic Ocean is characterized by the confluence of the Brazil and Malvinas Currents, which form very strong surface and subsurface fronts that can be detected from hydrographic and remote sensing observations. Three data sets, consisting of TOPEX/Poseidon‐derived sea height anomalies, the climatologically derived depth of the 10°C isotherm, and reduced gravity, are used in conjunction with a two‐layer dynamical ocean scheme to monitor the Brazil Current front and to investigate its variability during a 6 year period (1993–1998). Results reveal that the fronts exhibit motions that are larger zonally than meridionally, showing strong interannual variability with annual mean amplitudes that range from 1° to 6°. The annual and semiannual components account for more than 75% of the variability of the frontal oscillations. In the annual cycle the frontal motions appear to be related closely to fluctuations in the baroclinic transport of the Brazil Current and are only influenced by the Malvinas Current when the Brazil Current transport is very small.

Volume, heat and salt transport by the West Spitsbergen Current

During the summer of 2000 (June-July) 14 CTD and ADCP transects perpendicular to the West Spitsbergen Current and along the western border of the Barents Sea were made. The measurements covered the area between 69° 43’and 80° N and 01° and 20° E. The main purpose was to follow changes in volume, heat and salt content of Atlantic Water (AW) on its way north. The strongest and most stable flow of AW was located along the continental slope where northward flowing currents exceeding 40 cm/sec were measured. A few weaker northward branches were also found to the west of the slope. South-directed currents were recorded between them and eddy-like mesoscale structures were commonly observed. Measured by vessel-mounted acoustic Doppler current profiler (VM-ADCP), the net northward transport of AW volume in the upper 136 m layer decreased from nearly 6 Sv at the southernmost transect to below 1 Sv at a latitude of 78° 50’N. Similarly, heat transport drops from about 173 TW to about 9 TW and relative salt transport (over 34.92 psu) from 980 × 103 kg/sec to 14 × 103 kg/sec. Transport in the southern direction prevails at the transect located between 79° 07’and 79° 30’N. The calculated baroclinic geostrophic transport of AW volume, heat and salt in the upper 1000 m layer behaves similarly. East-directed transport dominates at the Barents Sea boundary while westward flow prevails on the western side of the West Spitsbergen Current.

Identification of the Kuroshio Extension, its bifurcation and northern branch from altimetry and hydrographic data during October 1992–August 1999: Spatial and temporal variability

A methodology is proposed using altimter‐derived upper layer thickness and baroclinic transport to identify the Kuroshio Extension, the Bifurcation Point and the Northern Branch, by combining TOPEX/POSEIDON altimeter and climatological data within a two‐layer reduced gravity model. Results obtained from the Japanese coast to 175°W show that the location of the Bifurcation Point presents interannual variability that is related with upstream conditions. The longitude of the Bifurcation Point ranged from 147 to 160°E. Estimates of baroclinic transport at the Kuroshio Extension and its Northern Branch decrease steadily to the east trough the region of study from 35 to 11 and from 10 to 3 Sv, respectively.

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.

Fronts, transport, and Weddell Gyre at 30°E between Africa and Antarctica

A detailed description of the frontal structure of major currents and estimates of transport between Africa and Antarctica at 30°E were made on the basis of a finely resolved hydrographic section made during the 1996 Civa‐2 cruise. Particular emphasis was put on a refinement of the eastern boundary of the Weddell Gyre by analyzing also supplementary hydrographic data from the 1993 Civa‐1 cruise and the best available historical hydrographic data in the Weddell and Enderby Basins. Contrary to the general belief, our results show that the Weddell Gyre extends far beyond 30°E, reaching at least as far east as the Enderby Land promontory (53°E). From this point, westward currents along the Antarctic continental margin constitute the gyre's southern boundary, thus closing the cyclonic circulation of the gyre at its easternmost end. As the most convincing evidence for this refined circulation scheme, significant westward transport south of 65°S has been identified from the Civa‐1 acoustic Doppler current profiler data. We have shown also, using an inertial jet model, that the blocked feature of the Weddell cold regime (∼57°S) to the west of 25°E is due to the topographic control of flow by local bathymetry. During the Civa‐2 cruise, ∼70% of the Antarctic Circumpolar Current (ACC) was concentrated near the Polar Front (51°S), with surface speeds reaching up to 70 cm s−1. The baroclinic current field in the subpolar area south of the Weddell Front (58°S for Civa‐2 and 57°S for Civa‐1) is featureless (<3 cm s−1), but yielding a nonnegligible eastward transport of ∼15 Sv (1 Sv = 106 m3 s−1). This bottom‐referenced baroclinic transport is entirely compensated by a substantial westward barotropic transport south of 65°S. The ACC transport across the 30°E section during the Civa‐2 cruise is estimated to be ∼160 Sv.

Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3)

Baroclinic transport variability of the Antarctic Circumpolar Current (ACC) near 140°E is estimated from six occupations of a repeat section occupied as part of the World Ocean Circulation Experiment (WOCE section SR3). The mean top‐to‐bottom volume transport is 147±10 Sv (mean ±1 standard deviation), relative to a deep reference level consistent with water mass properties and float trajectories. The location and transport of the main fronts of the ACC are relatively steady: the Subantarctic Front carries 105±7 Sv at a mean latitude between 51° and 52°S; the northern branch of the Polar Front carries 5±5 Sv to the east between 53° and 54°S; the southern Polar Front carries 24±3 Sv eastward at 59°S; and two cores of the southern ACC front at 62° and 64°S carry 18±3 and 11±3 Sv, respectively. The variability in net property transports is largely due to variability of currents north of the ACC, in particular, an outflow of 8±13 Sv of water from the Tasman Sea and a deep anticyclonic recirculation carrying 22±8 Sv in the Subantarctic Zone. Variability of net baroclinic volume transport is similar in magnitude to that measured at Drake Passage. In density layers, transport variability is small in deep layers, but significant (range of 4 to 16 Sv) in the Subantarctic Mode Water. Variability of eastward heat transport across SR3 is significant (range of 139°C Sv, or 0.57×1015 W, relative to 0°C) and large relative to meridional heat flux in the Southern Hemisphere subtropical gyres. Heat transport changes are primarily due to variations in the westward flow of relatively warm water across the northern end of the section. Weak (strong) westward flow and large (small) eastward heat flux coincides with equatorward (poleward) displacements of the latitude of zero wind stress curl.