Agulhas Current Region Research Articles

Indian Ocean sources of Agulhas leakage

AbstractWe examine the mean pathways, transit timescales, and transformation of waters flowing from the Pacific and the marginal seas through the Indian Ocean (IO) on their way toward the South Atlantic within a high‐resolution ocean/sea‐ice model. The model fields are analyzed from a Lagrangian perspective where water volumes are tracked as they enter the IO. The IO contributes 12.6 Sv to Agulhas leakage, which within the model is 14.1 ± 2.2 Sv, the rest originates from the South Atlantic. The Indonesian Through‐flow constitutes about half of the IO contribution, is surface bound, cools and salinificates as it leaves the basin within 10–30 years. Waters entering the IO south of Australia are at intermediate depths and maintain their temperature‐salinity properties as they exit the basin within 15–35 years. Of these waters, the contribution from Tasman leakage is 1.4 Sv. The rest stem from recirculation from the frontal regions of the Southern Ocean. The marginal seas export 1.0 Sv into the Atlantic within 15–40 years, and the waters cool and freshen on‐route. However, the model's simulation of waters from the Gulfs of Aden and Oman are too light and hence overly influenced by upper ocean circulations. In the Cape Basin, Agulhas leakage is well mixed. On‐route, temperature‐salinity transformations occur predominantly in the Arabian Sea and within the greater Agulhas Current region. Overall, the IO exports at least 7.9 Sv from the Pacific to the Atlantic, thereby quantifying the strength of the upper cell of the global conveyor belt.

Submesoscale cyclones in the Agulhas current

AbstractGliders were deployed for the first time in the Agulhas Current region to investigate processes of interactions between western boundary currents and shelf waters. Continuous observations from the gliders in water depths of 100–1000 m and over a period of 1 month provide the first high‐resolution observations of the Agulhas Current's inshore front. The observations collected in a nonmeandering Agulhas Current show the presence of submesoscale cyclonic eddies, generated at the inshore boundary of the Agulhas Current. The submesoscale cyclones are often associated with warm water plumes, which extend from their western edge and exhibit strong northeastward currents. These features are a result of shear instabilities and extract their energy from the mean Agulhas Current jet.

Sea surface salinity variability in the Agulhas Current region inferred from SMOS and Aquarius

Abstract Understanding of the Agulhas Current's unique retroflection and ring shedding has been greatly improved by satellite altimetry data over the last several decades, but salinity remote sensing technology has not been available to conduct similar studies with respect to regional sea surface salinity (SSS) variability. ESA's Soil Moisture and Ocean Salinity (SMOS) and NASA's Aquarius/SAC-D salinity missions can provide such data, but it remains unclear if the accuracy of these sensors is high enough to properly study this dynamic region. To address this issue, this study validates both SMOS and Aquarius against monthly in situ data from Argo floats. All available, corresponding data points over a three year period have root mean square differences less than 0.2 pss and correlation coefficients greater than 0.9. Additionally, over 80% of differences between the products fall between ± 0.5 pss. Surface salt transports were calculated to see how well the satellite products estimate SSS advection in the Agulhas region. Zonal and meridional surface salt transports have annual maxima in austral winter and minima in austral summer, although the seasonal range is small. Using Argo, SMOS, and Aquarius SSS for this calculation produces statistically identical results, suggesting the utility of using the satellites to monitor regional salinity advection at higher frequencies and horizontal resolutions than previously possible using only sparse in situ data. SSS gradients along the subtropical front reveal that SMOS and Aquarius are much more effective tools for observing small-scale structure prevalent in the meandering of the Agulhas Return Current. Finally, we conclude that when compared to Argo floats in situ observations SMOS and Aquarius are statistically accurate and may yet prove powerful tools for observing Agulhas salinity processes on smaller time and spatial scales.

Salt exchange in the Indian‐Atlantic Ocean Gateway since the Last Glacial Maximum: A compensating effect between Agulhas Current changes and salinity variations?

AbstractThe import of relatively salty water masses from the Indian Ocean to the Atlantic is considered to be important for the operational mode of the Atlantic Meridional Overturning Circulation (AMOC). However, the occurrence and the origin of changes in this import behavior on millennial and glacial/interglacial timescales remains equivocal. Here we reconstruct multiproxy paleosalinity changes in the Agulhas Current since the Last Glacial Maximum and compare the salinity pattern with records from the Indian‐Atlantic Ocean Gateway (I‐AOG) and model simulations using a fully coupled atmosphere‐ocean general circulation model. The reconstructed paleosalinity pattern in the Agulhas Current displays coherent variability with changes recorded in the wider I‐AOG region over the last glacial termination. We infer that salinities simultaneously increased in both areas consistent with a quasi interhemispheric salt‐seesaw response, analogous to the thermal bipolar seesaw in response to a reduced cross‐hemispheric heat and salt exchange during times of weakened AMOC. Interestingly, these hydrographic shifts can also be recognized in the wider Southern Hemisphere, which indicates that salinity anomalies are not purely restricted to the Agulhas Current System itself. More saline upstream Agulhas waters were propagated to the I‐AOG during Heinrich Stadial 1 (HS1). However, the salt flux into the South Atlantic might have been reduced due to a decreased volume transport through the I‐AOG during the AMOC slowdown associated with HS1. Hence, our combined data‐model interpretation suggests that intervals with higher salinity in the Agulhas Current source region are not necessarily an indicator for an increased salt import via the I‐AOG into the South Atlantic.

A 1000-Year Carbon Isotope Rainfall Proxy Record from South African Baobab Trees (Adansonia digitata L.).

A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab (Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.

Madagascar corals track sea surface temperature variability in the Agulhas Current core region over the past 334 years

The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere and is key for weather and climate patterns, both regionally and globally. Its heat transfer into both the midlatitude South Indian Ocean and South Atlantic is of global significance. A new composite coral record (Ifaty and Tulear massive Porites corals), is linked to historical AC sea surface temperature (SST) instrumental data, showing robust correlations. The composite coral SST data start in 1660 and comprise 200 years more than the AC instrumental record. Numerical modelling exhibits that this new coral derived SST record is representative for the wider core region of the AC. AC SSTs variabilities show distinct cooling through the Little Ice Age and warming during the late 18th, 19th and 20th century, with significant decadal variability superimposed. Furthermore, the AC SSTs are teleconnected with the broad southern Indian and Atlantic Oceans, showing that the AC system is pivotal for inter-ocean heat exchange south of Africa.

Seasonal mixed-layer dynamics in an eddy-resolving ocean circulation model

[1] Mean and seasonal mixed-layer depths (MLDs) derived from an eddy-resolving ocean general circulation model with a horizontal resolution of (1/10)° are validated with climatological observations. Associated heat budgets on seasonal timescales are analyzed for six boundary current regions with high eddy kinetic energy (Somali Current, Agulhas Current region, Kuroshio, East Australian Current, Gulf Stream, and Brazil-Malvinas/Falkland Confluence). In all of these regions and on seasonal timescales, (a) horizontal advection significantly contributes to the mixed-layer heat budget (MLHB) on eddy scales and locally exceeds ±5°C/month; (b) lateral mixing (calculated as a residual term) is similar in size to surface net heat flux, horizontal advection, and vertical entrainment in defining the mixed-layer temperature; (c) seasonal vertical entrainment has a cooling effect on mixed-layer temperature throughout the year in the regions investigated; and (d) a phase lag between MLD and changes in mixed-layer heat content exists such that local cooling (warming) in the mixed layer precedes maxima (minima) in MLD by 1–3 months. A rather complex picture emerges where the MLHB in ocean boundary currents on larger spatial scales is determined by net surface heat fluxes and entrainment, whereas local, eddy-related advection and stirring modulate the large-scale signals.

Processes driving thunderstorms over the Agulhas Current

Lightning occurs predominantly over land and is not common over the open ocean. We study here one oceanic region in which thunderstorms are frequently found, namely the warm Agulhas Current off the southeast coast of South Africa. The seasonal and interannual lightning variability is derived from satellite and terrestrial data sets. Favorable climatic conditions for lightning are investigated using both ERA‐Interim and NCEP/NCAR reanalysis data. We find peak lightning in austral autumn over the Agulhas Current but with low seasonality (i.e., there is considerable lightning throughout the year). While the climatological wind direction varies strongly with latitude and season, the wind direction is predominantly northerly throughout the region during thunderstorms. A composite of sea level pressure during thunderstorm days indicates that thunderstorms are related to eastward‐propagating synoptic‐scale wave trains passing through the Agulhas Current region. The strong convective activity during thunderstorms occur in the warm sector of a cyclone and is associated with horizontal convergence and lifting of warm, moist surface air originating over the warm Agulhas Current.

Mapping the Agulhas Current from space: An assessment of ASAR surface current velocities

Over 2 years of surface current information collected in the Agulhas Current region and derived from the Doppler centroid anomalies of Envisat's advanced synthetic aperture radar (ASAR) are examined. The sources of errors and potential use of ASAR surface current velocities for oceanographic research are assessed. ASAR surface current velocities are compared to surface drifter data and merged altimetry observations. Maps of sea surface temperature are used to establish the ASAR's capacity to capture the synoptic circulation. Discrepancies between observed and predicted ASAR velocities result predominantly from inadequate wind corrections combined with radar incidence angles below 30°. Occasionally observed wind‐induced outliers cause a bias in the estimated ASAR velocities but do not affect the ability of the ASAR to systematically image regions of strong surface current flow and shear. Time‐averaged maps of ASAR‐derived surface current velocity seem able to accurately capture the position as well as the intensity of the Agulhas Current. The ability of the ASAR to pick up the smaller features of the circulation along the shelf break also shows that variability along the Agulhas Bank is of the same order of magnitude as that observed in the Agulhas retroflection. ASAR surface current velocities offer a very good complement to altimetry in regions where the mean dynamic topography is poorly resolved. The quasi‐synoptic nature of ASAR acquisitions combined with the relatively high resolution of ASAR surface current velocities also make it attractive for studies of submesoscale processes and western boundary current dynamics.

Antarctic intermediate water at the Agulhas Current retroflection region

The inter-ocean transport of salty, warm Indian Ocean water to the Atlantic Ocean has been shown to stabilize the meridional overturning of the Atlantic. This salt and heat is mostly transported by Agulhas rings, shed at the Agulhas retroflection. To investigate the contribution of Antarctic Intermediate Water (AAIW) to this salt and heat flux, 7 hydrographic sections in the south-west Indian and south-east Atlantic Oceans were investigated using Optimum Multi-parameter Analysis. The mean contributions of AAIW from the South Indian Ocean (iAAIW) and that of the Drake Passage (dAAIW), over the density range σ n = 27.25–27.7, showed that iAAIW was the dominant AAIW type east of the Agulhas Current retroflection. The maximum contributions of iAAIW were confined to the Agulhas Current, the Agulhas Return Current and Agulhas rings. In the subdivided density ranges, iAAIW was always the dominant water type in the upper density range ( σ n = 27.25–27.4), whilst in the lower density range ( σ n = 27.4–27.7) the dAAIW/iAAIW mix was almost 50/50. West of the retroflection the mean iAAIW/dAAIW mix over the density range σ n = 27.25–27.7 was around 50/50 for the southern part of the Cape Basin. In the upper density range the average mix of dAAIW/iAAIW was 45/55, whilst for the lower density range the mix was 46/54. Section by section comparisons, however, showed considerable differences in the observed mix of dAAIW/iAAIW in both the upper and lower density ranges.

Summer and winter distribution of ƒÂ13CDIC in surface waters of the South Indian Ocean [20°S-60°S

This paper describes for the first time the summer and winter distributions of sea surface ¥ä13CDIC in the Southern Indian Ocean (20°S°60°S)° For this we used δ13CDIC measurements from 10 cruises conducted between 1998 and 2005. For summer and winter, the highest δ13CDIC values(>2‰) are observed in sub-Antarctic waters (40°S-50°S) and attributed mainly to biological activity, enhanced in the vicinity of Crozet and Kerguelen Archipelagoes. The lowest δ13CDIC values are found in subtropical waters (25°S-35°S), with a minimum (1) in the Agulhas Current region and in the Mozambique channel. On the seasonal scale, δ13CDIC is higher during summer than during winter in all regions. The largest seasonal amplitude of variation (∼0.15‰), observed in the region 35°S-40°S, is attributed to biological activity during summer and to deep vertical mixing during winter. In subtropical oligotrophic waters, the mean seasonal amplitude of variation (∼0.15‰) is mainly explained by air.sea CO2 fluxes. On the interannual scale, we also identified a large negative anomaly of δ13CDIC in the subtropical waters during austral summer 2002, associated to an anomalous ocean CO2 sink due to cold conditions during this period.

Sea level anomaly signals in the Agulhas Current region

In the Agulhas Current and its sources and sinks, variability in steric height derived from the 0.1° POP model is 3–11 times greater than the mass‐load signal at periods shorter than 20 days, the approximate limit of temporal resolution for the Jason series of satellite altimeters. Model output is evaluated for realism against in situ SLA data from a suite of moorings deployed in the southeastern South Atlantic Ocean in 2003–2005. The effects of temporal filtering and alongtrack smoothing on output sampled at 10‐day intervals is examined. In approximately 14% of the region, including much of the Agulhas Retroflection, aliasing is ≤10% only at periods of ≥120 days. Smoothing reduces that proportion by 40% but also sharply reduces variability at periods of ≤73 days indicating that use of Jason‐style altimetry for time series analysis of this current system should be confined to seasonal and longer scales.

Where two oceans meet: distribution and offshore interactions of great-winged petrels Pterodroma macroptera and Leach’s storm petrels Oceanodroma leucorhoa off southern Africa

Abstract During seabird surveys off southern Africa, great-winged petrel Pterodroma macroptera and Leach’s storm petrel Oceanodroma leucorhoa were widespread and relatively common. Similar aerial displays, frequent (social) interactions, a tendency to “huddle” in tight (mixed) gatherings and interactions during foraging inspired a study of their behaviour and distribution. Both species peaked at >2,000-m-deep ocean waters, with lower densities over the shelf and with Leach’s storm petrels (ca. 0.5 km−2) twice as abundant as great-winged petrels (ca. 0.25 km−2). The results suggest half a million great-winged petrels and well over a million Leach’s storm petrels occurring over deep waters off southern Africa. Active fishing vessels elevated background densities of petrels in some areas, indicating the utilisation of discarded material, but in most areas hardly any fisheries were encountered and on the shelf with no discernable effect on petrel densities. Both species showed an association with meso-scale hydrographic features in the Agulhas current retroflection region. High densities of Leach’s storm petrels occurred in offshore areas with steep salinity and sea surface temperature gradients. No such tendency was found in great-winged petrels. Great-winged petrels spent relatively little time feeding during daylight (10.3% of birds observed) in comparison to Leach’s storm petrels (66.2%), but were frequently seen to join feeding Leach’s storm petrels where they profited from the searching skills of Leach’s. Both species performed displays as individuals, in pairs, in larger groups and in mixed-species groups. Tight gatherings of petrels swimming at sea (huddling) occurred, consisting either of one species or both. When disturbed, such flocks would disperse after take off and often engage in aerial displays. It is speculated that the formation of these flocks is part of the anti-predator strategy of petrels against attacks from under water.

Sea surface temperature fronts in the midlatitude South Atlantic revealed by using microwave satellite data

The Subtropical Convergence or Frontal Zone has been little studied in the central South Atlantic Ocean. Despite limited data, some work has suggested that this feature exists continuously across the South Atlantic. Here, satellite microwave sensed sea surface temperature (SST) data, available from June 2002, are analysed for the existence of frontal gradients in the subtropical to subpolar South Atlantic with focus on the central to eastern region. The frontal locations and SST gradients in the southwest South Atlantic and in the Agulhas Current retroflection region obtained from this satellite data are consistent with those estimated previously from hydrographic observations. Evident in the satellite data is pronounced mesoscale variability in subtropical frontal features, particularly south of Africa, and extending west from the Agulhas Current retroflection to at least the Greenwich meridian, and in the southwest South Atlantic. Over the central to eastern part of the basin, the analysis suggests that there is substantial interannual variability in the strength and latitudinal location of the Northern and Southern Subtropical Front, as defined by Belkin and Gordon (1996). Some of this variability may be related to shifts in the winds, including that in the latitude of the strongest wind stress curl. On seasonal scales, this central to eastern region is also characterised by variations in the frontal intensity but shifts in the position of regions of strong gradients are less obvious. Thus the data indicate that there is no continuous subtropical frontal feature across the basin in terms of surface temperature.

Numerical simulations of a severe rainfall event over the Eastern Cape coast of South Africa: sensitivity to sea surface temperature and topography

The intense precipitation event over East London, South Africa, on 15–16 August 2002 is investigated using a mesoscale model (MM5) and its sensitivity to sea surface temperature (SST) and topography tested. Over 300 mm of rain fell in 24 h over East London as compared to the August average of 78 mm. The intense precipitation resulted from a cut-off low system aloft and the formation of a low-level jet (LLJ) near the surface.The MM5 control simulation indicated that the intense precipitation was due to the ascent of moist air at the leading edge of the LLJ, which advected warm moist air at low levels onto the coastal mountains and was enhanced by the formation of a meso-beta scale low-pressure system just off the coast. The ascent at the leading edge of the LLJ formed the ascending arm of a convective circulation cell.Sensitivity experiments with MM5 indicated that the warm core of the Agulhas Current contributed to the formation of the meso-beta scale low, while the mesoscale structure of the SST field was important in determining its location and that of the LLJ, and therefore the precipitation in the model. Most of the moisture originated from the Agulhas Current region. The South African topography was found to influence the location of the LLJ via trapping a near-surface warm trough on the eastern side of the Drakensberg mountains. We suggest that deficiencies in the control experiment may have been due to inaccuracies in using a weekly mean SST field to force the model at the lower boundary.

Spreading of the Indonesian Throughflow in the Indian Ocean*

The Indonesian Throughflow (ITF) spreading pathways and time scales in the Indian Ocean are investigated using both observational data and two numerical tracer experiments, one being a three-dimensional Lagrangian trajectory experiment and the other a transit-time probability density function (PDF) tracer experiment, in an ocean general circulation model. The model climatology is in agreement with observations and other model results except that speeds of boundary currents are lower. Upon reaching the western boundary within the South Equatorial Current (SEC), the trajectories of the ITF tracers within the thermocline exhibit bifurcation. The Lagrangian trajectory experiment shows that at the western boundary about 38%±5% thermocline ITF water flows southward to join the Agulhas Current, consequently exiting the Indian Ocean, and the rest, about 62%±5%, flows northward to the north of SEC. In boreal summer, ITF water penetrates into the Northern Hemisphere within the Somali Current. The primary spreading pathway of the thermocline ITF water north of SEC is upwelling to the surface layer with subsequent advection southward within the surface Ekman layer toward the southern Indian Ocean subtropics. There it is subducted and advected northward in the upper thermocline to rejoin the SEC. Both the observations and the trajectory experiment suggest that the upwelling occurs mainly along the coast of Somalia during boreal summer and in the open ocean within a cyclonic gyre in the Tropics south of the equator throughout the year. All the ITF water eventually exits the Indian Ocean along the western boundary within the Mozambique Channel and the east coast of Madagascar and, farther south, the Agulhas Current region. The advective spreading time scales, represented by the elapsed time corresponding to the maximum of transit- time PDF, show that in the upper thermocline the ITF crosses the Indian Ocean, from the Makassar Strait to the east coast of the African continent, on a time scale of about 10 yr and reaches the Arabian Sea on a time scale of over 20 yr.

Ocean–Atmosphere Interaction in the Agulhas Current Region and a South African Extreme Weather Event

Ocean‐atmosphere interaction above warm western boundary currents such as the Gulf Stream, the Kuroshio Current, and the Agulhas Current often leads to very high evaporation rates. In the case of the Agulhas Current, which flows just off the southeast coast of Africa, such high latent heat fluxes may lead to increased low-level advection of moisture onshore and local intensification of storm systems. Observational evidence for the significant latent heat fluxes in the Agulhas Current area was obtained during the Agulhas Current Air Sea Exchange Experiment, which showed that about 5 times as much water vapor is transferred to the atmosphere above the 80‐100-km-wide core of the current than from the neighboring waters. Using NCEP reanalyses, Meteosat, and Tropical Rainfall Measuring Mission (TRMM) satellite data and local station and radiosonde observations, this study investigates the evolution of a severe storm and flood event that occurred over the southern coastal regions of South Africa on 14‐15 December 1998. Heavy rainfall occurred in two widely separated locations, and tornadoes were reported. Moisture flux transects through the storm region and backward trajectories of air parcels suggest that low-level onshore flow of moisture from the Agulhas Current region played a significant role in the storm evolution. However, because the NCEP data on which these moisture fluxes are based are known to significantly underestimate the surface latent heat flux when compared with ship observations, it is suggested that the actual contribution of the Agulhas Current moisture source to the storm may have been even greater than is documented in this paper.

Lanternfish larvae from the Agulhas current (SW Indian Ocean)

This paper is presented as a guide to assist in identifying larval stages of lanternfish (family Myctophidae) of the Agulhas Current region. The work is based on the larvae collected during three cruises conducted off the eastern coast of South Africa from 29°S to 34°S in 1990 and 1991. The larval distributions obtained from these surveys are summarised briefly. Generally, only literature citations are given for species of the region whose larvae have been described previously. Descriptions are limited to those species for which complete larval development have not been published previously and to those species not known to spawn in the region, to compare larval features with those from regions where the species is known to spawn (i.e., Benthosema fibulatum, B. pterotum, Hygophum proximum, Lampadena luminosa, Myctophum selenops, Notoscopelus caudispinosus and Triphoturus nigrescens). Also, diagnostic characters and illustrations are presented for species which may be difficult to identify in some developmental stages, particularly the early larval stages (i.e., Benthosema suborbitale, Bolinichthys spp., Ceratoscopelus townsendi, Diogenichthys panurgus, Myctophum phengodes, Lobianchia dofleini, L. gemellarii and Scopelopsis multipunctatus).

The water masses and volumetry of the southern Agulhas Current region

It has been suggested that the southern termination of the Agulhas Current plays a crucial role in the global circulation of thermocline water and thus in global climate. Due to a lack of modern hydrographic observations in this region, no detailed description of water masses or a fine-scale volumetric census for this geographic area had been carried out. Such an analysis of a collection of recent high-quality hydrographic measurements shows that the warm, saline, surface water of Agulhas Current origin contributes very little to the overall volume of the upper 1500 m of the water column in the area. Occasional equatorward leakages from south of the Subtropical Convergence are represented by a range of low-salinity outliers, but they represent <1% of the total volume. The distribution of water volume in temperature/salinity space for the Agulhas Retroflection is less diverse that that of the world ocean as a whole, 25% of the total volume of the region being contained in only 21 fine-scale temperature/salinity classes. North Atlantic Deep Water is the dominant water mass, accounting for 40% of the total volume. Deep Water in general accounts for 60% of the total volume.

Satellite-Tracked Drifters between Africa and Antarctica

Trajectories and velocities are reported for six drifters launched in February 1976 in the South Atlantic Ocean mostly along the 10°W meridian between 35° and 57°S. These drifters had drogues centered on 7 or 57 m and the tracking durations were 33–302 days. In the Circumpolar Current the tracks were remarkably zonal, remaining generally within ±1° of latitude, except for one drifter deployed at ~42°S, which when it was south of the Cape curved equatorward in the Agulhas Current terminal region, and one at 57°S, which slanted equatorward along the Atlantic-Indian Ridge. The expected deflections occurred at the mid-ocean ridges, and generally the course of the drifters was consistent with the published streamlines of geostrophic transport. The trajectory of a drifter launched equatorward of the subtropical convergence, at 35°S, was one of complex eddying motion, accompanied by a slow drift to the northeast. In the Circumpolar Current, the latitudinal variation of zonal current velocity was qualitatively similar to the mean annual wind shear and (in the region of 0° meridian) to the geostrophic zonal surface velocities calculated by Ostapoff (1962). Drifter velocities were, however, considerably larger than the geostrophic velocities. Maximum mean velocities (~0.3 m/s) occurred along 49°50′.