Satellite-Tracked Drifters between Africa and Antarctica

1978 ◽  
Vol 59 (1) ◽  
pp. 51-59 ◽  
Author(s):  
T. F. W. Harris ◽  
C. C. Stavropoulos
Keyword(s):  
Atlantic Ocean ◽  
Current Velocity ◽  
Wind Shear ◽  
South Atlantic Ocean ◽  
Agulhas Current ◽  
Ocean Ridges ◽  
Slow Drift ◽  
The Mean ◽  
Circumpolar Current ◽  
Geostrophic Velocities

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′.

scholarly journals Relating Agulhas leakage to the Agulhas Current retroflection location

Ocean Science ◽  
2009 ◽  
Vol 5 (4) ◽  
pp. 511-521 ◽  
Author(s):  
E. van Sebille ◽  
C. N. Barron ◽  
A. Biastoch ◽  
P. J. van Leeuwen ◽  
F. C. Vossepoel ◽  
...  
Keyword(s):  
Time Series ◽  
High Resolution ◽  
Atlantic Ocean ◽  
Ocean Model ◽  
Altimetry Data ◽  
Agulhas Current ◽  
Numerical Ocean Model ◽  
The Indian Ocean ◽  
The Mean ◽  
Annual Time

Abstract. The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression.

scholarly journals Relating Agulhas leakage to the Agulhas Current retroflection location

2009 ◽  
Vol 6 (2) ◽  
pp. 1193-1221 ◽  
Author(s):  
E. van Sebille ◽  
C. N. Barron ◽  
A. Biastoch ◽  
P. J. van Leeuwen ◽  
F. C. Vossepoel ◽  
...  
Keyword(s):  
Time Series ◽  
High Resolution ◽  
Atlantic Ocean ◽  
Ocean Model ◽  
Altimetry Data ◽  
Agulhas Current ◽  
Numerical Ocean Model ◽  
The Indian Ocean ◽  
The Mean ◽  
Annual Time

Abstract. The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression.

scholarly journals An observed 20-year time series of Agulhas leakage

Ocean Science ◽  
2014 ◽  
Vol 10 (4) ◽  
pp. 601-609 ◽  
Author(s):  
D. Le Bars ◽  
J. V. Durgadoo ◽  
H. A. Dijkstra ◽  
A. Biastoch ◽  
W. P. M. De Ruijter
Keyword(s):  
Time Series ◽  
Large Scale ◽  
Ocean Model ◽  
Dynamic Topography ◽  
Return Current ◽  
Major Barrier ◽  
Agulhas Current ◽  
The Mean ◽  
The Difference ◽  
Geostrophic Velocities

Abstract. We provide a time series of Agulhas leakage anomalies over the last 20-years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between the Agulhas Current and Agulhas Return Current, which allows us to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows us to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow us to determine the mean leakage but that leakage anomalies can be accurately computed.

scholarly journals An observed 20 yr time-series of Agulhas leakage

2014 ◽  
Vol 11 (1) ◽  
pp. 171-196 ◽  
Author(s):  
D. Le Bars ◽  
J. V. Durgadoo ◽  
H. A. Dijkstra ◽  
A. Biastoch ◽  
W. P. M. De Ruijter
Keyword(s):  
Time Series ◽  
Large Scale ◽  
Ocean Model ◽  
Dynamic Topography ◽  
Return Current ◽  
Major Barrier ◽  
Agulhas Current ◽  
The Mean ◽  
The Difference ◽  
Geostrophic Velocities

Abstract. We provide a time series of Agulhas leakage anomalies over the last twenty years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between Agulhas Current and Agulhas Return Current which allows to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow to determine the mean leakage but that leakage anomalies can be accurately computed.

South Atlantic circulation in a world ocean model

1994 ◽  
Vol 12 (9) ◽  
pp. 812-825 ◽  
Author(s):  
Matthew H. England ◽  
Véronique C. Garçon
Keyword(s):  
Indian Ocean ◽  
Atlantic Ocean ◽  
North Atlantic ◽  
Drake Passage ◽  
South Atlantic ◽  
South Atlantic Ocean ◽  
Global Ocean ◽  
Intermediate Water ◽  
Agulhas Current ◽  
Benguela Current

Abstract. The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agreement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather barotropic Antarctic Circumpolar Current (ACC) through the Drake Passage, and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates of the mass transport shed westward at the Agulhas retroflection. Topographic steering of the ACC dominates the structure of flow in the circumpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and fresher Subantarctic surface water (originating in the ACC). The Benguela Current is seen to modify its flow and fate with depth; near the surface it flows north-westwards bifurcating most of its transport northward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the water column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the Benguela Current extension encounters the coast of South America. At intermediate levels, these northward currents trace the flow of Antarctic Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the model's rather coarse resolution, some subtle features of the Brazil-Malvinas Confluence are simulated rather well, including the latitude at which the two currents meet. Conceptual diagrams of the recirculation and interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and isobaric surfaces. This analysis shows how the return path of NADW is partitioned between a cold water route through the Drake Passage (6.5 Sv), a warm water route involving the Agulhas Current sheeding thermocline water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).

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