The greater Agulhas Current System: Intercomparison of altimetry and model results
Bjørn C. Backeberg(1) , Johnny A. Johannessen(2) , Laurent Bertino(1) , and Nina G. Winther(1)
(2) Nansen Environmental and Remote Sensing Center, Thormøhlensgt. 47, N-5006 Bergen, Norway
The Agulhas Current flows polewards along the South African East Coast, before retroflecting near 40 S and between 16 - 20 E. With average and peak velocities of approximately 1.36 m/s and 2.45 m/s, and volume transport estimated to lie between 60-65 Sv, it has been described as one of the strongest western boundary currents in the world's ocean. The Agulhas Retroflection describes a loop with a diameter of up to 340 km, and is associated with frequent ring shedding, or eddy formation, events. These eddies may have diameters exceeding 200 km and are thought to contribute to the Indo-Atlantic heat and salinity exchange, and thereby to the global thermohaline circulation and the South Atlantic overturning. Following the Retroflection the Agulhas Return Current flows eastward between 38 and 40 S with surface velocities of up to 2 m/s. Semi-permanent geographically trapped meanders have been observed associated with the flow around the Agulhas Plateau. The dynamics and characteristics of the greater Agulhas Current System exhibit strong spatial and temporal gradients, which makes the region particularly suitable for observations from satellites, in particular measurements from radar altimeters. The analysis of along-track sea level anomaly (SLA) data from Jason-1, shows that the Agulhas Current system dynamics seem to remain fairly stable over a 10-day orbital repeat cycle. A comparison of the repeat-track SLA analysis to gridded merged SLA maps for the region shows that the two data are well correlated, with seemingly no significant loss, or smoothing of information from the production of the 7-day merged maps and subsequent monthly mean calculations. This suggests that the mesoscale variability of the system is fairly well captured in the merged SLA maps, making these a powerful tool for assessing ocean current dynamics and variability. Furthermore, they provide a very good spatial picture of the flow regime and basis for comparison to modelling studies of the region. We compare results from a regional hybrid coordinate ocean model (HYCOM) for the Agulhas Current system to the aforementioned SLA data. As the merged (7-day / monthly mean) SLA data product captures the mesoscale dynamics of the system quite accurately, it becomes a powerful data source for model validation.