Sea surface slope as a proxy for Agulhas Current strength

The linear relation between the strength of the Agulhas Current at nominal latitude 34°S and the gradient in sea level height anomaly across the current is investigated in a 1/10° resolution regional numerical ocean model. Our results show that the strength of the current can be estimated with reasonable accuracy using altimeter data, once it has been calibrated using in-situ transport measurements. Three years of transport measurements provide a calibration with worst-case correlation R = 0.78. In that case the errors in proxy transport have a standard deviation of 9.8 Sv, compared to a 20.2 Sv standard deviation of the transport time series itself. From these results we conclude that the design of the Agulhas Current Timeseries (ACT) experiment, a three-year deployment of moorings across the Agulhas Current and along a TOPEX/Jason altimeter ground track, will likely produce a good quality multi-decadal time series of Agulhas Current strength.

Figure 1: The model mean sea surface height in the Agulhas Region (upper panel, in m) over the 25 years used in the analysis. The green triangles and yellow circles denote the locations of the moorings and the C-PIES, respectively. From the instruments on these moorings, the 25-year mean velocity profile across the Agulhas Current in the numerical model can be determined (lower panel, in m/s). The crosses are current meters, the green triangles are upward looking ADCPs, and the yellow circles are C-PIES. These latter yield only column-integrated transports between 200 m and 2000 m depth, and hence no vertical structure. Above and below that, there is no velocity information.