Relating Agulhas leakage to the Agulhas Current retroflection location

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.

Agulhas leakage using Lagrangian particles. The particles are released in the Agulhas Current in the fine-resolution AG01 model and then advected forward in time. The influence of mesoscale eddies is clearly visible.

Figure 1: A snapshot of dynamic topography (in meter) on 20 November 2002 from the AG01 model. The thick black line is the Agulhas Current path CA, starting at point ps, as detected by the algorithm. The dashed closed contour is the associated ring found by the ring detection algorithm. The westward extent of the current is indicated by φ_w.