Assessing Agulhas leakage

Abstract

Agulhas leakage, the water that flows from the Indian Ocean to the Atlantic Ocean, plays an important role in the circulation of the Atlantic Ocean. The magnitude of this flux of warm and saline Indian Ocean water into the much colder and fresher Atlantic Ocean can be related to the strength of the Atlantic meridional overturning circulation, both in numerical ocean models and in paleoceanographic records. However, estimating the magnitude of this Agulhas leakage is not easy. The Agulhas region is the locus of three circulation systems: the subtropical gyres of the South Atlantic and Indian Oceans, and the Subtropical Front of the Southern Ocean. Agulhas leakage is only a minor and intermittent flux in the region and due to the vigorous mixing in the Cape Basin the signal is quickly diluted beyond the Agulhas Current retroflection. The method of estimating the magnitude of Agulhas leakage which is closest to the definition of Agulhas leakage employs the trajectories of floats that start in the Agulhas Current and end in the Atlantic Ocean. In this way the Indian Ocean water in the Atlantic Ocean can be labeled and tracked allowing for the determination of, among other quantities, volumetric fluxes. For a statistical analysis on interannual time scales the number of floats required is in the order of millions and one has to turn to numerical ocean models. The feasibility and skill of three different strategies for assessing Agulhas leakage are tested in a numerical ocean model. For this, time series of quantities which can in principle be measured in the real ocean are constructed: the westward extent of the Agulhas Current retroflection as determined from sea surface height, the volume flux of the Agulhas Current, and the volume flux of warm and saline water in the Cape Basin. These three time series are related to a time series of Agulhas leakage as determined from a Lagrangian float experiment, in order to test the skill of the monitoring strategies. Because all strategies have advantages and disadvantages, the best monitoring strategy will be a combination of the three strategies proposed