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Spatio-temporal features of the Agulhas Current variability

 

Because of its impact on the global circulation of heat and salt, it is critical to understand the mechanisms of variability of the Agulhas leakage (advection of warm and salty water from the Indian Ocean towards the South Atlantic). The Agulhas leakage is caused by the unstable retroflection of the Agulhas current that generates large eddies at the southern tip of Africa. Such warm and salty core eddies, known as Agulhas rings, compose most of the Agulhas leakage. Observed and modelled intensity of the leakage do not converge, partly because the methods used to estimate the transport differ, but mainly because the retroflection is a complex phenomenon driven by non-linear processes.

 

The Agulhas leakage is primarily rendered by Agulhas rings, large eddies with warm and salty cores that detach from the retroflection and migrate north-westward to the South Atlantic. One would expect a convergence between models that represent the Agulhas rings properly, namely ocean models with high horizontal resolution. On the contrary, ocean models with low horizontal resolution, such as those included in climate models, are expected to reproduce the Agulhas leakage with different characteristics of spatio-temporal variability.

This has been precisely verified by inter-comparing the characteristics of spatial and temporal variability of the Agulhas leakage in a total of 6 ocean models of varying framework and horizontal resolution. First step of this study was to come up with a new method of quantification of the Agulhas leakage that could be applied, in a robust and efficient manner, over several model outputs. Such a method was developed and proved efficient when compared with other classical methods of quantification of the Agulhas leakage (those which are too expensive to be applied on a variety of model simulations). Then, the characteristics of the Agulhas leakage were compared in the different simulations using various statistical techniques.

Ocean simulations at high resolution (1/10° or finer) showed very similar characteristics of variability, related to the size and frequency of Agulhas Rings as observed, despite their use of radically different ocean frameworks (indeed, simulations of NEMO, ROMS and HYCOM were compared with each other). At coarser resolution (approximately 1/4°), those characteristics are slightly different as Agulhas rings are not properly represented by the models. When the horizontal resolution is further reduced, there is no sign of the variability imprinted by the Agulhas rings any more hence variability of the Agulhas leakage is not represented at all. This has dramatic implications on the way Earth System Models include the contribution of the Agulhas leakage to the past, present and future climate.

  

Collaborations: J. Deshayes (CNRS-IRD), B. Backeberg (NTC), B. Loveday (UCT), J. Hermes (SAEON), C. Reason (UCT), L. Holton (MSc UCT), I. Giddy (MSc UCT)

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