Towards Resolving the Role of Agulhas Leakage in 20th Century Global Climate Change

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Recent studies show that Agulhas leakage may be increasing in response to anthropogenic climate change, which would act to strengthen the Atlantic overturning circulation at a time when anthropogenic warming and associated ice melt are predicted to weaken it. Evidence from paleoceanographic proxies, theory, and models indicate that the Agulhas leakage may impact the climate system on a number of time scales. However, coupled climate models, including the IPCC models, do not resolve the dynamics that control Agulhas leakage and its impact on the Atlantic overturning circulation, and hence may be flawed in their ability to predict future climate.The Agulhas system and its leakage of warm and salty water from the Indo-Pacific into the Atlantic represents a choke point in the global thermohaline circulation, feeding the upper arm of the strong and deep Atlantic overturning cell via a non-linear exchange of thermocline waters within Agulhas Rings, eddies, and filaments. Coupled climate models do not simulate leakage correctly, because inertial mechanisms and Ring formation are not resolved. Climate models also tend to overestimate the effect of the Southern Hemisphere westerly winds on overturning, because of unresolved eddies. This means the interplay between winds, leakage, and the overturning circulation may be flawed in these models. Finally, almost all coupled climate models exhibit a northward overturning freshwater transport, while hydrographic observations point to a southward one. This effects the sensitivity and stability of the overturning circulation in the models, and is linked to the balance between Agulhas and Antarctic inputs into the South Atlantic.A coupled climate simulation of the 20th century with one tenth degree ocean resolution will be run using the CCSM4. For the first time, the Agulhas System and how its leakage responds to anthropogenic forcing in a coupled climate system will be examined. The project objectives are to: (1) diagnose the sensitivity of Agulhas leakage and the overturning circulation to changing winds in the presence of eddies versus in coarser resolution IPCC models; (2) analyze the relationship between Agulhas Current inertia, leakage strength, subtropical wind stress curl, and the latitude of the westerly wind maximum to test the constraints on leakage and whether they are as theoretical models predict; (3) isolate the response of Atlantic overturning to Agulhas leakage in the absence of a direct wind effect, and (4) consider the balance of the cold versus warm water routes into the surface arm of the overturning, its correlation with leakage, and its effect on the stability of the overturning. One of the broader impacts of this project is to provide qualitative uncertainty estimates of the predictive capability of coarse resolution coupled climate models (especially Intergovernmental Panel for Climate Change models) with respect to overturning strength and sensitivity in the presence of a resolved Agulhas leakage. This project will also support a graduate student and a postdoc who will be mentored by both a field-going oceanographer and a climate modeler. All simulation data will be freely shared with the community for further analyses.