Effect of Atlantic Meridional Overturning Circulation on Tropical Atlantic Variability: A Regional Coupled Model Study

A simplified coupled ocean-atmosphere model, where an atmospheric general circulation model (AGCM) is fully coupled to a 2 1/2-layer reduced-gravity ocean model (RGO) over the tropical Atlantic basin, is presented in the context of studying the role of the Atlantic meridional overturning circulation (AMOC) in tropical Atlantic variability (TAV). In the ocean model, the strength of the AMOC is controlled by specifying mass transport at open boundaries. The fidelity of the reduced-physics model in capturing major features of tropical Atlantic variability, as well as its response to the AMOC changes, is demonstrated in a series of model experiments. The results of the experiments reveal the relative importance of oceanic processes and atmospheric processes in AMOC-induced tropical Atlantic variability-change. It is found that the oceanic processes are a primary factor contributing to the warming at and south of the equator and the precipitation increase over the Gulf of Guinea, while atmospheric processes are responsible for the surface cooling of the tropical North Atlantic and southward displacement of ITCZ. A systematic investigation of the coupled system response to changes in AMOC strength indicates that the SST over the cold-tongue region responds nonlinearly to AMOC changes. The sensitivity of the SST response increases rapidly when AMOC strength decreases below a threshold value. Such nonlinear behavior is also found in precipitation response over the Gulf of Guinea. These results suggest that complex and competing atmosphere-ocean processes are involved in TAV response to AMOC changes and the nature of the response can vary from one region to another. This complexity should be taken into consideration in Atlantic abrupt climate studies.