Adjoint analysis of merdional overturning sensitivity to wind stress forcing

Sudden, precipitous shifts in thermohaline overturning strength have been documented in numerous ocean only modeling experiments. In this work we use an adjoint model to obtain a global picture of the sensitivity of the North Atlantic overturning strength to surface momentum and buoyancy fluxes. Two cases are presented, the first uses fixed climatological conditions at the ocean's surface, the second allows for feedbacks between the ocean and the atmosphere by coupling in a simple zonally averaged energy balance model.The adjoint sensitivity analysis is shown to be valid for periods of up to 400 years in this 4 degree lateral resolution ocean model, this is sufficient to get a picture of the climatologically relevant processes.

The results highlight strikingly that the North Atlantic overturning is strongly influenced, on centennial timescales, by global scale, interhemispheric and interbasin factors. In the standalone ocean, wind-stress forcing in the Indonesian through-flow region and around the southern tip of South America and primarily Africa stand out as strongly influencing the overturning (see attached figure). A simplified but interactive energy balance atmosphere acts to reduce the importance of these ``gateways'' to the Atlantic basin and emphasizes instead the role played by wind stress in the equatorial and tropical regions. The importance of the heat and freshwater fluxes is in all configurations confined to the high latitudes of the Atlantic basin, but the coupled model is more sensitive to change in forcing than the standalone ocean. An important advantage of the adjoint method is that it allows a quantification and a comparison of the impact on the thermohaline ciruculation of perturbations in each one of the important forcing mechanisms at each location of the surface of the ocean.

Our work supports the conclusion that including atmospheric feedback processes dramatically alters the model's sensitivity to most climate change relevant processes, even when the underlying oceanic circulation is similar with and without the atmosphere.

Figure: Sensitivity of overturning strength to zonal wind stress.