1.5 Ekman model response to the extratropical thermal forcing

We examine how the Ekman transport modulates the tropical response to hemispherically differential thermal forcing. The 1.5 layer ocean model is implemented to a gray atmospheric model, with no radiative feedbacks, coupled to an aquaplanet slab ocean. The control climate with no surface flux adjustment is perturbed by southern hemisphere cooling and northern hemisphere warming at the surface. We examine the sensitivity to the forcing amplitude, forcing location, and Ekman flux strength in the control climate. The tropics-to-midlatitude component of Ekman transport, which is a negative feedback that counteracts the prescribed forcing, dominates over the equatorial component, which is a positive feedback that reinforces the prescribed forcing. This leads to a reduction in the atmospheric energy transport, but by less than the Ekman heat transport, because the OLR response is also damped by the negative extratropical feedback. As a result, the damping effect of Ekman flux on the atmospheric energy transport is limited, with the atmospheric energy transport response always dominant over the Ekman heat transport. The results highlight the need to look into the role of deep circulation and subtropical gyres other than the Ekman transport in fully coupled models.