Impact of extra-tropical sea surface temperature fronts on the atmospheric Ekman layer and Ekman pumping

A corner stone of geophysical fluid dynamics is the transmission of a surface stress into the inviscid flow away from the boundaries by Ekman pumping. Recent satellite borne observations reveal a strong imprint of sea surface temperature (SST) on the surface stress in areas of strong SST fronts. This suggests that oceanic fronts impact the atmospheric circulation by modulating Ekman layer and pumping. We reconstruct the atmospheric Ekman layer over frontal regions and estimate the impact of SST on atmospheric Ekman pumping using a high resolution atmospheric analysis. We find that in the frontal regions of the Gulf stream Ekman pumping due to SST is of the same order as atmospheric vertical velocities on time scales of months and longer. SST fronts modify the rate of damping of the tropospheric geostrophic circulation, and generate curl of the wind stress due to the spatial variations of the mixing in the atmospheric boundary layer, and due to the imprint of the sea surface temperature on the atmospheric boundary layer temperature and pressure, as suggested previously. Atmospheric Ekman pumping may be a key process to couple the ocean meso-scale and atmospheric circulation in the extra-tropics.