The Effects of Eddy-Induced Ekman Pumping on Mesoscale and Large-Scale Ocean Circulation

The simultaneous availability of scatterometer measurements of surface winds, microwave measurements of sea surface temperature (SST) and altimeter measurements of sea surface height (SSH) allows a quantitative assessment of the two mechanisms for self-induced Ekman pumping by oceanic mesoscale eddies. Analysis of these combined satellite datasets reveals that the effects of eddy-induced surface currents on the surface stress usually dominate the effects of air-sea interaction from the influence of eddy-induced SST anomalies on the surface wind field (see figure). The former arises from the surface current effect on the relative wind that determines the surface stress. The latter arises from the effects of SST on vertical mixing in the atmospheric boundary layer. Although this SST influence is generally weaker than the surface current effect, it is not negligible.

While the effects of mesoscale eddies on Ekman pumping are clearly documented from the satellite data, it is not yet known how the feedback of this Ekman pumping on the ocean affect the kinematic properties and evolution of oceanic mesoscale eddies. This is being investigated from empirically coupled numerical model simulations in which the SST and surface current effects on eddy-induced Ekman pumping are imposed individually and together, and the resulting ocean circulation is compared with a Control run in which the eddy-induced Ekman pumping is turned off. Preliminary results have found that both SST and surface currents attenuate the mesoscale eddy field. Consistent with the analysis of QuikSCAT and other satellite data, surface current effects attenuate the eddy field more than SST effects. The SST effects are not negligible, however. They alter the eddy fields in the numerical simulations in more subtle but significant ways. The effects of eddy-induced Ekman pumping on the large-scale circulation will also be investigated from the various numerical simulations.

Figure Caption: Maps of Ekman pumping velocities w_E from SST and surface current effects (top and bottom rows, respectively) for an idealized but realistic anticyclonic (counterclockwise-rotating) eddy at 40°S in the Southern Hemisphere.