Importance of geostrophic shear on Eddy-induced Ekman pumping: Verification by mesoscale eddies in the Kuroshio Extension region

A new paper by Usui et al. published in Journal of Physical Oceanography develops an analytical equation for vertical motion associated with mesoscale eddies subject to wind forcing. Mesoscale eddies, the "weather" of the ocean, are characterized by swirling motions of ocean currents and associated temperature and salinity anomalies about 10-100 km in radius. These eddies serve an important role in the ocean by transporting properties such as heat, salt, and nutrients.

According to classic steady-state Ekman theory, wind-forced vertical velocity in the upper ocean, (known as "Ekman pumping velocity"), depends only upon wind stress. But since this theory assumes that ocean currents or density do not vary in the horizontal direction, it does not apply to mesoscale eddies. This new formulation, however, also depends upon sea surface height to account for swirling motions of mesoscale eddies, and sea surface temperature and sea surface salinity to account for density anomalies associated with eddies. Much like the classic theory, it relies solely on surface fields, and this facilitates oceanographic research using satellite observations. Further, a diagnostic equation can be used to calculate the component of vertical velocity in these mesoscale eddies that is not driven by wind stress.

As a test, Usui et al. computed a composite mesoscale eddy using the Japanese Ocean General Circulation Model (OGCM) For the Earth Simulator (OFES) output, and found good agreement between the model's vertical velocity and that estimated from this analytical Ekman pumping formulation.