Eddy-Induced Ekman Pumping from Sea-Surface Temperature and Surface Current Effects

Numerous past studies have discussed the biological importance of upwelling of nutrients into the interiors of nonlinear eddies. Such upwelling can occur during the transient stages of formation of cyclones from shoaling of the thermocline. In their mature stages, upwelling can occur from Ekman pumping driven by eddy-induced wind stress curl. Previous investigations of ocean-atmosphere interaction in regions of persistent sea-surface temperature (SST) frontal features have shown that the wind field is locally stronger over warm water and weaker over cold water. Spatial variability of the SST field thus results in a wind stress curl and an associated Ekman pumping in regions of crosswind temperature gradients. It can therefore be anticipated that any SST anomalies associated with eddies can generate Ekman pumping in the eddy interiors. Another mechanism for eddy-induced Ekman pumping is the curl of the stress on the sea surface that arises from the difference between the surface wind velocity and the surface ocean velocity. While SST-induced Ekman upwelling can occur over eddies of either polarity surface current effects on Ekman upwelling occur only over anticyclonic eddies The objective of this study is to determine the spatial structures and relative magnitudes of the two mechanisms for eddy-induced Ekman pumping within the interiors of mesoscale eddies. This is achieved by collocating satellite-based measurements of SST, surface winds and wind stress curl to the interiors of eddies identified and tracked with an automated procedure applied to the sea-surface height (SSH) fields in the Reference Series constructed by AVISO from the combined measurements by two simultaneously operating altimeters. It is shown that, on average, the wind stress curl from eddy-induced surface currents is largest at the eddy center, resulting in Ekman pumping velocities of order 10 cm day^-1. While this surface current-induced Ekman pumping depends only weakly on the wind direction, Ekman pumping from the wind stress curl associated with eddy-induced SST variations depends strongly on both the magnitudes of the SST anomalies and on the ambient wind direction. In some regions within the eddy interiors, the SST-induced wind stress curl acts to reduce the magnitude of the Ekman pumping from surface currents while in other regions it acts to reinforce the current induced Ekman pumping. In midlatitude regions, the Ekman pumping from eddy-induced SST variations is shown to be nearly an order of magnitude smaller than that associated with eddy-induced surface currents. Eddy-induced SST effects on Ekman pumping become more comparable to surface current effects at high latitudes, especially over the Antarctic Circumpolar Current