Observations of Ekman Spirals and Ekman Transport near the Kerguelen Plateau

V.W. Ekman's theory of wind forcing on the ocean is a corner stone of oceanography. By considering a balance between frictional and Coriolis forces and assuming a constant vertical eddy viscosity Ekman devised equations for the latitudinal and longitudinal velocity components as a function of depth. For steady winds the resulting solution is the Ekman spiral and has a characteristic exponential amplitude decay and anticyclonic (anticlockwise in the Southern Hemisphere) rotation. The net transport arising from Ekman currents is of significance in meridional overturning circulation, driving the upwelling of deep waters near 50°S and transporting them northward, and may also be significant in the formation of Mode Waters. Ekman currents and Ekman transport within the Southern Ocean has been the subject of few previous studies.

Using data from 8 EM-APEX floats deployed in late 2008 north of Kerguelen Island with vertical resolution of 3-6m, the first stage of this study examines the structure of Ekman spirals. Wind-driven velocities were isolated from the absolute velocity profiles by averaging pairs of velocity profiles separated by half an inertial period to remove inertial oscillations before subtracting a deep velocity to remove geostrophic currents. 285 Ekman-like spirals were identified along with 205 spirals with Ekman-like decay and reverse rotation, possibly consistent with cyclonic super-inertial wind forcing. For the observed Ekman and reverse spirals, estimates of eddy viscosity and e-folding depth were obtained by fitting exponential curves to current speeds and linear equations to current heading. Generally, estimates of e-folding depth from velocity amplitude decay were found to be inconsistent with estimates from current heading. Typically, spirals were found to be ‘compressed': the rotational e-folding depth being approximately twice the amplitude of decay e-folding depth. This is consistent with a number of previous studies.

The second phase of this study used the same EM-APEX velocity dataset to make estimates of cross track Ekman transport for each EM-APEX float. This problem was approached from two directions: firstly, cross-track components of wind driven velocities as derived above were calculated; secondly, the cross-track geostrophic velocity profiles were calculated from temperature and salinity profiles and subtracted from the absolute velocity observations taken by the floats to leave residual Ekman velocities in a similar manner to several previous studies. Transport at each station was then calculated by integrating the profiles of Ekman velocity and compared to transport estimates derived from the NCEP reanalysis wind stress. The vertical structure of transport was then calculated for each float and examined as a function of mixed layer depth and thermocline depth.