Estimates of wind energy input to the Ekman layer in the Southern Ocean from surface drifter data

摘要

The energy input to the upper ocean Ekman layer is assessed for the Southern Oceanby examining the rotary cross spectrum between wind stress and surface velocity forfrequencies between 0 and 2 cpd. The wind stress is taken from European Center forMedium-Range Weather Forecasts ERA-40 reanalysis, and drifter measurements from15 m depth are used to represent surface velocities, with an adjustment to account for thevertical structure of the upper ocean. The energy input occurs mostly through the nonzerofrequencies rather than the mean. Phenomenologically, the combination of a strongeranticyclonic wind stress forcing associated with a greater anticyclonic response makes thecontribution from the anticyclonic frequencies dominate the wind energy input. Thelatitudinal and seasonal variations of the wind energy input to the Ekman layer are closelyrelated to the variations of the wind stress, both for the mean and for the time-varyingcomponents. The contribution from the near-inertial band follows a different trend,increasing from 30°S to about 45°S and decreasing further south, possibly a consequenceof the lack of variance in this band in the drifter and wind stress data.