Variability of the Thermocline Due to a Sudden Change in the Ekman Pumping

Abstract

The thermocline variability caused by a sudden variation of the Ekman pumping is studied, using a 2.5-layer geostrophic model that represents the ventilated and the shadow zones of a subtropical gyre and baroclinic Rossby wave propagation. During spinup the propagation of the first baroclinic mode induces a large deepening of the thermocline on a timescale ranging between 2 and 15 yr, depending on latitude. The propagation is similar throughout the basin, and is not influenced by the geostrophic flow, save nonlinearly through the variation in layer depth. South of the subduction line, the adjustment is completed by the second baroclinic mode. In the ventilated zone, the latter is not very active, and there are only smaller (by a factor of 5) variations of the thermocline depth primarily linked to a slight imbalance between Ekman pumping and vertically averaged meridional advection. In the shadow zone, the second baroclinic mode plays a more important role since it primarily balances the Ekman pumping, although the variations of the second layer depth remain smaller than in the ventilated area. Spindown induces similarly a shoaling of the thermocline, but, because of nonlinearities, lower Rossby wave speeds, and decreased advection, the adjustment is everywhere slower than during spinup.