Variability of the thermocline due to a sudden change in the Ekman pumping

The thermocline variability caused by a sudden variation of the Ekman pumpi ng is studied, using a 2.5-layer geostrophic model that represents the vent ilated and the shadow zones of a subtropical gyre and baroclinic Rossby wav e propagation. During spinup the propagation of the first baroclinic mode i nduces a large deepening of the thermocline on a timescale ranging between 2 and 15 yr, depending on latitude. The propagation is similar throughout t he basin, and is not influenced by the geostrophic flow, save nonlinearly t hrough the variation in layer depth. South of the subduction line, the adju stment 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 imbalanc e between Ekman Dumping and vertically averaged meridional advection. In th e shadow zone, the second baroclinic mode plays a more important role since it primarily balances the Ekman pumping, although the variations of the se cond layer depth remain smaller than in the ventilated area. Spindown induc es similarly a shoaling of the thermocline, but, because of nonlinearities, lower Rossby wave speeds, and decreased advection, the adjustment is every where slower than during spinup.