Thermocline Forced by Varying Ekman Pumping. Part II: Annual and Decadal Ekman Pumping

Abstract

Thermocline variability forced by zonally uniform Ekman pumping with annual to decadal periods is investigated. Both analytical and numerical solutions are obtained by the method of characteristics. As found in Part I, there is little thermocline variability in the ventilated zone or pool zone. In contrast, strong variability may exist in the shadow zone. For annual forcings, nonlinearity is negligible. However, the linear solution is influenced substantially by the basic-state thermocline structure. As a result, local responses dominate for a shallow interface, while remote Rossby waves dominate for a deep interface. Under a strong decadal forcing, nonlinearity may become important. The time-mean thermocline in the shadow zone is shallower than the steady thermocline under the mean Ekman pumping, particularly in the western part of a shadow zone where the mean deviation may reach the order often meters. This shallower mean thermocline is caused by the nonlinear Rossby wave.