Nonlinear effects in the seasonal cycle of the tropical Atlantic ocean

Nonlinearities have a large effect on the circulation of the tropical Atlantic Ocean within a few hundred kilometers of the equator, both in the surface layers and at depth. Qualitative features of a nonlinear model that are absent from a linear model include energetic unstable waves in the western equatorial Atlantic and a westward surface jet that penetrates to considerable depths between the equator and 3°N. The largest quantitative difference between the nonlinear and linear models is the intensity of the westward surface flow at the equator. In a linear model it can be twice as fast as in a nonlinear model. Motion below the equatorial thermocline, though sufficiently slow to be linear, is related to the surface forcing in a nonlinear manner because it is forced to a large extent by vertical movements of the thermocline. (Linear models assume that disturbances reach the deep ocean by propagating through a fixed thermocline.) In addition to the equatorial zone, nonlinearities affect the coastal zone of Africa south of the equator, where alongshore currents and zonal pressure gradients in linear and nonlinear models are different.