The generation of equatorial currents

Abstract

In response to the sudden onset of zonal winds the surface layers of the ocean accelerate in the direction of the wind. Motion is most intense near the equator where a jet forms within a week. The next stage in the evolution of equilibrium conditions is associated with wave fronts, excited initially at the coasts, that propagate across the ocean basin and establish zonal density gradients. Wave modes trapped in and above the strong shallow tropical thermocline because of internal reflection there are responsible for the adjustment of the upper ocean in low latitudes. These thermocline‐trapped modes extend over a depth greater than that of the wind‐driven surface currents and hence give rise to an undercurrent in the thermocline. This undercurrent is zonal and particularly intense near the equator, where it appears in the wake of an eastward traveling Kelvin or westward traveling Rossby wave after about 1 month. In the case of eastward winds, nonlinearities intensify the eastward equatorial surface jet and weaken the westward undercurrent. In the case of westward winds a different nonlinear mechanisms intensifies the eastward Equatorial Undercurrent and weakens the westward surface flow. In a 5000‐km wide basin, equilibrium equatorial currents are established about 150 days after the onset of the winds. The response time of the ocean below a depth of a few hundred meters is much longer. Winds with no spatial and a simple temporal structure generate currents with a complex vert…