The Effects of Mean Shear Flow on Equatorial Basin Modes

Abstract

We test the sensitivity of energy trapping by equatorial basin modes to the presence of mean currents in a series of numerical experiments. At low frequency, trapping persists in the form of modified basin modes only when flow is unidirectional to the east and centered on the equator. If return flow at higher latitudes is westward, severe leakage occurs along the eastern boundary, though mean flows in wide boundary layers can recycle some of this energy back into the interior before it propagates up the coast. If the boundary layers are thin, however, or if flow along the equator is westward, variability is dominated by leaky unstable waves. By contrast, high-frequency basin modes are robust regardless of the mean flow configuration. Thus basin modes may be a more important source of oceanic variability at high frequencies than at low.