Oceanic adjustment in the presence of mean currents on an equatorial β plane

Abstract

The oceanic adjustment to variable winds in the presence of mean currents is studied by means of a shallow water model on an equatorial β plane. The waves that are available to affect the adjustment fall into two groups: a discrete, finite or infinite number of equatorially trapped modes, and a continuum of modes that are bounded by turning latitudes and/or critical latitudes. In response to variable winds, adjustment near the equator is very similar to the adjustment in the absence of the mean currents except for modifications to the speeds and structures of the waves that are involved. Off the equator, where the continuum of modes come into play critical layer absorption is a factor provided that the waves have a sufficiently large meridional group velocity to propagate to the critical layers in a reasonable time (before they cross the ocean basin, for example). Winds that are suddenly switched on excite waves with small zonal wave numbers and small meridional group velocities. The waves therefore succeed in effecting the adjustment. In the case of periodic winds, linear critical layer absorption modifies the shadow zones (Ekman response region) and regions of Sverdrup response that characterize the response in the absence of mean currents. In response to the annual harmonic of the wind the western part of the North Equatorial Countercurrent is unlikely to by affected by Rossby waves, because that is a shadow zone. The region between 10°N and 15°N approximately, the southern flank of the North Equatorial Current, is an interesting region because it is effectively a waveguide for modes with a turning latitude to the south and a critical latitude to the north. Wave trains are likely to be evident in that region.