Is the Cromwell current driven by equatorial Rossby waves?

Abstract

A second-order theory for equatorial, baroclinic Rossby waves in a bounded ocean yields, among other components, terms of zero frequency. We inquire whether the associated rectified flow (or ‘streaming’, or ‘mass transport’) can be related to the submerged equatorial jet observed by Cromwell, Knauss and others. For quite reasonable models of baroclinic Rossby waves, the resultant streaming can be sharply concentrated at the equator, and varies in depth as N(z), thus giving maximum flow at the thermocline (where the Väisälä frequency N(z) is largest). In this way the rectification hypothesis might account for the observed submerged equatorial steering of the Cromwell current. The observed direction of streaming calls for a predominance of equatorially symmetric Rossby waves. The observed magnitude implies that the r.m.s. fluctuations be of the same order as the mean current; that is to say, we have applied weakinteraction theory to a strong-interaction problem. There are other difficulties.