The Baroclinic Stability of the Atlantic North Equatorial Current

Abstract

POLYMODE Array III, Cluster C, was a collection of four current meter moorings that were placed in the Atlantic North Equatorial Current for one year in May 1978. They were designed to look for indications of baroclinic instability such as downgradient eddy heat fluxes and upward phase propagation. However, Fu, Keffer and other investigators find that the eddy heat fluxes tended to be more across-gradient and sometimes even upgradient. They also find only slight indications of upward phase propagation. In this study, the linear stability model of Gill et al. is applied to determine the expected heat fluxes, and the modal shapes and scales due to linear amplification. It appears that the expected fluxes should be detectable. Modal shapes are similar to the observed shapes but the fastest growing mode is only about half the size of the observed correlation scale. Although these discrepancies could be explained by simple statistical variability, the supercriticality of the mean flow, the large observed length scales, and the relatively high eddy energies all suggest that this is a region of fully developed finite-amplitude flow. Recent numerical model results of Holland tend to support this hypothesis.