A Dissertation Submitted to the Graduate Division of the University of Hawaii in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Oceanography

This study investigates the sensitivity of the dynamics of the equatorial ocean to the parameterization of vertical mixing and focuses on the circulation in the meridionalvertical plane driven by easterly winds: the Yoshida jet. A new high resolution, numerical model of a zonally independent, equatorial channel helps explore this question and includes three parameterizations, all of which increase mixing for decreasing Richardson numbers. It compares the smooth increase of eddy coefficients traditionally used in general circulation models, the dramatic increase of the eddy coefficients for small Richardson numbers recently observed in the equatorial Pacific, and a combination of a mixing mechanism based on the diagnostic adjustment of the water column to noncritical Richardson numbers and of a bulk mixed-layer model. The main numerical result is that the meridional and vertical velocity fields in the surface layer are very sensitive to the strength of mixing implied by the different parameterizations. For the smooth Richardson number dependence of the eddy coefficients. equatorial upwelling due to easterly winds reaches the surface. The dramatically increasing eddy coefficients for small Richardson numbers yield reduced equatorial upwelling rates in the surface layer. The diagnostic adjustment of the Richardson number shows downwelling in response to easterly winds! A simple model explains the wind drift at the equator in the presence of a meridional density gradient and reproduces this reversal of the meridional and vertical flows. The density gradient causes a vertically dependent pressure gradient force which is partially balanced by the meridional component of the Coriolis force, indicating that