On the role of mixing in a tropical ocean general circulation model

General circulation models (GCMs) of the equatorial Pacific have some common problems. Equatorial sea surface temperatures are too low in the east and the characteristic equatorial thermocline structure (rising isotherms in the east and a deep warm pool in the west) is not simulated very well. We have investigated the effects of two types of vertical mixing processes acting in the surface layers of these models. We consider mixing due to shear instability and wind stirring. Both processes are described by means of existing parameterization relations. They are represented in terms of vertical mixing both for temperature and momentum with functional relationships for the mixing coefficients. We have tested the impact of these mixing schemes on the surface temperatures or equivalently on the surface heat fluxes, on tie equatorial thermocline and undercurrent. We find that shear instability mixing has a strong influence on the simulation of the strength and depth of the equatorial undercurrent. The presence of a wind stirring parameterization strongly reduces the error in the surface heat fluxes at the equator. An optimal equatorial GCE must include parameterizations for both types of vertical mixing. Advanced functional relations for the mixing coefficients are not superior to extremely simple ones