Hydrographic patterns and vertical mixing in the equatorial pacific along 150°W

The WEC88 cruise sampled along a meridional transect from 15°N to 15°S along 150°W from February 17 to March 18, 1988, with a 6-day time series at the equator. The large-scale hydrographic patterns were typical for boreal spring. Equatorial maxima in dissipation of turbulent kinetic energy e, and of thermal variance x, were found between 2°N and 2°S for the top 60 m. The equatorial time series coincided with a shift from southward to northward velocity, which returned the zonal current system to the equator. This led to a decrease in temperature, and increases in salinity, nutrient, and chlorophyll concentrations in the surface layer. Vertical diffusivity as well as e and x increased with the observed intensification of the Equatorial Undercurrent. Maximum values of e and x were observed at around 55 m, and the temporal trends occurred first at depth. Turbulent heat flux out of the mixed layer was the same order of magnitude as the penetrative irradiance at that depth. Maximum vertical heat flux occurred at depth in response to large diffusivity coefficients. The Richardson number was useful in predicting the regions of enhanced mixing in the meridional transect. However, for the equatorial time series, where the Ri was less than 0.45, intensity of dissipation was not proportional to Richardson number.