Meridional variability of turbulence through the equatorial undercurrent

Four meridional transects of the turbulent dissipation rate ε and of shear were taken across the equator along 140°W in late 1984. Averaging the upper 110 m of the transects, we found that the vertically averaged ε as well as shear showed maximum values within ±1° latitude. Because a strong diurnal cycle of ε aliased the measurements, we cannot extract spatial variations with good accuracy. However, when ε is separated into the contributions from the thermocline and from the surface mixed layer, consistent patterns emerge. In the thermocline, ε was strongest where the Richardson number was lowest, i.e., near the equator—consistent with the production of turbulence by the shear of the undercurrent. In the surface mixed layer near the equator, ε was usually larger than predicted by similarity scaling, but the discrepancy varied with the diurnal cycle of shear. When the mean shear was low, after prolonged mixing by nighttime convection, ε was close to the similarity scaling. We therefore attribute the excess dissipation to shear production.