The diurnal cycle of the upper equatorial ocean: Turbulence, fine‐scale shear, and mean shear

Abstract

Physical processes within the diurnal cycle of the upper ocean in the Pacific Equatorial Undercurrent are investigated based on observations at 0°/140°W of April 1987. The cycle extended below the surface mixed layer into the pycnocline, where nighttime maxima and daytime minima occurred in turbulent dissipation rates ε, fine‐scale shear variance, and in “large‐scale,” low‐frequency zonal shear, buoyancy frequency, and gradient Froude number. The time‐of‐day average e varied by a factor of 40 at 20 m, near the base of the mixed layer. Its night/day ratio decreased with increasing depth, the diurnal signal extending to 45–60 m, compared to nighttime mixed layer depths of 10–25 m. The daily variation of ε extended throughout the layer of high shear and high gradient Froude number Fr, above the Undercurrent core, being cut off where the large‐scale Fr dropped below 1. The diurnal cycle of ε remained unaffected by changes in the large‐scale shear and stratification. The nighttime fine‐scale shear variance in the 5–10 m vertical wavelength band exceeded daytime minima by a factor of 2 on average, the daily variation decaying rapidly with increasing depth. Turbulent overturns were smaller than 5 m, so that we suspect that the fine‐scale shear was related to internal waves. Fine‐scale shear was large enough to be a factor in local shear instability. A zone some 20 m thick below the mixed layer showed weak nighttime maxima in the large‐scale, low‐frequency zonal shear, stratification, and Froude number. We suspect that these variations were a product of turbulent mixing. They could become a discriminative tool in a future comparison of our observations with upper ocean models. The application of such models is encouraged by indications of a local control of the upper ocean heat budget by the surface heat flux. In apparent response to a weaker Undercurrent, stronger stratification, and weaker trade winds, mean and nighttime dissipation rates were lower in 1987 than in November 1984. The night/day ratio, however, was larger in 1987 owing to the absence of daytime high‐ε events.