THE DIURNAL CYCLE OF THE UPPER EQUATORIAL OCEAN - TURBULENCE, FINE-SCALE SHEAR, AND MEAN SHEAR

Physical processes within the diurnal cycle of the upper ocean in the Pacific Equatorial Undercurrent are investigated based on observations at 0-degrees/14-degrees-W of April 1987. The cycle extended below the surface mixed layer into the pycnocline, where nighttime maxima and d aytime minima occurred in turbulent dissipation rates epsilon, fine-sc ale shear variance, and in ''large-scale,'' low-frequency zonal shear, buoyancy frequency, and gradient Froude number. The time-of-day avera ge epsilon varied by a factor of 40 at 20 m, near the base of the mixe d layer. Its night/day ratio decreased with increasing depth, the diur nal signal extending to 45-60 m, compared to nighttime mixed layer dep ths of 10-25 m. The daily variation of epsilon extended throughout the layer of high shear and high gradient Froude number Fr, above the Und ercurrent core, being cut off where the large-scale Fr dropped below 1 . The diurnal cycle of epsilon remained unaffected by changes in the l arge-scale shear and stratification. The nighttime fine-scale shear va riance 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. Fi ne-scale shear was large enough to be a factor in local shear instabil ity. A zone some 20 m thick below the mixed layer showed weak nighttim e maxima in the large-scale, low-frequency zonal shear, stratification , and Froude number. We suspect that these variations were a product o f turbulent mixing. They could become a discriminative tool in a futur e comparison of our observations with upper ocean models. The applicat ion of such models encouraged by indications of a local control of the upper ocean heat budget by the surface heat flux. In apparent respons e 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-epsilon events.