FY 1987

Fine-scale variability at 140°W in the equatorial Pacific

Chereskin, T.K., J.N. Moum, P.J. Stabeno, D.R. Caldwell, C.A. Paulson, L.A. Regier, and D. Halpern

J. Geophys. Res., 91(C11), doi: 10.1029/JC091iC11p12887, 12,887–12,897 (1986)

In November-December 1984 we carried out an intensive 12-day upper ocean sampling program on the equator at 140°W as part of the Tropic Heat Experiment. From our observations we constructed hourly averaged profiles of temperature, salinity, s_t, turbulent kinetic energy dissipation rate, and horizontal velocity. These data were used to examine the correspondence between hydrographic and velocity fields and to compare the measured turbulent dissipations with the calculated Richardson numbers. We found that the core of the Equatorial Undercurrent tracked a density surface (σ_t = 25.25) on times as short as 1 hour. The variability in both hydrographic and velocity fields was greatest at the semidiurnal frequency. The supertidal energy was not significantly different from the Garrett-Munk mid-latitude level once latitudinal scaling was removed from the Garrett-Munk model parameters. Horizontal velocity spectra were found to be contaminated by displacement of the background shear. Turbulent dissipation was dominated by a diurnal cycle, with high values of dissipation occurring at night above the undercurrent core. Shear and buoyancy frequency, calculated over 12-m vertical scales, were observed to track each other above the core and were dominated by a diurnal period above 40 m and by a semidiurnal period below 40 m. When shear and buoyancy frequency were combined to form a Richardson number, neither diurnal nor semidiurnal cycles were present. Above the undercurrent core, the Richardson numbers were uniformly small (0.3 to 0.6).