LARGE-EDDY SIMULATION OF THE EQUATORIAL OCEAN BOUNDARY-LAYER - DIURNAL CYCLING, EDDY VISCOSITY, AND HORIZONTAL ROTATION

Oceanic boundary layers (OBL) at the equator are studied using a large -eddy simulation (LES) model. The model has no equatorial undercurrent , and the;vertical component of Earth's rotation is zero Omega(z) = 0. It is forced with a constant westward-zonal wind stress and with a co nstant surface heat flux and, in some cases,a diurnal cycle of solar h eating. Averaged over day 4, temperature and velocity are vertically m ixed deeper for the diurnal cycling case, the surface temperature is c ooler by 0.04 degrees C, and the surface velocity is weaker by 0.08 m/ s, compared to a constant surface heating case with the same daily ave raged surface heat flux. There is no general relationship between eddy viscosity (or diffusivity) and the gradient Richardson number, especi ally for the case of diurnal cycling, where nonlocal transports can le ad to countergradient fluxes during nighttime convection. Sharp gradie nts of eddy viscosity are found near the critical Richardson number, R (i) = 0.25, and in the lower part of the entrainment layer where R(i) approximate to 1.0. The effect of the horizontal component of Earth's rotation Omega(y), on an entraining equatorial OBL is also investigate d. It is found in our entraining cases that with and without Omega(y), mean quantities (temperature, velocity) and turbulent fluxes differ n o more than a few percent, in accord with the second-order closure res ults of Galperin et al. [1989] and Kantha et al. [1989]. The LES resul ts are used to extend the bulk mixed layer theory of Garwood et al. [1 985a, b] to the entraining case. With no entrainment, Garwood et al. [ 1985a, b] show that Omega(y), effects can be substantial. The effects of Omega(y) are small when the entrainment heat flux is significant.