Heat budgets and diurnal mixing in the western equatorial Pacific Ocean

Abstract

This thesis presents a study of upper ocean processes effecting the heat content and sea surface temperature of the western equatorial Pacific Ocean. The study uses high resolution ship-borne measurements of upper ocean hydrography and current structure, and simultaneous measurements of air/sea fluxes of heat, moisture and momentum. Two periods are investigated, (1) April 1988 and, (2) December 1992- February 1993. Methods of analysis are heat budget estimation of the two data sets and one dimensional numerical mixing model experiments. The second heat budget shows that over the period December 1992-February 1993 the heat balance is between the net surface heat flux and the local change in heat content of the upper 40 m to within 10 W m~2. Exceptions to this local balance are of short duration, order(days), and of large amplitude, order(100 W m~2). During April 1988 the local change in heat content is found to be dominated by horizontal advection down to 200 m. The net surface heat flux is found to be balanced by an estimate of vertical turbulent diffusion of heat to within 10%. Evidence is presented, using a combination of observations and numerical simulations, showing sub- surface density structure to interact with the mixed layer in such a way as to effect the sea surface temperature. One dimensional mixing models are shown to be able to simulate the observed diurnal cycle of mixing in the upper ocean. Sensitivity studies show the diurnal momentum cycle to be sensitive to vertical velocity shear and the value of the Coriolis parameter, /, and relatively insensitive to the zonal pressure gradient. The diurnal thermal cycle is shown to be sensitive to short wave radiation, and to a lesser extent latent heat loss. Mixing models are used to investigate the effects of precipitation on the sea surface temperature. Rain is shown to depress the sea surface temperature by up to 0.3 degree C immediately following a rain event, but after a period of weeks there is an increase in sea surface temperature, caused by local barrier layer formation. For a simulation of the period December 1992-February 1993 the increase is small (0.05 degree C). Implications of the work for general circulation modelling are discussed.

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