Numerical Simulation of Equatorial Ocean Circulation. Part I: A Basic Case in Turbulent Equilibrium

Abstract

An equatorial ocean experiment has been carried out, using the primitive equation model of Semtner and Mintz (1977) with a highly conservative differencing scheme, with high horizontal resolution (?x = 0.50°, ?y=0.25°) and with 14 levels in the vertical. A turbulent equilibrium state has been reached for a 3300 km ? 2200 km equatorial ocean, driven by constant 0.5 dyn cm?2 wind stress, heated at the surface and cooled at the northern and southern walls. The predicted surface temperature field shows an upwelling-induced cold region along the equator. The temperatures at the equator near the eastern wall are as much as 6°C colder than in the subequatorial regions. Westward moving waves occur in the temperature field a few degrees north and south of the equator. These waves have periods of 33 days, wavelengths of 800 km, and are symmetric about the equator. Their structure is similar to that of equatorially trapped Rossby waves with n=1 in the vertical and m=1 in the horizontal. Shorter wavelength disturbances are found throughout the thermocline near the equator, and these have periods typical of equatorially trapped inertia-gravity waves. The horizontal temperature field at depth suggests that a number of high baroclinic modes are superposed. The surface flow in the model is characterized by Ekman drift plus transient geostrophic flow off the equator and by weak and variable flow at the equator. A pressure gradient due to the tilt of the sea surface along the equator largely balances the wind stress on the surface layer. Below the surface, this pressure gradient drives an equatorial undercurrent, which slopes upward to the east and intensifies to a maximum of about 100 cm s?1. The undercurrent meanders, with periods of 100 days or more, by as much as 100 km on either side of the equator. Below the current, westward moving cross-equatorial flows with periods of about 44 days sometimes link up the quasi-geostrophic circulations on opposite sides of the equator. These flows appear to be associated with an antisymmetric (in u and p) Rossby wave of the same period having m = 2. An analysis of energetics shows that the disturbances on either side of the equator are maintained by baroclinic instability, whereas the equatorial undercurrent exhibits mainly barotropic instability. These instabilities lead to transient circulations whose characteristics are similar to those of equatorially trapped neutral waves. Frictional dissipation is concentrated at the equator, and most of the loss of energy is from the eddy circulations rather than from the mean flow.