Variability of the Tropical Atlantic Ocean Simulated by a General Circulation Model with Two Different Mixed-Layer Physics

Abstract

The embedment of a 1.5 turbulence closure model in an ocean general circulation model of the equatorial Atlantic is presented. The eddy viscosity and diffusivity involved in the vertical mixing are defined as the product of a characteristic turbulent velocity—the root square of the turbulent kinetic energy—and a characteristic mixing length. The turbulent kinetic energy is defined through a prognostic equation while the turbulent length scales are defined by a diagnostic formulation. The results of an experiment that includes this closure scheme are compared to the results issued from another experiment that includes a Richardson number-dependent parameterization of the mixing coefficients. The two simulations were performed over the tropical Atlantic during the 1982–1984 period, which allows direct comparisons with data from the FOCAL and SEQUAL experiments. Obvious contrasts between the two experiments on the sea surface temperature and on the dynamics indicate that the turbulent vertical diffusion plays a major role in the surface processes simulated by the model. Comparisons with available observations show that the introduction of the 1.5 closure scheme improves the ability of the general circulation model to represent the sea surface temperature, the vertical mixed-layer structure, the equatorial meridional circulation cell, as well as the equatorial undercurrent, which becomes more energetic. Despite strong assumptions in the turbulent vertical mixing scheme, the turbulent fields provided by the turbulent kinetic parameterization allow a comparison with direct measurements of turbulence performed in the tropical oceans and highlight the complex behavior of turbulent mixing in the ocean.