SENSITIVITY OF AN EQUATORIAL PACIFIC OGCM TO THE LATERAL DIFFUSION

An OGCM is used to investigate the importance of lateral mixing in the tropical Pacific Ocean circulation. Horizontal subgrid-scale physics is parameterized by the usual Laplacian operator. Three simulations ar e performed using three different orders of magnitude for lateral eddy viscosity and diffusivity coefficients: 10(4), 10(3), and 10(2) m(2) s(-1). The upper layer response is found sensitive to lateral diffusio n as well as the rest of the general circulation. Decreasing lateral m ixing coefficients raises the mean kinetic energy level and the input of energy by the wind, and enhances the vertical dissipation. This wea kens the equatorial meridional cell and induces a reduction of 20 Sv i n tile transport of the equatorial upwelling. These results are due to the nonlinear interplay between horizontal and vertical diffusion. Th e nature of the Equatorial Undercurrent (EUC) is found particularly se nsitive to the relative importance of the diffusive conditions. Latera l mixing dominates the different regimes of the EUC when the strongest diffusion coefficient is used. Under these conditions, the EUC dynami cs is similar to a boundary layer regime where strong meridian and ver tical circulation insulates the equatorial dynamics. Conversely, a wea kness of horizontal diffusion leads to enhancement of the vertical dif fusion role, even in the core of the EUC. In the eastern inertial regi me, vertical diffusion can replace horizontal diffusion when dissipati on is needed. While the dynamics is severely altered between the simul ations, the SST pattern errors with the climatology are found quite si milar, in conflict with the results reported by Stockdale et al. Never theless, at the basin scale, lateral mixing conditions affect the meri dian heat transport. Similar diffusive and advective heat transport am plitudes are found with the strongest lateral coefficient, while advec tive meridian hear transport dominates in both other experiments. More over, the different terms of the surface heat budget are found sensiti ve to the lateral diffusion: when lateral diffusion coefficient is suf ficiently low, the balance implies the heating by the transient heat t ransport and the cooling bg bath monthly mean advection and vertical d iffusion. Indeed, in the equatorial cold tongue, vertical diffusion is large enough to distribute heat down to the thermocline below the mix ed layer.