Sensitivity of an Equatorial Pacific OGCM to the Lateral Diffusion

Abstract

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 are performed using three different orders of magnitude for lateral eddy viscosity and diffusivity coefficients: 104, 103, and 102 m2 s−1. The upper layer response is found sensitive to lateral diffusion as well as the rest of the general circulation. Decreasing lateral mixing coefficients raises the mean kinetic energy level and the input of energy by the wind, and enhances the vertical dissipation. This weakens the equatorial meridional cell and induces a reduction of 20 Sv in the transport of the equatorial upwelling. These results are due to the nonlinear interplay between horizontal and vertical diffusion. The nature of the Equatorial Undercurrent (EUC) is found particularly sensitive to the relative importance of the diffusive conditions. Lateral mixing dominates the different regimes of the EUC when the strongest diffusion coefficient is used. Under these conditions, the EUC dynamics is similar to a boundary layer regime where strong meridian and vertical circulation insulates the equatorial dynamics. Conversely, a weakness of horizontal diffusion leads to enhancement of the vertical diffusion role, even in the core of the EUC. In the eastern inertial regime, vertical diffusion can replace horizontal diffusion when dissipation is needed. While the dynamics is severely altered between the simulations, the SST pattern errors with the climatology are found quite similar, in conflict with the results reported by Nevertheless, at the basin scale, lateral mixing conditions affect the meridian heat transport. Similar diffusive and advective heat transport amplitudes are found with the strongest lateral coefficient, while advective meridian heat transport dominates in both other experiments. Moreover, the different terms of the surface heat budget are found sensitive to the lateral diffusion: when lateral diffusion coefficient is sufficiently low, the balance implies the heating by the transient heat transport and the cooling by both monthly mean advection and vertical diffusion. Indeed, in the equatorial cold tongue, vertical diffusion is large enough to distribute heat down to the thermocline below the mixed layer.