The Effects of Resolution and Viscosity in an Isopycnic-Coordinate Model of the Equatorial Pacific

Abstract

The circulation of the equatorial Pacific is simulated with an implementation of the Miami Isopycnic-Coordinate Ocean Model in a Pacific basin extending from 40°N to 40°S. At a horizontal resolution of 1.25° the annual mean temperature structure on the equator in the model is close to the climatology, with a realistically sharp thermocline, but the speed and mass transport of the Equatorial Undercurrent (EUC) are only 50%?60% of those observed. Decreasing the Smagorinsky shear-dependent viscosity coefficient at this resolution does not increase the EUC speed or transport, but instead increases the grid-scale noise in the eastern part of the undercurrent. Sensitivity studies are carried out in which the resolution is increased in stages up to 0.3125° and the model's two viscosity parameters are reduced. Once the resolution is higher than 1°, the EUC becomes narrower, faster, and stronger as the viscosity is reduced, with the transport increasing more strongly with the decreasing viscosity than with increasing resolution. At 0.3125° resolution the width, depth, and speed of the EUC are simulated realistically, as are the South Equatorial Current and the North Equatorial Countercurrent. In particular, the EUC attains a realistic maximum speed of over 1 m s?1, and its mass transport rises to 33 Sv, consistent with observations. The temperature on the model layers and the SST differences from climatology are relatively insensitive to the viscosity and resolution, the former being tentatively ascribed to the near constancy of potential temperature on the isopycnal layers. However, temperature differences on levels do occur, and are attributed to vertical migration of the isopycnal layer interfaces. The effects on the SST and the EUC transport of using a wind climatology with stronger zonal wind stress in the Tropics are examined, as are the consequences of placing either a zonal or a meridional velocity point on the equator. Overall, the work shows that a realistic circulation in the equatorial Pacific may be simulated in a numerical model without diapycnal momentum diffusion below the base of the mixed layer, in accord with observations which suggest that the EUC is largely adiabatic.