Progress in the 3-D circulation of the eastern equatorial Pacific in a climate ocean model

The Pacific Equatorial Undercurrent (EUC) displays important seasonal, interannual and decadal variability. The correct representation of the EUC in ocean general circulation models (OGCM) is of primary importance for an adequate simulation of the Tropical Pacific Ocean, and for the ENSO (El Nino Southern Oscillation) cycle. This study aims at better understanding the EUC termination and its interactions with the three-dimensional circulation in the eastern Pacific in a climate-type OGCM. It also aims at improving the realism of the EUC in this region where some deficiencies were previously noticed. Several sensitivity experiments have been performed from 1993 to 2000, and the model results are compared to TAO observations. The sensitivity to vertical and lateral eddy mixing coefficients is explored, and we find that the 3-D equatorial circulation is highly sensitive to a reduction of the background vertical eddy coefficients in the surface layers. This reduction is only possible because of the change in the advection scheme and leads to a great improvement in the annual mean and the seasonal cycle of the modelled EUC, increasing both the EUC and the tropical cells transports. However, it appears that in the eastern Pacific, the increase in EUC transport is largely independent of the increase in the 3-D circulation. This is also true for the seasonal cycle transport. Reducing the isopycnal mixing coefficients further improves the mean EUC speed, but does not modify the associated 3-D circulation. Adding the Galapagos Archipelago to the model bathymetry only induces very local changes in the equatorial 3-D circulation. However, it leads to important changes in the pathways of EUC waters that reach the South American coast.