Improvements to the equatorial Pacific cold tongue in an OGCM: possible implications for the NCEP GODAS and CFS

Kris Karnauskas

A reduced gravity OGCM of the tropical Pacific Ocean is used to determine the improvements to the simulated equatorial Pacific cold tongue region through choices in horizontal resolution and coastline geometry, particularly the Galápagos Islands. Four simulations are performed, forced by identical climatological forcing. Results are compared between model grids with and without the Galápagos Islands, with coarse and fine resolutions. It is found that a more realistic treatment of the Galápagos Islands results in a complete termination of the Equatorial Undercurrent (EUC), which leads to improvements in the simulated spatial structure of the cold tongue, in addition to a basin-wide warming of up to 2°C in the central Pacific. The termination of the EUC is directly responsible for the improvements east of the Galápagos Islands, and for the basin-wide reduction of the tropical cold bias through dynamical and surface heat flux adjustments.

It is thought that such an improvement will have a considerable impact on the ability of coupled ocean-atmosphere and ocean-ecosystem models to produce realistic clouds, precipitation, and biological activity in the tropical Pacific Ocean. It is clear that the effects of the Galápagos Islands on the Pacific Ocean beginning with equatorial currents cannot be ignored on the annualmean. The next logical step is to exploit these effects on interannual variability and the implications for the predictability of the couple ocean-atmosphere system. In particular, the potential benefits to the predictive skill of, for example, future iterations of the NCEP GODAS and CFS by implementing the Galápagos Islands at sufficient resolution should be given serious consideration.