Modeling the tropical ocean response to westerly wind forcing

Abstract

A primitive equation ocean general circulation model, with mixed layer physics, has been developed and applied to an investigation of the equatorial ocean. The major physical problem addressed with this model is the response of the upper ocean to westerly wind events, such as those that occur during tropical cyclones and during El Nino events. In the model development phase, several configurations of the mixed layer physics, domain size, and wind stress were tested. The best overall simulations were produced when both Richardson number dependent mixing and a bulk mixed layer model were included. Small model domains were found to be especially sensitive to the prescribed lateral boundary conditions and wind stress. Smaller domains required more realistic wind stress fields in order to achieve reasonable current structures. In addition, the off-equatorial currents were particularly responsive to changes in the zonal gradient of the wind stress. The final configuration of the model produces realistic simulations of climatological threedimensional temperature and current structures in the equatorial ocean. In the model application phase, synoptically varying winds for the tropical Pacific were used to force the model. The winds came from the Navy's Operational Global Atmospheric Prediction System's (NOGAPS) daily analyses for the 1991-92 El Nino, which included several strong westerly wind events. The strong temporal and spatial variability in these winds produced complex fluctuations of the model's temperatures, currents, and internal waves, including reversals of the South Equatorial Current and equatorially trapped Kelvin waves. Model verification was performed by comparison with an observational study of in situ equatorial Pacific buoy data. This comparison showed that synoptic scale variations in the wind stress are needed to simulate the ocean's strong responses to westerly wind events.