A study of the seasonal cycle of sea surface temperature in the tropical Pacific Ocean using reduced gravity models

Abstract

A simple model of the upper ocean is used to simulate the seasonal cycle of sea surface temperature (SST) in the tropical Pacific Ocean. The model is an extension of the conventional 1½‐layer reduced gravity system that includes the physics of the surface mixed layer and allows prediction of the sea surface temperature. Two types of mixed layer models, a constant depth mixed layer model of Cane and Zebiak and a variable depth mixed layer model of Kraus and Turner, are examined. The numerical simulations indicate that the simple models are capable of capturing the essential seasonal SST variability in the tropical Pacific when forced with climatological surface forcing. The sensitivity study of model SST response to changes in the surface forcing demonstrates that although the response to surface heat flux forcing dominates the seasonal cycle of SST in most areas of the tropical Pacific, the dynamical response of the ocean to changes in the wind also makes a significant contribution to the seasonal variation of SST in the eastern equatorial Pacific. The seasonal variation of the winds is particularly important in maintaining the phase structure of the annual SST signal along the equator. Moreover, the sensitivity of the SST response depends on the form of vertical entrainment parameterization in the model. It is shown that the Cane‐Zebiak type of model has a bias toward the dynamical response of the ocean to the winds, whereas the Kraus‐Turner type of model has bias toward the surface heat flux variations. The simple model results are also compared with those from an ocean general circulation model.