Upper ocean dynamics and its role in maintaining the annual mean western Pacific warm pool in a global GCM

Abstract

This study provides a description of the large-scale aspects of upper ocean dynamics and its role in balancing the annual mean surface heat budget in the tropical western Pacific Ocean, using the results from an ocean general circulation model (GCM) combined with existing observations. A comparison with observations shows that the model simulates major aspects of the observed upper-layer thermal structure and circulation, and it has a reasonable representation of net surface heat flux. The heat flux in the model is of the order of 10 W m−2 into the ocean near the Equator and less at high latitudes, which supports the previous inference that fluxes in the region are overestimated in most climatologies. The annual mean surface heat budget of the model averaged over a large region (20°S–20°N and 110°E–160°E) indicates that heat is generally transported downward to the deeper levels by vertical motion and mixing, which agrees with earlier studies. However, close inspection of six subregions within the large region indicates that different mechanisms are balancing the surface heat budget in different subregions. Horizontal advection is important in some subregions. Upper-layer convergence induced by the equatorward western boundary currents in the region of the North Equatorial Countercurrent (NECC) is equivalent to a surface heat flux of 17 W m−2 into the ocean, about 5 W m−2 larger than the net exchange of heat between ocean and atmosphere in the model. This provides a reasonable explanation for why the warmest (>28°C) water of the global oceans exists in the tropical western Pacific and an independent evidence for Wyrtki’s hypothesis of accumulation of heat in the region. The residence time of the warm pool water is about 8 months in the model, shorter by a factor of about two than Wyrtki’s estimate of 1·3 years. ©1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 711–724 (1997) (No. of Figures: 9. No. of Tables: 1. No. of Refs: 40.)