UPPER OCEAN DYNAMICS AND ITS ROLE IN MAINTAINING THE ANNUAL MEAN WESTERN PACIFIC WARM POOL IN A GLOBAL GCM

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 a n ocean general circulation model (GCM) combined with existing observa tions. A comparison with observations shows that the model simulates m ajor aspects of the observed upper-layer thermal structure and circula tion, 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 oce an near the Equator and less at high latitudes, which supports the pre vious inference that fluxes in the region are overestimated in most cl imatologies. The annual mean surface heat budget of the model averaged over a large region (20 degrees S-20 degrees N and 110 degrees E-160 degrees E) indicates that heat is generally transported downward to th e deeper levels by vertical motion and mixing, which agrees with earli er studies. However, close inspection of six subregions within the lar ge region indicates that different mechanisms are balancing the surfac e heat budget in different subregions. Horizontal advection is importa nt in some subregions. Upper-layer convergence induced by the equatorw ard western boundary currents in the region of the North Equatorial Co untercurrent (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 degrees C) water of the global o ceans exists in the tropical western Pacific and an independent eviden ce 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 s horter by a factor of about two than Wyrtki's estimate of 1.3 years. ( C) 1997 by the Royal Meteorological Society.