The Seasonal Cycle of Meridional Heat Transport in a Numerical Model of the Pacific Equatorial Upwelling Zone

Abstract

The seasonal heat transport mechanisms important in the Pacific equatorial upwelling zone are investigated using the primitive equation, reduced gravity model developed by Gent and Cane. Mechanisms of meridional heat transport are shown and discussed with respect to the heat budget of a box about the equator containing the upwelling. There is a horizontal cell in which warm water enters the upwelling box in the west in strong equatorward currents located near the, western boundary, which feed the eastward flowing undercurrent. To compensate, water leaves the section as a colder and weaker poleward thermocline flow in the eastern basin. The meridional-vertical cell comprises additional equatorward geostrophically balanced inflow in the upper thermocline, which is compensated by the warmer poleward outflow by Ekman divergence in the surface layer. In the annual mean, the magnitude of the net heat exported by the meridional-vertical cell exceeds the net heat import due to the gyre exchange so that the net heat transport is poleward. This annual mean net heat export is compensated by the surface heat flux. The transient eddy heat transport is equatorward and much smaller. It is noted that in the winter seasons, boreal December?February and austral June?August, a large amount of heat is lost by a net excess of heat transport by meridional overturning. In the transition seasons, March?May and September?November, there is an equatorward heat transport anomaly in the upwelling box, either related to an excess of heat equatorward transport by gyre exchange in March?May or a reduction in the poleward heat transport by meridional overturning in September?November. March-May is the season during which the undercurrent has its maximum transport, and the strength of the gyre exchange is largest. During September?November, the season of strongest zonal wind when maximum overturning transport is expected, the poleward heat transport by meridional overturning is a minimum. This is partly because the temperature difference between the divergent surface water and convergent subsurface water is smallest in this season, which is the season of lowest SST in the cold tongue and the shallowest and warmest subsurface flow. Seasonally, the variations in the surface heat flux are much smaller than the variations in the heat transport. Thus, the seasonal heat content changes are compensated by the heat transport anomalies.