UPPER-OCEAN HEAT-BALANCE IN THE KUROSHIO EXTENSION REGION

A horizontally two-dimensional mixed-layer model is used to study the upper-ocean heat balance in the Kuroshio Extension region (30-degrees- 40-degrees-N, 141-degrees-175-degrees-E). Horizontal dependency is emp hasized because, in addition to vertical entrainment and surface therm al forcing, horizontal advection and eddy diffusion make substantial c ontributions to changes in the upper-ocean thermal structure in this r egion. By forcing the model using the wind and heat flux data from ECM WF and the absolute sea surface height data deduced from the Geosat ER M, the mixed-layer depth (h(m)) and temperature (T(m)) changes in the Kuroshio Extension are hindcast for a 2.5-year period (November 1986-A pril 1989). Both phase and amplitude of the modeled T(m) and h(m) vari ations agreed well with the climatology. The horizontal thermal patter ns also agreed favorably with the available in situ SST observations, but this agreement depended crucially on the inclusion of horizontal a dvections. Although the annually averaged net heat flux from the atmos phere to the ocean (Q(net)) is negative over the Kuroshio Extension re gion, the effect of the surface thermal forcing, when integrated annua lly, is to increase T(m) because the large, negative Q(net) in winter is redistributed in a much deeper mixed layer than it is in summer whe n Q(net) > 0. This warming effect is counterbalanced by the vertical t urbulent entrainment through the base of the mixed layer (35% when ann ually integrated), the Ekman divergence (16%), the geostrophic diverge nce (12%), and the horizontal eddy diffusion (35%). Though small when averaged in space and time, the temperature advection by the surface f lows makes a substantial contribution to the local heat balances. Whil e it warms the upstream region of the Kuroshio Extension (west of 150- degrees-E), the current advection tends to cool the upper ocean over t he vast downstream region due to the presence of the recirculation gyr e.