Upper-Ocean Heat Balance in the Kuroshio Extension Region

Abstract

A horizontally two-dimensional mixed-layer model is used to study the upper-ocean heat balance in the Kuroshio Extension region (30°?40°N, 141°?175°E). Horizontal dependency is emphasized because, in addition to vertical entrainment and surface thermal forcing, horizontal advection and eddy diffusion make substantial contributions to changes in the upper-ocean thermal structure in this region. By forcing the model using the wind and heat flux data from ECMWF and the absolute sea surface height data deduced from the Geosat ERM, the mixed-layer depth (hm) and temperature (Tm) changes in the Kuroshio Extension are hindcast for a 2.5-year period (November 1986?April 1989). Both phase and amplitude of the modeled Tm and hm variations agreed well with the climatology. The horizontal thermal patterns also agreed favorably with the available in situ SST observations, but this agreement depended crucially on the inclusion of horizontal advections. Although the annually averaged net heat flux from the atmosphere to the ocean (Qnet) is negative over the Kuroshio Extension region, the effect of the surface thermal forcing, when integrated annually, is to increase Tm because the large, negative Qnet in winter is redistributed in a much deeper mixed layer than it is in summer when Qnet > 0. This warming effect is counterbalanced by the vertical turbulent entrainment through the base of the mixed layer (35% when annually integrated), the Ekman divergence (16%), the geostrophic divergence (12%), and the horizontal eddy diffusion (35%). Though small when averaged in space and time, the temperature advection by the surface flows makes a substantial contribution to the local heat balances. While it warms the upstream region of the Kuroshio Extension (west of 150°E), the current advection tends to cool the upper ocean over the vast downstream region due to the presence of the recirculation gyre.