Heat Budget in the Kuroshio Extension Region: 1993–99

Abstract

Processes responsible for the seasonal and interannual variations of the sea surface temperature as well as of the heat content of the upper ocean (0?400 m) in the Kuroshio Extension region are examined from a 3D advection?diffusion model in finite elements, with an embedded bulk mixed layer. The geostrophic velocity is specified externally from TOPEX/Poseidon altimeter data, and Ekman velocity is specified from NCEP wind stress. The thermal field from the model shows good agreement with observations. While both atmospheric and oceanic processes are required to explain observed nonseasonal SST changes, the interannual heat storage rate is dominated by horizontal advection. In particular, the transition between an elongated and a contracted state of the Kuroshio caused by geostrophic advection has a clear signature in the SST. There is an indication that this process is accompanied by consistent changes in nonseasonal entrainment: when the Kuroshio is in an elongated state and warmer waters are present below the mixed layer, entrainment appears less efficient in exporting heat out of the mixed layer.