Heat balance and regime shifts of the mixed layer in the Kuroshio Extension

We investigated the seasonal variations of the mixed layer depth and temper ature in the Kuroshio Extension region (145-180 degreesE, 30-36 degreesN), and studied the causes of the mixed layer depth and temperature 'regime shi fts' which occurred in the late 1980s, using upper ocean thermal and heat f lux datasets incorporated with a bulk mixed layer model. The mixed layer fr om fall to winter is cooled by the net surface heat flux, the Ekman transpo rt and the entrainment, and warmed by the horizontal heat convergence resul ting from the Kuroshio heat advection. The mixed layer depth is controlled by the entrainment and the horizontal transport divergence/convergence whic h act to slow the mixed layer deepening from fall to early winter and then in winter deepen the mixed layer. The entrainment velocity is significantly influenced by the temperature difference between the mixed layer and the l ayer below. The mixed layer shift from its deep to shallow phase occurred i n 1985. This shift is preceded the SST-shift in 1988 by three years, and wa s caused by the horizontal transport divergence anomaly. The horizontal hea t convergence of the Kuroshio Extension caused the SST-shift in 1988, where as its anomaly had been already positive since 1983. The delay from 1983 to 1987 can be attributed to the effects of the negative fall-SST anomaly, st ronger surface heat flux and Ekman cooling and the shoaling of the mixed la yer depth. It is suggested that the Kuroshio current system plays a major r ole in forcing the SST-shift and thus the subsequent climate regime shift. (C) 2000 Elsevier Science Ltd. All rights reserved.