Abstract We investigated the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment, and warmed by the horizontal heat convergence resulting from the Kuroshio heat advection. The mixed layer depth is controlled by the entrainment and the horizontal transport divergence/convergence which 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 layer below. The mixed layer shift from its deep to shallow phase occurred in 1985. This shift is preceded the SST-shift in 1988 by three years, and was caused by the horizontal transport divergence anomaly. The horizontal heat convergence of the Kuroshio Extension caused the SST-shift in 1988, whereas 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, stronger surface heat flux and Ekman cooling and the shoaling of the mixed layer depth. It is suggested that the Kuroshio current system plays a major role in forcing the SST-shift and thus the subsequent climate regime shift.