Seasonal Variations Of The SST Fronts In The Kuroshio And Kuroshio Extension And Their Causes

The seasonal variations of the sea surface temperature (SST) fronts in the East China Sea (ECS), Kuroshio and Kuroshio extension east of Japan (EJ), and their causes are investigated based on a suite of high-resolution oceanic and atmospheric reanalysis datasets. The main results are as follows:(1) Both the ECS Kuroshio front and the EJ Kuroshio and Kuroshio extension fronts, show significant seasonal variations, while their extreme values appear not exactly the same. The ECS Kuroshio SST front increases to the strongest in spring and recedes to the weakest in autumn, while the EJ Kuroshio and Kuroshio extension SST fronts achieve the maximum intensity in late winter and the minimum in late summer. On the time scale, the extremal intensity of the Kuroshio extension fronts appears a little earlier than the Kuroshio front. Therefore, the oceanic dynamics and thermodynamics together result in the seasonal variations of the SST fronts in the East China Sea and the sea area east of Japan, in which the horizontal advection and sea surface net heat flux play a major role, and the impact of vertical temperature advection is so small that can be negligible.(2) By diagnosing the thermodynamic equation in the mixed layer, it is found that the horizontal temperature advection increases the temperature gradient in the front zone, leading the ECS front to be strongest in spring and the EJ fronts to be strongest from January to February. The interface net heat flux has different influence on the Kuroshio SST fronts in the ECS and the EJ sea. It acts to enhance the SST front in autumn and winter, and restrains the front in spring and summer in the ECS. Differently, the net heat flux mainly plays a negative role on the EJ Kuroshio and Kuroshio extension SST fronts and its inhibition effect is significant in summer. In other seasons, the influence of the net heat flux is comparatively slight and varies according to the position. Moreover, the effect of the vertical temperature advection is relatively small and negligible. It dampens the SST front in the ECS, but affects not exactly the same in the EJ sea area. In a word, the joint action of the horizontal temperature advection and the sea surface net heat flux makes the ECS Kuroshio front to be strongest in spring, while the net heat flux and the vertical temperature advection together lead to the ECS front’s weakness and disappearance. As for the EJ Kuroshio and Kuroshio extension fronts, the horizontal temperature advection makes a major promotion to their strongest intensity in January and February, while the interface heat flux is the dominant reason for their weakest in late summer and early autumn.