Why is the Kuroshio Current widest and fastest during summer?

The Kuroshio current is at its widest and fastest during the summer primarily because it is strengthened by monsoon winds.1

During the summer, the Asian landmass becomes significantly warmer than the surrounding sea, which creates areas of low pressure and generates monsoonal winds that blow predominantly from the southeast. These winds influence the water circulation of the Kuroshio, causing it to increase in speed and width, while also displacing its axis well into the East China Sea. In contrast, during the winter, the circulation is modified by northerly monsoon winds, which reduce the strength of the north-flowing current.1