To understand the formation of North Pacific subtropical mode water (STMW) in the Kuroshio recirculation gyre region, the cause of STMW thickness variation is investigated using temperature profiles in a historically archived data set. The thickness variation is predominantly controlled by the main thermocline depth (MTD). When the main thermocline deepens (shoals), the wintertime mixed layer depth can develop (not develop), and consequently, thicker (thinner) STMW is observed in summer. The large-scale atmospheric forcing controlling the MTD is explored using a wind-driven hindcast ocean model. The MTD variation stems primarily from a baroclinic response in the ocean to the Aleutian Low (AL) activity; especially, the meridional movement of the AL exerts a remarkable influence.