Change in the extratropical wintertime-mean mixed layer has been widely studied, given its importance to both physical and biogeochemical processes. With focus on the south of the Kuroshio Extension region where the mixed layer is deepest in March, this study shows that variation of the synoptic-scale extreme mixed layer depth (MLD) is a better precursor than the monthly mean (or nonextreme) MLD for change in the subtropical mode water formation in spring, based on the NCEP Climate Forecast System Reanalysis (1979-2010). It is found that the extreme MLD events are attributable to the accumulation of excessive surface cooling driven by the synoptic storms that characterize cold air outbreaks. Particularly, difference between the extreme and nonextreme MLD is primarily related to difference in the cumulative synoptic heat flux anomalies, while change in the preconditioning upper-ocean stratification contributes almost equally to both cases. Relative contributions of oceanic and atmospheric forcing to the interannual variation of the extreme MLD are quantified using a bulk mixed layer model. Result shows comparable contributions: the preconditioning stratification change accounts for ∼44% of total variance of the extreme MLD, whereas the convective mixing by surface heat flux and the mechanical stirring by wind stress account for ∼35% and ∼13%, respectively. In addition, both the reanalysis and observational data reveal that the extreme/nonextreme MLD has been shallowed significantly during 1979-2010, which is accounted for by the strengthened stratification due to the enhanced ocean surface warming by the Kuroshio heat transport.