Winter Extreme Mixed Layer Depth South of the Kuroshio Extension

Changes in the extratropical wintertime-mean mixed layer depth (MLD) have been widely studied, because of their important implications for both physical and biogeochemical processes. With focus on the south of the Kuroshio Extension (KE) region where the deepest mixed layer is located, however, this study shows that variation of winter (March) extreme MLD is a better precursor than the time-mean MLD (or nonextreme MLD) for change in the North Pacific Subtropical Mode Water formation, based on the NCEP Climate Forecast System Reanalysis (1979-2010). Extreme MLD events are found to be associated with accumulated cooling effects of synoptic-scale (2-8 days) air-sea heat flux, which characterizes cold air outbreaks. Further analyses suggest that differences between extreme and nonextreme MLD events are closely related to differences in the cumulative synoptic heat flux anomalies, while changes in the preconditioning upper-ocean stratification contribute almost equally to both events. Relative contributions of oceanic and atmospheric forcing to the interannual variability of winter extreme MLD are quantified using the one-dimensional bulk mixed layer model. Result shows a comparable relative contribution: the preconditioning stratification changes accounts for 43.5% of the total interannual variance of extreme MLD, whereas convective mixing by surface heat flux and mechanical stirring by surface wind stress account for 35.3% and 13.1%, respectively. In addition, it is found that extreme/nonextreme MLD has been shallowed significantly over 1979-2010 in both the reanalysis and observational data, which can be explained by the stratification enhancement resulting from the intensification of the zonal heat advection by KE.