Frontogenesis and frontolysis of the subpolar front in the surface mixed layer of the Japan Sea

The frontogenesis and frontolysis processes of the subpolar front (SPF) in the surface mixed layer of the Japan Sea are investigated using state-of-the-art oceanic reanalysis data. The SPF experiences a 9 month weakening period from January to September, which shifts to a strengthening period in October. Our analysis shows that horizontal advection consistently contributes to the intensification of the SPF. After September, as the weakening effect of surface heat flux diminishes, horizontal advection becomes the dominant factor that contributes to changes in the SPF strength. Thus, the SPF enters a 3 month strengthening period. The geostrophic component of horizontal advection provides the most important contribution to strengthening the SPF, acting to intensify the SPF year-round. Ekman advection also promotes SPF strengthening with a smaller but still important contribution. During the weakening period, SPF strength is largely controlled by heat flux. The heat flux, especially the shortwave radiation component, is the primary cause of the surface front disappearance in the summer.