Processes that influence the mixed layer deepening during winter in the North Pacific

The contribution of various forcing sources to the mixed layer depth (MLD) growth during winter in the North Pacific was investigated by analyzing the Simple Ocean Data Assimilation and NCEP/NCAR reanalysis data during 1959-2004. It was found that the MLD growth can be mainly estimated from the heat budget analysis in the mixed layer, in which both the surface heat flux and the ocean heat transport are included. The remaining difference is explained in terms of the effects of entrainment by wind stress and the error in determining MLD due to a diffused thermocline. The contribution to the heat budget from the ocean heat transport is positive in the Kuroshio region owing to geostrophic advection and eddy diffusion, but in the outer ocean it is negative north of 27 N and positive south of it, influenced by Ekman advection. It was also found that the transition to the stronger ocean heat transport and smaller MLD growth in the Kuroshio Extension region occurs in 1985.