Residual Overturning Circulation and Associated Water-Mass Transformation in the East/Japan Sea

Abstract The East/Japan Sea (EJS), a marginal sea in the northwest Pacific, has marked isopycnal slopes, especially during winter, implying a store of potential energy available for baroclinic instability and hence, perhaps, a significant role for eddy processes in shaping the residual (mean plus eddy) overturning circulation. Here, for the first time, we compute the residual overturning circulation in the EJS in a high-resolution simulation and relate it to water-mass transformation processes due to air–sea fluxes and interior mixing. A sizable eddy-driven circulation is indeed found in the vicinity of tilted isopycnals. Wintertime surface buoyancy loss facilitates a volume flux toward higher-density classes, with latent heat loss being the main contributor and sensible heat loss also playing a role. The densification of northward subsurface flow is associated with diapycnal mixing. The water-mass formation rate, derived from the transformation rate, identifies upwelling and downwelling to the south and north of the subpolar front near 39.5°N, respectively, consistent with the broad pattern of residual overturning circulation. Significance Statement In the East/Japan Sea (EJS), we computed the residual overturning circulation for the first time and linked it to water-mass transformation through the air–sea fluxes and interior mixing. We found a significant eddy-driven circulation with tilted isopycnals, especially during winter. The residual overturning circulation orchestrates with the surface buoyancy flux. In particular, the wintertime buoyancy loss, mainly driven by latent heat, relates the volume flux to higher-density classes. Furthermore, we identify upwelling and downwelling patterns around the subpolar front based on the water-mass formation rate, which is consistent with the overall residual overturning circulation. These findings are crucial for understanding the role of eddies and air–sea buoyancy fluxes in the EJS circulation and provide a guide to the projected changes in the ocean circulation under a warming climate.