Alternate rotating layered circulation in response to extrinsic and intrinsic forcing in the Japan sea

The three-dimensional circulation in the Japan Sea (JS) plays an important role in its water mass and biogeochemical substances exchange with neighboring oceans. However, characterizing the spatiotemporal circulation pattern in the JS, and diagnosing its complex forcing mechanism between intrinsic flow-topography interaction and extrinsic flux through the straits connected with adjacent seas remain challenge. Combined observations with numerical modeling and a novel Stokes-based layer-integrated vorticity equation (LIVE) dynamics, we discovered a three-layer circulation with alternating cyclonic, anti-cyclonic, and cyclonic circulation in the upper (0–150 m), middle (150–250 m), and bottom (>250 m) layers in the JS, respectively. The strong cyclonic and weak anti-cyclonic circulations in the upper and middle layers show similar seasonal phase: the domain-integrated vorticity anomaly is positive during winter and negative from summer to early autumn. In contrast, cyclonic circulation in the bottom layer remains relatively stable throughout the year. We diagnosed that besides vorticity input from wind stress curl in the upper layer, the lateral planetary vorticity fluxes from inflow/outflow through the straits surrounding the JS lead to vortex stretching in all layers and extrinsically control the structure of the layered circulation. The joint effects of baroclinicity and relief (JEBAR) arising from flow–topography interaction is an intrinsic dynamic response to the extrinsic forcing and dynamically shapes the layered circulation. Based on Stokes circulation theorem, this study characterizes the layered circulation pattern, and based on LIVE dynamics, effectively identifies intrinsic and extrinsic forcing mechanisms for the layered circulation in the JS and other marginal seas.