Synoptic forcing of the Korea Strait transport

Korea Strait transport variations in the synoptic frequency band (2–20 days) are examined using results of a numerical 3-D primitive equation model, satellite observed sea-level variations, a linear barotropic adjoint dynamic model, and observed transports. The 3-D numerical model does not assimilate observations, and the agreement with the observed transport implies that wind forcing is one of the main contributors to variations in the synoptic band. The satellite-observed and 3-D model sea-level indicate a sea-level response to wind stress along the east Korean coast that propagates toward the Korea Strait and changes the sea-level slope across the strait. The adjoint results indicate that wind stress is most influential in the area east of Korea along with secondarily important area along the East China Sea shelf break south of Japan. The mechanism connecting wind stress to transport variations is a Kelvin wave propagation that changes sea-level slope across the strait, leading to the altered geostrophic transport through the strait. A strong southerly wind initially produces a sea-level set down along the east Korea coast and a sea-level increase along the shelf break. The set down propagates to the Korea Strait as a Kelvin wave, sea level across the strait changes, and the transport through the strait increases. Similarly, northerly wind stress produces a set up along the Korea coast and subsequent decreased transport. Wind stresses across the Yellow and East China Seas are not a significant forcing mechanism since Kelvin waves would propagate away from the strait. Barotropic transport response to wind stress is rapid (on the order of 3 h), but the relatively slow development of the atmospheric forcing (on the order of 1–2 days) modulates the response.