Causes of Interannual Variation of Summer Mean Alongshore Current Near the East Coast of Korea Derived From 16-Year-Long Observational Data

Interannual variation in the summer surface current near the east coast of Korea was analyzed using long-term (16years) time series measurements. Significant interannual variations in the summer alongshore current, which have a magnitude comparable to that of the climatological summer mean (equatorward) current, were observed. Positive anomalies (more poleward) occurred during 2001, 2009, 2010, and 2014; negative anomalies (more equatorward) occurred during 2002, 2006, and 2012. A linear combination of equatorward propagating coastal-trapped waves, forced primarily by remote winds off the Russian coast, and an upwelling/downwelling response to local wind forcing, though contribution of the latter is not as significant as that of the former, successfully reproduced (46%) the observed anomalies in the alongshore current (correlation coefficient of 0.68). The influence of cross-shore movement of the poleward flowing offshore current explains 35% of the total variance in the interannual anomalies of the alongshore current (correlation coefficient of 0.59), yielding less equatorward or slight poleward current near the coast in 2001 and 2009 with onshore movement of the offshore current. The combined effects of wind and offshore current explain 72% (correlation coefficient of 0.85) of the anomalies in the alongshore current observed during summer near the east coast of Korea, providing significant implications for interannual current dynamics near the midlatitude western boundary. Plain Language Summary Winds blowing over the western part of the East Sea (Japan Sea) varies over time, shaping year-to-year variations of coastal current, along with meandering of offshore currents, near one midlatitude western boundary, Korean east coast. We analyzed 16-year long in situ time series data of surface currents and water properties, collected near the coast, and found wind-forced long waves propagating from Russian coast to Korean coast and meandering of offshore current are main drivers for the interannual variations of summer current. The wind-forced wave and offshore current explain most (72%) of the observed current variations in summer, providing significant implications for interannual current dynamics near the midlatitude western boundary.