Seasonal Variation of Semidiurnal Internal Tides and Their Impact on Shelf-Slope Water Exchanges in the Southwestern East/Japan Sea

Abstract:

Sudden environmental changes along the southeast coast of Korea due to the appearance of cold water in summer, sometimes less than 10^oC, has seriously damaged on fisheries, aquaculture, and even ship traffics due to the sea fog caused by the extreme air-sea temperature difference. This cold water, originating from the bottom cold water in the Korea Strait connecting the East/Japan Sea and the East China Sea, has often been observed mainly during the summer months. Although previous studies have suggested possible causes why the bottom cold water intrusion is stronger during summer, its mechanism is not clear yet. Here we investigate the influence of tides on the bottom cold water intrusion using numerical simulations. A real-time ocean forecasting system with 1/12-degree grids, which assimilates in-situ data and includes both atmospheric and tidal forcings, reveals a significant seasonal variation of semidiurnal internal tides around the continental shelf slope between the Korea Strait and the southwestern East/Japan Sea. The internal tide energy during summer shows larger energy than during winter by a factor of about three in this region. Shipboard and Argo profiler measurements near the internal tide generation region and in the open basin support the simulated results. The seasonal stratification change induced by the Asian monsoon causes the seasonal internal tide energy change. This result leads us to investigate the influence of semidiurnal internal tides on the bottom cold water intrusion into the shallow Korea Strait. A series of numerical simulations using a 1/108-degree fine-resolution regional ocean model with different configurations demonstrates that the semidiurnal internal tides play a crucial role in enhancing the intrusion of the Korea Strait bottom cold water by an internal tidal pumping effect. Our results provide not only an explanation why the intrusion of the Korea Strait bottom cold water is strengthened during summer, but also an implication for the role of internal tides in water exchanges between the shelf seas and deep oceans.