Research papersWind-driven intensification of the Tsushima Warm Current along the Japanese coast detected by sea level difference in the summer monsoon of 2013

- A large coastal sea level difference was found in the southwestern Japan Sea.
- The time and horizontal scales were about 4 months and 1000 km, respectively.
- The strong southwesterly monsoon winds blew during the hot summer of 2013.
- The coastal sea level rose by Ekman transport toward the coast.

This study found an unusually large variation in the coastal sea level difference (SLD) in the southwestern Japan Sea from July to October 2013. The time and horizontal scales were about 4 months and 1000 km along the coast, respectively. During the exceptionally hot summer of 2013, strong monsoon winds blew through the Tsushima Straits into the southwestern Japan Sea, because the North Pacific High was located farther west than average. The southwesterly winds averaged 7 m s−1 parallel to the coast in July 2013, resulting in Ekman transport toward the coast. The most noticeable SLD was measured between Omijima and Mishima, normal to the 250 km rectilinear coastal line. The velocity of the first branch of the Tsushima Warm Current increased to about 50 cm s−1 between Omijima and Mishima from late July to early August. Although the SLD seemed to simultaneously increase and decrease along the Japanese coast with large time scale, the variation in coastal SLD propagated along the Japanese coast, reaching Niigata-nishi in the eastern Japan Sea. The propagation speed in the wide continental shelf of the southern Japan Sea was much higher than that in the narrow eastern shelf, which was consistent with the behavior of continental shelf waves.