Tsushima current

A warm current flowing northward through the Japan Sea along the west coast of Kyushu and Honshu.

The Tsushima Current is a branch of the western boundary current of the North Pacific subtropical gyre, which is split by the Japanese islands. It branches off the Kuroshio near 30°N to enter the Japan Sea through Korea Strait, where it carries 1.3 Sv (1.3 × 10⁶ m³ s^-1, about 2% of the total Kuroshio transport) with speeds near 0.4 m s^-1 in summer (August) but only 0.2 Sv with less than 0.1 m s^-1 in winter (January). Most of the summer transport is fed into the East Korea Warm Current but rejoins the Tsushima Current after the East Korea Warm Current separates from the coast at 36°–38°N. The seasonal variability of the Tsushima Current effects the hydrography of the southern Japan Sea greatly, reducing surface salinity from 35 psu in winter to below 32.5 psu in summer when the current carries low salinity water from the Yellow Sea. Most of the Tsushima Current rejoins the Kuroshio through the eastward flowing Tsugaru Warm Current, which passes through Tsugaru Strait (the passage between Honshu and Hokkaido). This current runs into the Oyashio near 42°N, which forces it to flow southward on the shelf along the east coast of Honshu to meet and join the northward flowing Kuroshio near 35°N. Another part of the Tsushima Current continues farther north, pushing the polar front to its most northern position in the Pacific, to enter the Sea of Okhotsk between Hokkaido and Sakhalin. This water traverses the Sea of Okhotsk as the Soya Warm Current, a rapid current with speeds reaching 1 m s^-1 that stays close to the coast. Current shear between the fast-flowing coastal water and the offshore region persistently produces eddies of between 10 and 50 km in diameter. The water leaves the Sea of Okhotsk near 46°–47°N to flow south between the Oyashio and the east coast of Hokkaido.

The western boundary current of the North Pacific subpolar gyre.
Fed by the Alaskan Stream and Kamchatka Current, the Oyashio flows southward along the Kuril Islands and the east coast of northern Japan to 39°–40°N and occasionally as far south as 36°N. Near southern Hokkaido (43°N) the current splits into two branches known as the First and Second Oyashio Intrusion. In the region east of Tsugaru Strait (41°N) it is dominated by eddies; one or two cyclonic (warm core) eddies are formed each year. Every six years or so one of them grows into a giant eddy that then dominates the region for a year. Farther downstream the Oyashio meets the northward flowing Kuroshio to form the Arctic Polar Front. Both currents maintain their own frontal systems, the Oyashio Front being characterized by temperatures of 2°– 8°C and feeding into the Aleutian Current, while the Kuroshio Front has temperatures of 10°– 15°C and feeds into the North Pacific Current.