Sea of Japan - Hydrology

Hydrology

The sea currents circulate in the counterclockwise direction. The Kuroshio (Japan Current), the Tsushima Current and the East Korea Warm Current bring warmer and more saline water to the north. There they merge into the Tsugaru Current and flow into the Pacific Ocean through the Tsugaru Strait. They also feed the Sōya Current and exit through the La Perouse Strait to the Sea of Okhotsk. The returning branch is composed of the Liman, North Korea and Central (or Mid-) Japan Sea currents which bring fresh and cold water along the Asian coast to the south.

Water temperature is mostly affected by exchange with the atmosphere in the northern part of the sea and by the currents in the southern part. Winter temperatures are 0 °C or below in the north and 10–14 °C in the south. In this season, there is a significant temperature difference between the western and eastern parts owing to the circular currents. So at the latitude of Peter the Great Gulf, the water temperature is about 0 °C in the west and 5–6 °C in the east. This east-west difference drops to 1–2 °C in summer, and the temperatures rise to 18–20 °C in the north and 25–27 °C in the south.

As a result of the enclosed nature of the sea, its waters form clearly separated layers which may show seasonal and spatial dependence. In winter, the temperature is almost constant with the depth in the northern part of the sea. However, in central-southern parts, it may be 8–10 °C down to 100–150 m, 2–4 °C at 200–250 m, 1.0–1.5 °C at 400–500 m and then remain at about 0 °C until the bottom. Heating by the sun and tropical monsoons increases the depth gradient in spring–summer. So in the north the surface layer (down to 15 m) may heat up to 18–20 °C. The temperature would sharply drop to 4 °C at 50 m, then slowly decrease to 1 °C at 250 m and remain so down to the seabed. On the contrary, the temperature in the south could gradually decrease to 6 °C at 200 m, then to 2 °C at 260 m and to 0.04–0.14 °C at 1000–1500 m, but then it would rise to about 0.3 °C near the bottom. This cold layer at about 1000 m is formed by sinking of cold water in the northern part of the sea in winter and is brought south by the sea currents; it is rather stable and is observed all through the year.

The hydrological isolation of the Sea of Japan also results in slightly lower average water salinity (34.09‰, where ‰ means parts per thousand) compared with the Pacific Ocean. In winter, the highest salinity at 34.5‰ is observed in the south where evaporation dominates over precipitation. It is the lowest at 33.8‰ in the south-east and south-west because of frequent rains and remains at about 34.09‰ in most other parts. Thawing of ice in spring reduces water salinity in the north, but it remains high at 34.60–34.70‰ in the south, partly because of the inflow of salty water through the Korea Strait. A typical variation of salinity across the sea in summer is 31.5‰ to 34.5‰ from north to south. The depth distribution of salinity is relatively constant. The surface layer tends to be more fresh in the sea parts which experience ice melting and rains. The average water density is 1.0270 g/cm3 in the north and 1.0255 g/cm3 in the south in winter. It lowers in summer to 1.0253 and 1.0215 g/cm3, respectively.

Few rivers flow into the Sea of Japan from mainland Asia, the largest being Tumen, Rudnaya, Samarga, Partizanskaya and Tumnin; all of them have mountainous character. In contrast, numerous large rivers flow from Honshū and Hokkaidō into the sea, including Japan’s four largest rivers in the Shinano, Ishikari, Agano and Mogami. The total annual river discharge into the sea is 210 km3 and is relatively constant through the year, except for a minor increase in July. Most water (97% or 52,200 km3) flows into the sea through the Korea Strait and discharges through the Tsugaru (64% or 34,610 km3), La Pérouse (10,380 km3) and Korea straits. Rainfall, evaporation and riverine inflow make only 1% of the water balance. Between October and April, the outflow exceeds the inflow due to the lower income through the Korea Strait; this balance reverses between May and September.

The sea has complex tides, which are induced by the tidal wave of the Pacific Ocean penetrating through the Korea Strait and Tsugaru strait. The tides are semi-diurnal (rise twice a day) in the Korea Strait and in the northern part of the Strait of Tartary. They are diurnal at the eastern shore of Korea, Russian Far East and the Japanese islands of Honshū and Hokkaidō. Mixed tides occur in Peter the Great Gulf and Korea strait. The tidal waves have a speed of 10–25 cm/s in the open sea. They accelerate in the Korea Strait (40–60 cm/s), La Pérouse Strait (50–100 cm/s) and especially in the Tsugaru Strait (100–200 cm/s). The amplitude of the tides is relatively low and strongly varies across the sea. It reaches 3 meters in the south near the Korea Strait, but quickly drops northwards to 1.5 meters at the southern tip of Korean Peninsula and to 0.5 meters at the North Korean shores. Similar low tides are observed in Hokkaidō, Honshū and south Sakhalin. The amplitude however increases to 2.3–2.8 meters toward the north of the Strait of Tartary due to its funnel-like shape. Apart from tides, the water level also experiences seasonal, monsoon-related variations across the entire sea with the highest levels observed in summer and lowest in winter. Wind may also locally change the water level by 20–25 cm; for example, it is higher in summer at the Korean and lower at the Japanese coasts.

The sea waters have blue to green-blue color and a transparency of about 10 meters. They are rich in dissolved oxygen, especially in the western and northern parts, which are colder and have more phytoplankton than the eastern and southern areas. The oxygen concentration is 95% of the saturation point near the surface, it decreases with the depth to about 70% at 3,000 meters.

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