Kuroshio Current

THE KUROSHIO IS a warm northeasterly ocean current off the coast of JAPAN. This current is also called the gulf stream of the Pacific or Japan Current. Kuroshio means “the black stream” in Japanese, named after the deep ultramarine color of the high salinity water, which is found flowing north of the current’s axis. The system includes the following branches: Kuroshio, up to 35 degrees N; Kuroshio extension, extending eastward into two branches up to 160 degrees E longitude; North Pacific current, a further eastward continuation, which throws branches to the south as far as 150 degrees W; Tsushima current, branches of the main current that run into the Japan Sea, along the west coast of JAPAN; and Kuroshio counter-current, the large swirl or eddy on the east and south east of the Kuroshio.

The Kuroshio originates from the greater part of North Equatorial current, which divides east of the PHILIPPINES. The Kuroshio is the current running from Formosa to about 35 degrees N latitude. It continues directly as a warm current known as the Kuroshio Extension, from there it is continues as the North Pacific current. Water enters the Kuroshio over a broad front, 621 mi (1,000 km) in width, which then accelerates and narrows. A narrow band less than 62 mi (100 km) in width and about .6 mi (1 km) of maximum depth runs for 1,864 mi (3,000 km) along the western edge of the Pacific, between the Philippines and the east coast of Japan. A narrow, intense flow persists for 930 mi to 1,240 mi (1,500 to 2,000 km) after the current leaves Japan’s east coast, after which there is a marked drop in velocity. Here, there is no land boundary on the lefthand side to generate a fractional boundary layer.

Kuroshio is a fast ocean current (2 to 4 knots). Every second, the current carries some 50 million tons of sea water past Japan’s southeast coast, a flow equal in volume to about 6,000 rivers the size of the DANUBE or the VOLGA. On the whole, there are two distinct types of water in the current: warm, saline water on the right and cold, dilute water on the left The current undergoes marked changes in speed in the location of its axis, which varies from place to place and with seasons. Apart from changes resulting from tides, shortterm changes from a major shift in the axis of the Kuroshio can occur as it flows past southern Japan. When meanders develop, cold water is brought up toward the surface between the Kuroshio and the coast, and the temperature drops to as much as 50 degrees F (10 degrees C) below normal. This change has a profound effect on coastal and offshore fisheries.

The Kuroshio Current plays a vital role in the circulation of the North PACIFIC OCEAN. The current involves great volumes of water capable of carrying large amounts of heat. The heat, which is carried north by this flow, has an effect on climate of the adjacent land areas. Water temperature offshore strongly influences cloud cover and rainfall. On the southern coast of ALASKA, the effect of the Kuroshio Extension creates a somewhat more temperate climate.

Kuroshio Current

The Kuroshio Current is the second-largest ocean current in the world. It flows in the western Pacific from the east coast of Taiwan island, northwards to Japan, and then beyond Hokkaido past the Kuril Islands, where it merges with the North Pacific Current. In many ways, it is similar to the Gulf Stream in the Atlantic Ocean, which also carries warm tropical waters toward the polar region. The Japanese call it the Kuroshio Current, with some early English geographers translating this to the Black Stream, the name coming from the deep blue of the water; a few writers refer to it as the Japan Current. A branch of the Kuroshio Current, known as the Tsushima Current, passes through the Tsushima Straits between Japan and Korea, into the Sea of Japan.

The warm waters of the Kuroshio Current are largely responsible for the abundance of coral reefs off the east coast of Japan, bringing coral reef larvae from other coral reefs in the South China Sea and off the coast of the Philippines. As a result, the coral reefs off Japan are the northernmost coral reefs in the world; the waters off the coast of Japan are as warm as 68 degrees F (20 degrees C) off the Ryukyu Islands, and even reach 60 degrees F (16 degrees C) in winter off the southern coast of Honshu Island. It has been the center of the Japanese fishing industry since prehistoric times. Since the 1940s, there has been a significant decline in the fish in the Kuroshio Current; this can be attributed to overfishing, or to global warming and climate change since the 1960s. In 1988, studies of the coral reefs showed heavy coral bleaching, which resulted in the deaths of many of the reefs. The pressure of population, with some waste dumped in the sea, has been a major contributory factor, as well as the rise in temperature of the Kuroshio Current.

This rise in temperature is the major actor in coral bleaching off the Osasawara Islands, which have 460 hectares of coral reefs, especially around Muko, Chichi, and Haha islands. These losses cannot be simply attributed to overpopulation. The local fishing industry has been unable to sustain itself by fishing in Japan’s territorial waters, leading to increasing numbers of Japanese fishing and whaling ships operating throughout the Pacific. Between 1993 and 1998, the fishing catch off Okinawa dropped by 21 percent. Off the Ryukyu Islands, although the number of fish has increased, the species diversity has dramatically fallen. The worst losses in coral reefs have occurred near Okinawa, where overpopulation has led to pollution. An aerial survey by the Environment Agency of Japan from 1990 to 1992 revealed that some 90.8 percent of the coral communities near Okinawa had less than 5 percent cover. In the summer of 1998, some 97 municipalities reported bleaching of coral reefs in their communities, especially of the Acropora species, which appears the worst affected.