The Kuroshio (Japanese for "Black Stream") is a major western boundary current of the Pacific Ocean. It stretches as wide as a megacity and carries more water than the Amazon river. Like its Atlantic cousin the Gulf Stream (see also AMOC), it carries tropical heat northward, helping keep East Asia habitable.
The Kuroshio gathers strength east of the Philippines, courses northward past Taiwan, swerves along the Ryukyu islands and bifurcates around Japan. It transports nutrients, sustains fisheries and has shaped regional societies for millennia. Near the Korean peninsula it branches into the Tsushima current.
Its darkness comes, paradoxically, from its clarity. A dearth of plankton and sediment at the surface means the water absorbs most light rather than scattering it. Despite seemingly impoverished surface waters, the Kuroshio sustains rich fisheries -- the prevailing hypothesis attributes this to regular upwelling from deeper, nutrient-rich layers.
Spanish sailors discovered in 1565 that they could ride the Kuroshio from the Philippines to Mexico, underpinning the expansion of the Spanish empire and an early example of globalisation. In the Pacific basin where the Kuroshio empties, the 19th century saw the world's most prolific whaling region, the "Japan Ground" of Moby Dick fame.
In early 1841 a Japanese boat caught in the Kuroshio ended up on Torishima. An American whaling crew rescued the castaways, including 14-year-old Nakahama Manjiro, who returned to Massachusetts, adopted the name "John Mung" and eventually helped Japan navigate Commodore Perry's arrival in 1853.
The earliest traces of Homo sapiens on the Ryukyu islands date to some 30,000 years ago. Without a land bridge, the first settlers must have come by sea across the Kuroshio. In 2019 an interdisciplinary team at the University of Tokyo built a dug-out canoe using only stone tools and paddled from Taiwan's east coast to Yonaguni in 45 hours, demonstrating the crossing was possible.
Global ocean temperatures have risen 0.6 degrees C over the past century; the Kuroshio zone has risen 1.3 degrees. The current has moved 0.5 degrees north in a century. Coral reefs can now be found in Tokyo Bay. Traditional seaweed harvests near Kuroshio town have fallen from 10-20 tonnes annually to zero. The warming and shifting Kuroshio contributes to more chaotic weather on land, including record snowstorms in Hokkaido.
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