The Kuroshio

Hachijō and the Kuroshio Current

Due to its geographical position, Hachijō Island and neighbouring areas are directly affected by the Kuroshio Warm Water current. The positioning and direction of the Kuroshio Current and the movement of cold water masses are the main factor influencing sea conditions in the area.

Image source: NASA  https://svs.gsfc.nasa.gov/10841

The Kuroshio Current (黒潮, Kuroshio, literally “Black Current”,  is named after the deeper blue colour of its waters), is a warm water, north-flowing current, which transports warm, and tropical water northward toward the polar region.

It is by far the most important current affecting the Japanese archipelago, and directly affects the seas surrounding Hachijō Island.

The Kuroshio branches out into the following currents:

• The Kuroshio Extension, which is the name given to the Kuroshio as it branches east in the Pacific not far after Hachijō, from where it goes to meets the southwards flowing Oyashio Current.
• The Kuroshio Countercurrent, which flows southward to the east of the main Kuroshio Current, in the Philippine Sea, roughly 700 km off the coast.
• The Tsushima Current  is a secondary branch of the Kuroshio Current flowing along the west coast of Japan.

The Kuroshio not only brings warmer water from the tropics, it merges into a major cold water current, the Oyashio flowing from the north, which gives the area its remarkable richeness and biodiversity.

As it reaches the vicinity of latitude 35°N, roughly near Kantō’s Bōsō Peninsula (which forms the eastern edge of Tokyo Bay), the bulk of the Kuroshio follows a more eastward course, in a segment known as the Kuroshio Extension.

The Kuroshio Extension then flows right into the southward-flowing Oyashio (“Parental Current”, named its metaphorical role as a parent, nurturing marine organisms), which is a subarctic, cold-water oceanic current, originating in the Arctic Ocean.

If we place the Kuroshio in wider perspective, we see that it constitutes the western, subtropical boundary section of the clockwise flowing North Pacific Ocean Gyre.

It finds its origins in the Pacific North Equatorial Current, which first runs westward along the Equator, passing through the east side of the biodiverse Coral Triangle.

The Kuroshio forms off the coast of the Philippines, from where its powerful warm waters flow northwards towards the Japanese islands.

As it reaches the northern part of Honshū Island, the Kuroshio then meets a cold current, the Oyashio (or Kurile Current) that flows southward from the Okhotsk Sea, and this collision forms a highly productive, life supporting-mixture of warm and cold waters.
The main flow then merges and feeds into with the eastern flow of the North Pacific Current.

Image credit /sources NASA: https://earthobservatory.nasa.gov/

The Kuroshio Current is one of the largest and strongest ocean currents in the world, flowing 2 to 4 knots on average.
It can be considered as the Pacific Ocean’s equivalent of the Atlantic Ocean’s Gulf Stream, in that both currents transport the ocean’s warm, tropical and nutrient rich waters northward, up towards the Polar Regions, with a crucial effect on regional climate and human activities,  ocean conditions and marine life.

Similar to the Atlantic’s Gulf Stream, the Kuroshio current, with its average surface temperature of 24°C, has an important warming effect upon the south and southeast coastal regions of Japan, especially strategically positionned islands like Hachijō, that lie directly on the current’s path.

The warm waters of the Kuroshio Current also sustain diverse coral reefs across Japan, which develop at higher latitude than elsewhere, including on Hachijō-jima.

Another indirect effect of the Kuroshio-Oyashio current system is the creation of powerful coastal upwellings, which, combined with Japan unique and complex, mostly volcanic based underwater topography, will bring deep-sea dwelling organisms up with the water.

Phytoplankton growing in the surface waters become concentrated in the area, which greatly boosts marine populations.
Kuroshio-specific marine life has long supported Japan’s extensive fisheries, on both coasts of the archipelago.

Many species,  are entirely dependent on the flow of Kuroshio Current, and sometimes nicknamed Kuroshio Species.

It is largely because of the multiple effects Kuroshio current flow that the waters surrounding Japanese islands count among some of the most productive and diverse regions in the world in terms of marine life, which is great for the fishing industry, but also for more contemplative activities such as scuba-diving, to which it gives warm and clear waters, coral and biodiversity.

Click here for more detailed information on the Kuroshio and other currents of the Japanese archipelago, on BlueJapan.org’s geography pages