Kuroshio Current
The Kuroshio Current, also known as the Black Current or Japan Current, is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Kuroshio is a powerful western boundary current that transports warm equatorial water poleward and forms the western limb of the North Pacific Subtropical Gyre. Off the East Coast of Japan, it merges with the Oyashio Current to form the North Pacific Current.
The Kuroshio Current has significant effects on both physical and biological processes of the North Pacific Ocean, including nutrient and sediment transport, major pacific storm tracks and regional climate, and Pacific mode water formation. Additionally, the current's significant nutrient transport results in a biologically rich ecoregion supporting an important fishing industry as well as diverse marine food webs. The South China Sea for example has relatively low nutrient concentrations in its upper waters, but experiences enhanced biological productivity due to the input from the Kuroshio Current Intrusion. Ongoing research centered around the Kuroshio Current's response to climate change predicts a strengthening in surface flows of this western boundary current which contrasts the predicted changes in the Atlantic Ocean's Gulf Stream.
History
It was discovered in 1565 by Andrés de Urdaneta, a native of Guipuzcoa, colonial administrator, supervisor of nautical expeditions, Corregidor, Augustinian monk and loyal navigator in the service of King Philip II, when, aboard the nao San Pedro, he was the first to open the "tornaviaje" between Cebu and the coasts of Old California. The secret of the tornaviaje gave Spain absolute hegemony over the Pacific Ocean for centuries, a hegemony that was embodied in the so-called "Manila galleon".The existence of the Kuroshio Current was known to Europeans from as early as 1650. This was shown in a map drawn by Bernhardus Varenius. As well as this, it was also noted by Captain J. King during a British expedition under Captain James Cook.
Physical properties
The Kuroshio is a relatively warm ocean current with an annual average sea-surface temperature of about, is approximately wide, and produces frequent small to meso-scale eddies. The Kuroshio originates from the Pacific North Equatorial Current, which splits in two at the east coast of Luzon, Philippines, to form the southward-flowing Mindanao Current and the more significant northward-flowing Kuroshio Current. East of Taiwan, the Kuroshio enters the East China Sea through a deep break in the Ryukyu island chain known as the Yonaguni Depression. The Kuroshio then continues northwards and parallel to the Ryukyu islands, steered by the deepest part of the East China Sea, the Okinawa Trough, before leaving the East China Sea and re-entering the Pacific through the Tokara Strait. It then flows along the southern margin of Japan but meanders significantly. At the Bōsō Peninsula, the Kuroshio finally separates from the Japanese coast and travels eastward as the Kuroshio Extension. The Kuroshio Current is the Pacific analogue of the Gulf Stream in the Atlantic Ocean, transporting warm, tropical water northward toward the polar region.The Kuroshio's counterparts associated with the North Pacific Gyre are the: east flowing North Pacific Current to the north, the south flowing California Current to the east, and the west flowing North Equatorial Current to the south. The warm waters of the Kuroshio Current sustain the coral reefs of Japan, the northernmost coral reefs in the world. The part of the Kuroshio that branches into the Sea of Japan is called Tsushima Current.
Similar to the Atlantic Ocean's Gulf Stream, the Kuroshio Current creates warm ocean surface temperatures, and significant moisture in the atmosphere along the western Pacific basin, and thus produces and sustains tropical cyclones. Tropical cyclones, also known as typhoons, are formed when atmospheric instability, warm ocean surface temperatures, and moist air are combined to fuel an atmospheric low-pressure system. The Western North Pacific Ocean experiences an average of 25 typhoons annually. The majority of typhoons occur from July through October during northern hemisphere summer, and typically form where the Kuroshio Current is the warmest near the equator. Typhoons tend to track along the current's warm water poleward until they dissipate in colder waters.
The strength of the Kuroshio varies along its path and seasonally. Within the Sea of Japan, observations suggest that the Kuroshio transport is relatively steady at about 25Sv. The Kuroshio strengthens significantly when it rejoins the Pacific Ocean, reaching 65Sv southeast of Japan, although this transport has significant seasonal variability. The Kuroshio Current splits into Kuroshio Current extension and the Tsushima Current, as the currents wrap around Japanese Island and reconnects, changes in flow will impact the flows of the other currents.
The path of the Kuroshio may have been different in the geologic past based on historical sea level and bathymetry, however there is currently conflicting scientific evidence. It has been proposed that lower sea-level and tectonics may have prevented the Kuroshio from entering the Sea of Japan during the last glacial period, approximately c. 115,000 – c. 11,700 years ago, and remained entirely within the Pacific basin. However, other proxies and ocean models have alternatively suggested that the Kuroshio path was relatively unaltered, possibly as far back as 700,000 years ago.
Sediment transport
The magnitude of the Kuroshio Current and seafloor bathymetry results in deep sea erosion and sediment transport in multiple regions. Offshore of Southern Taiwan on the Kenting Plateau erosion is likely caused by the strong bottom currents which increase in velocity along the rise on this plateau. The bottom water accelerates as it travels from a depth of 3500 m to a depth around 400–700 m. The increase in current velocity exacerbates erosion revealing the Kuroshio Knoll, a 3 km × 7 km bean-shaped elevated flat area 60–70 m below surface levels in comparison to the rest of the Plateau which located at around 400–700 m. The Plateau is being uplifted and is balanced with erosion.The granulometry of the Kenting Plateau and surrounding area demonstrates the eroding qualities of the Kuroshio Current. The sediment grain size of the sand varies along the edge of the Plateau. The deeper down the edge, the larger the grains as smaller grains are swept away by the current. Some of these fine sand particles have settled into a dune field while the remaining sediment is transported and deposited throughout the region by the Kuroshio Current.
The Kuroshio Current also transports Yangtze River sediment. The amount of sediment transport is highly dependent on the relationship between the Kuroshio Current intrusion, the China Coastal Current, and the Taiwan Warm Current. The Yangtze River sediment is being deposited on the East China Sea inner shelf rather than the deep sea due to the interaction of the three currents.
Distinct elemental characteristics of sediments from differing sources permits tracking sources of sediments within the Kuroshio. Taiwanese sediment notably contains illite and chlorite. These traceable compounds have been found all the way through the Kuroshio Current up into its branch through the Kuroshio Current Intrusion in the South China Sea. The South China Sea branch of the Kuroshio and the cyclonic eddy west of Luzon Island impact Luzon and Pearl River sediments. The Luzon sediment containing high levels of smectite is unable to travel northwestward. The Pearl River sediments contains high levels of kaolinite and titanium and is trapped above the abyssal basin between Hainan Island and the Pearl River mouth. These compounds allow scientists to track sediment transport throughout the Kuroshio Current.
Eddies
There are indications that eddies contribute to the preservation and survival of fish larvae transported by the Kuroshio. Plankton biomass fluctuates yearly and is typically highest in the eddy area of the Kuroshio's edge. Warm-core rings are not known for having high productivity. However, there is evidence of equal distribution of biological productivity throughout the warm-core rings from the Kuroshio Current, supported by the upwelling at the periphery and the convective mixing caused by the cooling of surface water as the rings move north of the current. The thermostad is the deep mixed layer that has discrete boundaries and uniform temperature. Within this layer, nutrient-rich water is brought to the surface, which generates a burst of primary production. Given that the water in the core of a ring has a different temperature regime than the shelf waters, there are times when a warm-core ring is undergoing its spring bloom while the surrounding shelf waters are not.There are many complex interactions within warm-core rings and thus, lifetime productivity is not very different from the surrounding shelf water. A study from 1998 found that the primary productivity within a warm-core ring was almost the same as in the cold jet outside it, with evidence of upwelling of nutrients within the ring. In addition, there was discovery of dense populations of phytoplankton at the nutricline within a ring, presumably supported by the upward mixing of nutrients. Furthermore, there have been acoustic studies in the warm-core ring, which showed intense sound scattering from zooplankton and fish populations in the ring and very sparse acoustic signals outside of it.