Kuroshio Current

The Kuroshio current 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 (Philippines) and the coasts of Old California (New Spain). 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. Historians have debated the current’s role in structuring the economic geography of Japan, especially at the time before steam navigation. The current’s path influenced the development of fisheries and whaling businesses that grew in the early modern period in specific regions. Its movement was also a factor in the development of maritime routes for domestic shipping as well as international navigation along the shores of Japan. In the mid-19th century, the current became an object of oceanographic interest both in Japan and abroad. The ideological effect of scientific conquest and industrial appropriation of the Kuroshio by the Japanese Empire has been likened to a frontier incorporation. == Physical properties ==

, and Sea of Japan. 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 . 8. Liman 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, The strength (transport) 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 (25 million cubic metres per second). The Kuroshio strengthens significantly when it rejoins the Pacific Ocean, reaching 65Sv (65 million cubic metres per second) southeast of Japan, 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. 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 (Ti) and is trapped above the abyssal basin between Hainan Island and the Pearl River mouth. 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 Organisms such as phytoplankton and algae use these newly introduced nutrients to grow. In 2003, two typhoons induced significant surface layer mixing as they passed through the region. This mixing directly produced two algal bloom events in the North Western Pacific Ocean that negatively affected Japan. ==Nutrient transport==

The Kuroshio Current is considered a nutrient stream because of high nutrient flux from surrounding oligotrophic waters with primary production of 150 to 300 grams of carbon per square meter per year based on SeaWiFS global primary productivity estimates. The current transports significant amounts of nutrients to support this primary production from the East China Sea continental shelf to the subarctic Pacific Ocean. The maximum chlorophyll value is found around depth. Nutrients are brought to the surface water from deeper layers where the Kuroshio Current flows over shallow areas and seamounts. This process occurs over the Okinawa Trough and the Tokara Strait. The Tokara Strait also has high cyclonic activity where the Kuroshio Current passes through. This in combination with the Coriolis effect causes intense upwelling along the continental shelf. This upwelling and nutrient transport into surface layers is essential for primary production because these vital nutrients would otherwise be inaccessible to phytoplankton which need to remain in upper layers where sunlight is available for them to perform photosynthesis. The constant transport of nutrient rich waters to regions with high levels of light therefore supports increased photosynthesis supporting the rest of the biologically diverse ecosystem associated with the Kuroshio current. ==Marine life==

colliding with the Kuroshio Current near Hokkaido. When two currents collide, they create eddies. Phytoplankton growing in the surface waters become concentrated along the boundaries of these eddies, tracing out the motions of the water. The transportation of nutrients, heat and plankton by the Kuroshio Current and the current's transection of multiple different waterbodies gives way to high species richness in and adjacent to this current. In addition, the Kuroshio is classified as a biodiversity hotspot, meaning the waters circulating through the region are host to many different species, yet many of its resident organisms are at risk of becoming endangered or are already at the brink of extinction as a result of local and/or global human activity. Overfishing and overharvest are the primary risks for many of the threatened or endangered species here. Photoautotrophs Phytoplankton Phytoplankton are responsible for the aforementioned high rates of primary productivity within the current. Warm sea surface temperatures and low turbidity in the region lead to clearer waters which allows for deeper penetration of sunlight and an extension of the epipelagic zone. These particular characteristics, along with lower nutrient availability within the current, correspond well with the requirements of two specific cyanobacteria: Prochlorococcus and Synechococcus. Prochlorococcus is the dominant species of picophytoplankton within the Kuroshio Current and these two species may be responsible for as much as half of the fixation of in the entire Kuroshio Current photic zone. During these events, dust clouds transport and deposit phosphate and trace metals which subsequently stimulate growth in both Prochlorococcus and Synechococcus as well as diatoms. This nitrogen fixation supplies a limiting nutrient (nitrate), to other photoautotrophs for growth and reproduction. Meanwhile, in areas influenced by upwelling with higher nutrient and carbon concentrations, diatoms are important contributors to carbon and nitrogen out of the euphotic zone due to the weight of their "glass houses" made of silica and their tendencies to sink. Macro-flora At least ten genera of seaweed reside in waters in and around the Kuroshio Current. This upwelling event, the Kuroshio Current intrusion through the Luzon Strait and South China Sea, and summer monsoons, represent the convergence of a multitude of oceanic waters of different origin. These water convergence zones and subsequent circulation and mixing, have a major influence on the transport and distribution of many zooplankton species causing zooplankton communities to be more nutritious, unique and diverse. High diversity in copepods in waters adjacent the Kuroshio Current have also been reported. Two dominant copepod species of the current, C. sinicus and E. concinna, are transported northward in high concentrations by the current from the East China Sea in winter. Like copepods and diatoms, tunicates, specifically salps and doliolids, also play an important role on the biogeochemical cycle as well as on the food web in the Kuroshio. Salps transport carbon to the region's bottom water with their carbon-rich, fast-sinking fecal pellets and carcasses. Thaliaceans (salps and doliolids) are known to feed a minimum of 202 marine species, however, these animal's blooms have been found to cause harmful feeding conditions for pelagic fishes in the region. These changes impacts the food chain below and above this trophic level. This can influence fish migration, fish population's at large and major fisheries. The Kuroshio Current has an influence of several species of foraminifera, including species G. ruber and P. obliquiloculate. G. ruber is normally a surface dweller and was found at depths of 1000 meters along the Kuroshio Current. P. obliquiloculate normally resides between 25 and 100 m, yet was found deep in the abyssal basin (>1000 m). The distribution of these species in comparison to their standard dwelling depths observed by Gallagher (2015) demonstrates the ability of this intrusion and the overall Kuroshio Current's to redistribute nutrients vertically An important reef-building coral to this area, Heliopora coerulea, has been listed as threatened due to anthropogenic stressors to its environment such as: warming sea surface temperatures from climate change, ocean acidification from anthropogenic greenhouse gas emissions and dynamite fishing. Acropora japonica, Acropora secale, and Acropora hyacinthus are 3 more reef-building corals in the region. These species utilize symbiotic relationships with zooxanthellae, peridinin and pyrrhoxanthin, as a source of carotenoids. The Kuroshio Current controls patterns of connectivity between coral reefs (as well as other marine organisms with a larval phase), transporting larvae from southerly coral reefs to downstream reefs along the Ryukyu Arc. Squid Western boundary currents are used by certain species of squid for rapid and easy transport, allowing mature squid to travel with minimum energy expenditure to exploit rich northern feeding grounds, while eggs and larvae develop in the warm current waters during winter. The Japanese flying squid (Todarodes pacificus), for example, has three populations that breed in winter, summer, and autumn. The winter spawning group is associated with the Kuroshio Current, because following spawning events in January to April in the East China Sea the larvae and juveniles travel north with the Kuroshio Current. They are turned inshore and are caught between the islands of Honshu and Hokkaido during the summer. The summer spawning is in another part of the East China Sea, from which the larvae are entrained into the Tsushima current that flows north between the islands of Japan and the mainland. Afterward, the current meets a southward flowing cold coastal current, the Liman Current. The group of squid spawned in the summer are traditionally found around the boundary between the two currents, sustaining rich fisheries. In fact, studies have reported that annual catches in Japan have gradually increased since the late 1980s and it has been proposed that changing environmental conditions have caused the autumn and winter spawning areas in the Tsushima Strait and near the Goto Islands to overlap. In addition, winter spawning sites over the continental shelf and slope in the East China Sea are expanding. Vertebrates '') are reef fish commonly found in the Kuroshio Current reef systems. Fish The Kuroshio Current is home to thousands of fish species occupying nutrient rich and diverse waters in this region. This expansive biomass is influenced by elevated rates of primary production leading to large biomass in the lower trophic levels, facilitated by warmer local oceanic and atmospheric conditions. Resident fish of this area include reef fish like rabbitfish and parrotfish, pelagic fishes such as sardines, anchovies, mackerel, and sailfish, and higher trophic predators such as sharks. Female sea turtles utilize the transport potential of the current to access the warm nesting beaches of Japan's shores, and adolescent green and hawksbill turtles utilize the current transport to access waters surrounding Japan. Three types of whales of the same genus (Balaenoptera) also use this rich area for feeding grounds, including the common Minke (Balaenoptera acutorostrata), the sei whale (Balaenoptera borealis) and Bryde's whale (Balaenoptera edeni). The availability of Japanese sardines and mackerel eggs, larvae, and juveniles are the baleen whales' primary food sources in these areas. Top-tier trophic predators can serve as units in developing conservation management in this region. == Carbonate chemistry ==

The ocean absorbs approximately one third of the CO2 produced by fossil fuel combustion, cement production, and deforestation. One of the more significant oceanic sinks for atmospheric CO2 is the Kuroshio Current. In its highly biologically productive regions, this uptake of CO2 is carbon burial is facilitated by a strong biological pump. In the less productive northern current transition, the Kuroshio remains an important CO2 sink, through high CO2 solubility. The Kuroshio Extension region is classified as the strongest sink for atmospheric CO2 in the North Pacific. This is especially true in the winter when higher amounts of human-produced CO2 are taken up in the Kuroshio Extension region when compared with the summer. This is likely explained by cooler temperatures facilitating the solubility of CO2 in ocean water. As CO2 levels continue to increase in the atmosphere, so does CO2 uptake in the Kuroshio, making this seasonality more dramatic. == Climate implications ==

Western boundary currents are integrated parts in the world's climatic balance. The Kuroshio Current plays an important role in influencing regional climate and weather patterns mainly through the input of warm waters from lower latitudes northward into the western edge of the Pacific basin. Along with the other western boundary currents in the world, the Kuroshio Current is subject to seasonal changes that manifest in different flow rates, bifurcation latitudes, and water salinity. Circulation within the Pacific Ocean is largely influenced by this northerly transport of warm salty water north along the Western boundary, concurrently providing structure to the western edge of the North Pacific Gyre. Mode water formation As the Kuroshio Current separates from the equatorial current and flows northward, warm water from the Western Pacific Warm Pool segues into the northwest Pacific Ocean Basin. Principal heat flux in the Kuroshio occurs via the Kuroshio Extension between 132°E and 160°E and 30°N to 35°N, depending on the latitude where the extension splits off from the Kuroshio Current along the coast of Japan. The process of warm water injection into the open ocean plays an important role in the formation of North Pacific Subtropical Mode waters and the regulation of sea surface temperatures, affecting moisture transport across the western Pacific Basin. North Pacific subtropical mode waters are created when Kuroshio Extension waters lose large amounts of heat and moisture to the cold and dry northerly winds during boreal wintertime months, creating dense salty surface waters prone to sink and cause convection. The temperature range of the sinking North Pacific Subtropical Mode Waters characteristically falls between 16 °C and 19 °C, however exact temperatures and depths to which these waters sink varies annually depending on the efficiency of water transportation by the extension, which is a function of atmospheric and mesoscale eddy conditions. Climate change Climate change, specifically with respect to increasing sea surface temperatures and decreasing salinity, has been predicted to strengthen the surface flow of the Kuroshio Current as well as other western boundary currents across the Pacific. == Economic considerations ==

. Jack mackerel represent a large fishing industry in the Pacific: around 1.5 million pounds were harvested in 2020 by California fisheries alone, creating $272,000 in revenue according to the NOAA Fisheries commercial fishing landings database. The Kuroshio Current can be useful as a shipping lane as the current can save time and fuel usage when underway with the current. However, ships that travel against the current will spend more time and fuel to compensate for the water flowing against the shipping vessel. The Kuroshio supports many important fisheries. Jack Mackerel populations are one of the most important fishery resources in Japan, Korea and Taiwan. As the Kuroshio flows northeastward from northeast of Taiwan along the shelf slope of the Eastern China Sea, it carries Jack Mackerel eggs and larvae to southern Japan and Honshu Island. These larvae are caught and then raised in aquaculture through adulthood and harvested. Other important fisheries include pollock, sardine, and anchovy. There are also many developing port cities along the Kuroshio Current. While the Kuroshio Current is historically known to support many fisheries where it meets with the Oyashio current, this region is still recovering from the Fukushima Daiichi Nuclear Power Plant accident. In 2011, a magnitude 9.0 earthquake triggered a devastating tsunami. This tsunami inundated more than 200 miles of Japan's coastline and drastically altered the sea level in some coastal areas by meters. It killed more than 18,500 people and set off a nuclear disaster at the Fukushima nuclear plant, releasing radiocesium into the surrounding waters. While local water bodies were the most severely affected, this radiocesium was transported as far as the entire North Pacific Ocean by the North Pacific Current which is formed by the collision of the Kuroshio and the Oyashio current. Local fisheries lost over 90% of their fleets and were unable to resume operations for up to a year after the accident. The local economy has been working to return to pre-tsunami levels but, even now, fishery yields have not reached nearly the levels they were before the accident. No catches are made within a 10 km radius to the accident site and even catches outside of that zone are subject to inspection for radioactive materials, costing fisheries both time and money. Minamisanriku had most of the town's port and aquaculture facilities restored by 2014, and as of 2018, reconstruction of key infrastructure in the prefectures of Iwate and Miyagi was near completion. Local Japanese fishing fleets hauled 5,928 tons of seafood product valued at over 2.21 billion yen (19.342 million U.S. dollars) in 2021. Management practices must consider protecting these animals and recognizing the potential economic impacts on local hunters. == References ==