KUROSHIO AND OYASHIO CURRENTS 1413 to provide better overwinter conditions for the krill. Further Reading Salps compete with krill for phytoplankton } in poor sea ice years salp numbers are increased and Constable AJ, de la Mare W, Agnew DJ, Everson I and Miller D (2000) Managing Rsheries to krill recruitment is reduced. Further north in their conserve the Antarctic marine ecosystem: practical range, E. superba abundance is dependent on the implementation of the Convention on the Conserva- transport of krill in the ocean currents as well as tion of the Antarctic Marine Living Resources Suctuations in the strength of particular cohorts. (CCAMLR). ICES Journal of Marine Science 57: Given the importance of euphausiids in marine 778}791. food webs throughout the world’s oceans, they are Everson I (ed.) (2000) Krill: Biology, Ecology and Fishe- potentially important indicator species for detecting ries. Oxford: Blackwell Science. and understanding climate change effects. Changes Everson I (2000) Introducing krill. In: Everson I (ed.) in ocean circulation or environmental regimes will Krill: Biology, Ecology and Fisheries. Oxford: Black- be reSected in changes in growth, development, well Science. recruitment success, and distribution. These effects Falk-Petersen S, Hagen W, Kattner G, Clarke A and Sargent J (2000) Lipids, trophic relationship, may be most notable at the extremes of their distri- and biodiversity in Arctic and Antarctic krill. bution where any change in the pattern of variation Canadian Journal of Fisheries and Aquatic Sciences will result in major changes in food web structure. 57: 178}191. Given their signiRcance as prey to many commer- Mauchline JR (1980) The biology of the Euphausids. cially exploited species, this may also have a major Advances in Marine Biology 18: 371}677. impact on harvesting activities. A greater under- Mauchline JR and Fisher LR (1969) The biology of standing of the large-scale biology of the eu- the Euphausids. Advances in Marine Biology 7: phausiids and the factors generating the observed 1}454. variability is crucial. Obtaining good long-term and Miller D and Hampton I (1989) Biology and Ecology of R large-scale biological and physical data will be the Antarctic Krill. BIOMASS Scienti c Series, 9. Cam- & fundamental to this process. bridge: SCAR SCOR. Murphy EJ, Watkins JL, Reid K etal . (1998) Interannual variability of the South Georgia marine ecosystem: See also physical and biological sources of variation. Fisheries Oceanography 7: 381}390. Antarctic Circumpolar Current. Baleen Whales. Siegel V and Nichol S (2000) Population parameters. In: Copepods. Phalaropes. Plankton. Sea Ice: Over- Everson I (ed.) Krill: Biology, Ecology and Fisheries. view. Seals. Sperm Whales and Beaked Whales. Oxford: Blackwell Science. KUROSHIO AND OYASHIO CURRENTS B. Qiu, University of Hawaii at Manoa, North PaciRc Ocean, on the other hand, is domin- Hawaii, USA ated by upwelling. The upwelled, nutrient-rich Copyright ^ 2001 Academic Press water feeds the Oyashio from the north and leads to its nomenclature, parent (‘oya’) stream (‘shio’). doi:10.1006/rwos.2001.0350 The existence of a western boundary current to compensate for the interior Sverdrup Sow is well Introduction understood from modern wind-driven ocean circula- tion theories. Individual western boundary currents, The Kuroshio and Oyashio Currents are the western however, can differ greatly in their mean Sow and boundary currents in the wind-driven, subtropical variability characteristics due to different bottom and subarctic circulations of the North PaciRc topography, coastline geometry, and surface wind Ocean. Translated from Japanese, Kuroshio literally patterns that are involved. For example, the bi- means black (‘kuro’) stream (‘shio’) owing to the modal oscillation of the Kuroshio path south of blackish } ultramarine to cobalt blue } color of its Japan is a unique phenomenon detected in no other water. The ‘blackness’ of the Kuroshio Current western boundary current of the world oceans. Sim- stems from the fact that the downwelling-dominant ilarly, interaction with the semi-enclosed and often subtropical North PaciRc Ocean is low in biological ice-covered marginal seas and excessive precipita- productivity and is devoid of detritus and other tion over evaporation in the subarctic North PaciRc organic material in the surface water. The subarctic Ocean make the Oyashio Current considerably 1414 KUROSHIO AND OYASHIO CURRENTS different from its counterpart in the subarctic North The Kuroshio Current Atlantic Ocean, the Labrador Current. Because the Kuroshio and Oyashio Currents exert Region Upstream of the Tokara Strait a great inSuence on the Rsheries, hydrography, and The Kuroshio Current originates east of the Philip- meteorology of countries surrounding the western pine coast where the westward Sowing North North PaciRc Ocean, they have been the focus of Equatorial Current (NEC) bifurcates into the north- a great amount of observation and research in the ward-Sowing Kuroshio Current and the southward- past. This article will provide a brief review of the Sowing Mindanao Current. At the sea surface, the dynamic aspects of the observed Kuroshio and NEC bifurcates nominally at 123N}133N, although Oyashio Currents: their origins, their mean Sow this bifurcation latitude can change interannually patterns, and their variability on seasonal-to- from 113N to 14.53N. The NEC’s bifurcation tends interannual timescales. The article consists of two to migrate to the north during El Nino years and to sections, the Rrst focusing on the Kuroshio the south during La Nina years. Below the sea Current and the second on the Oyashio Current. surface, the NEC’s bifurcation tends to shift north- Due to the vast geographical areas passed by ward with increasing depth. This tendency is due to the Kuroshio Current (Figure 1), the Rrst section the fact that the southern limb of the wind-driven is divided into three subsections: the region subtropical gyre in the North PaciRc shifts to the upstream of the Tokara Strait, the region south north with increasing depth. of Japan, and the Kuroshio Extension region east of Branching northward from the NEC, the the Izu Ridge. As will become clear, the Kuroshio Kuroshio Current east of the Philippine coast has Current exhibits distinct characteristics in each of a mean geostrophic volume transport, referenced to these geographical locations owing to the differing 1250 dbar, of 25 Sv (1 Sverdrup"106 m3 s\1). Sea- governing physics. sonally, the Kuroshio transport at this upstream 60˚ N Bering Sea of Sea . Okhotsk C . am 50˚ East tre Kamchatka S N kan Alas Western Subarctic Gyre Alaska Gyre a m C. 40˚ Subarctic Current N o Tsushiurrent C Mixed water region yashi O Kuroshio extension 30˚ N t Recirculation en North rr u C Pacific o i Subtropical Ocean h s countercurrent o 20˚ r u N K North Equatorial Current 10˚ Mindanao N Current 120˚E 130˚E 140˚E 150˚E 160˚E 170˚E 180˚ 170˚W 160˚W 150˚W Figure 1 Schematic current patterns associated with the subtropical and subarctic gyres in the western North Pacific Ocean. KUROSHIO AND OYASHIO CURRENTS 1415 location has a maximum (&30 Sv) in spring and monsoon prevails, the Kuroshio Current passes the a minimum (&19 Sv) in fall. Similar seasonal Luzon Strait without intrusion. cycles are also found in the Kuroshio’s transports In the latitudinal band east of Taiwan in the East China Sea and across the Tokara (223N}253N), the northward-Sowing Kuroshio Cur- Strait. rent has been observed to be highly variable in As the Kuroshio Current Sows northward passing recent years. Repeat hydrographic and moored cur- the Philippine coast, it encounters the Luzon Strait rent meter measurements between Taiwan and the that connects the South China Sea with the open southernmost Ryukyu island of Iriomote show that PaciRc Ocean (Figure 2). The Luzon Strait has the variability of the Kuroshio path and transport a width of 350 km and is 2500 m deep at its deepest here are dominated by Suctuations with a period of point. In winter, part of the Kuroshio water has 100 days. These observed Suctuations are caused by been observed to intrude into the Luzon Strait and impinging energetic cyclonic and anticyclonic eddies form a loop current in the northern South China Sea migrating from the east. The Subtropical Counter- (see the dashed line in Figure 2). The loop current current (STCC) is found in the latitudinal band of can reach as far west as 1173E, where it is blocked 223N}253N in the western North PaciRc. The by the presence of the shallow shelf break off the STCC, a shallow eastward-Sowing current, is highly south-east coast of China. The formation of the unstable due to its velocity shear with the under- loop current is probably due to the north-east mon- lying, westward-Sowing NEC. The unstable waves soon, prevailing from November to March, which generated by the instability of the STCC-NEC deSects the surface Kuroshio water into the north- system tend to move westward while growing in ern South China Sea. During the summer months amplitude. The cyclonic and anticyclonic eddies that from May to September when the south-west impinge upon the Kuroshio east of Taiwan are 34°N 100 32°N East China Sea Kyushu PN-line 30°N China Tokara St. 28°N 26°N Ryukyu Islands 24°N Iriomote Is. Taiwan 22°N 100 200 Luzon St. 20°N 1000 South China Sea 18°N Luzon 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 132°E Figure 2 Schematic representation of the mean Kuroshio path (solid thick line) along the North Pacific western boundary. The thick dashed line south of Taiwan denotes the wintertime branching of the Kuroshio water into the Luzon Strait in the form of a loop current. PN-line denotes the repeat hydrographic section across which long-term Kuroshio volume transport is monitored (see Figure 4).