KUROSHIOANDOYASHIOCURRENTS - SOESTKUROSHIOANDOYASHIOCURRENTS 1413 10˚ N 20˚ N 30˚ N 40˚ N 50˚ N 60˚ N 120˚E 130˚E 140˚E 150˚E 160˚E 170˚E 180˚ 170˚W 160˚W 150˚W Sea

to provide better overwinter conditions for the krill.Salps compete with krill for phytoplankton } inpoor sea ice years salp numbers are increased andkrill recruitment is reduced. Further north in theirrange, E. superba abundance is dependent on thetransport of krill in the ocean currents as well asSuctuations in the strength of particular cohorts.

Given the importance of euphausiids in marinefood webs throughout the world’s oceans, they arepotentially important indicator species for detectingand understanding climate change effects. Changesin ocean circulation or environmental regimes willbe reSected in changes in growth, development,recruitment success, and distribution. These effectsmay be most notable at the extremes of their distri-bution where any change in the pattern of variationwill result in major changes in food web structure.Given their signiRcance as prey to many commer-cially exploited species, this may also have a majorimpact on harvesting activities. A greater under-standing of the large-scale biology of the eu-phausiids and the factors generating the observedvariability is crucial. Obtaining good long-term andlarge-scale biological and physical data will befundamental to this process.

See also

Antarctic Circumpolar Current. Baleen Whales.Copepods. Phalaropes. Plankton. Sea Ice: Over-view. Seals. Sperm Whales and Beaked Whales.

Further ReadingConstable AJ, de la Mare W, Agnew DJ, Everson I

and Miller D (2000) Managing Rsheries toconserve the Antarctic marine ecosystem: practicalimplementation of the Convention on the Conserva-tion of the Antarctic Marine Living Resources(CCAMLR). ICES Journal of Marine Science 57:778}791.

Everson I (ed.) (2000) Krill: Biology, Ecology and Fishe-ries. Oxford: Blackwell Science.

Everson I (2000) Introducing krill. In: Everson I (ed.)Krill: Biology, Ecology and Fisheries. Oxford: Black-well Science.

Falk-Petersen S, Hagen W, Kattner G, Clarke A andSargent J (2000) Lipids, trophic relationship,and biodiversity in Arctic and Antarctic krill.Canadian Journal of Fisheries and Aquatic Sciences57: 178}191.

Mauchline JR (1980) The biology of the Euphausids.Advances in Marine Biology 18: 371}677.

Mauchline JR and Fisher LR (1969) The biology ofthe Euphausids. Advances in Marine Biology 7:1}454.

Miller D and Hampton I (1989) Biology and Ecology ofthe Antarctic Krill. BIOMASS ScientiRc Series, 9. Cam-bridge: SCAR & SCOR.

Murphy EJ, Watkins JL, Reid K et al. (1998) Interannualvariability of the South Georgia marine ecosystem:physical and biological sources of variation. FisheriesOceanography 7: 381}390.

Siegel V and Nichol S (2000) Population parameters. In:Everson I (ed.) Krill: Biology, Ecology and Fisheries.Oxford: Blackwell Science.

KUROSHIO AND OYASHIO CURRENTSB. Qiu, University of Hawaii at Manoa,Hawaii, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0350

Introduction

The Kuroshio and Oyashio Currents are the westernboundary currents in the wind-driven, subtropicaland subarctic circulations of the North PaciRcOcean. Translated from Japanese, Kuroshio literallymeans black (‘kuro’) stream (‘shio’) owing to theblackish } ultramarine to cobalt blue } color of itswater. The ‘blackness’ of the Kuroshio Currentstems from the fact that the downwelling-dominantsubtropical North PaciRc Ocean is low in biologicalproductivity and is devoid of detritus and otherorganic material in the surface water. The subarctic

North PaciRc Ocean, on the other hand, is domin-ated by upwelling. The upwelled, nutrient-richwater feeds the Oyashio from the north and leads toits nomenclature, parent (‘oya’) stream (‘shio’).

The existence of a western boundary current tocompensate for the interior Sverdrup Sow is wellunderstood from modern wind-driven ocean circula-tion theories. Individual western boundary currents,however, can differ greatly in their mean Sow andvariability characteristics due to different bottomtopography, coastline geometry, and surface windpatterns that are involved. For example, the bi-modal oscillation of the Kuroshio path south ofJapan is a unique phenomenon detected in no otherwestern boundary current of the world oceans. Sim-ilarly, interaction with the semi-enclosed and oftenice-covered marginal seas and excessive precipita-tion over evaporation in the subarctic North PaciRcOcean make the Oyashio Current considerably

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Figure 1 Schematic current patterns associated with the subtropical and subarctic gyres in the western North Pacific Ocean.

different from its counterpart in the subarctic NorthAtlantic Ocean, the Labrador Current.

Because the Kuroshio and Oyashio Currents exerta great inSuence on the Rsheries, hydrography, andmeteorology of countries surrounding the westernNorth PaciRc Ocean, they have been the focus ofa great amount of observation and research in thepast. This article will provide a brief review of thedynamic aspects of the observed Kuroshio andOyashio Currents: their origins, their mean Sowpatterns, and their variability on seasonal-to-interannual timescales. The article consists of twosections, the Rrst focusing on the KuroshioCurrent and the second on the Oyashio Current.Due to the vast geographical areas passed bythe Kuroshio Current (Figure 1), the Rrst sectionis divided into three subsections: the regionupstream of the Tokara Strait, the region southof Japan, and the Kuroshio Extension region east ofthe Izu Ridge. As will become clear, the KuroshioCurrent exhibits distinct characteristics in each ofthese geographical locations owing to the differinggoverning physics.

The Kuroshio Current

Region Upstream of the Tokara Strait

The Kuroshio Current originates east of the Philip-pine coast where the westward Sowing NorthEquatorial Current (NEC) bifurcates into the north-ward-Sowing Kuroshio Current and the southward-Sowing Mindanao Current. At the sea surface, theNEC bifurcates nominally at 123N}133N, althoughthis bifurcation latitude can change interannuallyfrom 113N to 14.53N. The NEC’s bifurcation tendsto migrate to the north during El Nin� o years and tothe south during La Nin� a years. Below the seasurface, the NEC’s bifurcation tends to shift north-ward with increasing depth. This tendency is due tothe fact that the southern limb of the wind-drivensubtropical gyre in the North PaciRc shifts to thenorth with increasing depth.

Branching northward from the NEC, theKuroshio Current east of the Philippine coast hasa mean geostrophic volume transport, referenced to1250dbar, of 25 Sv (1 Sverdrup"106 m3 s�1). Sea-sonally, the Kuroshio transport at this upstream

1414 KUROSHIO AND OYASHIO CURRENTS

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Figure 2 Schematic representation of the mean Kuroshio path (solid thick line) along the North Pacific western boundary. Thethick dashed line south of Taiwan denotes the wintertime branching of the Kuroshio water into the Luzon Strait in the form of a loopcurrent. PN-line denotes the repeat hydrographic section across which long-term Kuroshio volume transport is monitored (seeFigure 4). Selective isobaths of 100 m, 200 m, and 1000 m are depicted.

location has a maximum (&30Sv) in spring anda minimum (&19Sv) in fall. Similar seasonalcycles are also found in the Kuroshio’s transportsin the East China Sea and across the TokaraStrait.

As the Kuroshio Current Sows northward passingthe Philippine coast, it encounters the Luzon Straitthat connects the South China Sea with the openPaciRc Ocean (Figure 2). The Luzon Strait hasa width of 350km and is 2500m deep at its deepestpoint. In winter, part of the Kuroshio water hasbeen observed to intrude into the Luzon Strait andform a loop current in the northern South China Sea(see the dashed line in Figure 2). The loop currentcan reach as far west as 1173E, where it is blockedby the presence of the shallow shelf break off thesouth-east coast of China. The formation of theloop current is probably due to the north-east mon-soon, prevailing from November to March, whichdeSects the surface Kuroshio water into the north-ern South China Sea. During the summer monthsfrom May to September when the south-west

monsoon prevails, the Kuroshio Current passes theLuzon Strait without intrusion.

In the latitudinal band east of Taiwan(223N}253N), the northward-Sowing Kuroshio Cur-rent has been observed to be highly variable inrecent years. Repeat hydrographic and moored cur-rent meter measurements between Taiwan and thesouthernmost Ryukyu island of Iriomote show thatthe variability of the Kuroshio path and transporthere are dominated by Suctuations with a period of100 days. These observed Suctuations are caused byimpinging energetic cyclonic and anticyclonic eddiesmigrating from the east. The Subtropical Counter-current (STCC) is found in the latitudinal band of223N}253N in the western North PaciRc. TheSTCC, a shallow eastward-Sowing current, is highlyunstable due to its velocity shear with the under-lying, westward-Sowing NEC. The unstable wavesgenerated by the instability of the STCC-NECsystem tend to move westward while growing inamplitude. The cyclonic and anticyclonic eddies thatimpinge upon the Kuroshio east of Taiwan are


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Figure 3 Map of the root-mean-square (rms) sea surface height variability in the North Pacific Ocean, based on theTOPEX/POSEIDON satellite altimetric measurements from October 1992 to December 1997. Maximum rms values of '0.4 m arefound in the upstream Kuroshio Extension region south east of Japan. Sea surface height variability is also high in the latitudinalband east of Taiwan. (Adapted with permission from Qiu B (1999) Seasonal eddy field modulation of the North Pacific SubtropicalCountercurrent: TOPEX/Poseidon observations and theory. Journal of Physical Oceanography 29: 2471}2486.)

results of these large-amplitude unstable waves.Indeed, satellite measurements of the sea level(Figure 3) show that the Kuroshio east of Taiwanhas higher eddy variability than either its upstreamcounterpart along the Philippine coast or itsdownstream continuation in the East China Sea.

The Kuroshio Current enters the East China Seathrough the passage between Taiwan and IriomoteIsland. In the East China Sea, the Kuroshio pathfollows closely along the steep continental slope.Across the PN-line in the East China Sea (see Figure2 for its location), repeat hydrographic surveys havebeen conducted on a quarterly basis by the JapanMeteorological Agency since the mid-1950s. Basedon the measurements from 1955 to 1998, the vol-ume transport of the Kuroshio across this sectionhas a mean of 24.6 Sv and a seasonal cycle with

24.7 Sv in winter, 25.4 Sv in spring, 25.2Sv in sum-mer, and 22.8 Sv in fall, respectively. In addition tothis seasonal signal, large transport changes on lon-ger timescales are also detected across this section(see Figure 4). One signal that stands out in thetime-series of Figure 4 is the one with the decadaltimescale. SpeciRcally, the Kuroshio transport priorto 1975 was low on average (22.5 Sv), whereas themean transport value increased to 27.0Sv after1975. This decadal signal in the Kuroshio’s volumetransport is associated with the decadal Sverdruptransport change in the subtropical North PaciRcOcean.

Although the main body of the Kuroshio Currentin the East China Sea is relatively stable due to thetopographic constraint, large-amplitude meandersare frequently observed along the density front of


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Figure 4 Time-series of the geostrophic volume transport of the Kuroshio across the PN-line in the East China Sea (see Figure2 for its location). Reference level is at 700 dbar. Quarterly available transport values have been low-pass filtered by the 1-yearrunning mean averaging. (Data courtesy of Dr M. Kawabe of the University of Tokyo.)

the Kuroshio Current. The density front marks theshoreward edge of the Kuroshio Current and islocated nominally along the 200m isobath in theEast China Sea. The frontal meanders commonlyoriginate along the upstream Kuroshio frontnorth east of Taiwan and they evolve rapidlywhile propagating downstreamward. The frontalmeanders have typical wavelengths of 200}350km,wave periods of 10}20 days, and downstreamwardphase speeds of 10}25cm s�1. When reaching theTokara Strait, the fully developed frontal meanderscan shift the path of the Kuroshio Current in thestrait by as much as 100km.

Around 1283E}1293E and 303N, the KuroshioCurrent detaches from the continental slope andveers to the east toward the Tokara Strait. Noticethat this area is also where part of the Kuroshiowater is observed to intermittently penetratenorthward onto the continental shelf to feed theTsushima Current. The frontal meanders of theKuroshio described above are important forthe mixing and water mass exchanges betweenthe cold, fresh continental shelf water and thewarm, saline Kuroshio water along the shelf breakof the East China Sea. It is this mixture of the waterthat forms the origin of the Tsushima Current.The volume transport of the Tsushima Current isestimated at 2 Sv.

Region South of Japan

The Kuroshio Current enters the deep ShikokuBasin through the Tokara Strait. Combined surfacecurrent and hydrographic observations show thatthe Kuroshio’s volume transport through the TokaraStrait is about 30 Sv. Inference of transport from thesea level measurements suggests that the Kuroshio’s

transport across the Tokara Strait is maximum inspring/summer and minimum in fall, a seasonalcycle similar to that found in the upstream KuroshioCurrent. Further downstream, offshore of Shikoku,the volume transport of the Kuroshio has a meanvalue of 55Sv. This transport increase of theKuroshio in the deep Shikoku Basin is in part due tothe presence of an anticyclonic recirculation gyresouth of the Kuroshio. Subtracting the contributionfrom this recirculation reduces the mean eastwardtransport to 42 Sv. Notice that this ‘net’ eastwardtransport of the Kuroshio is still larger than itsinSow transport through the Tokara Strait. Thisincreased transport, &12Sv, is probably suppliedby the north-eastward-Sowing current that has beenoccasionally observed along the eastern Sank of theRyukyu Islands. Near 1393E, the Kuroshio Currentencounters the Izu Ridge. Due to the shallow north-ern section of the ridge, the Kuroshio Current exit-ing the Shikoku Basin is restricted to passing the IzuRidge at either around 343N where there is a deeppassage, or south of 333N where the ridge heightdrops.

On interannual timescales, the Kuroshio Currentsouth of Japan is known for its bimodal path Suctu-ations. The ‘straight path’, shown schematically bypath A in Figure 5, denotes when the KuroshioSows closely along the Japan coast. The ‘large-meander path’, shown by path B in Figure 5, sig-niRes when the Kuroshio takes a detouring offshorepath. In addition to these two stable paths, theKuroshio may take a third, relatively stable paththat loops southward over the Izu Ridge. This path,depicted as path C in Figure 5, is commonly ob-served during transitions from a meandering state toa straight-path state. As the meander path of theKuroshio can migrate spatially, a useful way of


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Figure 5 Schematic stable paths of the Kuroshio Current south of Japan. (Adapted with permission from Kawabe M (1985) Sealevel variations at the Izu Islands and typical stable paths of the Kuroshio. Journal of the Oceanography Society of Japan 41:307}326.) Selective isobaths of 1000 m, 2000 m, 4000 m, 6000m, and 8000 m are included.

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Figure 6 Time-series of the Kuroshio path index from 1955 to 1998, where the Kuroshio path index is defined as the offshoredistance of the Kuroshio axis (inferred from the 163C isotherm at the 200 m depth) averaged from 1323 to 1403E. Solid dots denotethe seasonal index values and the solid line indicates the annual average. (Adapted with permission from Qiu B and Miao W (2000)Kuroshio path variations south of Japan: Bimodality as a self-sustained internal oscillation. Journal of Physical Oceanography 30:2124}2137.)

indexing the Kuroshio path is to use the mean dis-tance of the Kuroshio axis from the Japan coastfrom 1323E to 1403E. South of Japan, the Kuroshioaxis is well represented by the 163C isotherm. Basedon this representation and seasonal water temper-ature measurements, Figure 6 shows the time-seriesof the Kuroshio path index from 1955 to 1998.A low index in Figure 6 denotes a straight path, anda high index denotes an offshore meandering pathof the Kuroshio. From 1955 to 1998, the Kuroshiolarge meanders occurred in 1959}62, 1975}79,

1982}88, and 1990. Clearly, the large-meanders oc-currence is aperiodic. Once formed, the meanderstate can persist over a period ranging from a yearto a decade. In contrast, transitions between themeander and straight-path states are rapid, oftencompleted over a period of several months. It isworth noting that development of the large me-anders is often preceded by the appearance ofa small meander south of Kyushu, which migrateseastward and becomes stationary after reaching1363E.


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Figure 7 Mean temperature map (3C) at the 300 m depth from 1976 to 1980. (Adapted with permission from Mizuno K and WhiteWB (1983) Annual and interannual variability in the Kuroshio Current System. Journal of Physical Oceanography 13: 1847}1867.)

Several mechanisms have been proposed to ex-plain the bimodal path variability of the Kuroshiosouth of Japan. Most studies have examined therelationship between the Kuroshio’s path patternand the changes in magnitude of the Kuroshio’supstream transport. Earlier studies of the Kuroshiopath bimodality interpreted the meandering path asstationary Rossby lee wave generated by the pro-truding coastline of Kyushu. With this interpreta-tion, the Kuroshio takes a meander path when theupstream transport is small and a straight pathwhen it is large. By taking into account the realisticinclination of the Japan coast from due east, morerecent studies have provided the following explana-tion. When the upstream transport is small, thestraight path is stable as a result of the planetaryvorticity acquired by the north-eastward-SowingKuroshio being balanced by the eddy dissipationalong the coast. When the upstream transport islarge, positive vorticity is excessively generatedalong the Japan coast, inducing the meander path todevelop downstream. In the intermediate transportrange, the Kuroshio is in a multiple equilibrium statein which the meandering and straight paths coexist.Transitions between the two paths in this case aredetermined by changes in the upstream transport(e.g. the transition from a straight path to a meanderpath requires an increase in upstream transport).

A comparison between the Kuroshio path vari-ation (Figure 6) and the Kuroshio’s transport in theupstream East China Sea (Figure 4) shows that the1959}62 large-meander event does correspond toa large upstream transport. However, this corre-spondence becomes less obvious after 1975, as therewere times when the upstream transport was large,but no large meander was present. Assuming that

the upstream Kuroshio transport after 1975 is in themultiple equilibrium regime, the correspondence be-tween the path transition and the temporal changein the upstream transport (e.g. the required trans-port increase for the transition from a straight pathto a meander path) is also inconclusive from thetime-series presented in Figures 4 and 6. Given thelow frequency and irregular nature of the Kuroshiopath changes, future studies based on longer trans-port measurements are needed to further clarify thephysics underlying the Kuroshio path bimodality.

Downstream Extension Region

After separating from the Japan coast at 1403E and353N, the Kuroshio enters the open basin of theNorth PaciRc Ocean where it is renamed theKuroshio Extension. Free from the constraint ofcoastal boundaries, the Kuroshio Extension hasbeen observed to be an eastward-Sowing inertial jetaccompanied by large-amplitude meanders and en-ergetic pinched-off eddies. Figure 7 shows the meantemperature map at 300m depth, in which theaxis of the Kuroshio Extension is well representedby the 123C isotherm. An interesting feature ofthe Kuroshio Extension east of Japan is the exist-ence of two quasi-stationary meanders with theirridges located at 1443E and 1503E, respectively. Thepresence of these meanders along the mean path ofthe Kuroshio Extension has been interpreted asstanding Rossby lee waves generated by the pres-ence of the Izu Ridge. A competing theory alsoexists that regards the quasi-stationary meanders asbeing steered by the eddy-driven abyssal meanSows resulting from instability of the KuroshioExtension jet.


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Near 1593E, the Kuroshio Extension encountersthe Shatsky Rise where it often bifurcates. The mainbody of the Kuroshio Extension continues eastward,and a secondary branch tends to extend north-east-ward to 403N, where it joins the eastward-movingSubarctic Current. After overriding the EmperorSeamounts along 1703E, the mean path of theKuroshio Extension becomes broadened and instan-taneous Sow patterns often show a multiple-jetstructure associated with the eastward-SowingKuroshio Extension. East of the dateline, the dis-tinction between the Kuroshio Extension and theSubarctic Current is no longer clear, and togetherthey form the broad, eastward-moving North PaciRcCurrent.

As demonstrated in Figure 3, the Kuroshio Exten-sion region has the highest level of eddy variabilityin the North PaciRc Ocean. From the viewpoint ofwind-driven ocean circulation, this high eddy varia-bility is to be expected. Being a return Sow compen-sating for the wind-driven subtropical interior

circulation, the Kuroshio originates at a southernlatitude where the ambient potential vorticity (PV)is relatively low. For the Kuroshio to smoothly re-join the Sverdrup interior Sow at the higher latitude,the low PV acquired by the Kuroshio in the southhas to be removed by either dissipative or nonlinearforces along its western boundary path. For thenarrow and swift Kuroshio Current, the dissipativeforce is insufRcient to remove the low PV anomalies.The consequence of the Kuroshio’s inability to effec-tively diffuse the PV anomalies along its path resultsin the accumulation of low PV water in its extensionregion, which generates an anticyclonic recirculationgyre and provides an energy source for Sow instabil-ity. Due to the presence of the recirculation gyre(Figure 8), the eastward volume transport of theKuroshio Extension can reach as high as 130Svsouth east of Japan. This is more than twice themaximum Sverdrup transport of about 50Sv in thesubtropical North PaciRc. The inSated eastwardtransport is due to the presence of the recirculating


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Sow to the south of the Kuroshio Extension. Al-though weak in surface velocity, Figure 8 showsthat the recirculating Sow has a strong barotropic(i.e. depth-independent) component. As a conse-quence, the volume transport of the recirculationgyre in this case is as large as 80 Sv.

In addition to the high meso-scale eddy variabil-ity, the Kuroshio Extension also exhibits large-scalechanges on interannual timescales. Figure 9A andB compares the sea surface height Reld in theKuroshio Extension region in November 1992 withthat in November 1995. In 1992, the KuroshioExtension had a coherent zonal-jet structureextending beyond the dateline. The zonal mean axis

position of the Kuroshio Extension from 1413E to1803E in this case was located north of 353N. Incontrast, the jet-like structure in 1995 was no longerobvious near 1603E and the zonal mean axis posi-tion shifted to 343N. Note that the changes in thezonal mean axis position of the Kuroshio Extensionhave interannual timescales (Figure 9C) and areassociated with the changes in the strength of thesouthern recirculation gyre. As the recirculation gyreintensiRes (as in 1992), it elongates zonally, increas-ing the zonal mean eastward transport of theKuroshio Extension and shifting its mean positionnorthward. When the recirculation gyre weakens (asin 1995), it decreases the eastward transport of the


Kuroshio Extension and shifts its zonal meanposition southward. At present, the cause of thelow-frequency changes of the recirculation gyre isunclear.

The Oyashio Current

Due to the southward protrusion of the AleutianIslands, the wind-driven subarctic circulation in theNorth PaciRc Ocean can be largely divided into twocyclonic subgyres: the Alaska Gyre to the east of thedateline and the Western Subarctic Gyre to the west(Figure 1). To the north, these two subgyres areconnected by the Alaskan Stream, which Sowssouth-westward along the Aleutian Islands as thewestern boundary current of the Alaska Gyre. Nearthe dateline, the baroclinic volume transport of theAlaskan Stream in the upper 3000m layer is esti-mated at about 15}20Sv. As the Alaskan StreamSows further westward, the deep passages between1683E and 1723E along the western Aleutian Islandsallow part of the Alaskan Stream to enter the BeringSea. In the deep part of the Bering Sea, the intrudingAlaskan Stream circulates anticlockwise and formsthe Bering Sea Gyre. The western limb of the BeringSea Gyre becomes the East Kamchatka Current,which Sows south-westward along the east coastof the Kamchatka Peninsula. The remaining part ofthe Alaskan Stream continues westward along thesouthern side of the Aleutian Islands and uponreaching the Kamchatka Peninsula, it joins the EastKamchatka Current as the latter exits the BeringSea.

As the East Kamchatka Current continues south-westward and passes along the northern Kuril Is-lands, some of its water permeates into the Sea ofOkhotsk. Inside the deep Kuril Basin in the Sea ofOkhotsk, the intruding East Kamchatka Currentwater circulates in a cyclonic gyre. Much of thisintruding water moves out of the Sea of Okhotskthrough the Bussol Strait (46.53N, 151.53E), whereit joins the rest of the south-westward-Sowing EastKamchatka Current. The East Kamchatka Currentis renamed the Oyashio Current south of the BussolStrait. Because of the intrusion in the Sea of Ok-hotsk, the water properties of the Oyashio Currentare different from those in the upstream East Kam-chatka Current. For example, the mesothermalwater present in the East Kamchatka Current (i.e.the subsurface maximum temperature water appear-ing in the halocline at a depth of 150}200m) is nolonger observable in the Oyashio. While high dis-solved oxygen content is conRned to above thehalocline in the upstream East Kamchatka Current,elevated dissolved oxygen values can be found

throughout the upper 700m depth of the Oyashiowater.

The baroclinic volume transport of the OyashioCurrent along the southern Kuril Islands and offHokkaido has been estimated at 5}10Sv from thegeostrophic calculation with a reference level of no-motion at 1000 or 1500m. Combining mooredcurrent meter and CTD (conductivity-temperature-depth) measurements, more recent observationsalong the continental slope south east of Hokkaidoshow that the Oyashio Current has a well-deRnedannual cycle: the Sow tends to be strong, reach-ing from surface to bottom, in winter/spring, andit is weaker and conRned to the layer shallowerthan 2000m in summer and fall. The total(baroclinic#barotropic) volume transport reaches20}30Sv in winter and spring, whereas it is only3}4Sv in summer and fall. This annual signal inthe Oyashio’s total transport is in agreementwith the annual signal in the Sverdrup transport ofthe wind-driven North PaciRc subArctic gyre.

After Sowing south-westward along the coast ofHokkaido, the Oyashio Current splits into twopaths. One path veers offshoreward and contributesto the east-north-eastward-Sowing SubArctic Cur-rent. This path can be recognized in Figure 10 bythe eastward-veering isotherms along 423N southeast of Hokkaido. Because the Oyashio Currentbrings water of subarctic origin southward, the Sub-arctic Current is accompanied by a distinct temperature-salinity front between cold, fresher waterto the north and warm, saltier water of subtropicalorigin to the south. This water mass front, referredto as the Oyashio Front or the Subarctic Front, hasindicative temperature and salinity values of 53Cand 33.8PSU at the 100m depth. Across 1653E,combined moored current meter and CTD measure-ments show that the Subarctic Current around 413Nhas a volume transport of 22 Sv in the upper 1000mlayer.

The second path of the Oyashio Current con-tinues southward along the east coast of Honshuand is commonly known as the Rrst Oyashio intru-sion. As shown in Figure 10, an addition to thisprimary intrusion along the coast of Honshu, thesoutherly Oyashio intrusion is also frequently ob-served further offshore along 1473E. This offshorebranch is commonly known as the second Oyashiointrusion. The annual mean Rrst Oyashio intrusioneast of Honshu reaches on average the latitude38.73N, although in some years it can penetrate asfar south as 373N (see Figure 11). In addition to theyear-to-year Suctuations, Figure 11 shows that thereis a trend for the Oyashio Current to penetratefarther southward after the mid-1970s. Both this


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Figure 11 Time-series of the annually averaged southernmost latitude of the first Oyashio intrusion east of Honshu. The dashedline shows the mean latitude (38.73N) over the period from 1964 to 1991. (Data courtesy of Dr K. Hanawa of Tohoku University.)

long-term trend and the interannual changes inthe Oyashio’s intrusions seem to be related to thechanges in the intensity of the Aleutian low atmo-spheric pressure system and the southward shift inthe position of the mid-latitude westerlies. It isworth noting that the anomalous southward intru-sion of the Oyashio Current not only inSuences thehydrographic conditions east of Honshu, it alsoaffects the environmental conditions in the Rshingground and the regional climate (e.g. an anomaloussouthward intrusion tends to decrease the air tem-perature over eastern Japan).

Concluding Remarks

Because the Kuroshio and Oyashio Currents trans-port large amounts of water and heat efRciently inthe meridional direction, there has been heightenedinterest in recent years in understanding the dynamicroles played by the time-varying Kuroshio andOyashio Currents in inSuencing the climate throughsea surface temperature (SST) anomalies. Indeed, out-side the eastern equatorial PaciRc Ocean, the largestSST variability on the interannual-to-decadal time-scale in the North PaciRc Ocean resides in the


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Figure 12 (A) Spatial pattern of the second empirical orthogonal function (EOF) mode of the wintertime sea surface temperatureanomalies (1950}1992) in the Pacific Ocean. This mode explains 11% of the variance over the domain. (B) Time-series of thewintertime sea surface temperature anomalies averaged in the Kuroshio-Oyashio outflow region (323N}463N, 1363E}1763W).(Adapted with permission from Deser C and Blackmon ML (1995) On the relationship between tropical and North Pacific seasurface variations. Journal of Climate 8: 1677}1680.)

Kuroshio Extension and the Oyashio outSow regions(Figure 12). Large-scale changes in the Kuroshio andOyashio current systems can affect the SST anomalyReld through warm/cold water advection, upwellingthrough the base of the mixed layer, and changes inthe current paths and the level of the meso-scale eddyvariability. At present, the relative roles played bythese various physical processes are not clear.

This article summarizes many observed aspects ofthe Kuroshio and Oyashio Current systems, al-though due to the constraints of space, importantsubjects such as the water mass transformationprocesses in regions surrounding the Kuroshio andOyashio and the impact of the Kuroshio and

Oyashio variability upon the oceanographic condi-tions in coastal and marginal sea areas have notbeen addressed. It is worth emphasizing that ourknowledge of the Kuroshio and Oyashio Currentshas increased signiRcantly due to the recent WorldOcean Circulation Experiment (WOCE) program(observational phase: 1990}1997). Fortunately,many of the observational programs initiated underthe WOCE program are being continued. With re-sults from these new observations, we can expect animproved description of the Kuroshio and OyashioCurrent systems in the near future, especially ofthe variability with timescales longer than thosedescribed in this article.


See also

Abyssal Currents. Okhotsk Sea Circulation.Paci\c Ocean Equatorial Currents. Wind DrivenCirculation.

Further ReadingDodimead AJ, Favorite JF and Hirano T (1963) Review of

oceanography of the subarctic PaciRc region. Bulletinof International North PaciTc Fisheries Commission13: 1}195.

Kawabe M (1995) Variations of current path, velocity,and volume transport of the Kuroshio in relation withthe large meander. Journal of Physical Oceanography25: 3103}3117.

Kawai H (1972) Hydrography of the Kuroshio Extension.In: Stommel H and Yoshida K (eds) Kuroshio } ItsPhysical Aspects, pp. 235}354. Tokyo: University ofTokyo Press.

Mizuno K and White WB (1983) Annual and interannualvariability in the Kuroshio Current system. Journal ofPhysical Oceanography 13: 1847}1867.

Nitani H (1972) Beginning of the Kuroshio. In: StommelH and Yoshida K (eds) Kuroshio } Its PhysicalAspects, pp. 129}163. Tokyo: University of TokyoPress.

Pickard GL and Emery WJ (1990) Descriptive PhysicalOceanography: An Introduction, 5th edn. Oxford:Pergamon Press.

Shoji D (1972) Time variation of the Kuroshio south ofJapan. In: Stommel H and Yoshida K (eds) Kuroshio} Its Physical Aspects, pp. 217}234. Tokyo: Universityof Tokyo Press.

Taft BA (1972) Characteristics of the Sow of theKuroshio south of Japan. In: Stommel H and YoshidaK (eds) Kuroshio } Its Physical Aspects, pp. 165}216.Tokyo: University of Tokyo Press.

Tomczak M and Godfrey JS (1994) Regional Oceanogra-phy: An Introduction. Oxford: Pergamon Press.


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