Transport of North Pacific Intermediate Water across Japanese WOCE sections

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Transport of North Pacific Intermediate Water across Japanese WOCE sections
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  Transport of North Pacific Intermediate Water across JapaneseWOCE sections Yuzhu You, 1,2  Nobuo Suginohara, 1,3 Masao Fukasawa, 4,5 Hiroyuki Yoritaka, 6 Keisuke Mizuno, 7 Yuji Kashino, 8 and Djoko Hartoyo 9,10 Received 27 September 2002; revised 14 January 2003; accepted 11 March 2003; published 19 June 2003. [ 1 ]  Under the auspices of the World Ocean Circulation Experiment (WOCE)Hydrographic Program, Japan carried out a total of four sections in the North Pacific, P2 at 30   N, P13 at 165  E, P9 at 137  E, and P8 at 130  E. These Japanese and several other WOCE sections are used to estimate the transport of North Pacific Intermediate Water (NPIW) in the subtropical gyre. Previous studies speculate about 3 Sv (1 Sv = 10 6 m 3 s  1 )of NPIW transport, srcinating mainly in Okhotsk Sea, entering the subtropical gyresouthward via the mixed water region in the east of Japan. The total subpolar input is 3.2Sv when another source of NPIW in the Gulf of Alaska, estimated at 0.2 Sv in this study,is added. This transport from the Japanese WOCE sections (a mean of 2.8 ± 0.2 Sv)compares well with several other sections (a mean of 2.5 ± 0.2 Sv). From a total of fivemeridional and two zonal sections, a mean transport of 2.7 ± 0.2 Sv is found in thedefining NPIW layer between  s  N  = 26.5 and 27.4 neutral density surfaces. The transport shows a steady southward and westward increase. About 3 Sv transport is found mainly inthe southern (south of 25   N) and western (west of the dateline) sections. Closer to thesubarctic-tropical frontal zone and in the eastern subtropical gyre, NPIW transport is lessthan 2 Sv. This implies that a lower subpolar input of about 2 Sv is sufficient for subtropical ventilation. Recirculation, entrainment with the upper and lower waters, andepineutral and dianeutral mixing likely add about 1 Sv to the NPIW transport. To interpret the transport distribution, a simple water mass mixing scheme is applied, which includestwo formation sources, Okhotsk Intermediate Water (OIW) and Gulf of AlaskaIntermediate Water (GAIW) in the subpolar regions, and a transformed aged NPIW(aNPIW) in the northwestern subtropical gyre southeast of Japan. The aNPIW mixingfraction increases southward and westward, while OIW and GAIW fractions decrease.This suggests that after NPIW source water parcels enter the subtropical gyre, they largelyrecirculate within the wind-driven gyre and thus become aged. Therefore one usually seesvery low oxygen and CFC values in the NPIW layer in the subtropical gyre.  I   NDEX  T   ERMS  :  4223 Oceanography: General: Descriptive and regional oceanography; 4283 Oceanography: General:Water masses; 4532 Oceanography: Physical: General circulation; 4536 Oceanography: Physical:Hydrography;  K   EYWORDS  :  North Pacific Intermediate Water, water mass mixing, NPIW transport, mixingfraction, WOCE, North Pacific Citation:  You, Y., N. Suginohara, M. Fukasawa, H. Yoritaka, K. Mizuno, Y. Kashino, and D. Hartoyo, Transport of North PacificIntermediate Water across Japanese WOCE sections,  J. Geophys. Res. ,  108 (C6), 3196, doi:10.1029/2002JC001662, 2003. 1. Introduction [ 2 ] North Pacific Intermediate Water (NPIW) is a well-studied salinity minimum layer (about 34.0–34.3 psu) at adepth range of 300–800 m in the subtropical gyre of the North Pacific [ Sverdrup et al. , 1942]. Its distinct salinityminimum follows the potential density surface  s q  = 26.8[  Reid  , 1965;  Talley , 1993] or a neutral density surface  s  N  = JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C6, 3196, doi:10.1029/2002JC001662, 2003 1 Center for Climate System Research, University of Tokyo, Tokyo,Honshu, Japan. 2  Now at University of Sydney Institute of Marine Science (USIMS),University of Sydney, Sydney, New South Wales, Australia. 3  Now at Frontier Observational Research System for Global Change,Kanazawa-ku, Yokohama, Japan. 4 Faculty of Marine Science and Technology, Tokai University, Shimizu,Shizuoka, Japan.Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JC001662 27  -  1 5  Now at Japan Marine Science and Technology Center, Yokosuka,Honshu, Japan. 6 Ocean Research Laboratory, Hydrographic Department, MaritimeSafety Agency, Tokyo, Honshu, Japan. 7  National Research Institute of Far Seas Fisheries, Shimizu, Honshu,Japan. 8 Japan Marine Science and Technology Center, Yokosuka, Honshu,Japan. 9 Badan Pengkajian Dan Penerapan Teknologi, Jakarta, Indonesia. 10  Now at CentreforMarineandCoastalStudies,SchoolofMathematics,University of New South Wales, Sydney, New South Wales, Australia.  26.9 [ You et al. , 2000]. The sources of NPIW are in thesubpolar regions, mainly the Okhotsk Sea and westernsubpolar gyre [ Talley , 1991;  Yasuda , 1997;  You et al. ,2000]. The Gulf of Alaska contributes to the NPIW salinityminimum to a much less extent and at shallower depth andmostly in winter [ Van Scoy et al. , 1991;  You et al. , 2000].The Okhotsk Sea may provide about 3 Sv (1 Sv = 10 6 m 3 s  1 ) of transport into the subtropical gyre via the mixedwater region (MWR) east of Japan [ Talley et al. , 1995; Talley , 1997]. The contribution from the Gulf of Alaska isunknown. In this paper, we use high-resolution World OceanCirculation Experiment (WOCE) sections to estimate NPIWtransport in the subtropical gyre, which has not been inves-tigated systematically yet. This study focuses on NPIW inthe subtropics, but also considers sources of NPIW in thesubpolar regions. Therefore we follow the second definitionof NPIW from  Talley  [1997]: the entire isopycnal layer in thesubtropical gyre, which is affected by input of water venti-lated in the northwestern subpolar gyre and Okhotsk Sea.[ 3 ]  Talley  [1991] points to the Okhotsk Sea as a major source of NPIW. The Kuril Islands provide a leaky bound-ary between the North Pacific Ocean and the Okhotsk Sea, permitting communication through numerous passages be-tween the islands. The deepest passage, the Bussol’ Strait, is2300 m deep. The second deepest passage is the Kruzensh-tern Strait, which is 1900 m deep. Even two shallower Friza and Chetvertyy Straits at 600 m deep are deep enoughfor the communication between the Pacific and Okhotsk intermediate waters. All ice in the Okhotsk Sea is first-year ice. Thus sea ice brine fluxes and associated vertical mixingmay be the principle mechanisms setting the density of  NPIW within the Okhotsk Sea. Considerable effort has beenmade to locate convection sites during winter in polynyasand outcrops of NPIW isopycnals.[ 4 ] The NPIW  s q  = 26.8 isopycnal likely outcrops inwinter because the summer water of upper 300 m on thenorthern and northwestern shelves of the Okhotsk Sea lies between  s q  = 26.6 and 27.0 [  Kitani , 1973;  Kitani and Shimazaki , 1971]. A pycnostad exists in the Kuril Basinof the Okhotsk Sea at   s q  = 26.6–27.0 [ Yasuda , 1997].During a survey of WOCE line P1W,  Freeland et al.  [1998]observed convection at   s q  = 26.8 in the Okhotsk Sea, but not at higher densities.  Martin et al.  [1998] identified polynyas not only on the northern and northwestern shelves, but also in Shelikhov Bay, the most northern and shallowest shelf (as counting for about 25% of the contribution todense water production). It is in these shelf regions that   s q  =26.8 outcrops.[ 5 ] The Turner angle indicates strong vertical diffusion inthe NPIW core at   s  N  = 26.9 in large regions of the Okhotsk Sea and northwestern subpolar gyre, and below on  s  N  =27.2 and 27.4, mainly within the Okhotsk Sea [ You et al. ,2000]. Although some Pacific water in the East KamchatkaCurrent flows into the Okhotsk Sea feeding the West Kamchatka Current, the East Kamchatka Current largelyfeeds the Oyashio Current along the Kuril-KamchatkaTrench without entering the Okhotsk Sea. Therefore  Youet al.  [2000] define the Okhotsk Intermediate Water (OIW)to include waters from both the Okhotsk Sea and north-western subpolar gyre.[ 6 ] Tritium observations suggest NPIWis ventilated in theGulf of Alaska in winter [ Van Scoy et al. , 1991]. However, Yuan and Talley ’s [1992] examination of the MarathonExpedition data at 152  W shows a shallow salinity mini-mum srcinating in the Gulf of Alaska, while the NPIWsalinity minimum comes from the west, namely the Okhotsk Sea.  Aydin et al.  [1998] use the WOCE P17N section tostudy modification of intermediate water in the Alaskan gyreand point to the importance of vertical mixing.  You et al. [2000] find strong diffusive convection on  s  N  = 26.5 andrelatively weak diffusive convection on the  s  N  = 26.9surface. Combined with a T-S analysis, they conclude the s  N  = 26.9 surface is the deepest penetration of Alaskan gyrewater or Gulf of Alaska Intermediate Water (GAIW). How-ever, the role played by GAIW in forming NPIW is not asstraightforward as OIW in the west because the wind-drivengyre theory requires streamlines to turn northward to closethe Alaskan gyre in the northeastern Pacific and southwardto close the subtropical gyre. If GAIW contributes to NPIW,then cross-frontal exchange of water between the two gyresmust occur. This is confirmed by a seasonal mapping of stream functions on neutral density surfaces [ You et al. ,2000]. In winter the Alaskan Stream recirculates southward.The recirculation of the Alaska Stream and intensificationof the Aleutian Low are responsible for meandering andfrontal mixing. Cross-frontal exchange occurs, as the zerowind stress curl line migrates meridionally. As water  parcels are exposed south of the zero wind stress curl line,Ekman suction produces downward motion, which is pro- posed for the formation of shallow and middle salinityminima [ Yuan and Talley , 1992]. In the eastern part of thesubtropical gyre (east of 170  W), upper NPIW (between s  N  = 26.5 and  s  N  = 26.9) consists of a mixture of OIW andGAIW [ You et al. , 2000]. However, a quantitative estimateof GAIW transport has not been made until now.[ 7 ] Various estimates of the transport of NPIW sourcewaters are summarized in Table 1. These estimates of NPIWtransport out of the Okhotsk Sea, including the dense water  produced on the north and northwestern shelves, range from0.2 to 0.5 Sv [  Alfultis and Martin , 1987;  Martin et al. ,1998]. The mixed water (mode water) exported from theKuril Basin into the western subpolar gyre in the Oyashio,transports about 1.1 ± 0.8 Sv [ Yasuda , 1997]. At theKuroshio-Oyashio interfrontal zone and MWR, wherenew NPIW is contained in the broad zonal currents, largetransports of 6.1–17.8 Sv (depending on the choice of reference levels) are found [ Yasuda et al. , 1996;  Talley et al. , 1995]. By applying a mixing ratio of 6:4 between theKuroshio and Oyashio,  Talley et al.  [1995] and  Talley [1997] infer 3 Sv of subpolar water enters the subtropicalgyre.  Yasuda  [1997] speculates a probably higher transport of 3–7 Sv. By adjusting epineutral and dianeutral diffusiv-ities to be K = 10 3.5 m 2 s  1 and D = 10  5 m 2 s  1 , 3 Sv of  NPIW transport can be produced by cabbeling throughdianeutral transport at the subarctic-tropical frontal zone(SATFZ) [ You , 2003]. These estimates of NPIW sourcetransports vary and depend on methods, data sources,reference levels, and chosen density ranges. Regardless of these differences, a transport of 3 Sv from the subpolar regions into the subtropical gyre is reasonable. Our studyfocuses on the NPIW transport within the subtropical gyre.Therefore our results can be used to determine if thesubpolar input of 3 Sv is consistent with the transport inthe subtropical gyre. 27  -  2 YOU ET AL.: TRANSPORT OF NORTH PACIFIC INTERMEDIATE WATER   [ 8 ] To make an accurate estimate of NPIW transport inthe subtropical gyre, we need to know both the spatial andvertical extent of NPIW in the subtropical gyre. The former involves the definition of various fronts, while the latter requires some density surfaces to bound the NPIW layer.Table 2 shows the definitions of fronts in the North Pacific:Subarctic Front (SAF), Kuroshio Bifurcation Front (KBF),Kuroshio Extension Front (KEF), Subtropical Front (STF),and Doldrum Front (DF). The definitions change dependingon data sources, methods, and depth ranges. Since the Table 1.  A Summary of NPIW Source Transports Data Resourceand MethodTransport,SvReferenceLevelDensityRange Note Author(s)Special sensor microwave/imager (SSM/I),Okhotsk Sea polynyas for 1990–1995 winters0.2–0.4 ice and dense water  production  Martin et al.  [1998]Satellite observation of icecover, northwestern shelf of Okhotsk Sea0.5 annual productionof dense water   Alfutis and Martin  [1987]CTD, summer 1991–93,east of Kuril Island andHokkaido, Kuril Basin,geostrophic1.1 ± 0.8> 3.0 bottom/1000 dbar 26.6–27.0  s q  Okhotsk Sea ModeWater speculated new NPIW Yasuda  [1997]CTD, May–June 1992,Kuroshio-OyashioInterfrontal zone (30   N–40   N,140  E–180  ), geostrophiccalculation9.5 1000 dbar 26.6–27.2  s q  new NPIW formed alongthe Kuroshio Extension Yasuda et al.  [1996]13.2 1500 dbar 26.6–27.2  s q 17.8 1500 dbar 26.6–27.5  s q CTD, MWR, April–June 1989,geostrophic calculation,mixing ratio 6:4 betweenKuroshio and Oyashio6.1 2000 dbar 26.64–27.4  s q  eastward out of MWR into subtropical gyre Talley et al.  [1995]3.0CTD section at 152  E in MWR,May 1981 and June 1982,geostrophic calculation andmixing ratio3.0 2000 dbar 26.64–27.4  s q  amount of subpolar water that enters thesubtropical gyre Talley  [1997]Hydrography, geostrophicseasonal transport difference,water mass mixing fraction  0.2 1300 dbar 26.5–26.9  s  N  Gulf of Alaska IntermediateWater (GAIW)this study1994 Levitus climatology[  Levitus , 1994], by dianeutraltransport calculation, andadjustment of diffusivities to be K = 10 3.5 m 2 s  1 andD = 10  5 m 2 s  1 3.0 26.5–27.4  s  N  transformed in thesubarctic-tropical frontalzone (36   –44   N,142  E–140  W) You  [2003] Table 2.  An Inventory of the Definitions of the Subarctic Front (SAF), Kuroshio Bifurcation Front (KBF), Kuroshio Extension Front (KEF), Subtropical Front (STF), and Doldrum Front (DF) Data Resources SAF KBF KEF STF DF Method Author(s)R/V  Thomas G. Thompson (1968–1974), STD/CTD,sound velocity40   –45   N(at 168  E)38  15 0  N a  (at 154E)28   –35   N(at 140  E)10   N  b upper layer T, S,density gradient,sound velocity,current/shear   Roden  [1975]TRANSPAC XBT(1976–1980),(0–450m), (30   –45   N,16  E–160  W)42   N c 39   N c 35   N 32   N temperature gradient at 300 m  Levine and White  [1983]MBT, XBT, JODC T/D(1976–1980), (130  E–170  W)40   N(6   –8  C)36   N(12  C)temperature variabilityand dynamics at 300 m  Mizuno and White  [1983]R/V  Thompson  (1983–1984)(at 165  E and 175  W), T, S,and density41   N 31.5   –33.5   N largest horizontaldensity gradient in the upper layer   Joyce  [1987]R/V  Oshoro Maru  etc.(since 1977), and Levitushydrography44.5   N(5  C)40.5   N d (6   –8  C)34.5   N d (12  C)water mass front at 300 m  Zhang and  Hanawa  [1993]72 cruise sections,  Levitus  [1982] 41   –44.5   N 34   N e 30.5   N T and S gradients at 10 m Yuan and Talley  [1996] NODC MBT, XBT, and SBT 41   N d (6   –8  C)35   N d (12  C)temperature range at 300 m Suga et al.  [1997] a  Called the Kuroshio Front.  b Also called the North Doldrum Salinity Front. c Called the North and South SAF, respectively. d Applying  Mizuno and White  [1983] definitions for KBF and KEF. e Called the ‘‘34   N front’’ [after   Lynn , 1986]. YOU ET AL.: TRANSPORT OF NORTH PACIFIC INTERMEDIATE WATER   27  -  3   NPIW layer at the SATFZ lies generally below the surfacelayer (at least 100 m), the fronts defined at 300 m by  Levineand White  [1983],  Mizuno and White  [1983], and  Zhang and Hanawa  [1993] are appropriate for the NPIW layer.The fronts move slightly northward with increasing depth(see Table 2). At 300 m, the fronts are relatively stable inlatitude [ You , 2003]. This depth roughly corresponds to the NPIW salinity minimum at   s  N  = 26.9 in the middle of theSATFZ. The SATFZ is bounded by the SAF (at about 44   N)and KEF (at about 36   N) with the KBF in the middle at 40.5   N. The STF and DF are located at 31   N and 10   N.Since the fronts are longitude dependent (more south in thewest and more north in the east), the defined latitude of thefronts is their mean position or their location in the central North Pacific.[ 9 ] Figure 1 provides a schematic view of the definingspatial area of NPIW (short dashed line) in the subtropicalgyre from water mass Turner angle [ You et al. , 2000], the NPIW sources (OIW, GAIW, and the aged NPIW(aNPIW)), the major frontal currents, the location of theMWR [ Talley , 1993], and transition domain (shaded)within the SATFZ (adapted from  You  [2003]). NPIW layer lies between  s  N  = 26.5 and 27.4 neutral density surfaces[  McDougall  , 1987] with the  s  N  = 26.9 surface followingthe salinity minimum. An additional surface  s  N  = 27.2 isdefined between  s  N  = 26.9 and 27.4, considering the deepeffect of NPIW on the water below (Figure 2). The upper  boundary at   s  N  = 26.5 separates NPIW from other water masses whose signatures are apparent short distancesabove  s  N  = 26.5 [ You , 2003]. The shallow signals arethe Shallow Salinity Minimum at   s q  = 25.8 [ Yuan and Talley , 1992], the North Pacific Subtropical Mode Water at  s q  = 25.4, and the North Pacific Central Mode Water at  s q  = 26.2 [  Nakamura , 1996;  Suga et al. , 1997]. The deepest signal above the NPIW is the Middle Salinity Minimum,which is shallower than  s q  = 26.4 with a mean density of  s q  = 26.35 [see  Yuan and Talley , 1992, Figure 3c], which isslightly less than  s  N  = 26.4. The  s  N  = 26.5 surface isdefined as the upper boundary of NPIW, which is also thedefined level of GAIW [ You et al. , 2000]. Further choicesof these neutral density surfaces and comparison with other definitions of NPIW, based on isopycnal surfaces, will begiven later. The objective of this study is to use WOCEsections to derive transports of NPIW in the subtropicalgyre and thus to examine the input from the subpolar sources. 2. Data and Methods [ 10 ] As a Japanese contribution to the WOCE program, atotal of four sections were carried out in the North Pacific, atranspacific zonal section, P2, and the three meridionalsections, P8, P9, and P13 (Figure 3). Almost every sectionis completed as a result of cooperative work by more thanone cruise carried out by different agencies.[ 11 ] P8 located in the western North Pacific along 130  Ecutting across the Kuroshio. This section was completedwith three cruises: R/V  Kaiyo Maru  in June/July 1996 bythe Japan Fisheries Agency, R/V  Kaiyo  in June/July 1996 by the Japan Marine Science and Technology Center (JAMSTEC), and R/V  Shoyo  in July/August 1996 by theHydrographic Department, Japan Maritime Safety Agency.The R/V  Kaiyo Maru  cruise took mainly bottle samplesfrom 8   N to 31   N south of Japan with sparse CTD stations.The R/V  Shoyo  cruise performed dense CTD sampling. Tomeasure the Kuroshio transport variability south of Japan,R/V  Shoyo  repeated stations between 26   and 31   N inOctober 1996. Both CTD and bottle stations were occupiedin the southern part of the section from the equator to near 10   N by R/V  Kaiyo . The equatorial region of the sectionextends into the Indonesian Exclusive Economical Zone andwas completed in cooperation with Indonesian scientists.[ 12 ] The P9 section, which lies 7   east of P8 at 137  E,was carried out on board R/V  Ryofu Maru  in boreal Figure 1.  A schematic of NPIW sources: Okhotsk Intermediate Water (OIW), Gulf of AlaskaIntermediate Water (GAIW), and aged NPIW (aNPIW); the major subarctic frontal current systems:Oyashio, Kuroshio, Subarctic Current (SAC), North Pacific Current (NPC), Alaska Current, AlaskaStream, California Current, Kuroshio Bifurcation, and Kuroshio Extension; and the mixed water region(MWR) and transition domain (shaded) (adapted from  You  [2003 and the extent of NPIW in thesubtropical gyre (dashed line) [ You et al. , 2000]. 27  -  4 YOU ET AL.: TRANSPORT OF NORTH PACIFIC INTERMEDIATE WATER   Figure 2.  A schematic of meridional section indicating North Pacific Subtropical Mode Water at   s q  =25.4, the shallow salinity minimum at   s q  = 25.8, North Pacific Central Mode Water at   s q  = 26.2, and themiddle salinity minimum at   s q  = 26.35 (about 26.4  s  N ); North Pacific Intermediate Water (NPIW)(shaded) bounded by the neutral density surfaces  s  N  = 26.5 (in the upper boundary),  s  N  = 26.9 (dashedline, following the NPIW salinity minimum), and  s  N  = 27.4 (in the lower boundary); and the latitudinallocations of the Subtropical Front (STF) at 32   N, Kuroshio Extension Front (KEF) at 36   N, KuroshioBifurcation Front (KBF) at 40.5   N, and Subarctic Front (SAF) at 44   N (marked in the top of the figure;the same for the following figures) (adapted from  You  [2003]). Figure 3.  Locations of four Japanese WOCE sections, P2, P13, P9, and P8, in the Pacific. P13 iscompleted with two legs, P13J (denoted by crosses) and P13C. YOU ET AL.: TRANSPORT OF NORTH PACIFIC INTERMEDIATE WATER   27  -  5
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