Stable volume and heat transports of the North Pacific subtropical gyre revealed by identifying the Kuroshio in synoptic hydrography south of Japan (original) (raw)
Related papers
Seasonal variation of heat and freshwater transports by the Kuroshio in the East China Sea
Journal of Marine Systems, 2000
The annual and seasonal means of total transport of volume, heat, and fresh water through a fixed section across the Ž . Kuroshio in the central East China Sea ECS are estimated. The estimation is done by integrating the absolute geostrophic volume transport for four water masses, using the hydrographic and sea surface current data obtained quarterly by the Japan Meteorological Agency during 1981-1992. While the total volume transport is dominated by Kuroshio Thermocline Water, Ž 6 3 y1 . ranging from 12.6 Sv in fall to 18.2 Sv in spring 1 Sv s 10 m s , the winter volume transport of 14.2 Sv is composed entirely of Kuroshio Thermocline Water. Both Kuroshio Surface Water and Kuroshio Intermediate Water contribute to the total transport in spring through fall with a spring transport of 25.9 Sv and a fall transport of 23.5 Sv. In summer, these three water masses plus ECS Shelf Water contribute to the total volume transport, causing a maximum value of 28.5 Sv. The northward heat transport is found to have a large seasonal variation with a maximum of 0.46 = 10 15 W in summer and a minimum of 0.25 = 10 15 W in winter, with an annual mean of 0.33 = 10 15 W. The northward freshwater transport also exhibits a large seasonal variation with a maximum of q1.7 = 10 6 kg s y1 in summer and a minimum of y2.0 = 10 6 kg s y1 in winter, about an annual mean of y0.2 = 10 6 kg s y1 . These results are based on the assumptions that the mean temperature and salinity in the southward return flow region have constant values of 15.118C and 34.6 psu and there is no w net meridional mass transport. The heat transport of the Kuroshio estimated for the same section by Bryden et al. Bryden,
For better understanding of interactions between the Kuroshio south of Japan and the climate system in the North Pacific, variations of the satellite-derived eddyremoved volume transport of the Kuroshio through-flow (KTVT) south of Japan in 1993 -2007 and the West Pacific Pattern index (WPPI) are compared with each other. Both of KTVT and WPPI with longer periods than 8 months have dominant components with about annual period. They have a maximum in every winter while their amplitudes change year by year. By the cross correlation analysis of their around annual period components, we found that interannual variations of WPPI in May and October lead to KTVT by 41-month while those of KTVT in February and August lead to WPPI by about 4 years, suggesting that the interactions between annual period components of WPPI and KTVT are dominated by seasonal different mechanisms.
Deep Sea Research Part II: Topical Studies in Oceanography, 2010
The East/Japan Sea (EJS) has unique water-mass characteristics in the western Pacific marginal seas due to limited exchange with open North Pacific. The major inflow of source water mass is North Pacific Subtropical Water (NPSW) carried by the Kuroshio branching and Tsushima Current. The locally formed cold/fresh waters from the Tatar Strait and Russia coast by winter convection mix with NPSW contributing to water mass transformation, especially during winter when upper isopycnal surfaces outcrop and thermocline is ventilated. The geographic limit of the Korea/Tsushima Strait (KTS) with a sill depth of about 120-140 dbar confines the inflow of lower NPSW, and so the EJS thermocline layer is somewhat truncated with a rather thin layer for about 100 dbar. This study uses high resolution conductivity-temperature-depth (CTD) and Argo data with a third decimal or higher accuracy for temperature and salinity obtained mainly by Research Institute of Oceanography, Seoul National University through domestic and international collaboration with several Korean ocean research institutes and Russia and USA partners since early 1990s. The basin covered data were divided into summer and winter half year representing seasonal difference since most CTD surveys were conducted purposely in summer and winter. Analysis is made for the upper layer, about 50-70 dbar, from the surface to the upper main thermocline and the thermocline layer down to about 150-180 dbar south of the subpolar front (SPF). The lower thermocline is defined slightly below the sill depth of the KTS, considering the deepening of NPSW after passing through the KTS. The thermocline layer is encompassed by three selected neutral density surfaces N =25.8, 26.4 and 27.0 with a distance of about 40 dbar between two neighboring surfaces. The core of thermocline is followed by the N =25.8 surface characterized by a salinity maximum and a minimum of potential vorticity. Winter convection is discussed and compared with three other major convection sites of the world's oceans, the Gulf of Lions, Labrador Sea and Greenland Sea, showing some common and distinctive features, especially the extremely low salinity of the EJS. Water-mass properties on neutral density surfaces are analyzed with the water-mass Turner angle (WTu) and circulation and transport are deducted from geostrophic calculations. From the 15 year mean hydrography, a basin-wide net annual mean transport of about 2.10±0.29 Sv (1 Sv=10 6 m 3 s -1 ) is estimated with summer and winter transports of 2.56±0.36 and 1.63±0.23 Sv, respectively. This transport is slightly less than the annual mean transport of the Tsushima Current at the KTS, 2.4 Sv from 3 cable and 2.3 Sv from other direct current meter and geostrophic methods but matches the ±14% error bar of ±0.29 Sv adjusted by ±150 dbar from the reference level of 800 dbar. This error bar is close to the error of ±0.34 Sv determined from water-mass conservation residual in a separated study. Three mechanisms are discovered to explain the seasonal difference in the Tsushima Current transports: the stronger winter Ekman pumping, outcropping and southward crossing flow. During winter, the Tsushima Current branches are imposed under strong wind stress curl in the Ulleung Basin and Yamato Basin, showing a doubling Ekman downwelling transport, partly weakening the Tsushima Current flow in the eastern boundary. Meanwhile the thermocline isopycnal surfaces outcrop in winter, reducing volume transport due to reduced space and thickness. The southward currents in the southern Ulleung Basin and Yamato Basin are perpendicular to the Tsushima Current branches west of Japan, which weakens the eastern boundary current in winter. Here NPSW is defined as the shallow component of North Pacific Central Water (NPCW). Since isopycnal surfaces in NPSW outcrop southeast of Japan where salinity and temperature maximum is found in the subtropical gyre center, NPSW is characterized by a distinct temperature and salinity maximum. NPSW is formed by Kuroshio recirculation in the first meander of the Kuroshio Extension. In contrast, the lower component of NPCW is formed east of Honshu, south of the subarctic-tropical frontal zone (SATFZ) with a slightly low salinity. Therefore, the latitudes of NPCW formation region are generally referred to about 35-40ºN (Tomczak and Godfrey, 2003), about 5º farther north of the NPSW formation region. The North Pacific Subtropical Mode Water (NPSTW) (Masuzawa, 1969) is a special case of NPSW, i.e., a freshly formed and well mixed component of NPSW characterized by a thermostad and pycnostad.
Progress in Oceanography, 2014
High-resolution reanalysis of heat transport in the northwestern Pacific marginal seas was conducted for the period January 1980-December 2009 using ensemble Kalman filter. An ocean circulation model with a grid of 0.1 Â 0.1°horizontal resolution and 20 vertical levels was used. Atmospheric forcing data from daily European Centre for Medium-Range Weather Forecasts were used in the ocean model. The assimilated data for the reanalysis were based on available observations of hydrographic profiles, including field surveys and Argo float and satellite-observed sea-surface temperature data. This study focused on mean and temporal variations in oceanic heat transport within the major straits among the marginal seas over 30 years. The mean heat transport in the Korea/Tsushima Strait and onshore transport across the shelf break in the East China Sea (ECS), Taiwan Strait, Tsugaru Strait, and Soya Strait were 182, 123, 82, 100, and 34 Â 10 12 W, respectively. The long-term trends in heat transport through the Korea/Tsushima Strait and Tsugaru Strait and onshore transport across the shelf break of the ECS were increasing, whereas the trend in heat transport through the Taiwan Strait was decreasing. There was little long-term change in heat transport in the Soya Strait. These long-term changes in heat transport through the Korea/ Tsushima Strait, across the shelf of the ECS, and through the Taiwan Strait may be related to increased northeasterly wind stress in the ECS, which drives Ekman transport onto the shelf across the shelf break.
Journal of Physical Oceanography, 2008
Middepth, time-mean circulation in the western North Pacific Ocean (28°-45°N, 140°-165°E) is investigated using drift information from the profiling floats deployed in the Kuroshio Extension System Study (KESS) and the International Argo programs. A well-defined, cyclonic recirculation gyre (RG) is found to exist north of the Kuroshio Extension jet, confined zonally between the Japan Trench (ϳ145°E) and the Shatsky Rise (ϳ156°E), and bordered to the north by the subarctic boundary along ϳ40°N. This northern RG, which is simulated favorably in the eddy-resolving OGCM for the Earth Simulator (OFES) hindcast run model, has a maximum volume transport at 26.4 Sv across 159°E and its presence persists on the interannual and longer time scales. An examination of the time-mean x-momentum balance from the OFES hindcast run output reveals that horizontal convergence of Reynolds stresses works to accelerate both the eastward-flowing Kuroshio Extension jet and a westward mean flow north of the meandering jet. The fact that the northern RG is eddy driven is further confirmed by examining the turbulent Sverdrup balance, in which convergent eddy potential vorticity fluxes are found to induce the cyclonic RG across the background potential vorticity gradient field. For the strength of the simulated northern RG, the authors find the eddy dissipation effect to be important as well.
Synoptic temperature structure of the East China and southeastern Japan/East Seas
Deep Sea Research Part II: Topical Studies in Oceanography, 2005
Air-deployed expendable bathythermograph (AXBT) surveys are used to investigate the synoptic temperature structure of the East China Sea (ECS), southern Yellow Sea, and southeastern Japan/East Sea (JES) from 1992-1995, in the depth range 0-400 m. The main results of this study are as follows. The regional winter mixed layer is shallow along the paths of the Kuroshio and Polar Front due to strong advection of warm water, which suppresses vertical mixing. The region where mixing is to the bottom is generally northwest of the 100-m isobath in the ECS and in the Tsushima Strait. Two September surveys reveal two different Kuroshio Front positions: in 1992 this front was far on-shelf of its mean position, preventing the formation of eddies at Mien Hwa Canyon; in 1993, the Kuroshio Front had migrated off-shelf, and eddies along the front had formed. The Kuroshio Current Branch West of Kyushu is not seen in summer at 60 m depth, only winter and spring; however, there is evidence of this branch current in all seasons at 100 m depth. Along the entire length of the ECS in all surveys except winter (where wind mixing had homogenized the water column), cross sections show that cold water (o18 1C) inshore of the Kuroshio reached westward of the 200-m isobath, onto the shelf, and as shallow as 60 m. From 1992-1995, the Nearshore Branch is present in all surveys, and the East Korean Warm Current is generally present in spring-fall, and strongest in each May survey. r
Temporal and spatial variability of volume transport of the Kuroshio in the East China Sea
Deep Sea Research Part I: Oceanographic Research Papers, 1993
f~ fill 4-000 Pnnted m Great Bream ~ tim3 Pergamon Pr~j Ltd Abstract--The temporal and spatial varmb,lnty of absolute geostrophnc volume transport of the Kuroshno us examined using h3drographtc and surface current data collected by the Japan Meteorological Agency on 39 transport sections made at six transects m the East China Sea and one transect lust outside the East China Sea during the three-year pernod 1986--1988. The Kurosh,o volume transport (KVT) ts estnmated by integration over each transect of the downstream (pos,tnve) part of the geostropMc velocity referenced to the observed surface current. Our analysts undncates that. on average, only about 25% of the KVT reside the East China Sea ts due to the baroclm,c or vertncally vary,rig component Whde KVTexhnbtts a small-scale temporal and spatial var0abdnty ~lth downstre,ml pha,.e speeds of order 8-19 km da,, -n and wavelengths of order 15(I---375 kin. mean Kt,'Tts spatmll 3, uniform v, ithm the East China Sea KVThas significant annual and anterannual v,trn,ttnons KVTnslargemsunm~er. is po,,tttvely correlated wuth local downstream v, lnd stre,,,,, and ts large during large meander event,, of the Kuroshio south of lap,in. Our bert estmutte of the mean KVT in the Ea,,t Ch,na Sea during IQS¢'~IgSS is 23 7 Sv __. 2.ll Sv. Th~s represents abottt one-h,df of the mean mternor S~.erdrup tr.,nsport at flus I,ttttude. Tile Kurostln+ quickly M~sorbs m~st of the rent,tmnng 0nternor S,,crdrt,p transl~rt alter nt exnts the East Chin,I Sea through the Tokara Str:ut. I. INTRODUCTION As rH~ western boundary current in the North l'acilic subtropical gyre, the Kuroshio plays an important role in the meridional transports of mass, momentum, heat and fresh water. . Therefore, in order to develop a better understanding of the interaction of the Kuroshio with continental shelf water within the ECS, and to estimate the net meridional transports of heat and freshwater of the subtropical gyre of the North Pacific, it is useful to examine first the temporal and spatial variability of volume transport of the Kuroshio throughout the ECS. "Faculty of Ftsherncs. Kago5hima University. 4--5(b-20, Shimoar:tt:t. Kagoshnma, 8(~), Japan t Department of Physical Oceanogr:lphy. Woods Flole Oce~,nographtc Instntution. Woods Hole. MA 02543. USA 5~3 58-1. H ICHI~t.'*A and R C. BEARDSLEY 120" 125" 130"E 35" 30" 25" 120" 125" 130"E F,g l. Map'J~owmg locatut~ns of the tross-strcam transects of thu Kuro,,hto in ttnc E,t,,t Chma Sea ,malyscd ill ttus ,,tudy. Al,,o shov, n t,, the location td the trt,mgul,lr ('FD/ADCP ,,urvcy mddc oil the !'~ V l'honlp~on Ill J,tnu,try 1986. The p,tth ol the Kuro',hl¢~ ,tntl other ¢t.rrcnt,, arc ~.ho~,~ll in ,.chore,me Iorm taken horn NH x'~t (It~72)
Response of ocean dynamics to multiple equilibria of the Kuroshio path South of Japan
Dynamics of Atmospheres and Oceans, 2019
Variability of the Kuroshio path to the south of Japan plays a central role in the local climate change and exerts tremendous influences on the local atmosphere and ocean. In this study, the response of ocean dynamics, in terms of the eddy kinetic energy (EKE), potential vorticity (PV), relative vorticity, and eddy-mean flow interaction, to the Kuroshio path change is discussed. Kuroshio path south of Japan includes the near-shore non-large meander (nNLM), the offshore non-large meander (oNLM), and the typical large meander (tLM). Analyses reveal that the distribution of EKE, PV, relative vorticity, and energy exchange between the eddy field and the mean flow respectively varies with the Kuroshio path: (1) The tLM has the maximum EKE along the path; (2) The positive and negative PV are located at the onshore and offshore side of Kuroshio axis, respevetively; (3) The distributions of anomalous relative voritcity of nNLM, oNLM, and tLM are consistent with sea surface height anomalies (SSHAs); (4) The tLM has the largest energy exchange between the eddy field and the mean flow in terms of the rate of barotropic energy conversion. On the other hand, the stability analysis of ocean currents suggests that the three Kuroshio paths south of Japan have their own intrinsic properties of the instability.
Journal of Geophysical Research, 2001
Ocean heat transport near the tropical/subtropical boundary of the North Pacific during 1993-1999 is described, including its mean and time variability. Twenty-eight trans-Pacific high-resolution expendable bathythermograph (XBT)/expendable conductivitytemperature-depth (XCTD) transects are used together with directly measured and operational wind estimates to calculate the geostrophic and Ekman transports. The mean heat transport across the XBT transect was 0.83 + 0.12 pW during the 7 year period. The large number of transects enables a stable estimate of the mean field to be made, with error bars based on the known variability. The North Pacific heat engine is a shallow meridional overturning circulation that includes warm Ekman and western boundary current components flowing northward, balanced by a southward flow of cool thermocline waters (including Subtropical Mode Waters). A near-balance of geostrophic and Ekman transports holds in an interannual sense as well as for the time mean. Interannual variability in geostrophic transport is strikingly similar to the pattern of central North Pacific sea level pressure variability (the North Pacific Index). The interannual range in heat transport was more than 0.4 pW during 1993-1999, with maximum northward values about 1 pW in early 1994 and early 1997. The ocean heat transport time series is similar to that of European Centre for Medium-Range Weather Forecasts air-sea heat flux integrated over the Pacific north of the XBT line. The repeating nature of the XBT/XCTD transects, with direct wind measurements, allows a substantial improvement over previous heat transport estimates based on one-time transects. A global system is envisioned for observing the time-varying ocean heat transport and its role in the Earth's heat budget and climate system.