seasonal variation of heat and freshwater transports by the kuroshio in the east china sea

Ž .Journal of Marine Systems 24 2000 119–129www.elsevier.nlrlocaterjmarsys

Seasonal variation of heat and freshwater transports by theKuroshio in the East China Sea

Hiroshi Ichikawa ), Masaaki ChaenFaculty of Fisheries, Kagoshima UniÕersity 4-50-20, Shimoarata, Kagoshima 890-0056, Japan

Received 1 January 1996; received in revised form 1 July 1996; accepted 14 December 1998

Abstract

The annual and seasonal means of total transport of volume, heat, and fresh water through a fixed section across theŽ .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 JapanMeteorological 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 Svs10 m s , the winter volume transport of 14.2 Sv is composedentirely of Kuroshio Thermocline Water. Both Kuroshio Surface Water and Kuroshio Intermediate Water contribute to thetotal transport in spring through fall with a spring transport of 25.9 Sv and a fall transport of 23.5 Sv. In summer, these threewater masses plus ECS Shelf Water contribute to the total volume transport, causing a maximum value of 28.5 Sv. Thenorthward heat transport is found to have a large seasonal variation with a maximum of 0.46=1015 W in summer and aminimum of 0.25=1015 W in winter, with an annual mean of 0.33=1015 W. The northward freshwater transport alsoexhibits a large seasonal variation with a maximum of q1.7=106 kg sy1 in summer and a minimum of y2.0=106 kgsy1 in winter, about an annual mean of y0.2=106 kg sy1. These results are based on the assumptions that the meantemperature and salinity in the southward return flow region have constant values of 15.118C and 34.6 psu and there is no

wnet meridional mass transport. The heat transport of the Kuroshio estimated for the same section by Bryden et al. Bryden,xH., Roemmich, D., Church, J., 1991. Ocean heat transport across 24 N in the Pacific. Deep-Sea Res. 38, 297–324. for June

1985 is shown to be 20% larger than the annual mean. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: East China Sea; water mass; transport; Kuroshio; seasonal change

1. Introduction

Ž .The Kuroshio enters the East China Sea ECSfrom the east of Taiwan, and flows northeastward

) Corresponding author. Tel.: q81-99-286-4101; Fax: q81-99-286-4103.

ŽE-mail address: [email protected] H..Ichikawa .

Ž .along the continental shelf margin Fig. 1 . As thewestern boundary current in the North Pacific sub-tropical gyre, the Kuroshio is considered to play animportant role in the meridional transports of heatand fresh water. In order to estimate the meridionalheat transport, hydrographic and ADCP measure-ments were made in June 1985 in the Kuroshioregion northwest of Okinawa in the ECS as part of atranspacific section made along 248N in May–June

0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0924-7963 99 00082-2

( )H. Ichikawa, M. ChaenrJournal of Marine Systems 24 2000 119–129120

Fig. 1. Map showing the fixed hydrographic and sea surfacecurrent station in the schematic of current system along the PN

Ž .line in the ECS taken from Nitani 1972 .

1985 aboard the R.V. Thompson. Using these data,Ž .Bryden et al. 1991 estimated the volume transport

Ž 6 3 y1.of the Kuroshio to be 28.3 Sv 1 Svs10 m s .The transport-averaged mean temperature is esti-mated at 18.468C, from which we can estimate the‘‘temperature’’ transport to be 2.14=1015 W bymultiplying mean temperature times volume trans-port. They also estimated the ‘‘heat’’ transport of theKuroshio to be 0.39=1015 W by multiplying thedifference in mean temperature in the Kuroshio re-gion in the ECS and the southward return flowregion times the volume transport in the Kuroshioregion. They showed that the heat transport of theKuroshio is about half the ocean heat transport across248N in the North Pacific.

However, the water transported by the Kuroshioin the ECS is modified by mixing with shelf water ofwhich salinity and temperature have large seasonalvariations, and the volume transport of the Kuroshioin the ECS has both large seasonal variation and

Ž8–30 day period fluctuations e.g., Ichikawa, 1993;.Ichikawa and Beardsley, 1993 , suggesting that the

volume and heat transports estimated by Bryden etŽ .al. 1991 may not represent an annual mean value.

In order to estimate the annual mean meridional

transports of heat and fresh water of the subtropicalgyre in the North Pacific with some certainty, it isnecessary to elucidate their seasonal change. As theKuroshio warm saline water originates from east ofTaiwan while the less saline shelf water from riverrunoff from China, it is useful to first examine thecontributions from different water masses to the totalvolume transport in the ECS. This will help inidentifying the mechanisms that contribute to sea-sonal variability in the heat and freshwater trans-ports.

One method for estimating the seasonal variationsof volume, temperature and salt transport of the

Ž .Kuroshio in the ECS is to a estimate the seasonalmean values of sea surface current, temperature andsalinity, using data obtained quarterly at geometri-

Ž .cally fixed section; b compute the seasonal meanabsolute geostrophic current referred to the seasonal

Ž .mean sea surface current; and c compute the vol-ume transport by integrating the velocity itself overthe section, and the temperature and salt transportsby multiplying temperature and salinity times veloc-ity and integrating over the section, respectively.However, this procedure results in the artificial hori-zontal smoothing of the sea surface current andequivalent artificial mixing of water masses espe-cially in the frontal region where the front canchange its position significantly over time scales lessthan 1 month. Therefore, the seasonal mean velocity,temperature and salinity obtained by the methoddescribed above are not appropriate to be used in theestimation of annual and seasonal means of tempera-ture and salt transports. In this paper, we first definethe TS segments, estimate next the absolutegeostrophic volume transport for each TS segmentand transect, and finally estimate the seasonal meanvolume transport for each TS segment. The annualmean is estimated by averaging the seasonal means.The volume transports for individual water massesare obtained by integrating the annual or seasonalmean volume transport for each TS segment over allthe TS segments composing the water mass.

The data and analysis method used in this paperare described next in Section 2. Section 3 presentsthe annual and seasonal mean total transport ofvolume, temperature and salt together with the vol-ume transports of individual water masses, and the

Ž .meridional heat and salt freshwater transports by

( )H. Ichikawa, M. ChaenrJournal of Marine Systems 24 2000 119–129 121

the Kuroshio in the ECS. The results are summarizedin Section 4.

2. Data source and analysis method

The hydrographic and sea surface current datawere obtained quarterly by the Japan MeteorologicalAgency during 1981–1992 at 11 fixed stations alongthe PN line across the Kuroshio in the central ECS

Ž .from 30.08N, 124.58E to 27.58N, 128.258E Fig. 1 .The surface current were measured by geomagnetic

Ž .electrokinetograph GEK or Doppler current mea-Ž .surement system DCMS and reported with 18 reso-

Ž y1 .lution for direction and 0.1 knot 5 cm s resolu-tion for speed. It should be mentioned that theuncertainty of the current speed used in this study issmaller than 0.1 knot because the inherent uncertain-ties in the GEK and DCMS measurements weresmaller than 0.1 knot even though the current datawere reported with only 0.1 knot resolution.

The geometric segments are defined by the hori-zontal distance between adjacent hydrographic sta-tions and the vertical width of 1 dbar from seasurface to the maximum depth which is fixed foreach station pair. The TS segments are defined by18C width in potential temperature and 0.1 psu widthin salinity. The absolute geostrophic current is deter-mined at the center of each geometric segment withreference to the mean value of sea surface currentsobserved at each of adjacent hydrographic stations.

ŽThe geostrophic volume transport positive for.northeastward for each geometric segment is esti-

mated by multiplying the absolute geostrophic cur-rent at the center of each geometric segment timesthe area of each geometric segment. The mean watertype for each geometric segment is defined by thepair of mean potential temperature and salinity val-ues that are determined from four interpolated valuesat two adjacent hydrographic stations with a 1 dbarinterval. The positive and negative components oftotal volume transport for each geometric segment inevery transect are accumulated at corresponding TSsegment separately by its water type. The seasonalmean positive and negative volume transports foreach TS segment are estimated by averaging thevolume transport for each TS segment in the same

season in various years. The annual mean volumetransport for each TS segment is estimated by aver-aging the seasonal mean volume transport for eachTS segment.

The annual and seasonal mean positive and nega-Ž .tive components of total transport V and V forp n

particular water mass crossing the section are esti-mated by:

V sÝV ,p p i

V sÝV ,n n i

where V and V are the annual or seasonal meanp ni i

positive and negative components of total volumetransport in each TS segment which defines theparticular water mass.

The annual and seasonal means of total volumeŽ . Ž .transport VT , temperature transport TT and saltŽ .transport ST of particular water mass crossing the

section are estimated by:

VTsÝV ,i

sÝ V qV ,Ž .p ni i

TTsrC Ý V T ,Ž .p i i

STsrÝ V S ,Ž .i i

where V is the annual or seasonal mean total vol-i

ume transport for each TS segment defining theparticular water mass, T and S the mean potentiali i

temperature and salinity of each TS segment, respec-tively, r the sea water density approximated to havea constant value of 103 kg my3, and rC thep

specific heat content for unit volume approximatedto have a constant value of 4.1=106 J my3 Ky1.

The transport-averaged annual and seasonal meanŽ . Ž .temperature T and salinity S for each water mass

are defined by:

TsTTrVT,

SsSTrVT.

In order to avoid the effects of differences ofsection area and month of observation among tran-sects on transport estimation, individual transectswere not used if the hydrographic or sea surface


current observation was not made at more than onestation, or if the maximum depth of the hydrographicobservation was shallower than the standard value atmore than one station, or if the observations were notmade in the months of January, April, July andOctober. Therefore, the seasonal mean transport foreach TS segment is estimated by averaging the trans-port for each transect in the same month over 6–8years. Table 1 shows the data sets used in the presentstudy, and Table 2 shows the location and waterdepth of each station and the fixed maximum depthsfor each pair of stations.

It should be mentioned here that the station spac-ing of both the hydrographic and current data used inthis study is 30 km in the strong current region.Using the hydrographic and surface current dataobtained by the Japan Meteorological Agency,

Ž .Ichikawa and Beardsley 1993 computed the abso-lute geostrophic volume transport of the Kuroshio inthe ECS by the same calculation method of absolutegeostrophic velocity as adopted in this study. Theyestimated that when the minimum station spacing forsurface current data is larger than 15 km, the com-bined experimental uncertainty in the total volumetransport due to station spacing and tidal and wind-driven surface current contamination reaches up to10–15 Sv. As the total volume transports estimatedin this study represent the seasonal mean values for

Table 1List of data sets used in the estimation of seasonal mean trans-ports. Open circles indicate the month and year in which data setis used

Year January April July October

1981 ` `1982 ` ` `1983 ` ` `1984 ` ` `1985 ` `1986 ` `1987 `1988 `1989 ` ` ` `1990 ` `1991 ` ` `1992 ` `Total 8 6 6 8

Table 2The nominal location and water depth of each station and thefixed maximum depth for each station pairs

Station Latitude Longitude Water Maximumnumber N E depth depth

Ž . Ž .m m

1 27–30 128–15 1015800

2 27–48 128–48 948800

3 28–06 127–21 1024800

4 28–16 127–08 970250

5 28–25 126–54 277125

6 28–33 126–41 153100

7 28–42 126–27 128100

8 28–59 126–00 11775

9 29–18 125–32 9775

10 29–36 125–05 8750

11 30–00 124–30 68

6–8 years, the uncertainty in the seasonal mean totalvolume transport is reduced to 3–7 Sv.

3. Volume, heat and salt transports

3.1. Total Õolume transport

Figs. 2 and 3 show the annual mean volumeŽ .transport for each TS segment T–S–V diagram and

the annual mean volume transport for a unit tempera-Ž .ture width of 18C T–V diagram . Fig. 2 shows that

the water flowing across the section during the yearcovers a wide range of temperature and salinity withtotal volume transport from y0.2 to q0.2 Sv perunit TS segment. However, most of the water trans-ported northeast by the Kuroshio are, in general,confined in a very narrow TS region with the maxi-mum total volume transport of 1.18 Sv for TS seg-ment centered at 188C and 34.8 psu. The existence ofthree maxima of volume transport per unit tempera-ture width found in Fig. 3 indicates that three watermasses can be defined by potential temperature, i.e.,T F11.58C, 11.5-T -22.58C, and T G22.58C.u u u


Ž .Fig. 2. Annual mean total volume transport Sv for each TS segment. Color scales in positive and negative values are different from eachother, and zero volume transport is shown by green.

Table 3 shows the annual and seasonal meanvalues of total, negative, and positive volume trans-port. The total volume transport in winter is muchsmaller than in other seasons. The total volumetransport of 28.3 Sv in June 1985 estimated by

Ž .Bryden et al. 1991 is nearly equal to the seasonalmean summer value found in this study. The total

Fig. 3. Annual mean volume transport for temperature width of18C. Full line: total transport; broken line: negative component;dotted line: positive component.

Žvolume transport has a larger seasonal variation 14.4. Ž .Sv than the positive component 12.1 Sv due to the

seasonal variation of negative transport.It should be noted here that the annual mean

positive component of the volume transport esti-mated in this study is nearly equal to that estimated

Ž .by Ichikawa and Beardsley 1993 from hydro-graphic and sea surface current data at each of ninetransects on the PN line across the Kuroshio in the

Table 3The annual and seasonal mean values of total, negative, andpositive volume transport in the ECS

Total Negative Positivetransport transport transportŽ . Ž . Ž .Sv Sv Sv

aŽ .Annual mean 23.03 y4.35 27.38 27.6Ž .Winter January 14.17 y5.87 20.04Ž .Spring April 25.88 y4.65 30.52Ž .Summer July 28.54 y3.60 32.14

Ž .Fall October 23.52 y3.28 26.80Ž .June 1985 28.3 Bryden et al., 1991

a Ž .Ichikawa and Beardsley 1993 : estimated from hydrographicand sea surface current data at each transect during 1986–1988 onthe PN line.


Fig. 4. Definitions of water masses in the ECS. KSW: theKuroshio Surface Water; KTW: the Kuroshio Thermocline Water;KIW: the Kuroshio Intermediate Water; ESW: the Shelf Water inthe ECS.

ECS during 1986–1988. This result supports thevalue of 27.4 Sv as an accurate, well-fixed estimateof the Kuroshio annual mean positive volume trans-port even though the data used in this study have lessthan ideal spatial sampling.

3.2. Volume transport for indiÕidual water masses

In order to examine the contributions of particularwater masses to the seasonal variation of total vol-ume transport, we have estimated the volume trans-port for the following four water masses defined inFig. 4 by the dependency of volume transport ontemperature and salinity shown in Figs. 2 and 3;

Ž .Kuroshio Surface Water KSW , Kuroshio Thermo-Ž .cline Water KTW , Kuroshio Intermediate Water

Ž . Ž .KIW and ECS Shelf Water ESW .

Table 4The annual and seasonal mean values of total, negative, andpositive volume transport in the ECS for the warm Kuroshio

Ž .Surface Water T G22.58C, SG34.15 psuu


Annual mean 4.36 y0.22 4.58Ž .Winter January 0.39 0.00 0.39Ž .Spring April 3.67 y0.06 3.72Ž .Summer July 7.01 y0.48 7.50

Ž .Fall October 6.38 y0.33 6.71

Table 5The annual and seasonal mean values of total, negative, andpositive volume transport in the ECS for the saline KuroshioThermocline Water defined in Fig. 4


Annual mean 15.06 y1.55 16.62Ž .Winter January 14.15 y2.73 16.88Ž .Spring April 18.22 y1.50 19.72Ž .Summer July 15.31 y1.25 16.56

Ž .Fall October 12.57 y0.73 13.30

Tables 4–7 show the annual and seasonal meanvalues of total, negative and positive volume trans-port for the warm KSW, the saline KTW, the lesssaline KIW, and the ESW, respectively. Fig. 5 showsthe seasonal variation of contributions in each watermass to the total volume transport. The total volumetransport for the KSW has a large seasonal variation,with large values of 6.4–7.0 Sv in summer to falland a minimum of 0.4 Sv in winter. In contrast, theKTW has a smaller seasonal variation with a maxi-mum of 18.2 Sv in spring and a minimum of 12.6 Svin fall. According to Kutsuwada and TeramotoŽ .1987 , the interior Sverdrup transport at 26–288Nhas an annual cycle with a maximum value of 90.2Sv in February and a minimum value of 20.4 Sv inSeptember. The annual signals of the total and posi-tive volume transport for the KTW are nearly inphase with their result, suggesting that only theseasonal variation of the transport of KTW is domi-nated by basin-wide variation of wind stress.

Table 6The annual and seasonal mean values of total, negative, andpositive volume transport in the ECS for the less saline Kuroshio

ŽIntermediate Water 3.58CFT F11.58C, 34.05 psuFSF34.45u

.psu


Annual mean 2.65 y1.82 4.42Ž .Winter January y0.21 y2.63 2.43Ž .Spring April 3.33 y1.89 5.22Ž .Summer July 3.85 y1.41 5.26



Table 7The annual and seasonal mean values of total, negative, andpositive volume transport for the ECS Shelf Water defined in Fig.4


Annual mean 0.95 y0.76 1.71Ž .Winter January y0.17 y0.51 0.33Ž .Spring April 0.66 y1.20 1.86Ž .Summer July 2.37 y0.45 2.82


The total volume transport of KIW occupying thelower layer in the Okinawa Trough has a smallnegative value of y0.2 Sv in winter but a positivevalue of 3.3–3.9 Sv in the other seasons due to theminimum positive transport and maximum negativetransport in winter, suggesting that the winter deepcirculation in the Okinawa Trough differs greatlyfrom the other three seasons. The total volume trans-port of ESW has a large positive value of 2.4 Svonly in summer, with rather smaller values betweeny0.2 and 0.9 Sv in the other seasons. This resultcoincides with the fact that the river discharge from

Ž .the Changjian Yangtze River has the largest valueŽ .in summer Chen et al., 1994 .

These results indicate that the total volume trans-port is dominated by KTW in all seasons, and it is

Fig. 5. Seasonal variation of contribution of each water mass tothe total volume transport. Dash–dot line indicates the totalvolume transport across the section. The areas between full anddotted lines, dotted and broken lines, and broken and dash–dotlines are the contributions of KIW, KSW and ESW, respectively.

the sole contribution in winter. Both KSW and KIWcontribute from spring through fall, with ESW mak-ing a significant contribution only in summer.

3.3. Temperature and salt transports

Table 8 shows the annual and seasonal meanvalues of total transport of temperature and salt, andthe transport-averaged mean values of temperatureand salinity. Note that the transport-averaged meantemperature is the highest in winter, not in summer.This seems to contradict the general thought that thetemperature should have its lowest value in winterwhen the heat loss from the sea surface to theatmosphere is largest. While the section-averagedmean temperature has a minimum in winter and amaximum in summer, the transport-averaged meantemperature depends not only on the section area andtemperature but also on the current. Except in winter,the low-temperature KIW contributes to the totalvolume transport and causes a decrease in the meantemperature. The transport-averaged mean salinity isthe highest in winter and the lowest in summer. Thesaline KTW is the sole contributor to the total vol-ume transport in winter, while the less saline KSWand much less saline ESW contribute in addition toKTW to the total volume transport in summer.

As the ratios of seasonal means to annual mean oftemperature have very small variation, ranging from94.8% in spring to 104.7% in winter, the seasonalmean total temperature transport is nearly propor-

Table 8The annual and seasonal mean values of temperature, total tem-perature transport, salinity, and total salt transport

Mean Total Mean Totaltemperature temperature salinity saltŽ . Ž .8C transport psu transport

15 9 y1Ž . Ž .10 W 10 kg s

All seasons 18.60 1.76 34.609 0.797Ž .Winter January 19.47 1.13 34.740 0.492Ž .Spring April 17.64 1.87 34.655 0.897Ž .Summer July 19.06 2.23 34.542 0.986

Ž .Fall October 18.57 1.79 34.562 0.813a bJune 1985 18.46 2.14 34.590 0.979

a Interpolated value from mean temperature.b Estimated by multiplying the interpolated mean salinity times

Ž .total volume transport 28.3 Sv .


tional to the total volume transport. For the samereason, the seasonal mean total salt transport isnearly proportional to the total volume transport. Asthe mean temperature and the total volume transportestimated for the same section by Bryden et al.Ž .1991 are nearly equal to the summer mean valuesfound here, the total temperature transport in June1985 is nearly equal to the summer mean value.

3.4. Meridional heat and freshwater transports

Ž .Bryden et al. 1991 showed the meridional heattransport by the Kuroshio in June 1985 to be 0.39=

1015 W by multiplying the difference between themean temperature of 18.468C in the Kuroshio regionin the ECS and that of 15.118C in the southwardreturn flow region times the volume transport of 28.3Sv in the Kuroshio region. The current and tempera-ture may vary seasonally only in the upper severalhundred meters in the interior southward currentregion. Since the total water depth there is more than5000 m, we can assume that the mean temperature ofthe southward return flow region has a nearly con-stant value during the year. Table 9 shows the annualand seasonal mean values of meridional heat trans-port by the Kuroshio in the ECS estimated by adopt-ing the constant value of 15.118C as the mean tem-perature in the southward return flow region. Thistable indicates that the meridional heat transport bythe Kuroshio in the ECS has a large seasonal varia-tion, with a maximum of 0.46=1015 W in summerand a minimum of 0.25=1015 W in winter, aboutan annual mean of 0.33=1015 W. While the total

Table 9The annual and seasonal mean values of meridional heat transportin the ECS for using 15.118C as mean temperature of southwardreturn flow region

Mean Total Meridionaltemperature volume heatŽ .8C transport transport

15Ž . Ž .Sv 10 W

All seasons 18.60 23.0 0.33Ž .Winter January 19.47 14.2 0.25Ž .Spring April 17.64 25.9 0.27Ž .Summer July 19.06 28.5 0.46

Ž .Fall October 18.57 23.5 0.33June 1985 18.46 28.3 0.39

volume transport in winter is much smaller than inspring, the heat transport in winter and spring arenearly equal to each other due to the difference ofmean temperature. These results indicate that theheat transport estimated for the same section by

Ž .Bryden et al. 1991 is 20% larger than the annualmean value in the ECS found here.

Using the same approach, we can estimate themeridional salt or freshwater transport by theKuroshio in the ECS by assuming a constant salinityvalue for the southward return flow region. Beforeshowing the results, the relation between salt andfreshwater transports will be described next.

The total volume transport across the 248Ntranspacific section is approximately zero so that wecan take:

V qV yV s0,1 2 0

where V is the volume transport of the southward0

return flow, V the total volume transport of the1

Kuroshio in the ECS, and V the Ekman transport2

positive northward across the section. NeglectingŽ .density variations, the total salt transport TST and

Ž .total freshwater transport TFT across the sectioncan then be computed using:

w xTSTsr S V qS V yS V ,1 1 2 2 0 0

sr S yS V q S yS V ,Ž . Ž .1 0 1 2 0 2

TFTsr 1yS V q 1yS V y 1yS V ,Ž . Ž . Ž .1 1 2 2 0 0

sr S yS V q S yS V ,Ž . Ž .0 1 1 0 2 2

syTST,

where S , S , S are the transport-averaged mean0 1 2

salinities of the southward return flow, the Kuroshioin the ECS, and the Ekman flow, respectively. Theseequations show that the freshwater transport of theKuroshio in the ECS is equal but opposite in sign tothe salt transport of the Kuroshio. It should be notedhere that the TST and TFT defined here do notinclude the salt and freshwater fluxes across thesection due to net meridional mass flux discussed by

Ž .Wijffels et al. 1992 .Ž .Bryden et al. 1991 estimated the salt transport

across the 248N section to be 5.5=106 kg sy1

northward in June 1985 by multiplying salinity timesvelocity and integrating over the entire transpacificsection including the Kuroshio. The transport-aver-


aged mean salinity in the ECS in June 1985 isestimated to be 34.590 psu by interpolation using thedependency of mean salinity on mean temperature asshown in Table 9. Using these values and the Ekmanvolume transport of 12.0 Sv estimated by Bryden et

Ž .al. 1991 , we can estimate the mean salinity S of2

the surface Ekman flow from:

34.59yS 28.3q S yS 12.0s5.5.Ž . Ž .0 2 0

Assuming S equals the section-averaged salinity of0Ž .34.6 psu Bryden et al., 1991 , S is 35.09 psu,2

which is a reasonable estimate of the mean salinityŽof the Ekman flow along the 248N section Roem-

.mich et al., 1991 . These results support the assump-tions that the mean salinity in the ECS in June 1985was 34.590 psu and that of the southward return flowwas 34.6 psu, respectively.

Table 10 shows the annual and seasonal meanvalues of meridional salt transport by the Kuroshioin the ECS estimated using the constant value of34.6 psu as the mean salinity of the southward returnflow region. This table indicates that the salt trans-port by the Kuroshio in the ECS has a large seasonalvariation with a minimum of y1.7=106 kg sy1 insummer and a maximum of 2.0=106 kg sy1 inwinter, about an annual mean value of 0.2=106 kgsy1. While the total meridional salt transport acrossthe 248N section in June 1985 was about 5.5=106

kg sy1, the salt transport in the ECS is only y0.3=

106 kg sy1, which means that the salt transport bythe Ekman flow across the 248N section was a

Table 10The annual and seasonal mean values of meridional salt transportin the ECS for using 34.6 psu as mean salinity of southward returnflow region

Mean Total Meridionalsalinity volume saltŽ .psu transport transport

6 y1Ž . Ž .Sv 10 kg s

All seasons 34.609 23.0 0.2Ž .Winter January 34.740 14.2 2.0Ž .Spring April 34.655 25.9 1.4Ž .Summer July 34.542 28.5 y1.7

Ž .Fall October 34.562 23.5 y0.9aJune 1985 34.590 28.3 y0.3

a Interpolated value from mean temperature.

dominant contribution to the total with a value of5.8=106 kg sy1 northward.

The negative northeastward salt transport in theECS in summer and fall corresponds with the posi-tive northeastward freshwater transport by the lesssaline KSW and ESW, while the northeastward salttransport is positive in winter and spring due to thedominant volume transport of the saline KTW.

4. Discussion and conclusions

In this paper, hydrographic and sea surface cur-rent data obtained quarterly by the Japan Meteoro-logical Agency during 1981–1992 along the PN lineacross the Kuroshio in the central ECS have beenused to estimate the mean and seasonal variation oftotal transports of volume, heat and freshwaterthrough this section. Using TS segments of 18C by0.1 psu, the contributions of various water masses totransports were estimated by integrating over thetransect the product of the absolute geostrophic ve-locity and the values of temperature and salinityrepresenting each TS segment composing the watermass. The four water masses considered are the

Ž .Kuroshio Thermocline Water KTW , the KuroshioŽ .Surface Water KSW , the Kuroshio Intermediate

Ž . Ž .Water KIW and the ECS Shelf Water ESW .The results obtained in this study are summarized

as follows.Ž .1 While the total volume transport is dominated

by KTW in all seasons, KTW is the only contributorin the winter when the volume transport has a mini-mum value of 14.2 Sv. The KSW and KIW con-tribute to the total volume transport from springthrough fall. In summer, all four water masses con-tribute to the total volume transport when it has themaximum value of 28.5 Sv.

Ž .2 As the ratios of seasonal means to annualmeans of temperature and salinity have very smallvariations, the seasonal mean total transports of tem-perature and salt are nearly proportional to the totalvolume transport.

Ž .3 The heat transport of the Kuroshio in the ECShas a large seasonal variation, with a maximum of0.46=1015 W in summer and a minimum of 0.25=

1015 W in winter, about an annual mean of 0.33=15 Ž10 W assuming the mean temperature in the


southward return flow region has a constant value of.15.118C .

Ž .4 The northward freshwater transport by theKuroshio in the ECS also has a large seasonal varia-tion, with a maximum of q1.7=106 kg sy1 insummer due to contributions from the KSW andESW to the total volume transport, and a minimumof y2.0=106 kg sy1 in winter when the totalvolume transport is due solely to the KTW. Theannual mean freshwater transport is y0.2=106 kg

y1 Žs assuming the mean salinity in the southwardreturn flow region has a constant value of 34.6 psu

.and there is no net mass transport .The annual mean positive component of the vol-

ume transport computed by Ichikawa and BeardsleyŽ .1993 for 2 years with better spatial resolution givesthe same mean as this study, giving confidence inthese results with clear improvement, due to time-averaging over many sections despite less spatialresolution.

As the mean temperature and the total volumetransport estimated for the same section by Bryden et

Ž .al. 1991 from a one-time survey in June 1985 arenearly equal to the summer mean values obtained inthis study, respectively, their total temperature trans-port of 2.14=1015 W is nearly equal to the summermean value of 2.23=1015 W. The heat transport of

15 Ž .0.39=10 W estimated by Bryden et al. 1991 is20% larger than the annual mean value of heattransport by the Kuroshio in the ECS found here.

It must be mentioned here that Bryden et al.Ž .1991 discussed the annual variability of the com-bined heat transport by the Kuroshio and Ekmanflow across the 248N transpacific section, and con-cluded that the annual variation in the northwardheat transport is of the order of 0.2=1015 W aboutthe mean heat transport of 0.76=1015 W with amaximum in July, as long as the deep circulationbelow 1000 m exhibits little variation in water masstransport. The heat transport by the northward Ek-man flow is estimated to have a maximum of 0.51=

1015 W in November and a minimum of 0.14=1015

W in January about an annual mean of 0.37=1015

W. These values are estimated from the seasonalmean values of Ekman transport estimated by Bry-

Ž .den et al. 1991 and the mean temperature of Ekmanflow which is assumed to have a constant of 22.568C

Ž .determined by Bryden et al. 1991 . It is concluded

that the annual variation range in the northward heattransport across the 248N transpacific section is theorder of 0.4=1015 W, two times larger than that

Ž .estimated by Bryden et al. 1991 , about the annualmean of 0.70=1015 W.

Finally, we must say that the present results onthe seasonal mean transport of heat and fresh waterin the ECS depend largely on the mean values oftemperature and salinity in the southward return flowregion. For more refined estimates of seasonal andannual mean transport of heat and fresh water by theKuroshio in the ECS, better information is requiredon the seasonal cycle of transport-averaged meantemperature and salinity in the southward return flowregion.

Acknowledgements

The authors wish to express their sincere thanksto R.C. Beardsley for his invaluable help and sugges-tions in revising the manuscript. They also thank I.Yamaguchi and M. Miyamoto for their help in dataprocessing. The comments from two anonymous re-viewers were very helpful. A part of this study wassupported by a Grant-in-Aid for Scientific Researchand an International Cooperative Research Pro-gramme on Global Ocean Observing System, bothsponsored by Ministry of Education, Science, Sportsand Culture, Japan.

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seasonal variation of heat and freshwater transports by the kuroshio in the east china sea

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