DTIC ADA480005: Japan Sea Thermohaline Structure and Circulation. Part 1: Climatology

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by  Defense Technical Information Center

244 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME31
Japan Sea Thermohaline Structure and Circulation. Part I: Climatology
PETER C. CHU, JIAN LAN, AND CHENWU FAN
DepartmentofOceanography,NavalPostgraduateSchool,Monterey,California
(Manuscriptreceived17August1999,infinalform9April2000)
ABSTRACT
Inthisstudy,theU.S.Navy’sGeneralizedDigitalEnvironmentalModel(GDEM)climatologicaltemperature
and salinity data on a 0.5(cid:56) (cid:51) 0.5(cid:56) grid is used to investigate the seasonal variabilities of the Japan/East Sea
(JES) thermohaline structure and circulations. The GDEM for the JES was built up on historical (1930–97)
136509 temperature and 52572 salinity profiles. A three-dimensional estimate of the absolute geostrophic
velocity field was obtained from the GDEM temperature and salinity fields using the P-vector method. The
seasonal variabilities of the thermohaline structure and the inverted currents such as the Subpolar Front, the
salinityminimumandmaximumintheJapanSeaIntermediateWater,andtheTsushimaWarmCurrentandits
bifurcationareidentified.
1. Introduction The JES thermohaline structure and general circula-
tion have been investigated for several decades. The
The Japan Sea, known as the East Sea in Korea, has
TsushimaWarmCurrent(TWC),dominatingthesurface
steepbottomtopography(Fig.1)thatmakesitaunique
layer, flows in from Tsushima Strait and carries warm
semienclosed ocean basin overlaid by a pronounced
water from the south up to 40(cid:56)N where a polar front
monsoon surface wind. The Japan/East Sea (hereafter
forms (Seung and Yoon 1995). Most of the nearly ho-
JES) covers an area of 106 km2, has a maximum depth
mogeneous water in the deep part of the basiniscalled
in excess of 3700 m, and is isolated from open oceans
the Japan Sea Proper Water (Moriyasu 1972) and is of
except for small (narrow and shallow) straits that con-
lowtemperatureandlowsalinity.AbovetheProperWa-
nect the JES to the Pacific Ocean. The JES contains
ter, warm and saline water flows in through Tsushima
three major basins called the Japan Basin(JB),Ulleng/
Strait, transports northeastward, and flows out through
Tsushima Basin (UTB), and Yamato Basin (YB),anda
the Tsugaru and Soya Strait.
highcentralseamountcalledtheYamatoRise.TheJES
The TWC separates north of 35(cid:56)N into western and
is of great scientific interest as a miniature prototype
eastern channels (Uda 1934; Kawabe 1982a,b; Hase et
ocean. Its basinwide circulation pattern, boundary cur-
rents, Subpolar Front (SPF), mesoscale eddy activity, al. 1999; Senjyu 1999). The flow through the western
anddeep-waterformationaresimilartothoseinalarge channelcloselyfollowstheKoreancoast[calledtheEast
ocean. Korean Warm Current (EKWC)] until it bifurcatesinto
TheJESexperiencestwomonsoons,winterandsum- twobranchesnear37(cid:56)N.Theeasternbranchfollowsthe
mer, every year. During the winter monsoon season, a SPF to the western coast of Hokkaido Island, and the
cold northwest wind blows over the JES as a result of western branch moves northward and forms a cyclonic
the Siberian high pressuresystemlocatedovertheEast eddy at the eastern Korean Bay. The flow through the
Asian continent. Radiative cooling and persistent cold eastern channel follows the Japanese coast, called the
airadvectionmaintaincoldairovertheJES.Thenorth- ‘‘Nearshore Branch’’ by Yoon (1982a–c). More accu-
west–southeast oriented jet stream is positioned above rately,wemaycallittheJapanNearshoreBranch(JNB).
the JES. Such a typical winter monsoon pattern lasts The JNB is usually weaker than the EKWC. TheTWC
nearlysixmonths(Nov–Apr).Duringthesummermon- at both channels decreases with depth. The EKWC
soon, a warm and weaker southeast wind blows over meets the southward coastal current, the North Korean
the JES. Such a typical summer monsoon pattern lasts Cold Current (NKCC), at about 38(cid:56)N with some sea-
nearly 4 months (mid-May to mid-Sep). sonal meridional migration. After separation from the
coast, the EKWC and the NKCC converge and form a
strong front (i.e., SPF) that stretches in a west–eastdi-
rection across the basin. The NKCC makes a cyclonic
Corresponding author address: Dr. Peter C. Chu, Departmentof
recirculation gyre in the north but most of the EKWC
Oceanography,NavalPostgraduateSchool,Monterey,CA93943.
E-mail:[email protected] flowsoutthroughtheoutlets(Uda1934).Theformation
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JANUARY2001 CHU ET AL. 245
FIG.1.Geographyandisobathsshowingthebottomtopography(inmeters)oftheJapan/East
Sea(JES).
of the NKCC and the separation of the EKWC are due JIW used by Japanese oceanographers); and Kim and
to a local forcing by wind and buoyancy flux (Seung Kim (1999) found the high salinity water with high
1994). Large meanders develop along the front andare oxygenintheeasternJB(i.e.,northofSPF)andnamed
associated with warm and cold eddies. the water ‘‘High Salinity Intermediate Water’’ (HSIW;
Between the TWC water and the Japan Sea Proper (cid:46)34.07 psu).
Water,averticalsalinityminimum(SMIN)southofthe The seasonal variabilitiesof boththeJEScirculation
SPF, usually accompanied by a dissolved oxygen max- and thermohaline structure have been studied basedon
imum, was first found and named Japan Sea Interme- limited datasets such as the seasonal sea surface tem-
diateWater(JIW)byMiyazaki(1952,1953),andfurther perature(SST)variability(IsodaandSaitoh1988,1993;
depicted by Kajiura et al. (1958) and Moriyasu(1972). Kano1980;MaizuruMar.Observ.1997).Basedonsat-
The collocation of the SMIN and the dissolvedoxygen elliteinfraredimagesinthewesternpartoftheJESand
maximumimpliesthattheIntermediateWateroriginates theroutinehydrographicsurveybytheKoreaFisheries
fromthedescendingsurfacewateraroundtheSPF(Mi- Research and Development Agency in 1987, Isodaand
yazaki1952,1953;MiyazakiandAbe1960).Later,Kim Saitoh (1993) found SST patterns in winter and spring
and Chung (1984) found a very similar property in the thatcharacterizedasfollows:asmallmeanderofather-
UTBandproposedtheEastSeaIntermediateWater(i.e., mal front first originated from Tsushima Strait nearthe
246 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME31
Korean coastandgraduallygrewintoanisolatedwarm coefficients for the curves found for individualprofiles
eddywithahorizontalscaleof100km.Thewarmeddy (Teagueetal.1990).Differentfamiliesofrepresentative
intruded slowlynorthwardfromspringtosummer.Chu curves have been chosen for shallow, mid, and deep
et al. (1998a) reported the seasonal occurrence of JES depth ranges, with each chosen so that the number of
eddiesfromthecompositeanalysisontheU.S.National parametersrequiredtoyieldasmooth,meanprofileover
Centers for Environmental Prediction (NCEP) monthly the range was minimized. As mentioned by Teague et
SST fields (1981–94). For example, they identified a al. (1990), large-scale oceanographic features are gen-
warmcenterappearinginlaterspringintheEastKorean erally found to be similarly represented in both the
Bay. GDEM and the 1982 NOAA Climatological Atlas of
What are the seasonal variabilities of the three-di- the World Ocean temperature and salinity. TheGDEM
mensional JES circulation and thermohaline structure? appearstorenderbetterrepresentationsofseasonalvar-
WeusetheU.S.Navy’sunclassifiedGeneralizedDigital iability and regions of high current shear because of a
EnvironmentalModel(GDEM)temperatureandsalinity different smoothing method and a finer grid spacing.
data on a 0.5(cid:56) (cid:51) 0.5(cid:56) grid to investigatetheproblemin The GDEM data contains monthly mean temperature
this study. The outline of this paper is as follows: A and salinity (T,S) and annual mean temperature and
description of the GDEM data is given in section 2. A salinity (T,S) fields. Interested readers are referred to
depiction of the inversion of the absolute geostrophic Teague et al. (1990) for more information.
velocity is given in section 3. Explanations of monthly
mean and anomaly fields are given in section 4. The
3. The P-vector inverse method
seasonal variabilities of the three-dimensional thermo-
Recently, we developed the P-vector inversemethod
haline structure and the inverted velocity field are dis-
(Chu 1995; Chu et al. 1998b,c; Chu 2000; Chu and Li
cussedinsections5–7.Insection8wepresentourcon-
2000) to calculate the absolute geostrophic velocity.
clusions.
Thismethodcontainstwosteps:(i)determinationofthe
velocity direction and (ii) determinationofthevelocity
2. The U.S. Navy’s GDEM dataset magnitude. Two necessary conditions for the inversion
are easily implemented into this method: (i) the iso-
The U.S. Navy’s (GDEM) global climatological
pycnal surface does not parallel the potential vorticity
monthlymeantemperatureandsalinitydatasetisafour-
surface and (ii) the velocity has a vertical spiral (Chu
dimensional(latitude,longitude,depth,andmonth)dis-
et al. 1998b,c).
play.DataforbuildingthepresentversionoftheGDEM
climatology for theJES wereobtainedfromtheNavy’s
Master Oceanographic Observational Data Set a. Reduced physics
(MOODS), which has 136509 temperature and 52572
As pointed out by Wunsch and Grant (1982), in de-
salinity profiles during 1930–97. The main limitation
termining large-scale circulation from hydrographic
of the MOODS data is its irregular distribution intime
datawecanbereasonablyconfidentoftheassumptions
and space. Certain periods and areas are oversampled,
of geostrophic balance, mass conservation, adiabatic,
whileotherslackenoughobservationstogainanymean-
and no major cross-isopycnal mixing (except for water
ingful insights. Vertical resolution and data quality are
masses in contact with the atmosphere). Under these
alsohighlyvariabledependingmuchoninstrumenttype
conditions, the density of each fluid element would be
and sampling expertise (Chu et al. 1997a–d). The
conserved, which mathematically is given by
monthly distributions of the JES temperature (Fig. 2a)
V·(cid:61)(cid:114)(cid:53) 0, (1)
and salinity (Fig. 2b) stations show that the number of
temperaturestationsistwotothreetimesmorethanthe where (cid:114)is the potential density and V (cid:53) (u,(cid:121),w) is
numberofsalinitystations.Januaryhastheleastprofiles the geostrophic velocity. The conservation of potential
and August the most. Yearly temperature (Fig. 3a) and vorticityequationcanbeobtainedbydifferentiating(1)
salinity (Fig. 3b) profile numbers show temporally un- withrespecttoz,usinggeostrophicandhydrostaticbal-
even distribution with almost no observations in the ances,andincludingthelatitudinalvariationoftheCor-
whole JES in certain years (e.g., 1944, 1989 for tem- iolis parameter to give
perature,and1944,1987–93forsalinity)andmanyob- V·(cid:61)q (cid:53) 0, (2)
servationsin other years(e.g.,nearly6500temperature
profiles in 1969, and 3700 salinity profiles in 1967). where q (cid:53) f(cid:93)(cid:114)/(cid:93)z. Equations (1)and (2)implythatthe
Spatial and temporal irregularities along with the lack velocity V is parallel to (cid:61)q (cid:51) (cid:61)(cid:114).
of dataincertainregionsmustbecarefullyweightedin
order to avoid statistically induced variability. b. The P-vector method
The basic design concept of the GDEM is the deter-
Consider the unit vector P (Chu 1995) defined by
mination of a set of analyticalcurvesthatrepresentthe
mean vertical distributions of temperature and salinity P (cid:53) (cid:61)(cid:114)(cid:51) (cid:61)q . (3)
for grid cells (0.5(cid:56) (cid:51) 0.5(cid:56)) through the averaging of |(cid:61)(cid:114)(cid:51) (cid:61)q|
JANUARY2001 CHU ET AL. 247
Theexistenceofthisunitvectorimpliesthesatisfaction order to see the seasonal variation of the vertical ther-
of necessary condition (1). mohalinestructure,wepresentthelatitudinalandzonal
Thevelocity,V(cid:53)(u,(cid:121),w),parallelstheunitvectorP, cross sections of the monthly mean fields.
To identify the variability in both monthly meanand
V (cid:53) r(x, y, z) P, (4)
monthly mean anomaly fields, we use a cold (warm)
where r is the proportionality. Applying the thermal center to represent the temperature minimum (maxi-
wind relation at two different depths z and z , a setof mum),andafresh(salty)centertorepresentthesalinity
k m
algebraic equations for determiningthe parameterrare minimum (maximum).
obtained
r(k)Px(k) (cid:50) r(m)Px(m) (cid:53) (cid:68)ukm 5. Temperature
r(k)P(k) (cid:50) r(m)P(m) (cid:53) (cid:68)(cid:121) , (5) a. Sea surface
y y km
whicharetwolinearalgebraicequationsforr(k)andr(m)
Although the monthly SST field (Fig. 4) shows an
[r(i) (cid:53) r(x, y, z)]. Here
i (cid:69) evident seasonal variation, the SPF existscontinuously
g zk (cid:49)(cid:93)(cid:114)ˆ (cid:93)(cid:114)ˆ(cid:50) throughout the year. Its position is quite stationary,but
((cid:68)u , (cid:68)(cid:121) ) (cid:53) , (cid:50) dz(cid:57), (6) its intensity strengthens in winter (especially the east
km km f(cid:114) (cid:93)y (cid:93)x
0 zm part) and weakens in summer. Such a pattern issimilar
where (cid:114)ˆ is the in situ water density, and (cid:114) is the char- to an earlier description (Maizuru Mar. Observ. 1997).
0
acteristic value of the density. The location of the SPF in spring is quite consistent
Thedeterminantofthecoefficientmatrixof(5)isthe with Isoda and Saitoh’s (1988) estimations using ten
sine of the vertical turning angle between P(k) and P(m) NOAA-8 satellite Advanced Very High Resolution Ra-
h h
(Chu et al. 1998b,c; Chu and Li 2000). The existence diometer images in spring 1984.
of solutions of (5) implies the satisfactionofnecessary TheSSTisfoundalwayshigherthan5(cid:56)Cyear-round
condition (2). in the UTB and the YB, consistent with Kim et al.’s
For water columns satisfying thetwonecessarycon- (1999) observational studies. The maximum SST gra-
ditions, we may solve (5) to obtain r(k) for the levelz . dientisfoundas16(cid:56)C/l00kmnear137(cid:56)E,40(cid:56)NinFeb-
k
There are N (cid:50) 1 sets (m (cid:53) 1, 2, k (cid:50) 1, k (cid:49) 1, ..., ruary and March, and the minimum SST gradient is
N) of equations (5) for calculating r(k). Here N is the found as8(cid:56)C/100kmfromJulytoSeptember.TheSST
total number ofverticallevelsofthewatercolumn.All gradientacrosstheSPFistwotimesasstronginwinter
the N (cid:50) 1 sets of equations are compatible under the as in summer. The weakening of the SPF in summeris
thermal wind constraint and should provide the same caused by the faster warming of the water mass north
solution. However, because of errors in measurements of rather than south of the SPF in spring. North of the
(instrumentation errors) and computations (truncation SPF a second front occurs (bifrontal structure) during
errors), the parameters r(k) may vary with m. A least the fall-to-winter transition season, especially in No-
squares error algorithm is used to minimize the error. vember and December. This front parallelstheRussian
For further details and validation of the algorithm pre- coast with the maximum SST gradient around 4(cid:56)C/100
sented here see Chu et al. (1998b). (Interested readers km in November.
can obtain the software by contacting C. Fan at The monthly mean SST anomalyfieldobtainedfrom
[email protected] or by visiting our Web site at http:/ theGDEMdata(Fig.5)showsabasin-scalecoldanom-
/www.oc.nps.navy.mil/(cid:59)chu.) aly from December to May (winter monsoon) and a
basin-scale warm anomaly from June to October(sum-
mermonsoon).Thestrongestbasin-scalecoolingoccurs
4. Monthly mean and anomaly fields in February with the coldest anomaly center (T (cid:44)
an
The GDEM’s monthly mean data and inverted ve- (cid:50)9(cid:56)C) appearing in the central JB (40(cid:56)–42(cid:56)N, 135(cid:56)–
locity field show anevidentseasonalvariation.Theav- 137.5(cid:56)E);andthestrongestbasin-scalewarmingoccurs
erage of the 12 monthly mean fields (T,S) leads to the inAugustwiththewarmestanomalycenter(T (cid:46)11(cid:56)C)
an
annual mean field (T,S). The GDEM dataset also pro- appearing near Peter the Great Bay (PGB).
videstheannualmeanfields.Themonthlyminusannual The typical winter anomaly pattern is featuredby(i)
mean field is defined as the monthly anomaly field, allnegativevaluesand(ii)eastwardexpansionofacold
anomalyfromthePGB.Afterthewintermonsoononset,
T (cid:53) T (cid:50) T, S (cid:53) S (cid:50) S,
an an thecoldSSTanomalyfirstoccursinthenorthtonorth-
which represents the seasonal variation relative to the westernboundaryfromTartarStraittoPGBinNovem-
annual mean field. ber with a minimum value around (cid:50)1(cid:56)C.InDecember,
TheseasonalT,Svariationreducesasdepthincreases. the cold anomaly sweeps the whole JES basin withthe
At 300-m depth there is almost no seasonal variability. coldest center ((cid:50)4(cid:56)C) at the north to northwestern
Thus,wepresentonlythehorizontalfieldsatthesurface boundary. The PGB cold center becomes evident with
and the intermediate level (150 m) for illustration. In a minimum value of (cid:50)6(cid:56)C in January. The PGB cold
248 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME31
FIG.2.SpatialdistributionsofMOODSstationsduring1930–97:(a)temperatureand(b)salinity.
JANUARY2001 CHU ET AL. 249
FIG.2.(Continued)
250 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME31
theyear.SouthoftheSPF,thetemperaturechangesfrom
5(cid:56)C to 9(cid:56)C. The SPF meandering at 131(cid:56)E, 134(cid:56)E, and
138(cid:56)E forms several mesoscale eddies. The SPF me-
andering near Okin Gunto (134(cid:56)E) in spring was pre-
viously reported by IsodaandSaitoh(1988,1993).Be-
sides, a second front occurs (bifrontal structure) south
of the SPF along the west coast of Japan during the
winter and spring seasons.
Monthly mean temperature anomaly T (Fig. 7)
an
shows(i)anorth–southasymmetricpatternwithweaker
seasonal variability north of rather than south of the
SPF, and (ii) weakening of the SPF in the winter and
strengthening of the SPF in the summer.
An evident cooling occurs south of the SPF from
January to May (winter monsoon) with a large cold
anomalycenter(T (cid:44)(cid:50)2(cid:56)C)appearingintheYB(36(cid:56)–
an
39(cid:56)N,132.5(cid:56)–136(cid:56)E)fromMarchtoApril.Muchweak-
er cooling appears north of the SPF during the same
period (Jan–May). Such a differential cooling causes
the northward increase of T across the SPF, which
an
implies the reduction of the SPF strength.
An evident warming occurs south of the SPF from
June to October (summer monsoon) with a large warm
anomaly center (T (cid:46) 2(cid:56)C) appearing in the YB from
an
August to September. Much weaker warming appears
northoftheSPFduringthesameperiod(Jun–Oct).Such
adifferentialwarmingcausesthenorthwarddecreaseof
T across the SPF, which implies the enhancement of
an
FIG.3.TemporaldistributionsofMOODSstationsduring1930– the SPF strength.
97:(a)temperatureand(b)salinity.
c. Zonal cross sections (37(cid:56)and 43(cid:56)N)
centerexpandseastwardto138(cid:56)Eandoccupiesthemid-
JESnorthoftheSPF(39(cid:56)–43(cid:56)N)withthecoldestcenter The zonal cross sections (37(cid:56) and 43(cid:56)N) of monthly
of (cid:50)9(cid:56)C in February. The spring (Mar–May) patternis mean temperature show a strong seasonal/permanent
featured by weakening of the mid-JES cold center. A thermocline structure south of the SPF (Fig. 8a) and a
typical summer anomaly pattern is featured by (i) all strong seasonal/weak permanent thermocline structure
positive values and (ii) eastward expansion of a warm north of the SPF (Fig. 8b).
center from PGB. After the summer monsoon onset, a SouthoftheSPFat37(cid:56)N(Fig.8a),apermanentther-
strongSSTwarmcenter(8(cid:56)C)occursnearPGBinJuly. moclineislocatedat80–125mandappearsyear-round
ThePGBwarmcenterstrengthensandexpandseastward with the maximum strength (0.12(cid:56)C m(cid:50)1) in August.
to 138(cid:56)E and occupies the mid-JES north of the SPF Aboveit,theseasonalthermoclineoccursfromthesur-
(39(cid:56)–43(cid:56)N)withthewarmestcenterof11(cid:56)CinAugust. face to 50-m depth in June (0.15(cid:56)C m(cid:50)1), intensifies
Theautumn(Sep–Oct)patternisfeaturedbyweakening duringthesummermonsoonseasontoamaximumvalue
of the mid-JES warm center. The fact that the coldest of around 0.36(cid:56)C m(cid:50)1 in August, and weakens in Sep-
(warmest)centeroccupiesthemid-JESnorthoftheSPF tember.InOctober,theseasonalthermoclineerodesand
(39(cid:56)–43(cid:56)N) in winter (summer) suggests that the SST an ocean mixed layer (OML) is formed. In November,
seasonal variability is greater in the north than in the the OML is well established with the temperaturenear
southoftheSPF.Thisresultisconsistentwithanearlier 18(cid:56)Cand thedeptharound75m.Duringtheprevailing
analysis on the NCEP SST data (Chu et al. 1998b). winter monsoon season (Dec–Mar), the OML deepens
to80–130mwithawestwardupliftoftheOMLdepth:
80mneartheKoreancoastand130mneartheJapanese
b. Intermediate level (150 m) coast.TheOMLtemperatureisaround10(cid:56)C.TheOML
The seasonal thermal variability at the 150-m depth starts to warm at a rate of 2(cid:56)C month(cid:50)1 from March to
is much weaker than that at surface (Fig. 6). The SPF May, and its depth shoals, respectively. For example,
still exists throughout the year and is located at almost the OML temperature increases from 12(cid:56)C in April to
the same location as at the surface. North of the SPF, 14(cid:56)C in May, and the OML depth decreases from 50–
the temperature is uniformly cold (1(cid:56)–3(cid:56)C) throughout 70 m in April to less than 50 m in May. This process
JANUARY2001 CHU ET AL. 251
FIG.4.Monthlymeantemperature((cid:56)C)fieldattheoceansurface.
252 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME31
FIG.5.Monthlymeantemperatureanomaly((cid:56)C)fieldattheoceansurface.Negativevaluesareshaded.