JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 102, NO. C5, PAGES 10,499-10,507, MAY 15, 1997
A parametric model for the Yellow Sea thermal variability
Peter
C. Chu and Charles
R. Fralick
Jr.
Naval PostgraduateSchool,Monterey, California
StevenD. Haeger and Michael J. Carron
Naval Oceanographic
Office,StennisSpaceCenter,Mississippi
Abstract. A thermal parametricmodel hasbeen developedfor analyzingobserved
regionalseatemperatureprofilesbasedon a layeredstructureof temperaturefields
(mixedlayer,thermocline,and deeplayers).It containsthree major components:
(1) a
first-guess
parametricmodel,(2) high-resolution
profilesinterpolatedfrom observed
profiles,and (3) fitting of high-resolution
profilesto the parametricmodel.The output of
this parametricmodel is a set of major characteristics
of eachprofile: sea surface
temperature,mixed-layerdepth, thermoclinedepth, thermoclinetemperaturegradient,
and deeplayer stratification.Analyzingnearly 15,000Yellow Sea historical(1950-1988)
temperatureprofiles(conductivity-temperature-depth
station,4825; expendable
bathythermograph,
3213;bathythermograph,
6965) from the Naval Oceanographic
Office's
Master OceanographicObservationData Set by this parametricmodel, the Yellow Sea
thermalfield revealsdual structure:one layer (verticallyuniform) duringwinter and
multilayer(mixedlayer,thermocline,sublayer)duringsummer.Strongseasonalvariations
were alsofound in mixed-layerdepth,thermoclinedepth, and thermoclinestrength.
the Naval OceanographicOffice (NAVOCEANO)'s Master
OceanographicObservationData Set (MOODS) from the
The Yellow Sea is a semi-enclosedbasin coveringroughly years 1950 to 1988 and to transform each temperature profile
295,000
km2andisoneofthemostdeveloped
continental
shelf into a set of characteristicparameters such as sea surface
areasin the world seas[Yanagiand Takahashi,1993]. While temperature(SST), mixed-layerdepth (MLD), thermocline
the Yellow Sea coversa relativelylargearea, it is quite shallow, depth, temperaturedifferenceacrossthe thermocline,and the
reachinga maximumdepth of about 140 m (Figure 1). The deep layer stratification.
water depth over most of the area is less than 50 m. The
1.
Introduction
deepest water is confined to a north-south oriented trench
which runs from the northern boundary south to the 100-m
isobath, where it fans out onto the continental break. The
gradientsin slope acrossthe bottom are very small. Such a
broad and shallow continental
shelf leads to the fact that the
water is readilyaffectedby seasonally
varyingatmosphericconditionssuchasheating,cooling,andwind stress.Therefore the
seasonalvariation of the water massesis remarkably large.
Another feature of the depth distributionis the east/westasymmetry. Extensiveshoals(<20 m) are locatedin the western
Yellow Sea along the Chinese coast and are not generally
found in the South Korea coastal regions. Also, the 50-m
isobath is located
more than 100 km from the Chinese
coast
2. Master Oceanographic Observation Data Set
(MOODS)
The MOODS is a compilationof observedoceandataworldwide consisting
of (1) temperature-only
profiles,(2) both temperatureandsalinityprofiles,(3) sound-speed
profiles,and (4)
surface temperatures from drifting buoys. These measurementsare, in general,irregularin time and space.In this study
we analyze temperature profiles measuredfrom a variety of
instruments.Due to the shear size (more than six million
profiles)and enormousinfluxof datato NAVOCEANO from
various sources,quality control is a difficult task. Our study
domain includes the area 32ø-41øN and 118ø-127øE; the data
but only about 50 km from the South Korea coast.This asym- set within this region consistedof nearly 15,000profiles after
metry in bottom depth is important for the shoalingmixed- rejecting certain data during quality control. These primary
layer depth.Furthermore,the hydrographiccharacterof water editingproceduresincludedremovalof profileswith obviously
massesin the Yellow Seaalsodependson the degreeof mixing erroneouslocation,profileswith large spikes,and profilesdisof freshwater originatingfrom the China continentriver runoff playingfeaturesthat do not match the characteristicsof surroundingprofiles.In shallowwater this procedurecan be parwith the intrusion of East China Sea and Kuroshio waters.
tially
automated but also involves subjectiveinterpretation
Before we sketch vertical structures of the Yellow Sea shelf
becauseof the undersamplingof MOODS comparedto the
watermassesfrom the profiles,we shouldaskourselves
how can
spatialand temporal variability of the oceanography.
the thermal fields be described in terms of a set of characteristic
parameters?In this studywe developeda thermal parametric
model to analyze 15,000 historicaltemperatureprofiles from 3.
This paper is not subjectto U.S. copyright.Publishedin 1997 by the
American GeophysicalUnion.
Paper number 97JC00444.
Seasonal Variation of the Atmospheric
Forcing
The Asian monsoonstronglyaffectsthe Yellow Sea thermal
structure. During the winter monsoon season a very cold
10,499
10,500
CHU ET AL.: A PARAMETRIC
MODEL
FOR YELLOW
41
(thermal forcing),togetherwith the strongwind stress(mechanicalforcing),generatesturbulenceand mixesthe surface
water with the deeperwater. The mixedlayer is at its deepest
(usuallyfills the wholewater column)duringwinter owingto
both convectionandwind mixingby the strongnortheastmonsoonwinds.The changeof seasonbeginsin March when the
surfaceair temperatureis 5øCwarmerthan in February.Rapid
weakeningof the Siberianhigh progressesinto April.
In late April the polar front has moved northward toward
Korea with warm, moist air followingbehind.Numerousfrontally generatedeventsoccur,makinglate April and May highly
variablein termsof windsand cloudamount.By May the daily
high surfaceair temperaturesrise to 15ø-16øC.During this
period,stormsoriginatingin Mongoliamaycausestrong,warm
westerliescarryingyellow desert sand (termed the "Yellow
Wind"). By late May and early June the summersurfaceatmosphericlow-pressuresystembeginsto form over Asia. Initially, this low-pressuresystemis centerednorth of the Yellow
Sea, producingwesterlywinds.In late June this low beginsto
migrate to the west, settingup the southwestmonsoonthat
39
38
$
VARIABILITY
sphere.The upward buoyancyflux at the air-ocean interface
40
z
•
SEA THERMAL
37
36
35
34
33
dominates
the summer
months.
The
winds
remain
variable
through June until strengtheningof the Manchurian low118 119 12o 121 122 123 124 125 126 127
pressuresystemoccurs.The jet streamis just southof Korea,
and the polar front is just south of Kyushuand Shikoku.DeLongitude(E),Depth=O(m)
spite the very active weather systemsthe mean surfacewind
Figure 1. The Yellow Sea bathymetry.The data were obtained from the Naval OceanographicOffice DBDB5 world speedoverthe centralYellow Sea in summeris between3 and
4 m/s, which is weaker than in winter (Figure 2b). June also
bathymetrydatabase.Numbersshowthe depth(meters).
marks, historically,a jump in precipitation associatedwith
warm, moist air southof the polar front [Watts,1969]. Occanorthwest wind blows over the Yellow Sea as a result of the
sionally,the Okhotskhighblocksthe northerlyprogressionof
Siberianhigh-pressuresystem.The jet streamis positionedto the polar front. By July, however,high pressure(the Bonin
the southof the Yellow Seaand the polar front to the north of high) to the southand low pressureover Manchuriaproduce
Philippines.The mean surfacewind speedoverthe Yellow Sea southerlywindscarryingwarm, moist air over the Yellow Sea.
in Januaryis nearly 6 m/s (Figure 2a). The sea surfacetem- The summer monthly mean SAT is quite uniform, around
perature is 6øCat the northern extent and 10øCat the south- 24ø-26øC,and is usually 1.5ø-2øCwarmer than the mean SST
easternextent. The Januarysurfaceair temperature(SAT) [VanLoon, 1984].The warmer air causesa downwardheat flux
variesfrom 0øto 8øCin the Yellow Sea,roughly2ø-6øCcooler at the air-oceaninterface.This heat flux plusthe strongdownthan SST. The Yellow Sea surface loses heat to the atmoward net radiation stabilizesthe upper layer of the water and
41
1 MONTH
(A)
WIND
7 MONTH
(B)
WIND
4O
38
'•37
',•
$ 36
........
i..
.\
x....x.....i..i t..
..l. .i .. t...
....
I'"I'"I
118
119
120
121
122
123
124
125
Longitude(E), Depth=O(m)
126
127
118
119
120
121
122
123
124
125
126
127
Longitude(E), Depth=0(m)
Figure 2. Mean atmosphericsurfacecirculations
in the vicinityof the Yellow Seafor (a) Januaryand (b)
July (computedfrom the EuropeanCentrefor Medium-RangeWeatherForecasts(ECMWF) data).
CHU ET AL.: A PARAMETRIC
JAN 50-88
45
MODEL
FOR YELLOW
4.
0
MOODS
SEA THERMAL
VARIABILITY
10,501
SeasonalVariation of Temperature
Profiles
-20
-r -40
(a)
_5
<
-60
-80
4•;15 120 125 130JUN
50-88 00
MOODS
(b)
•'35 •
•30:i
115
120
125
LONGITUDE (E)
2'0
,
30
-20
-40
•
20
10
-60
-8O
130
0
10
20
The Yellow Sea shelfwater massesexperiencea strongseasonalvariation(Figure 3). Taking the easternpart of the Yellow Sea around 36øN as an example,the Januaryhistorical
(1950-1988) temperatureprofiles(Figure 3a) showa singlelayer structure(i.e., verticallyuniform temperaturefrom surface to bottom). The differentlengthsof theseprofilesin the
vertical are causedby the different water depths where the
observations
were taken (seeFigure 1). This single-layerstructure meansa very deep mixed layer extendingfrom the surface
to bottom.On the other hand,the Junehistorical(1950-1988)
temperatureprofiles(Figure 3b) indicatemultilayerstructure
(i.e., mixed layer, thermocline,and deep layer). There also
existsa shallowsurfacemixed layer in the summer profiles.
Sucha strongseasonalvariation in vertical thermal structureis
causedby a strong surface cooling in winter and a strong
surfacewarming in summer.
30
TEMPERATURE
5.
Thermal
Parametric
Model
Figure 3. Eastern Yellow Sea (around 36øN) temperature
During the summermonsoonseason,most profilesin the
profilesduring 1950-1988: (a) Januaryand (b) June. Solid
Yellow Sea exhibit a mixed layer, a thermocline, and a deep
squaresshowthe observationstations.
layer (Figure 3b), whichcan be outlinedby a "typical"profile.
To make the model more general,we assumetwo deep layers
causesthe surfacemixed layer to shoal,creatinga multilayer below the thermocline(Figure 4). When the two deep layers
structure. Below the thermocline, there is a cold water mass, have the sameverticalgradients,they becomeone deep layer.
commonlyreferred to asYellow SeaBottom Cold Water (YS- If two transition layers are added, the entrainment zone beBCW), that remains unchanged and nearly motionless tween the mixed layer and thermoclineand the transitionzone
throughoutthe summer [Li and Yuan, 1992]. October is the between the thermocline and the deep layer, the Yellow Sea
beginning of the transition back to winter conditions. The
southerlywindsweaken,lettingthe seasurfaceslopereestablish
the winter pattern. The SST steadilydecreasesfrom October
to January.
thermal structureduring summercan be well resolved.We use
a parametric model with six layers [Chu, 1995] to diagnose
shallow-watermultilayer structurefrom observedYellow Sea
temperatureprofiles.Each observedprofile is modeledby a set
OT/az
T(z)
SST
(-)
o
(+)
I
I
I
d4I
(t•-)
T
H
Figure 4. The temperature and gradient spacerepresentationsof the features or profile characteristics
modeled by the feature model.
10,502
CHU ET AL.: A PARAMETRIC
MODEL FOR YELLOW SEA THERMAL
of parameters,mostof whichhavephysicalmeaning,including
SST, MLD, depth of the baseof the thermocline,gradientin
the thermocline and deep layers, and additional parameters
describingcurvaturebetweenthe mixedlayer and thermocline
and curvature below the thermocline. Among them, SST is
taken as the observedvalues.The model parametersfor each
observedprofile are computedin gradientspace;more specifically,the depthsand gradientsof the modeledfeaturesare fit
to the verticalgradientof the observedprofile.The parametric
model depictsthe multilayerstructure.Determinationof layer
numberis basedon overallfeaturesof the profiles(Figure3b).
In this studythe thermal parametricmodel consistsof seven
depths(i.e., sixlayers)and sixgradientsas shownin Figure 4.
The first and last depthsare assumedto be at the surfaceand
bottom,respectively,and the gradientswithin the mixedlayer
and thermoclineare constrainedto be constant.The gradients
for the four other layers are assumedto vary with depth linearly. The mean gradient is taken as the representativevalue
for these layers.The model parametersare calculatedin the
gradientspace,whichwill bring larger numericalerrorsdue to
the differentiation.We will use the optimizationto filter out
the noise.
5.1.
Thermal
Parametric
If we considerprofiles in the gradient space,i.e., G r =
OT(z)/Oz, eachprofilecanbe representedby the surfacevalue
(SST) plusthe gradients,e.g.,
[SST, Gr(0, Zl), Gr(zl, z2), '''
thermocline, the transition zone, and the first and the second
deeplayers,respectively.
H is the water depth,d • is the mixedlayer depth,d2 is the depth of the thermoclinetop, d3 is the
depthof the thermoclinebottom,d4 is the depthof the top of
the first deeplayer, and ds is the bottom of the first deep layer
(Figure4). Here we assumeconstantverticaltemperaturegra-
dients
in theoceanmixedlayer(verysmall,G(rm) • 0) andin
thethermocline
(verylargeG(•h)) andlinearlyvarying
withz
in the entrainmentzone, the transitionzone, and the two deep
layers
addwithaverage
values
•(re") •(•r) •(rd•) and•(d2)
,
,
,
r
ß
Here the mean gradientin the entrainmentzoneis the average
of the mixed-layer
andthermocline
gradients,
i.e., (•(e") =
(G(T
m) + G(•h))/2.Byforcing
thisparametric
model(la)-(lf)
T
to eachobservedprofile,we shouldhavea firstguessof the five
depths(d•, d2, d3, d4, ds) and a highresolutionof temperature/depthpoints in the vertical in order to obtain the five
temperature
gradients
(G(T
m), G(•h), •(•r), •(Tdl), •52)).
Sucha treatmentprovidesthe most importantfeaturesfrom
the observational
data.
5.2.
High-Resolution Profiles (HP) Interpolated
From
Observations
z C I-d1, 0]
NAVOCEANO's Digital BathymetricData Base5 (DBDB5)
datasetis usedto obtainwater depthH. If the five depths(d •,
d2, d3, d4, ds) are known,we can dividethe data set HP =
cline,transitionzone,first deeplayer, and seconddeeplayer).
(la)
(Z +dl)
Foreachlayerwefit (T/, z/) to theparametric
model(la)-(lf)
r
,
andobtaina setoftemperature
gradients
(G(rm),G(•h), •(tr)
•(Tdl)' •(Td2)).
5.3. Modeled Profile (MP) Obtained by the Iteration
Method
A modeledprofile (Mr) with 0.5-m resolutioncanbe established by using the parametricmodel (la)-(lf) if the five
depths(d•, d2, d3, d4, ds) are given.In reality thesedepths
are not knownprior to processingthe data and vary from one
profile to the other. We usethe iteration methodto obtainthe
optimal Mr.
First, we start with a set of first-guessvaluesof the depths
and the five gradients(two constantsand three meanvalues),
•)(z) =2(d2-dl)
ß[(G•h)+ G(Tm))(d2
-dl) - (G•h)- G(T
m))
ß(Z+ d2)]+ •F(m)(-dl) z C I-d2, -dl]
m, Ti = r(zi), wherezi = Z/_l - 0.5 m (Zo = 0). The
(z/, Ti) intosixparts(mixedlayer,entrainment
zone,thermo-
, Gr(zn-1, zn)]
for the temperatureprofiles.Here, n + 1, is the number of
data points, and zi(i = l, 2,''',
rt) are the depthsof the
subsurfacedata points. For example, 100 temperature/depth
pointswouldproduce99 gradientvalues.If the surfacevalueis
included, we have the same amount of data in the gradient
spaceas in the original data set.
On the basisof the continuityof T and OT/Oz at interfaces
of any two layers,a parametricmodel can be constructedas
©=
(lb)
•(th)(z): G•h)(z+ d2)+ •"(en)(-d2)z • I-d3, -d2] (lc)
ø),d(2
ø),d?, d(4
ø),d;ø)),
(2)
G(T0)
= (G(Tmo),
G(tho)?7.(tro)O(dlo)
r ,'-'r , r ,Gr
-(d2o)
).
In this studywe choose
(Z + d3)
•(tr)(Z)
= (d4-d3)
[(G?)-•?)z+d4G?
)- d3•?•]
+ •"(th)(--d3) Z • I-d4, -d3]
(ld)
G(rø)= (0, 0.5øC/m,0.005øC/m,0, 0).
model (la)-(lf)
ds- d4
z G [-ds, -d4]
D ©= (20m, 24m, 32m, 38m, H)
For eachHP profilewe fit thedata(z•, Ti) to theparametric
(Z + d4)(z + ds)
•(dl)(z)---•(tr)(-d4)d-(z d-d4)•(T
dl)d-
ß[G?(-d4) - •(rdl)]
temperaturesin the mixed layer, the entrainmentzone, the
Each MOODS profile is linearly interpolated to Az = 0.5
Model
•(m)(z)--G(Tm)Z
+ SST
VARIABILITY
sincewe know the depthsand the gradients
and obtaina zeroth-order
modeledprofile,calledMP© =
(le)
(z•,•r•ø)).TheRMSerrorformismatch
ofHPandMP© is
computedby
•(d2)(Z): •(dl)(--d5)q-(Z q-d5)•(rd2)+
(z + ds)(z + H)
H-ds
RMS© =
ß[G?l)(-d5) - •(rd2)] Z G I-H, -ds]
(lf)
n•J
k'Tj)
2
•(
-
J=l
where•r(m),•r(•), •r(•h),•r(•r),•r(d•), and•(d2) aremodeled wherek = 0. We expectRMS© to be large.
(3)
CHU ET AL.: A PARAMETRIC
MODEL
FOR YELLOW
January
SEA THERMAL
VARIABILITY
May
38 .•
10,503
September
....................
39" •.,.i.i'
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18 120Lo•,de(;•4126
38
118 120Lo•de(;½4
126
Longi•de(E)
July
41 ........................
:..............
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..........
::•:•:i:iiiiiiiii•iiii!ii•½i•i•iii•{{{({i•{{{{{({i}}}•2•[•
18
120
1•
124
126
118
Longitude(E)
Ei•re
120
41 ..........................................................
•:•:::::1•1:1:•:1:1:•:•:i:•:•:1:•:•:•:i:•:1:1:1:1:1:1:•:1:
1•
124
126
118
120
Longitude(E)
1•
124
126
Long•tu• (E)
5. Bimonthlyvariation of SST. Data were obtainedfrom the MOODS for the years ]950-]988.
Second,we use the iteration method to obtain optimal MP
for each HP profile. Each depth can only be adjusted one
vertical grid (Az or -Az) for one iteration. From the kth-
order(k starting
from0, thefirstguess)
setof depths,
D ©, we
have242(---35 - 1) differentcombinations
of the depthadjustment,
D/•+•)= D © + aD• )
(4)
where
= (zx, o, o, o, o),
(-zXz, o, o, o, o),
aD242
© = (0, 0, 0, 0,-Az)
ß
the iteration until k = kmax. If the RMS error at the kmax
iteration is still greater than R•., we shouldreject the parametric model (la)-(lf); that is, the HP profile cannotbe fitted by
the parametricmodel. The rejectedprofilesare discarded.In
this study we chosekmax = 400, R•. : 0.4ø(2. This RMS
criterion (0.4ø(2)was chosendue to the accuracyof the temperature/depth(_+0.2øC,+2 m) measuredby the bathythermograph (BT). If the data are obtainedby more accurateinstru-
ments (e.g., thermometer), this criterion can be greatly
reduced. The number of rejected profiles is 1232. The six
temperaturegradientswere updatedat eachiteration.A set of
the six optimal gradientsis obtainedwhen the RMS error of
the temperature profile is lessthan the criterion.
6.
Yellow
Sea Shelf
Thermal
Features
The parametricmodel (la)-(lf) transformsany profile (if
We use (la)-(lf) to obtain 242 modeled profiles, among
which we pick a profile with minimum RMS error as the not rejected)into a setof five depths(d•, d2, d3, d4, ds) and
(G?), G(•h) &(en), O(tr) 0(Tdl) O(Td2))
(k + 1)th-ordersetof depths,
D (/•+•). We repeatthispro- six gradients
,
cedure until the minimum
RMS error is achieved. We have two
UT
T
,
,
ß
These parametersrepresentthe most importantphysicalfeacheckpointsto terminatethe iteration:the maximumnumber tures and are insensitiveto the first-guessvalues. The firstof iterationskmax and the RMS error criterion Re. At each guessvaluesonly affect the number of iterations.A good seiteration(k < kmax)
, RMS© iscompared
to a user-specifiedlection of first-guessvalues reducesthe number of iterations.
criterionR•. If RMS© < R•, we terminatethe iterationand Sincewinter (December-April) profilesreveal a single-layer
obtainanoptimalsetof depths.
If RMS© > Re,wecontinue structure [Fralick, 1994], the set of characteristicparameters
10,504
CHU ET AL.: A PARAMETRIC
MODEL FOR YELLOW SEA THERMAL
layer duringthe summermonsoonseason.The heat gain (or
loss)at the oceansurfacewarms (or cools)only the mixed
layer, causinga rapid changeof SST.
4. The seasonalvariationof seasurfacetemperaturehasits
the largestvalue(-20øC) in the northernYellow Sea(northof
37øN)and decreases
southward.
The SST over the Yellow Sea has a strongannualcycle.In
the centralYellow Sea (123ø-124øE,36ø-37øN)the maximum
25
•o20
e15
E
SST (25.2øC)appearsin late July and earlyAugust,and the
minimumSST (7øC)occursin March (Figure6a).
1C
6
8
10
12
Month
o
o
6.2. Mixed-Layer Depth
By late win_terthe water throughoutthe Yellow Sea is very
isothermal.During onsetof the summermonsoonan increase
in the solar insolationand weakeningwindsbegin the stratificationprocesson the Yellow Sea Shelf. This is supportedby a
strongannualcycleof MLD in the centralYellow Sea (123ø124øE,36ø-37øN).During the wintermonsoonseason(Decem-
6O
50( )
VARIABILITY
o
4O
ber-March),MLD hasthe samevalue(-50 m) as the water
depth(single-layerstructure).During the onsetof the summer
monsoon,MLD reducesdrasticallyfrom 50 to 8 m (multilayer
structuredue to stratification)and then keeps small values
(lessthan 8 m) throughoutthe wholesummerseason(Figure
Q 30
20
10
øo
•
Month
8
10
12
The Yellow Sea bimonthly progressionof the MLD plots
(Figure 7) revealssomeinterestingfeatures:
1. There are two stagesfor the time evolution of mixedlayer depth: a quasi-steady(and shallow) stage from May
Figure 6. Monthlyvariationof the centralYellow Sea(123øthroughJulyandanevidentdeepening
stagefromJulythrough
124øE,36ø-37øN)thermalfeatures:(a) SST (degreesCelsius) November.
and (b) MLD (meters).
2. The contoursof the mixed-layerdepth almostfollow the
bathymetry(Figure1). The centralYellow Seahasa relatively
reducesto (SST,H). Thereforethe thermalparametricmodel deep mixed layer during the whole summermonsoonseason,
stage(under
was used to processonly summer monsoonseason(May- whosedepthis from 5 to 10 m for the quasi-steady
November) profiles. Due to the data sparsity,the summer veryweak surfaceforcing)and deepensto 20-25 m duringan
profileswith the samemonth and differentyearswere grouped evidentdeepeningstagecausedby reducedinsolationand intogetherto obtainthe characteristicparametersof the monthly creased wind.
variations.
3. The mixed-layerdepth is very shallow(-5 m) in the
northYellow Sea(northof 37øN)duringthe quasi-steady
stage
6.1. SeasonalVariation of Sea Surface Temperature
and increasesto 15-20 m during the evidentdeepeningstage.
Bimonthlyevolutionof SST(Figure5), constructed
from the The shallowmixedlayer in the first stageisvery sensitiveto the
MOODS data (1950-1988), showssomeinterestingfeatures net surface heat flux.
which are summarized as follows.
4. Large mixed-layer depth occursduring the transition
1. During the winter monsoonseason,SST has a weak month of November, when solar insolation has decreased
temporal variation. In the central Yellow Sea, SST decreases markedlyand windshavepickedup.
5. The mixed-layerdepth is also affectedby the large disslowlycoolingrate --4øC/month) from Novemberthrough
January,staysalmostthe same(6ø-8øC)from Januarythrough charge from surroundingrivers (Yangtze River, Huangho
River, etc.). As reportedby Yanget al. [1983], the discharge
March, and increasesslowly (warming rate --3øC/month)
from March throughMay. The heatloss(or gain)at the ocean from the Yangtze River is maximumin July (16% of annual
andminimumin January(3% of annualdischarge).
surfacecools(or warms)the wholecolumnof the water,caus- discharge)
Near the Chinesecoast the mixed-layer depth is minimum in
ing a slow changeof SST.
2. The winter SST (January,March) showsa tongue-like July(lessthan 2 m).
distribution which extends from southeastto northwest, which
6. The mixed-layerdepth is stronglyinfluencedby tidal
coincides with observationscollected during a single year mixing in shallowregionssuchas along the Chinesecoastin
(1986) in the Yellow Sea[Chenetal., 1994].Sucha tongue-like water lessthan 20 m deep. The amplitudesof M 2 and S2 tides
structureimplies the existenceof the Yellow Sea Warm Cur- there are larger southof 34øNthan north of 34øN[Yanagiand
Inoue, 1994],which impliesstrongertidal vertical mixing(in
rent (YSWC) in winter [Hsueh,1988].
3. During the summermonsoonseason,on the other hand, turn thicker mixed layer) southof 34øN.This can be clearly
SST hasa strongtemporalvariation.In the centralYellow Sea, seenin Septemberand NovemberMLD plots(Figure 7).
SST increasesrapidly from May throughJuly (warmingrate
6.3. Prominence of the Seasonal Thermocline
-7øC/month) and stayswarm (above20øC)throughSeptemJulyis a month of strongsurfaceheatingaswitnessedby the
ber. SST noticeablycoolsfrom SeptemberthroughDecember
(--4øC/month). The rapidchangeis dueto a shallowermixed sharp increasein SST shown in Figure 5. As a result the
CHU ET AL.: A PARAMETRIC
MODEL FOR YELLOW SEA THERMAL
120
122
10,505
September
May
1 8
VARIABILITY
124
118
126
120
122
124
126
Longitude(E)
Longitud©(E)
November
July
41 .:::.::::::::x::::::::::::..::::::::::::::::::::
...........
!..............
•::::::,•,•,•,•
........
[•:;!!:•!11:•iii//glii!iii•i!//!i?
':*i::.-',::..•i•i•m•m,•,•:•
.................................. 41 ..........
---38•iii}i?:fiiii:
\,:':'.'m.:...:...':..::•'
::..h
•
:i!i•i?:?:ii•::!::•iii::•
......
118
120
122
124
126
118
Longitude(E)
120
122
124
126
Longitude(E)
Figure 7. Bimonthlyevolutionof MLD (meters) in the Yellow Sea. Data were gridded and horizontally
interpolated from the MOODS for the years 1950-1988.
prominenceof the thermoclineis expected.With the presence
of YSBCW in the centralregiona positivetemperaturegradient acrossthe thermoclineshouldbe apparentand appearsin
the central part of the Yellow Sea shelf. The presenceof this
centrallymanifestedgradient is more obviousin August and
showsa strongergradientas summerprogresses.
September
showsa ridgein the temperaturegradientdistributionoriented
north-southin the central region which suggeststhe deeper,
colder water servesto maintain the temperature difference
maximum
AT(th) alwaysappears
in the centralYellowSea.
This phenomenoncanbe explainedby the tidal verticalmixing
[Yanagiand Inoue, 1994; Takahashiand Yanae, 1995]. The
circulationsin the Yellow Sea during summerare mainly inducedby the sea surfaceheating and are affectedby the vertical mixing of the tidal current and the bottom topography.
The homogeneouswater which is formed by the sea surface
cooling in winter begins to stratify due to the sea surface
heatingin spring.However,due to the horizontaldifferenceof
even into the later summer. As solar insolation decreases in
the tidal verticalmixingeffect(strongertidal mixingin shallow
October and wind speedsbegin to increase,entrainmentbe- shelves),the intensityof the stratificationin the centralYellow
gins at the mixed-layerbase to break down the thermocline. Seabecomesstrongerthan the surroundingpart. Besides,surThis is borne out by the October map where the north-south face heatingduring the summermonsoonseasonalso shoals
orientedridge in temperaturegradientis still presentbut the the mixed layer and makesthe mixed-layertemperature(i.e.,
magnitudeof the gradienthasdecreasedsignificantly.
SST) relativelyuniformhorizontally.Sincethe minimumtemThetemperature
difference
across
thethermocline,
AT(th), peraturein the deeperlayer appearsin the centralYellow Sea
is computedfrom the output of the parametricmodel, d2, d3, (i.e., YSBCW), a strongthermoclineshouldbe locatedthere.
and G (th)
2. The thermoclinestrengthensduringthe first half of the
AT(th)= G(th)(d3-d2)
(5)
summer
monsoon
season
(May-July).The maximum
AT(th)
increasesfrom 5øCin May to 14øCin July.During the second
The bimonthly
evolution
of AT(th)field(Figure8) indicates half of the summermonsoonseason(July-September)the
some important features.
thermoclineweakensexceptin the central Yellow Sea where
1. Thecontourof AT(th)almost
followsthebottomtopog- the thermocline strength remains high value (maximum
raphy (Figure 1). No matter which month in the year, the AT(th) "" 14øC).The strengthof the thermocline
further
10,506
CHU ET AL.: A PARAMETRIC MODEL FOR YELLOW SEA THERMAL VARIABILITY
May
September
41
4O
39
39 ':•
..........
;.:i::::•i•."
;•ii!ii?:iii!!::i::!i!::iii::111ilili
38
'"' 37
35
34
33
32
118
120
122
124
126
118
120
Longitude(E)
July
41
40
40
39
39
38
38
..•37
•' 37
ß
• 36
'• 36
35
35
34
34
33
33
32
120
122
124
126
November
41
118
122
Longitude(E)
124
126
32
118
Longitude(E)
120
122
124
126
Longitude(E)
Figure 8. Thermoclinetemperaturedifference(degreesCelsius)in the YellowSeafromMay to November.
Data were obtainedfrom the MOODS for the years 1950-1988.
reduces from September through November (maximum
of the summermonsoonin May the oceanmixed layer shoals
and
AT(th) "• 5øC).Duringthewintermonsoon
season
(Decem- and experiencestwo stagesof evolution.A quasi-steady
ber-April) theYellowSeawatersarewellmixedvertically,and shallowstage(MLD --•10m at the centralYellowSea)appears
there is no thermoclineduring that period.
from May throughAugust, and an evident deepeningstage
7.
(MLD --• 20-25 m at the centralYellow Sea) occursfrom
AugustthroughNovember.During the first stagethe mixed
layer has the least thermal inertia and is sensitiveto surface
Conclusion
1. The thermal parametricmodel depictedin this paper
demonstratesa good capabilityto compressa large data set
into a smallenoughvolumeof data suchthat the investigators
may readily assimilateand interpret it. The model clearly
showslargethermalvariabilitiesand the dual structureof the
Yellow Seashelf:a one-layerstructureduringthe winter monsoonseason(December-April)anda multilayerstructuredur-
ing the summermonsoonseason(May-November).
2. The Yellow Sea surfacemixed layer has more thermal
inertia duringwinter monsoonseasonthan summermonsoon
season.SST stays almost the same from January through
March (winter monsoon)and, increasesdramaticallyfrom
May throughJuly (summermonsoonseason).The maximum
warmingrate (7øC/month)in summeris nearly2 timeshigher
than the coolingrate (<-4øC/month) in winter.The seasonal
variation of SST has its largestvalue (20øC)in the northern
thermal forcing.
4. A strongthermoclineoccursduring the summermon-
soonseason,
withthelargestAT(th)occurring
in September.
The thermoclinestrengthenscontinuouslyafter the onsetof
the summer monsoon season.The maximum AT (th) for the
Yellow Sea increasesfrom 5øCin May to 14øCin July and is
maintained
(maximum
AT(th) "• 14øC)throughSeptember.
The strengthof the thermoclinedecreasesfrom September
throughNovember
(maximum
AT(th) • 5øC).Duringthe
winter monsoonseasonthe Yellow Sea waters are vertically
well mixed, with little or no stratification.
5. The thermal structureis affectedby the air-seasurface
heat flux, the wind, the tidal vertical mixing, and the large
dischargefrom the mainland of China (Yangtze River,
HuanghoRiver, etc.). The Yellow Sea lies within the Asian
Yellow Sea and decreases southward.
monsooncirculationand experiences
hot, humidsummersand
3. During the winter monsoonseasonthe surfacemixed long,cold,dry winters.In winter the Yellow Seais dominated
with the Siberianhigh.
layer extendsfrom the surfaceto the bottom.After the onset by the cold northerlywind associated
CHU ET AL.: A PARAMETRIC
The SST is 6øC at the northern
MODEL
FOR YELLOW
extent and 10øC at the south-
east extent. The Januaryto surfaceair temperature(SAT)
variesfrom 0ø to 8øC in the Yellow Sea [Chu et al., 1997],
roughly2ø-6øCcoolerthan SST.The Yellow Seasurfaceloses
tremendousheat to the atmosphere.The upward buoyancy
flux at the air-oceaninterface(thermalforcing),togetherwith
the strongwind stress(mechanicalforcing),generatesturbulence and mixesthe surfacewater with the deeperwater. The
mixed layer is at its deepest(it usuallyfills the whole water
column) during winter owing to both convectionand wind
mixingby the strongnortheastmonsoonwinds.In summerthe
Yellow Seais dominatedby the warm and moistsouthwesterly
wind associated
with the atmosphericlow-pressuresystemover
Asia (the Manchurianlow) and high-pressure
systemin the
southeast
(the Boninhigh).The summerSAT is quiteuniform
and around24ø-26øC[seeChu et al., 1997].It is usually1.5ø2øCwarmerthan SST [VanLoon, 1984].The warmerair brings
downward heat flux at the air-ocean interface.
This heat flux
SEA THERMAL
VARIABILITY
10,507
References
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6. This parametricmodelcan alsobe usedto processoutput from anydynamicalmodelandto obtainmeaningfulproducts,i.e., MLD, thermoclinedepth,thermoclinestrength,and
deep layer stratification.
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P. C. Chu and C. R. Fralick Jr., Naval PostgraduateSchool, Code
OC/CU, Monterey, CA 93943. (e-mail: [email protected])
Acknowledgments. The authors are grateful to Laura Ehret,
Michael Cook, and C. W. Fan at the Naval PostgraduateSchoolfor
their programmingassistance.
This work was funded by the Naval
Oceanographic
Office,the Officeof Naval ResearchNOMP and PO
Programs,and the Naval PostgraduateSchool.
(ReceivedAugust13, 1996;revisedJanuary3, 1997;
acceptedJanuary23, 1997.)