Post on 30-Oct-2021
気象研究所研究報告 第52巻第2号 31-66頁 平成14年2月
Papers in Meteorology and GeophysicsVo1.52,No。2,pp。31-66,February2002
31
The Processes of SST CooHng by Typhoon Passage and Case Study ofTyphoon Rex with a Mixed layer Ocean Model
by
AldyoshiWada
ハ4α80名oJogづαzZ R6s6αzじh1%s房オzμ6。
乃%ゐ%δα,1bα名α1寵 3050052,ノ4ρα%
(Received November1,2000; Revised October25,2001)
Abstract
The mechanisms by which sea surface temperatures(S凹s)(1ecrease by passage of typhoons under
various initial oceanic con(litions and the atmospheric boundary con(1itions were investigated.A stronger
wind stress,slower typhoon translation,excessive heat且ux from the sea to the atmosphere,a thimer
initial血xed layer,and a greatervertical gradient ofthe sea temperature in the the㎜ocline layer,all have
an effecton mbζed layertemperature(MH)cooHng。MLTcooling occurs through intemlingled processes
by enhanced entrainment and upwelling.In contrast,horizontal advection produced by near-inertial
oscillation has a s五ght ef6ect on deteminingthe distributions ofMLTs。The depth in the mtxed layemear
the area along the typhoon track is(letemlined by the magnitudes ofthe wind stresses and the translation
speeds irrespective ofthe initialthic㎞ess ofthe mixedlayer,In addition,under dhferent曲d stresses,
translation speeds,andverticalpro丘1es ofseatemperatures,the MLTcooling is closelyrelated to the ratio
ofm漉mumvadation ofdepthbyupwe11ingto the m頷dmum thic㎞ess ofthe mixed layer。The ratio of
ma虹mumvadationofdepthbyupwellingtothem面mumthic㎞essofthem盈edlayerunderconditionsofexcessiveheatfluxes,isthe same as thatunderconditions ofno heatnux.However,MLTcoolingunder
condi廿ons in whichthe ma面mumheatnuxreaches800W/m2is O.7。Cgreaterthanthose in no heat旦ux.
Inthis case,seawatercoolingatthetransitionlayercausedbyentrainmentin∬uences MLTcooling.
Numerical simulations were conducted to elucidate the3。C S釧「cooling by Typhoon Rex that was
observed by R/V Ke血Mam of the Japan Meteorological AgencyσMA).The S訂variation obtained by
numerical simulation captures the aspects ofobservational S㎝s reported by R/VKeifu Mam,in thatthe
SSTrapidly(lecreases and the ma}dmum S釧「cooling reaches about3。C.This rapid S団「cooling is mainly
caused by the stronger wind stresses an(l slower translation speeds of Typhoon Rex。The ocean heat
contents,based on the mode1-computed sea temperature,are closely related to the intensities ofTyphoon
Rex.
1.In1■oduction
Some physical mechanisms of(lecreases of the
sea surface temperature(hereafter referre(l to as SST)
due to typhoon passage have been stu(1ie(1through
many observations an(l numerical experiments。It is
known that S釧「s decrease from1。C to about6。C with
the passa.ge of typhoons(Black,1983).Black(1983)
investigated the maximum SST cooling that occurs
primarilybehind and to the rightofthe hurricane trackl
the grea』test area of S㎝cooling was Iocated between
1~卿、罪and21~別礁,where1~吻、エwas the ra(lius ofmaximum
◎2002bytheMeteorologicalResearchInstitute
wind velocities.Benderαα」.(1993)distinguished three
ca』tegories of SST cooling observed after the passage of
16tropical cyclones.These SST(1ecreases weregroupe(1according to slow一,medium一,an(l fasしmoving
storms,with average cooling for the three groups of
5.30C,3.5。C,and1.8。C.
Decreases of the SST after the passage of
取phoons are consi(lered to affect the intensity and the
movement of typhoons.From a thermodynamic
viewpoint,Emanuel(1998)maintained that the
hurricane intensity (1epen(led strongly on the
microscale processes goveming the enthalpy,the
momentum flux across the sea surface at gale,and
entrainment of the cold water across the thermocline
32 WadaA VoL52,No.2
1ayer.Negative feedback characterizes the relationship
between the intensity of the typhoon and cold water.
The intensity・of the typhoon is suppresse(1when SST
cooling occurs(G呈nis,1995),since emission of the
latent heat圭n the atmosphere inside the typhoon eye is
suppressed.Thewamcoreofthe切phoonintheupperatmospheric layer then becomes cooler.Suppression of
the second.circulation weakens wind.velocities near the
surface.The heat fluxes near the surface are
suppresse(i because of the weaker win(1stresses.
Mesoscale circulations can affect not only the intensity
but also the movement oftyphoons.However,not only
the air-sea interaction but also the static s惚bilhy of the
planetary boun(1ary layer and environmental flows can
㎡ectthetyphoon development.
Hongαα」.(2000)examined the development and
the movement of Hurricane Opal using a hurricane-
ocean coupled mo(1el,which consisted of the Naval
Research Laboratoryls Coupled Ocean/Atmosphere
Mesoscale Prediction System(COAMPS)for theatmospheric part and the hydrostatic Geophysical Fluid
Dynamics Laboratoryls Modular Ocean Model version
2(MOM2)for the ocean part.When Hurricane Opal
was generated,warm core rings(WCR)separated from
the Gulf Stream already existed in the Gulf of Me}dco
(Shayαα」.2000).Hurricane Opal suddenly develope(i
during passage over the WCR,while the upper trough
e》dstednorthwestoftheWCRand Hurricane Opalwasmoving on the right si(ie ofthe upper trough.Bosart召」
α」.(2000)asserte(1that this trough was related to the
development ofHurr}cane Opal.The case ofHurricane
Opal demonstrates that not only the air-sea interaction
but also environmental flows and mesoscaleconvection,such as rainban{ls,can a丘ect the intensi取
of tyl)hoons.
The ocean response to hurricanes,particularly
SST cooling by stoms,has been previously studie(l by
numerical experiments(Chang an(1Anthes,1978;
Price,1981;Ginis6≠‘zJ.,1993;and others).One
conclusion was that SST cooling and deepening of the
mixed layer were noticeable on the right rear of the
running typhoon.Near-inertial oscillations then
appeared on the right si(1e of the running typhoon after
its passage.Another conclusion was that SST cooling
increases as the translation speed of the typhoon
decreases。The SST cooling ten(1s to apPear the nearby
typhoon atlowertyphoon translation speeds.
The purpose of this study is to clarhiy how much
the mixed layer temperature(hereafter referred to as
MLT)cooling by passage of typhoons depends on
atmospheric and oceanic conditions,such as the
maximum magnitude of wind stresses,translation
speed ofatyphoon,ma⊃dmum magnitude ofheatfluxes
fromthe seato the atmosphere,thickness ofthe mixed
1ayer,and the vertical gradient of sea temperatures in
the thermocline layer.In addition,some physical
processes,such as entrainment,upwelling,andhorizontal advection,are thought to cause a decrease of
SST.Among these physical processes,entrainment
depends partly on the initial conditions of the
atmosphere and the ocean.Here,we investigate how
entrainment rates are detemined and how dominant
theyareintemsofMLTcooling.Oursecondobjectiveis to examine the cause of SST cooling observed by
R/V Keifu Maru after the passage ofTyphoon Rex
south of Okinawa on August24,1998.In a(1dition,we
examine the relationship between SST cooling(or
ocean heat capacity)an〔l the intensity of封phoon Rex.
The mixed layer ocean model used in this paper is
summarized in section2.Some numerical experiments
are performed in section3to investigate the ocean
response under different wind stresses,typhoon
translation spee(1s,heat且uxes,initia1(1epth in the
mixed Iayer,and the vertical profiles of the sea
temperatures.The roles of physical processes such as
entrainment,upwelling,and horizontal advection are
discussed in section4.In particular,the relationship
between SST cooling and the ratio of maximum
vahationduetoupwellingtothema溢mumthic㎞essofthe mtxed layer is examined.In section5,we peぜorm a
numerical simulation of the lowe血g of the SST by
T夕phoon Rex un(1er the realistic initial conditions an(i
boundary conditions.In addition,we discuss how the
heat capacity(Leipper and.Volgenau,1972)of the
ocean contributed to the intensity of Typhoon Rex.
Some problems with our simulation for this SST
cooling are(iiscussed in section6.Our conclusions and
the summary are provided in section7.
2.Mixed Layer Ocean Model
The equations use(l in our mixed layer ocean
model are based on those of Benderθ麺」.(1993).
However,,some physical processes were constmcted or
mod迅ed.Kondols bulk aero(1ynamic formulas(1975)
were applied to estimate the momentum and the heat
(sensible and latent)fluxes.The density ofthe seawater
was calculated by UNESCO formulas(Gi11,1982).The
topographical distribution(1an(1and ocean)was
considered inthis model.We applied the time-spli廿ing
metho(10f Ginis an(l SutyFrin(1995)to calculate the
non-1inear term.The essential details of the numerical
algorithm are described in Ginis and Sutyrin(1995).
The region covered is from10。N to500N and
from120。E to160。E.This mixed layer ocean model
consists of eight layers.The ocean is assumed to be
且at-bottom and the depth ofthe ocean is initially set to
1500m.The first layer is de丘ned as an oceanic mbζed
2002 The Processes ofS訂CooHngbyTyphoon Pass&ge and Case StudyofTyphoon Rexwith a Mixed layer Ocean Mode1 33
layer where the sea temperature is vertically miform.
The secon(11ayer is defined as a thermoCline layer,
which is located below the oceanic mixed layer and is
characterセed as having the greatest vertical gra(lient of
sea temperatures.The portion ofturbulent energy that
is available for the generation of the ocean cun℃nt is
proportional to the difference of the wind stress
between that on the surface and that on the base of the
mixed layer.It is assumed that all turbulence is
con且ned to the mixed layer,and thus the mod迅cation
of the thermocline layer is due only to horizontal
advection an(1upwelling.The sea temperature below
the thermocline layer remains constantl i.e.,our mbくed
layer ocean model can only forecast the sea
temperatures ofthe two upPerlayers.
The Boussinesq and the hydrostatic appro⊃dmation
are assumed in this model.In addition,a reduced gravity
approx㎞ation h}which the pressure gra(1ient is reduce(l
to O in an infinite depth is also assumed.Note that the
effect of horizontal d血1sion is neglected in this model
becausewew皿1predictocean con(litionsup to afewweeks
at most.The thickness,the momentum,and the
themodynamicequaHons冴easfoUows.
Thethic㎞essequationsare,
並+▽・(h1玩)一ω,
∂渉
∂h2+▽・(h2瑞)一一ωe
∂診
∂h → ∫+▽・(h必)=0∂孟
(1-1)
(1-2)
(1-3)
Suf五x lづl in(licates the ith Iayer,except the丘rst and
the second layers,h(m)is the thic㎞ess ofthe layer,V
(m/s)is the ocean current,and物,1(m/s)is the
entrainmentrate.
The momentum equations are,
∂響+▽・(幅)+(∫+∂1穿θ)だ×h・味蘇+ω・ち(2一・)
∂響+▽・(幅)+(∫+び2穿θ)雇×h2畦蘇一ω・ち(32)
∂響+▽・(繭+(∫+禦)る×礁語 (2-3)
where∫is the Coriolis parameter,which is2Ωsinθ,θis
thelatitude,Ωistheangularvelocityoftheearthrotation(7.2×10{5s-1),σis the radius of the earth
(6370㎞),andρoistherelerencedensi敏(1023kg/m3).
P indicates the pressure gradient shown by Ben(1er召∫
α」.(1993).τ(N/m2)is thewind stress.
Thethemodynamicequationsare,
∂h・θ・+▽・(脅h1θ1)=B+ω,θ2
∂孟
∂h2θ2+▽・(彦h2θ2)一一2ω,θ2
∂診
(3-1)
(3-2)
θ1is T1-T3,andθ2is T2-T3.T3(K)is the
temperature atthe bottom ofthe thermocline layer and
is a constantvalue in this model.Suf6x l represents the
MLT,which is included in the S訂.Suf且x2indicates
the temperature between the mixed layer and the
thennocline layer.
The salini取equations are(lefined as replacing the
water temperature and the heat flux with the salinity
and the water flux.The entrainment rate that indicates
the degree of turbulent mixing is defined as the
empiricalfomulasofDeardor丘(1983).Theseempirical
formulas are calculated based on the Richardson
number for the frictional velocity(1~」τ)caused by win(1
stress,the buoyancy ef価ect(1~σ.)in the mixed layFer that
occurs by emission of heat fluxes on the sea surface,
an(l the vertical shear of currents(1~ん)between the
mtxe(11ayer an(1the thermocline layer.The essential
details of the numerical algorithm are described in
Deardorff(1983).The entrainment rate is described as
afunctionofR歪τ,1~づ薯,1~ル.
肌一肌(1~στ,1~づ捧,1~劾 (4)
Jacobαα」.(2000)examine(i three bulk
entrainment parameterizations for obselvational data
on Hurricane Gilbert.The Price(1981)scheme(iepen(1s only on the bulk Richardson number Riv.
According to Price (1981),the external bulk
Richardson number1~歪τwas also considered.The
Kraus and Tumer(1967)scheme(1epends on themagnitu(1es of the frictional velocity and the heat nux。
The Deardorff(1983)scheme depends on1~」,,1~∫.,and
Riv.Jacob6緬」.(2000)estimated the entrainment
mbdng across the base in the mtxed Iayerby Hurricane
Gilbert at different time stages.After the passage of
storms,the ma}dmum contribution to the dynamics in
the mb【ed layer was associated with the shear-induced
entrainment mbdng forced by near-inertial motions up
to the third day,as indicated by the bulk Richardson
numbers that remaine(i below criticality.Ginis(1995)
also estimated the features of three bulk entrainment
closure schemes,such as Price(1981),Deardorff,
(1983)an(1Elsberryαα」.(1976),an(l these kinds of
parameterizations were dhferent from those in Jacobθ孟
α」.(2000).The amount of SST cooling by Deardorff
(1983)was the greatest in Ginis(1995),and SST
cooling also sprea(1thewidestofthe three schemes.
34 W&(1aA Vol.52,No.2
3.Nume近ca1取pe血nents underIdeal Conditions
In a(ldition to the heat loss to the atmosphere,the
oceanic processes of vertical an(l horizontal advection
plus turbulent mixing at the top of the thermocline
layer contribute to cooling the upPer layer in the ocean
(Elsberryθ麺1.1976).Since these mechanisms are
intemingled,it is d置icult to resolve the magnitude of
each of the processes by observation.Price(1981)
discussed the parametric dependence of the upper
ocean response.He examined the ocean response for
parameters such as the hurricane wind speed,
hurricane transla.tion speed,hurricane size,the initial
thickness of the mtxed layer,the vertical gradient of
sea temperatures atthe transition layer,and the inertial
period.In his conclusion,the SST response is a lively
function of the hurricane st1●ength and translation
speed,the initial mixe(11ayer depth,and the vertical
gradient ofsea temperatures at the transition layer,but
a weak function of the latitude and the hun・icane size.
In this section,we quantitatively estimate the
parametric dependence of the upper ocean respqnse,
such as on the typhoon wind speed and size,the
取phoon translation speed,the heat且uxes,the initial
thic㎞ess of the mixe(i layer,and the vertical gradient
ofthe seatemperatures.
3.1 1nitial Conditions
We conducted numerical experiments under
simp1迅ed initial conditions to estimate the amount of
MLTcoolingundervarious initial conditions.Here,the
ocean responses to typhoon forcing are examined from
the stan(1points of the various parameters,such as the
size and the intensity of the wind stress,the speed of
the typhoon,the heat nuxes on the sea surface,the
initial thickness of the mixed layer,and the vertical
structure of the sea temperatures.It is assumed that
the wind distribution is determined by the Rankin
vortex,which is applied to Chang andAnthes(1978).
The distribution of wind stress(Fig.1)is
calculated as follows:
τ7(7,θ)=
τθ(7,θ)=
一1τ,1卿7巖蟻、πθノ
7min≦7≦7max
-1τ,1規グ孟n 7≦7min (5’1) 7>Zmax
O
1τ、1吻7無荒、寮θノ
1τθ1卿劣煮nπθノ
0
名min≦7≦7m眠7≦名min (5-2)
7>7max
The outer edge of the typhoon is now7卿、.(this
(listance correspon(ls to6.25。in this paper),and the
radius of the取phoon eye is7別砿(0.5。in this paper)。
Both are defined as the number ofg且ds in this model.
However,because of the northwar(l translation,the
zona1(1ist段nces a』re not const且nt(1ue to the change of
Iatitude.The distribution of win(1velocities was
assumed using Eqs.(5-1)and(5-2).The wind stresses
were then calculated by Kon(lols formula.Ginis and
Sutyrin(1995)reported that the shape of the wind
stress outside of the radius of the maximum win(1is
importantin detemining the horizontal stmcture ofthe
barotropic currents.
Weassumedthattheratioofthemmingdirectionof the typhoon is-1to the west an(12to the north in
these experiments.Therefore,this typhoon moves
north-northwestwar(i(Fig.1).
28N
26N
24N
22N
20N
/8N
∵∵∴∵緊辮
・::・::ン2 徊器隻沙
べ 、 、 ↑ ↑ チ メ 廓 4 { 叩 っ マ
\\~↑fヂプ 劉 貿 ゴ 7
\\\↑ノノ〆 胃.77 7.
ブ!/ - ” } ▼ 7 7 7 ’
\\N -’”ヲ77 77■
矯濃ヨ蝦㍑…
!/
1↓\\\、、う夢)》》》》 ・
Fig.1
154E 156E 窪38E 340E 142E 144E 一『(N/㎡)
The horizontal distribution of wind stresses at
the initial time an(l at48hours integration.The
contours indicate the wind stresses(N/m2)at the
initial time;the contour intelvals are O.5N/m2.
The vectors show the win(l stresses(N/m2)after
48hours integrationl the unit vector is5N/m2.
The typhoon mark is located in the typhoon
center at the initial time and at48hours
integration;the line represents the取phoon track
over48hours.
The coefficient∫(θ),which indicates
asymmetry ofatyphoon,is(1efined below.
∫(θ)=1+γ(cos(σ)cos(わ)一sin(α)sin(わ))
the
(6)
c・s(a)一(1-cx)/r,sin(a)一σ一cy)/r,c。s(b)一2/佑,sin(b)一一
1/お.
The point(cx,cy)is the center position of the
壇)hoon。The initial center position of the typhoon is at
140。Eand200N.Thecoe伍cientγusuallytakesavaluebetween O and O.3and is now applied to O.1.I and J
2002 The Processes ofSSrCoohngbyTyphoon Passage and Case StudyofTyphoon Rexwith aM盈edlayer Ocean Mode1 35
represent the indexes of the longi加de an(l latiUlde.r is
the(1istance be伽een(cx,cy)an(1(1,J).Price (1981),
Greatbatch(1983),and Chang and Anthes(1978)
invest圭gated the ef£ect of asymmetry in storm forcing,
but it is not important compare(l with the effect of the
nonlinear dynamics.
The d圭stribution of the atmospheric pressure is
given accor(ling to the fonow血g fonnula(Holan(11980).
且ραか=瓦+(P.一jP、)exp(一一万)
7
(7)
P。indicates the minimum central pressure of the
typhoon(960hPa).P.is the environmental pressure
(1010hPa).7is the distance丘om the typhoon center.A
is the radius of the tyl)hoon(6.25。in this paper).B is a
constant value that is equal to1.5.
It is assumed that the variation of the air
temperature always agrees with that of the SST.
Therefore,hea』t fluxes across the sea su㎡ace an(l the
entrainment rates(1ue to buoyancy can be neglected
when there are no differences between the air
temperature and the SST.
Each vertical layer thickness was initially set to
50m in the mixed lay・er,150m in the thermocline layer
and100,100,100,200,300,and500m below the
themoclinelayer.Theoceandepthwasinitially1500min the whole area.The sea temperatures at the top of
each layer were28。C in the SST,27。C in the transition
layer,180C at the bottom ofthe thermocline layer,and
120C,8。C,60C,5。C,and40C at the top of the bottom
layer.The transition layer was located between the
mixed layer and the thermocline layer.The vertical
profiles of the sea temperatures are shown in Fig.2.
The above-mentioned profile corfesponds to the lbl
profile in Fig.2and is used as a st3ndard profile in this
study.The lal profile of th6vertical sea temperature in
Fig.2has a30m mixed layer and a170m themlocline
layer,the lbl profile has a50m mtxe(l layer and a150m
thermocline layer,and the℃l profile has a70m mixed
layer an(1a130m thermocline layer.The ldl and lel
profiles have the same depth in the mb【ed layer and
thermocline layer as the lbl profile has,but the sea
temperature at the bottom of the thermocline layer in
the ldl profile is warmer than that in the lbl profile an(1
the sea temperature at the bottom of the thermocl圭ne
layer in the lel profile is cooler than that in the lbl
profile.The salinity is assumed to be uniform(35ppt)
throughout the whole ocean.It is assumed that ocean
currents are motionless atthe initial stage.
A summary of our numerical experiments is
shown in Table1.Experiments l to9were canied out
under differentmaぬmumwind speeds.Experiments10to21were then canie(l out un(1er(i迂ferent hea』t fluxes,
expenments22to27under蔽erentthic㎞essesofthe
m嬢edlayerandthemoclinelayer,andexperiments28to 29 un(1er different vertical profiles of sea
temperatures.All experiments for the different
conditions were pe㎡ormed un(1er(1ifferent translation
speeds,exceptexpehments28and29.
degree(OC)
05雀015202530depth O
(m)
100
200
300
400
500
ーメ
!
β×
/
〆×
∠
一ノニ
…丑a(30m)一(>一b(50m) l
l+c(70m)… d(5・m-W)i
I一〇一e(50m_G)1
Fig.2 Five vertical pro五1es of sea temperatures were
used in this sUIdy.The『a『pro丘1e of the vertical
sea temperature has a30m mixed layer and a
170m thermocline layer,the lbl profile has a50m
mixe(11ayer and a150m thermocline layer and
the lcl profile has a70m n通xe(i layer and a130m
themoclinelayer.Theldlandlelpro五lesarethe
same as pro丘1e lbl in the thic㎞ess ofthe mixed
layer and the the㎜ocline layer.The temperature
is20。C at200m,15。C at300m,and10。C at400m
in the ldl profile,and is15。C at200m and10。C at
300min the lel profile.
3.2MLTCooling Caused by Di押erentWind Stresses
In this section,we discuss how wind stresses
contribute to MLT cooling.Three kinds of wind
stresses are examined in experiments2,5,and8(Table
1).Figure3(a)shows the distribution of MLT
deviations from the initial condition at48hours
integration in experiment5.The dot represents the
丘xed point where a variation of MLT,a deepening of
the mixed layer an(1a variation of depth at the bottom
of the thermocl董ne layer by upwelling,was observed.
MLTcoohng on the right side along the typhoon track
is clearly seen under and behind the tyl)hoon.The area
of ma虹mum MIT cooling is seen around21.5。N and
the distance from the typhoon center(about26.50N)to
the area of maximum MI∫r cooling is from500km to
600km.Maximum deepening of the mixed layer is
about45m on the right rear of the typhoon,10cate(1
aroun(i22。N(Fig.3(b)).Except for this position,
deepening of the mixed layer is about30m and is
clearly seen near the typhoon center.The(leepening of
the mixed layer is smaller on the left side of the
running嚇)hoon.
Divergent currents are dominant on the right side
in the running(1irection,and then near-inertial currents
apPear on the right rear of the typhoon,especially near
36 WadaA. Vol.52,No.2
Experiments mixed-layer translation maximum wind sea temperature sea temp.一airtemp. maximum SSTthickness(m)s eed(m/s) s eed(m/s) at3rd b er (OC) decrease(OC)(48h)
1 50 3 30 18 0 一1.33
2 50 5 30 看8 0 一〇.97
3 50 10 30 18 0 一〇.3
4 50 3 35 18 0 一2.7
5 50 5 35 18 0 一2.18
6 50 10 35 璽8 0 一1.06
7 50 3 40 18 0 一4,34
8 50 5 40 18 0 一3.89
9 50 10 40 18 0 一2.2
Experiments mixed-layer translation maximum wind sea temperature sea temp.一airtemp. maximum SSTthickness(m)s eed(m/s) s eed(m/S) at3rd la er (℃) decrease(。C) (48h)
10 50 3 35 18 1 一2.85
11 50 5 35 18 1 一2.47
12 50 10 35 18 1 一韮.22
13 50 3 35 18 2 一2.97
14 50 5 35 18 2 一2.57
15 50 10 35 18 2 一1.31
16 50 3 35 18 3 一3.09
17 50 5 35 18 3 一2.68
18 50 10 35 18 3 一1.4
19 50 3 35 18 4 一3.2
20 50 5 35 18 4 一2.78
21 50 10 35 18 4 一1.5
Experiments mixed-layer translation maximum wind sea temperatUre sea temp,一airtemp. maximum SSTthickness(m)s eed(m/s) s eed(m/s) at3rd ia er
(。C) decrease(OC)(48h)
22 30 3 35 18 0 一4.09
23 30 5 35 18 0 一3,06
24 30 10 35 18 0 一1.94
25 70 3 35 18 0 一2.2
26 70 5 35 18 0 一1.36
27 70 10 35 18 0 一〇.49
Experiments mixed-layer translation maximum wind sea temperature sea temp.一airtemp. maximum SSTthickness(m)s eed(m/s) s eed(m/s) at3rd Ia er
(。C) decrease(OC)(48h)
28 50 5 35 20 0 一2.06
29 50 5 35 15 0 一2.67
Table1. A summary of the numerical experiment.There are29experiments that are divide(1into
五vecategohes,theinitialthic㎞essofthemb【edlayer,thetyphoontranslationspeed,the
maximum、瀬ndspeed,the seatemperatureatthebo枕omofthethemoclinelayer(=3「d
layer),and the deviations from the sea temperature to the air temperature.MLT cooling
for each isgiven in the7th column.
the typhoon center(Fig.3(b)).This is because
circulations of clockwise rota』tion are strengthene(l on
the right side in the running direction with the
movement of the wind stress(Price,1981).The
divergent currents are formed by counterclockwise
strong wind stresses near the typhoon center(Price,
1981).The depth atthe bottom ofthe thermocline layer
is remarkably shallower behind the typhoon,located at
25。N(Fig.3(c)).Figure3(c)indicates that the
magnitude of the shallower depth due to upwelling is
donオnant behind the typhoon.
Figure3((i)shows the horizontal(1istribution of
MLT advection.The amount of advection is calculated
by一か▽T,in which∂represents the mixed layer
currents,Tis the MLT,and▽is the gradient operator.
Advection plays a role in warming the MLT directly
mder the typhoon,while the MLT becomes cooler on
the right side of the typhoon center.The amount of
MI∫r advection directly under the typhoon is greater
than that behind the typhoon.However,thecontribution of MLT advection to the ma】dmum MUr
cooling is somewhαtsmaller.
Figure3(e)shows a vertical section along O.5。
eastward of the typhoon track.”Typhoon mark”
indicates the location of the typhoon center.Sea
temperature cooling is evident at the transition layer.
Figure3(e)also shows the existence of near-ine域ial
oscillations near the surface and convergent currents
belowthe thermocline layerbehin(l the typhoon.
The entrainment rate is locally great behind the
typhoon and has the important role of supplying cooler
water from below the thermocline layer to the sea
su丘ace.The ma痘mum entrainment appears where the
wind stress is e:田ciently working to the kinetic energy
ofthe mixed layFer(Price,1981,Fig.16b).This area also
has substantial current shears between the mlxed layer
andthermoclinelayer.WesupposethatthislargeshearmakeS the entrainment rateS greater Un(ler COnditiOnS
of no heat flux.However,maximum MLT cooling
cannot be determined by entrainment alone.In fact,
maximum MI∫r cooling appeared around220N behind
the typhoon in experiment5.Elsberryασ」.(1976)
reporte(10n the(iistribution of nea1●一surface cooling,
subsurface cooling in(1uced by upwelling,and an
intermediate layer of warming due to entrainment and
convective mixing.It is thought that SST cooling by
2002 瓢ePr・cesses・fS訂C・・HngbyT四h・・nPassageandCaseS加dy・fT”h・・nRex舳aM盈edlayerOce㎜M。de1 37
entrainment is included孟n these processes of Elsberly
etal.(1976)
28N
26N
24N
22N
20N
18N /54E
Fig.3(a)
、 Qヴ2粛
つ
一〇.
一〇.6
-0,5
28N
26N
24N
22N
20N
18N
》“})②=F 』みレ}“一壷一1壱一54レ}壷↓ ∠) ~ 甲Ψ耐 モ ↓ 曳 島 ^ 卜 斌\t舎癌4
き↓4\ 蹄ノメ胃)壷帝ψレ↑ ■π矛
、ジ\ 、 も
ペキ の
i魅藻、三灘 、〕15
-10
-5
5ゆ ψ 4 》
〆 一 ( ふ
〆 ∫ ▼ 4
賃56E て』38E 140E ↑42E 144E
The decrease of the MLT in experiment5is
shown horizontally from134。E to144。E an(l
from18。N to28。N.The contours indicate the
deviations ofthe MLTfromthe initialvalues.The
contour intervals are O.3。C.The line represents
the取phoon track over48hours.The取phoon
mark is locate(1in the取phoon center at the
inidal time and at48hours integration.The dotis
the丘xe(l point used in Fig.4.
454E 窪56E 138E 140E 142E 144E
訂 (m/s)
Fig.3(c) The variation of the thickness at the bottom of
the thermocline layer in experiment5is shown
horizontallyfrom134。E to144。E and from180N
to280N.The contours indicate the variation of
the thickness at the bottom of the thermocline
layer.The contour intervals are5m.The vectors
show the currents below the thermocline layer;
the unitvector is O.3m/s.The line represents the
typhoon track over48hours.The typhoon mark
is located in the typhoon center atthe initial time
and at48hours integration.The dot is廿1e fixed
point use(1in Fig.4.
28N
26N
24N
22N
20N
織欝耀 / ~ゆ夢 “寸》v /与→ , “ } v 》
\ 、 司 ソ レ 幽 乙
眠\ ψレ‘4くてマ
\ ≠ 4 卜トハ
1\勇5一漸;窺
\O灘多
ヘ ノ ノ
、 / ノ 刀ノ ノー
. ノ9∫!イグノノ“ >1ンノ ノ ノノ認
二’砂ノ ノ ノ ノr ~、 、 、
窪54E 156E 158E /40E 142E 344E -7 (m/s)
Fig,3(b)Deepening of the mbくed layer in experiment5is
shown horizontally from134。E to1440E and
from18。N to28。N.The contours illustrate the
deepening of the thickness of the mixed layer
from the initial value。The contour intervals are
5m.The vectors show the mb【ed layer currents;
the unit vector is lm/s.The line represents the
typhoon tr&ck over48hours.The typhoon mark
is located in the typhoon center at the initial time
and at48hours integration.The dot is the fixe(l
point used in Fig.4.
18N
28N
26N
24N
22N
20N
ノF 4↓ ↓ ‘ 一 解 畠 ム ム 』 》 》 貿
』↑〆/〆!,‘‘‘勝齢轟‘乙幽レ}}
154E 藷56E 158E 140E 142E 144E 「→ (m/s)
Fig.3(d)The MLT advection in experiment5is shown
horizontallyfrom134。E to144。E and from18。N
to280N.The contours indicates the amount of
MLT advection per time step(1200seconds)(一
θ・▽T,θis the mixed layer currents,T is the
MLT,and▽is the gradient operator).The
contour intervals are O.003。C/timestep(1200
secon(1s).The vectors show the mixe(11ayer
currentsl the unit vector is lm/s.
18N
38 WadaA. VoL52,NQ。2
Fig.3(e)
depth o(m) →50
100
150
200
250
500
550
400
450
500
、,.錦プ
......=q,モYM》♪79,蕪.、、...Tq.9〉♪〉τ9.蚤.
20N 22N 24N 26N 28N 50N 1 (m/s)
The vertical section along O.5。eastward of the typhoon track in experiment5,from20。N
to30。N and from the surface(Om)to500m depth.The vectors show the ocean currents;
the unitvector is lm/s.The contours indicate the(leviations ofthe sea temperatures;the
contourintervals are O.3℃.The typhoon markislocated inthe typhoon centerat72hours.
Figure4shows the time series for MLT cooling,
the deepening of the mb【e(l layer,~md the vahation of
the depth at the bottom of the themaocline layer at the
丘xed point139。E,22。N.This position is located on the
right si(1e of the running typhoon,where the wind
stress is somewhat greater.This position also can be
observed both under and behin(i the running typhoon.
Figure4indicates thatthe phases ofMLTcooling may
be distributed in three stages.From in圭tiation to12
hours,the mixed layer becomes deeper an(1the depth
at the bottom ofthe thermocline layer becomes a little
deeper(downwelling).In this first stage,the MLT
suddenly decreases after six hours,although MLT
coolingis onlyminimal atfirst.The(1epth atthebottom
of the thermocline layer becomes shallower
(upwelling)from12hours to30hours,an(l the mb【e(l
Iayer becomes a little shallower.In this second stage,
the MIT decreases as rapidly as that in the first stage
丘om6to12hours,and the ma虹mum MH’cooling is
detemined inthis stage.The depths inthe mbくedlayer
and at the bottom of the thermocline layer change
periodically after30hours.However,the MI∬’doesnlt
change as dramatically in the third stage as in the first
and secon(l S惚ges.
Experiment8is a case of stronger win(l stress.
The maximum MLT cooling is greater than that in
experiment5(Fig.5(a)).Ma⊃dmum deepening of the
mixed layer is around70m and is deeper than that in
Fig.4
㍗0
0
α
一〇.40
一〇.80
一1.20
一壌.60
一2.00
P P P E
髪t ‘
p峠 『1 1 1 1 1 1 1
…『-ll,,1噌IIIIIlliーーー5
l 1
PーーーPー“
一一
60.脆
40.00
20.00
一e-SST DEVIAT【ON(deg)P
i舟MiX LAYERO.oo DEEPEMNG(m)
20・00、赤BOπOMOF T短ERMOGU旺i SHALLOWER(m)1
40.001、
1
-2.40 三 一一」60.00 0 6 12 18 24 30 36 42 48 54 60 66 72
ho鱗r
The time series ofMLT cooling(。C),deepening ofthe mbくed layer(m),and variation of
the depth at the bottom ofthe the㎜ocline layer(m)at1390E and22。N from the initial
time to72hours integration in experiment8.The left axis indicates the temperature
measurement(。C),an(1the righta⊇ds shows the metermeasurement.
2002 The Processes ofSSTCoolingbyTyphoon Passage and Case Study ofTyphoon Rexwith aMixedlayer Ocean Mode1 39
28N
26N
24N
22N
20N
18N
畿謹繋、
、、⑲
那殿、 \
134E
28N
26N
24N
22N
20N
18N
駄
55 N\ ↓
~5 ↓
逗3肴》:-椿
一5
136E 158E 140E 142E 144E
鳶1Q、
、と遭尊1つ^》》 ↓ 曹 噸 噌 ▲ 》
∫ ‘ ψ や ぜ ▲
〆 ∫ 睡 Ψ 噌
154E 136E 138E 140E 142E 144E 耐 (m/s)
Fig.5(a)
28N
The decrease of the MLT in experiment8is
shown horizontally from134。E to144。E and
from18。N to28。N.The contours indicate the
devia丘onsoftheMLTfromtheinitialvalues.The
contour intervals are O.3。C.The line represents
the typhoon track over48hours.The typhoon
mark is located in the typhoon center at the
inidal dme an(l at48hours integration.The dot is
the fixed point used in Fig.6.
ク 瀟
蕪…灘・.灘
遠野6 ↓、
Z
恥 》 、
著 \
~ ぞ
ヤ
あ
疹 〆↓
強 誉痴〆〆㍑ 〆・
叡、振響θ激楽
Fig.5(c)
26N
24N
22N
20N
18N
玉ノノノノノ!
〆ノ
F 甲
/ノ
134E 156E 158E 140E 142E 144.E 一マ (m/s)
Deepening of the mb【ed layer in experiment8is
shown horizontally from1340E to144。E and
from18。N to28。N.The contours indicate the
deepeningofthemixedlayerthic㎞ess丘omtheinitial value.The contour intervals are5m.The
vectors show the mixe(l layer currents;the unit
vector is lm/s.The line represents the typhoon
track over48hours.The typhoon mark is located
in the typhoon center at the initial time and at48
hours integration.The dot is the fixe(1point used
in Fig.6.
28N
The variation of the thickness at the bottom of
the thermocline layer in experiment8is shown
horizontallyfrom134。E to1440E and from180N
to28。N.The contours indicate the var玉ation of
the thickness at the bottom of the thermocline
layer.The contour intervals are5m.The vectors
show the currents below the thermocline layer;
the unitvector is O.3m/s.The line represents the
typhoon track over48hours.The typhoon mark
is located in the typhoon center at the initial time
and at48hours integration.The dot is the fixed
point used in Fig.6.
26N
24N
22N
20N
Fig.5(b)18N
嶽騎α.
蝦〃〆房、、、u
麟擁磐諺駿認
154E 136E 158E 140E 14・2E 144E -7 (m/s〉
Fig.5(d)The MLT advection in experiment8is shown
horizontallyffom134。E to1440E and from18。N
to280N.The contours in(licate the amomt of
MLT advection per time step(1200seconds)(一
∂・▽T,∂is the mixe(l layer currents,T is the
MLT,and▽is the gradient operator).The
contour intervals are O.005。C/timestep(1200
secon(1s).The vectors show the mixed layer
currents;the unitvectoris lm/s.
40 Wa(iaA Vo1.52,No.2
experiment5(Fig.5(b)).Deepening ofthe mixed layer
near the typhoon center is also greater.The(1epth at
the bottom ofthe thermocline layer becomes shallower
behind the typhoon(Fig.5(c)).The amount of M皿’
a(1vection in Fig.5((1)is greater than that in Fig.3((1),
particularly near the typhoon center.Positive and
negative advections are greater due to strong wind
stresses,Even in this case,the effect of advection on
MLT cooling per time step is smalL Figure5(d)
indicates that the sea temperature is lower in the
vertica1(lirection.Sea temperature cooling at the
bottom of the mixed layer is particularly evident.
Figures5(c)an(i5(e)also indic&te that the variation
due to upwellingis greaterthan in experiment5.Figure
6shows the time se貞es for MLTcooling,deepening of
the mixed layer,and variation of the(1epth at the
bottom of the thermocline layer at the丘xed point139。
E,22。N un(ler strongerwin(i stresses.The m圭xed layer
(1eepening is greater and the depth atthe bottom ofthe
mixed layer is shallower than in experiment5.The
greater entrainment rates presumably make these
amplitudes higher.
In contrast,entrainment rates are lower in
experiment2,due to the weak wind forcing,so the
MH’cooling,deepening of the mixed Iayer,variation
byupwelling,and hohzontal advection are smaller.
Fig.5(e)
depth o(m)
50
100
150
200
250
300
350
400
450
500
→...,1篇一!1誌一茄》一1夏
.....一鵠題1』☆驚1、
/
一 TQ・5鳩と6.描q’裾ζ9・臥
20N 22N 24N 26N 28N 50N 1 (m/s)
The verdcal section along O.5。eastward ofthe typhoon track in exper㎞ent8from20。N to
30。N and from the surface(Om)to500m depth.The vectors show the ocean currents;the
unit vector is lm/s。The contours indicate the deviations of the sea temperaturesl the
contourintelvalsare O.5。C。The1yphoonmarkislocatedinthetyphooncenterat72hours.
Fig.6
驚』ゆ
0
一〇。50
一肇.00
樋.50
一2.00
一2.50
一3.00
一3.50
一4.00
一4.50
E臼 i
L ;1}
・」 一[
- I I l
i l
、、、-」
一
よ
』1
0 6 12 18 24 30 36
hour
42 48 54 60 66 72
m80.00
60.00
40.00
20.00
0.00
20.00
i令SST DEVIATION(deg)
一40。ooi
一60.00
→ヨーMIX LAYER DEEPE卜謎NG
ナ のロの ド
翻員臨騰)…
80.00
100.00
The time series of MLT cooling(。C),deepening ofmtxed Iayer(m)and variation ofthe
depthatthebo廿omofthethemocHnelayer(m)at139QEand22。N丘omtheinitialtimeto72hours integration in experiment8。The left axis indicates the temperature
measurement(℃),and the righta}ds shows the metermeasurement.
2002 TheProcessesofS訂CoolingbyTyphoonPassage and CaseStudyofTyphoon Rexwith aMixedlayer OceanModeI 41
3・3MLT Coo”ng Caused by Different Typhoon Trans置ation Speeds
Chang an(1Anthes(1978)reported that SST
cooling increased as the typhoon moved more slowly.
Here,numerical experiments were con(lucted with two
speeds under constant wind forcing.The typhoon
moved slowly(3m/s)in one an(1faster(10m/s)in the
other.Experiments l to9revealed that the slower
translation and the stronger wind forcing increase(1
MLT cooling at48hours integration(Table1)。
Experiment4,the slower case,and experiment6,with
fastermovementare described in this section.
A slower-moving typhoon makes the decrease of
S3[’greater in the hohzontal distribution(Fig.7(a)).
The location of MLT cooling was almost the same as
thatinexperiment5.Ma》dmumdeepeningofthemixedIayer(Fig.7(b))was at about30m,1ess than that in
experiment5after48hours integration.However,the
(1epth atthe bottom ofthe themocline layer(Fig.7(c))
was40m shallower than that in experiment5,and the
distribution was clearly d置erent from that in the5m/s
case.The area of shallower depth due to upwelling
appeared onlybehind the typhoon in experiment4.The
d迂ferent distribution ofthe shallower area by upwelling
seems to be related to the transferred distance and the
wavelength ofthe near-inertial oscillations.Figure7(d)
shows the amount of MLT advection.The amount of
maximum positive and negative MLT advection near
the typhoon center is greater than those in experiment
5.However,the contribution of MLT advection to the
ma⊇dmum MLT cooling is as small as the contribution
in experiments5and8.The near-inertial oscillation
doesnlt appear in the3m/s translation spee(1during48
hours,so the hodzontal distribution of MLT advection
differs from that in expehments5and8.The verticaI
section(Fig.7(e))in(1icates that sea temperature
cooling Penetrates in the vertical direction near the
typhoon center.Since integral time doesnlt reach a
period of near-inertial time,divergent currents in the
mixe(11ayemearthe typhoon center are conspicuous in
experiment4.Figure8depicts the time series of MH’
cooling,(1eepening of the mb【ed layer,and variation of
the depth at the bottom ofthe thermocline layer at the
fixe(l point139。E,220N for a3m/s translation speed.
The墨eatures of each stage of MLT cooling are similar
to those in experiment5.However,the amplitude due
to upwelling is greater,the wavelength of the near-
inertial oscillation is shorter,and the maximum MUr
cooling is greater.Currents below the thermocline in
this slower typhoon translation can be balanced
geostrophically,an(1subsequently generate greater
upwellingthan thatin experiment5.
28N
26N
24・N
22N
20N
18N
磯一
154E
Fig.7(a)
28N
156E 158E 140E 142E 144E
The decrease of the MLT in experiment4is
shown horizontally from134。E to144。E and
丘om18。N to28。N.The contours indicate the
deviations oftheMLrfromtheinitialvalues.The
contour intervals are O.3。C.The line represents
the typhoon track over48hours.The typhoon
mark is located in the typhoon center at the
initial time and at48hours integration.The dotis
the fixed point used in Fig.8.
26N
24N
22N
20N
18N
撃畢覧K k、~↑↑↑ ・調、マ,7》郊喉乳\\\ 6 ↑プ1メメ”τ▼7) 凧、 \\\~↑ア ノノ男...._\ や !ノ ー刀..γ←_~\ 1 ノ 、刃亭7 15
← ~ 櫓 - 」々7← ↑ ノ 身 マ
ゲ ゼ す ノド げ
一〆匝漉 24 ”〆 〆〆! 絃 --
11繊, 、誘ノグ -
■
■
ノ
ノ
/
オ
ψ↓↓↓い ウ24 グ7Ψ ↓ も 、、 →21 ■1
134E
Fig.7(b)
156E 158E 140E 142E 344E -7 (m/s)
Deepening of the mbζed layer in experiment4is
shown horizontally from1340E to1440E and
from180N to28。N.The contours indicate the
deepening of the thickness of the mixed layer
from the initial value.The contour intervals are
3m.The vectors show the mixed layer currents;
the unit vectoI’is lm/s。皿e line represents the
typhoon track over48hours.The typhoon mark
is locate(l in the typhoon center at the initial time
an(1at48hours integration.The dot is the fixed
point used in Fig.8.
42 Wa(iaA VoL52,No.2
28N
26N
24N
22N
20N
18N
,ζNV
㌧\ \
ぢ 〆 〆
∠
\、㌧↓↓↓↓↓
旨 、 、 ↓↓↓↓↓
岬
、 、 モ ↓ ↓
ノ
ヤ む
、ノ㍉、、、㍉モ
5、斗、、↓↓↓
154E 156[ 138E 140E 142E 144E 万ア (m/s〉
Fig.7(c)The variation of the thickness at the bottom of
the thermocline layer in experiment4is shown
horizontallyfrom134。E to1440Eξmd from18。N
to28。N.The contours indicate the variation of
the thickness at the bottom of the thermocline
layer.The contour intervals are5m.The vectors
show the cu1Tents below the themlocline layer;
the unitvector is O.1m/s.The line represents the
取phoon track over48hours.The typhoon mark
is locate(l in the卿hoon center at the initial time
and at48hours integration.The dot is the丘xed
point use(l in Fig。8.
28N
26N
24N
22N
20N
18N
llここ厭串獅驚噺凧~\\\\ ~ ↑ ア ア ノ ノ メ ■ ” . .<
綴1沼髄1・・〃//〃 。■ノ// /ノー♂’’”↓↓暫 ゆーノ/ノ/ノゆ ゆ ゆ も む
噛 ゆ ↓ も モ 、 、 、 →ノノノノ//∠=逗
134E 156E 158E 140E 142E 一「’ (m/s)
144[
Fig.7(d)The MLT advection in experiment4is shown
horizontallyfrom134。E to144。E and from18。N
to28。N.The contours indicate the amount of
MH’advection per time step(1200seconds)(一
θ・▽T,∂is the mixe(11ayer currents,T is the
MLT,and▽is the gradient operator).The
contour intervals are O.0030C/timestep(1200
seconds).The vectors show the mixed layer
currents;the unitvectoris lm/s.
depth o(m)
50
100
150
200
250
500
350
400
450
500
→ 一 一 ・ 2.
一〇.5 ノ20N 22N 24N 26N
1(m/s)
Fig.7(e)The verdcal section along O.5。eastward ofthe typhoon track in expehment4from20。N
to26。N and from the surface(Om)to500m depth.The vectors show the oceIm currents;
the unitvector is lm/s.The contours indicate the deviations ofthe sea temperatures;the
contour intelvals are O.3。C.The typhoon mark is located in the typhoon center at72
hours.
2002 The Processes ofSST Cooling byTyphoon Passage and Case Study ofTyphoon Rex with a Mb【ed layer Ocean Mode1 43
OC
O.00
一〇.25
一〇.50
一〇.75
一匪.00
一1.25
一1.50
一屡.75
一2.00
一2.25
一2.50
E5き
E…
E
ヨ引 ヨ
i
一 一 『
4i
…ヲi
一「 Ei
…
E E
0 6 12 18 24 30 36 42 48 54 60 66 72
h◎ur
OmOO
4
30.00
20.00i一喚一BQτTOM OF トほ ロれヒ
30。ooi SH肌OWER(m)
一40.00
50.00
60.00
Fig.8 The time sedes ofMLTcooling(。C),deepening ofmb【ed layer(m),and vanation ofthe
depth at the bottom ofthe themaocline layer(m)at139。E and22。N from the initial time
to72hours integration in experiment4.The left axis indicates the temperature
measurement(℃),and the righta》ds shows the metermeasurement.
A faster-moving typhoon results in less MLT
cooling in the horizonta1(listribution(Fig.9(a)).The
slower translation speeds of the typhoon have an
oceanic response in a circular paUlern(Fig.7(a)).In
contrast,the faster translation speeds of the typhoon
have a narrow oceanic response formed in a long
ellipse pattem(Fig.9(a)).These features are also
observed in the deepening of the mbくed layer(Fig.9
(b))and the variation ofthe depth at the bottom ofthe
thermocline layer(Fig.9(c)).Geisler(1970)
demonstrated that the interface between the upper and
lower Iayers in a two-1ayer model tends to experience
an oscillatory response rather than general vertical
a(1vection as the translation spee(l increases.The
vertical ocean response in experiment6was weaker
than that in experiments4and5.In addition,
Greatbatch(1983)investigate(l the ocean response to
storms with different translation speeds.In the
(1iscussion of Greatbatch(1983),two Rossby numbers
were used.んis(1etermine(l by WU,which indicates
the cross-section parameter for the typhoon translation.
U is the moving spee(l of the storm an(17is the
ma虹mum current aromd the area of MLT cooling.P渉
is determined by W互which indicates the along-
section parameter for the typhoon translation.L is the
scale of the wi(1th of the ocean response and the
constantvalueiぜespecUveofthespeedofthestom.∬
the typhoon moves slowly,the ratio of P必4∫is
somewhatsmall.This suggeststhatthe response inthe
cross-track direction is fairly don∬nant.In contrast,the
ratio of Pレ44∫is somewhat greater if the typhoon is
moving fast.In this case,the ocean response in the
along-track direction is moderately(lominant.
Greatbatch(1983)o丘ere(1concrete values ofノ箋an(1P≠
in Table2in his paper.His result indica』tes that P≠4≠is
O.36/0.54for a constant speed U=5m/s and O.27/0.20
for a constant speed U=10m/s.Therefore,the above
discussion is consistent with the results of Grea』tbatch
(1983).
Figure9(b)shows that remarkable near一圭nertial
currents could be seen on the right side behind the
typhoon.The wavelength ofthe near-inertial cun・ents is
Ionger than that in experiments2and5。Maximumdeepening ofthe mb【ed layerwas30m,butthe depth at
the bottom of the thermocline layer was only18m
shallower behind the typhoon.Upwelling may
minimally contribute to the MH’lowering,because
entrainment does not work we11.Figure9(d)depicts
the horizontα1distribution of MLT advection.The
amount ofmaximum positive and negative advection is
smaller than that in experiments2,5,and8.The
contribution of advection to ma}dmum MI∫r cooling is
logically as small as the contribution in experiments2,
5,and8.Figure9(e)in(licates that the along-track
wavelength of the oscillations left behind by passage of
the typhoon is close to the local inertial wavelength2π
αゲ(Greatba』tch1984).Here,Uisthetranslationspeed
of the storm an(1∫is the Coriolis parameter.Figure10
showsthetime series ofMLTcooling,deepe血gofthemixe(11ayer,and the variation of the(iepth at the
bottom ofthe thermocline layer at the fbくed point1390
E,22。N for a10m/s translation spee(1.Deepening of
the mbくe(i layer and the shallower depth at the bottom
ofthe thermocline layerreachthe maximum amplitude
(20m and30m)earlier than in experiments2and5,so
MLT cooling reaches a maximum early in the
44 WadaA. VoL52,No.2
integration.
Greatbatch(1985)examined the relationship
between SST lowering an(1the ratio of shallower
vahation by upwelling to the(1epth in the mixed layer,
which estima』tes the initial thickness ofthe mixed layer
and(leepeningby entrain血ent.1~uis defined as follows:
Ru一%. ) (8)
πuin(1icates the amplitu(le byupwelling,and E別is
the initial thickness of the mixed layer.△h represents
the deepening by entrainment.This parameter
measures the importance ofupwelling in any particular
case.Upwellingplays an essentialrole for MLTcooling
if1~u has a greater value.1~u is about O。75for slower
translation speeds(3m/s),but O.4for faster translation
speeds(10m/s).The d価erence of1~u suggests that the
effectby upwellingwas smaller in experiment8.1~uwas
about O.67in stronger win(l stresses,and O.43in
weaker wind stresses.Therefore,upwelling contributes
greatlyto MLTcoolingin slowertranslation speeds and
stronger wind stresses Ea.ch1~u in this paper was
estimated as somewhatgreaterthan the1~uin the paper
of Greatbatch(1985)because Greatbatch(1985)
estimated entrainmentrates using the parameterization
of Kraus an(1Tumer(1967),neglecting the variation of
the heat budget.
56N
52N
28N
24N
20N
\ ・・4V‘Ψ(4咽
・ “ } レ く ? 卜 牛ノ
》 ↓ V 』 《 Ψ k
S
. 》 ↓ 寸 》 レ 髪 臥
りPP一.L- 黙
(貿」↓ 、
^ア∠ 81
い↑5購捜・…
凡薯 - 、
や転 ’.みτ ^
↑ ↑ ♂ ~ 己 て ト 、
4 を ー 〆 メ 解 レ 4 マ ト 、
1
Fig.9(b)
/28E 132E 156E 1耳OE 144E
r→ (m/s)
Deepening of the mtKe(11ayer in experiment6is
shown hohzontally from1260E to146。E and from
180N to38。N.The contours in(1icate the
thickening ofthe面xed layerfヒom the initialvalue。
The contourintervals are3m.Thevectors showthe
mixed layer currents;the mit vector is lm/s.The
line represents the嚇)hoon track over48hours.
The typhoon markislocated in the typhoon center
at the initial time and at48hours integration.The
dot is the fixed point used in Fig.10.
36N
52N
28N
24N
20N
O
β
畿
0
O。
鑛
O) O=O
o、 OQ
O。_
搬驚.、
」』
ヰ
㏄)..
一 一
. 8謙。8、
\無丑
と ヤぽロ
惚旦 \
↑28E 152E 136E 140E 144E
56N
52N
28N
24N
20N
Fig.9(a)
レ κ
\ ・ ・ “ v 4 マ ( 4 質
・ ム 噌 レ 《 } 卜 卒 ノ
) “ v 轟 《 ▽ k&
.》』-》レ左 臥
.、じ寺L〆 黙
4帽」↓ ¥
(チ∠ 81
い↑5購§・.
鵡∠姻療
1
15
9ゆ ψ ↓ 、
7 ↓ 、 \ 5 ’→
The decrease of the MLT in experiment6is
shown horizontally from1260E to146。E an(l
ffom18。N to38。N.The contours indicate the
deviations ofthe MLTfromthe initialvalues.The
contour intervals are O,2。C.The line represents
the typhoon track over48hours.The typhoon
mark is located in the typhoon center at the
initia1廿me and at48hours integration.The dot is
the fixed point used in Fig.10.
Fig.9(c)
128E 152E 136[ 140E 144E 一『 (m/s)
The variation of the thickness at the bottom of
the thermocline layer in experiment6is shown
horizontallyfrom126。E to1460E and from18。N
to38。N.The contours indicate the variation of
the thickness at the bottom of the thermocline
layer.The contour intervals are3m.The vectors
show the currents below the thermocline layer;
the unitvector is O.1m/s,The line represents the
typhoon track over48hours.The typhoon mark
is located in the typhoon center at the initial time
and at48hours integradon.The dot is the五xed
point used in Fig.10.
2002 The Processes ofS訂CoohngbyTyphoon Passage and Case Study ofTyphoon Rexwith aMixed layer Ocean Mode1 45
56N
52N
28N
24N
20N
糞奪灘
128E 132E 156E 140E 144E 「’→ (m/s)
Fig.9((1)The advection in experiment6is shown
horizontallyfrom126。E to1460E and from18。N
to38。N.The contours indicate the amount of
MLT advection per time step(1200seconds)(一
砂・▽T,∂is the mixed Iayer currents,T is the
MLT,and▽is the gradient operator).The
contour intervals are O.0030C/timestep(1200
seconds)。The vectors show the mixed layer
cunfents;the unit vector is lm/s.
deρth o(m) →50
100
150
200
250
500
550
4・OO
450
500
照瓢繊鰍瞥㎜
一〇.1
一…一4…謀酬へ厨?ゐレv・
O.1
20N 22N 24N 26N 28N こ50N 52N 54N 56N 58N 40N 1 (m/s)
Fig.9(e)The vertical section along O.5。eastward of the typhoon track in experiment6from20。N
to260N and from the surface(Om)to500m depth.The vectors show the ocean currents;
the unit vector is lm/s.The contours indicate the deviations of the sea temperaturesl the
contourintelvals are O.1。C.Thetyphoonmarkislocate(linthe取phooncenterat72hours.
46 Wada A. Vo1.52,No.2
Fig.10
OC
O.00
一〇.20
一〇。40
一〇、60
一〇.80
一1.00
一1.20
一1.40
】』ヨ
ーー1
↑
0 6 12 18 24 30 36
hour
42 48 54 60 66 72
m30.00
20.00
10.00
1奇謡Aτ1。N(DEG)1
→ヨーM置XεDしAYERO.00 DEEPENING(m)
30.00
40.00
The time series ofMLT cooling(oC),deepening ofthe mixed layer(m),and variation of
the depth atthe bottom ofthe themocline layer(m)at1390E an(122。N from the initial
time to72hours integration in experiment6.The left a)ds indicates the temperature
measurement(oC),and the且ghta》ds shows the metermeasurement.
3.4 MLT Cooling Caused by Heat Fluxes
In this section,we examine how amounts and
differences of heat fluxes affect MLT cooling under
constant wind stresses.Here,four kinds of heat fluxes
are examined in experiments10to21with(lifferent
translation speeds.Convective overturning contributes
to SST¢ooling ifthe column of seawater is losing heat.
No deepening of the mixed layer will occur if the
mechanical term is sma11er than the convective term
when the column is being heated(Elsberryθ麺」.,
1976).In these experiments,the differences in the heat
且uxes are expresse(i as differences between the air
temperature and the MLT under constant wind
stresses.The(1ifference temperature of loC between
the air temperature and the MLT corresponds to
200W/m2,which is calculated by adding the sensible
heat to the latent heat under the maximum wind speed
of35m/s in this experiment.Similarly,the dhference
temperature of4。C between the air temperature and
the MLTcorresponds to800W/m2.Jacobαα」.(2000)
reportedthataheat且uxof1200W/m2was observed in
the directly forced region of Hurricane Gilbert.Price
(1981)an(1Benderαα」.(1993)suggested that the
direct effect of SST cooling by heat nuxes is about10%
ofthe total cooling,but the effect ofentrainment(1ue to
the buoyancy caused by heat flux is over80%.
Therefore,the heat flux is considered to play an
important part ill SST cooling.
Figures11(a)and(b)depict the distribution of
the entrainment rαtes in experiments5and20.The
ma》dmum entrainment rates are located in nearly the
same position in both cases,although the area of
entrainment rates in experiment20,under the
condition of excess量ve heat nux,spreads wider than
that in experiment5under the condition of no heat
丘ux.The entrainment rate in the greater flux cases is
greater than those in no heat且ux,particularly ahead of
the typhoon.The(1ifferent entrainment rates produced
by different heat flt【xes among experiments2,5,and8
and10to21,cause d1任erences in MI∫rcooling.Figure
12(a)indicates that the location of MLT cooling三s
almost the same as that in experiment5,but the MLT
cooling is about O.7QC greater.The maximum
deepening of the mixed layer(Fig.12(b))is about
55m,10m deeper than that in experiment5.The
shallower var玉ation of the depth at the bottom of the
thermocline layer is35m(Fig.12(c)),almost the same
as that in experiment5,but the magnitude of the
shallower(lepth at the bottom ofthe thermocline layer
is dif『erent at139。E,21。N.The distribution of MLT
ad』vection(Fig.12(d))is similar to that in Fig.3(d).
Therefore,we consider that the heat fluxes on the
surface have little or no af五ect on any aspect of MLT
advection.Figure12 (e) in(iicates that the sea
temperature atthe transition lαyer is cooler than that in
experiment5.The ma}dmum(1逝erence of about O.7。C
at the transition layer between Fig.3(e)an(1Fig.12(e)
corresponds to the dhference in MUr cooling between
Fig.3(a)an(1Fig.12(&).The dhference in SST cooling
by heat fluxes depends primarily on this seawater
cooling at the transition layer,since the contribution of
ho貞zontal a(lvection(loes not change un(1er the same
wind stress even lf the heat fluxes change.Figure13
depicts the time series of MLT cooling,deepening of
2002 The Processes ofSSTCoolingbyTyphoon Passage and Case StudyofTyphoon Rexwith a Mixed Iayer Oceξm Model 47
the mixed layer,an(l variation of the(1epth at the
bottom of the thermocline layer at the fixed point139。
E,22。N when the maximum heat flux reaches800W/m2.Noted that the ma}dmum deepening ofthe
mixed layer and thevariation byupwelling are a similar
value.The excessive deepening of the mixe(11ayer
compare(l with the deepening of the mixed layer in
experiment5causes the greater entrainment rates.
Other physical processes are similar to those in
experiment5.
RuisaboutO.45inan800W/m2heat且ux,andO.53
in no heat flux,because while entrainment rates are
greater(1ue to heat fluxes,the kinetic energy given by
28N
wind stresses(10es not change.The reason for the
cooler MLT is that cooler water produced by greater
entrainment at the transition layer is transported to the
mixed layer by upwelling.
28N
26N
24N
22N
26N
24N
22N
20N
18N
の
論、、鵜ノ、
o’o男
20N
18N
一〇,9
-1,2 -1.5 r1.8 \ 一2.1
じヰ
二11
-1.
一1.2
-0.9
136E 158E 140E 142E 144E
154E 156E 138E 140E 142E 144E
Fig.12(a)The decrease of the MLT in experiment20is
shown horizontally from134。E to144。E and
from18。N to28。N.The contours indicate the
deviations ofthe MLTfromthe initialvalues.The
contour intervals are O.3。C.The line represents
the typhoon track over48hours.The typhoon
mark is located in the typhoon center at the
initial time and at48hours integration.The dotis
the fixed point used in Fig.13。
154E
Fig.11(a)The entrainment rates are shown horizontallyfor
experiment5from134。E to144。E an(1from18。
N to28。N.The contours in(1icatethe entrainment
rates(m/s);the contourintelvals are O.0004。
28N
28N
26N
26N
24N
22N
20N
α・・瓠瀞・24N
22N
20N
22㌧\、、2A
bー- :
! ’4ψ夢\
154E 156E 138E 1耳OE 142E 144E -7 (m/s〉
Fig.12(b)Deepening of the mixed layer in experiment20is
shownhorizont証yfrom134。Eto144。E andffom
18。N to280N.The contours indicate the
thickening ofthe mixedlayerfromthe initialvalue。
The contour intervals are5m.The vectors show
the mixed layer currents;the unit vector is lm/s。
The line represents the typhoon track over48
hotぱs.The typhoon markis located in the typhoon
center at the initial time and48hours integration。
The dot is the fixe(i point used血Fig.13。
18N
N\、 ム て
諦 ㌧↓ τ 卜 恩 応 〆 ヘ タ
黛鐙灘ll
俺 0 ノ
ヘ ノ
、→~ 9 ノ
の ヤヤリ オ ノ ノ ノ
き
1、_算_““
154E 1こ56E 158E 140E 142E 144E
Fig。11(b)The entrainment rates are shown horizontally for
experiment20from1340Eto144。E and from18。
N to28。N.The contours indicate the entrainment
rates(m/s)l the contourintervals are O.0004,
18N
48 WadaA Vol.52,No.2
28N
26N
24N
22N
20N
18N
→) Q 産.↓一1も ドび 誉
捻§
」讃“ー遂
廓 、醇 N、
、 \↓
あ、、 \し
》 ネ \\、2
フ ヤ セ
ー15 マ ン シ ら ム ぜ
一10
《 《 ( 《 て τ ▼ ^ 卜一モ5マ
メ 胃 - , 4
、 麺 笥 畿 画
キ キ ル む の
↓’一↓5 ジ ー ψ
〆 ノ ゼ 皆
154E 156E 138E 140E 142E 144E π (m/s)
Fig.12(c)The variation of the thickness at the bottom of
the themlocline layer in experiment20is shown
horizontallyfrom134。E to1440E and from18。N
to28。N.The contours indicate the variation of
the thickness at the bottom of the thermocline
layer。The contour intervals are5m.The vectors
showthecurrentsbelowthethemoclinelayerl the unitvector is O.3m/s.The line represents the
typhoon track over48hours.The typhoon mark
is located in the typhoon center atthe initial time
and at48hours integration.The dot is the丘xed
point used in Fig.13.
28N
26N
24N
22N
20N
18N
vレレ《p卜44、胃〆・1111
冴
v:‡卜^41・・ノーノノノニ
v:ー :;ニニノ~コ
i…iノ蚤イ参ゴノ
154E 156E 138E 140E ↑42E
一「→ (m/s)
144E
Fig.12(d)The MLT advection in experiment20is shown
horizontallyfrom134。E to1440E and from18。N
to280N.The contours indicate the amount of
MLT advection per time step(1200seconds)(一
∂・▽T,∂is the mixed layer currents,T is the
MLT,and▽is the gradient operator).The
contour intervals are O.002。C/timestep(1200
seconds)。The vectors show the mixed layer
currents;the unitvectoris lm/s.
depth o(m)
50
100
150
200
250
500
550
400
450
500
一…謀☆マQ・丼記・^》_刈79・曇〉ヤτQ・蚤
hκ/2島た・プ/
20N 22N 24N 26N 28N 50N1(m/s)
Fig.12(e)The vertical section along O.5。eastward ofthe typhoon track in experiment20from20。N
to30。N and from the surface(Om)to500m depth。The vectors show the ocean currents;
the mitvector is lm/s.The contours indicate the deviations ofthe sea temperatures;the
contour intervals are O.3。C。The typhoon mark is located in the typhoon center at72
hours.
2002 The Processes ofS訂CoohngbyTyphoon Passage and Case StudyofTyphoon Rexwith aMixed layer Ocean Mode1 49
。C
O。00
一〇.50
一1.00
一1.50
一2.00
一2、50
一3.00
i
き
1
㎜ “ 一 一 甲㎜ P { 皿
■
111 I
lI
06121824 30 36 42
hour
48 54 60 66 72
m60.00
40.00
20.00
0.00
20、00
40.00
一G-SST DEV夏ATION(deg)
一{…トMIX LAY旺R
DEEPE卜繧NG(m)
一西BOτ丁OM OF
THERMOCUNE SHALLOWER(m)
60.00
」
Fig.13 The time series ofMLT cooling(oC),deepening ofthe m盈ed layer(m),and variation of
the depth at the bottom ofthe the㎜ocline layer(m)at139。E and22。N from the initial
time to72hours integration in experiment8.The left a》ds indicates the temperature
measurement(oC),and the righta》ds shows the metermeasurement.
3.5MLT Coo”ng Under Different Mixed Layer Thickness and Different Vertical Profiles of
Sea Temperatures
In this section,we discuss how veltical profiles of
the sea temperatures affect MLT cooling under a
constantwind stress.We examined two thic㎞esses of
the mixed layer(profiles lal and lcl in Fig.12)in
experiments22to27with different translation spee(1s
and two vertical profiles of sea temperatures(profiles
ldl an(11el in Fig.2)with a5m/s translation speed.
Among experiments22to27and4to6,MUr coolingwas generally larger(smaller)where the mixed layer
was thinner(thicker).
Figures14(a)an(1(b)depict the time series of
MLT cooling,deepening of the mixed layer,and
variation ofthe(1epth at the bottom ofthe thermocline
layer at the fbくed point139。E,22。N in the30m and
70m mb【ed layers.The lal profile in Fig.2was applied
in experiment23and Fig.14(a),while the lcl profile
was applied in experiment26and Fig.14(b).
Maximumdeepening ofthe mtxedlayerwas about70m
in experiment23an(125m in experiment26,and the
maximum depth ofthe m圭xed layer was about100m in
both experiment23and experiment26.The variation of
the(iepth at the bottom ofthe themocline layFer due to
upwelling in experiment23was almostthe same as that
in experiment26.Since the entrainment rates in
experiment23were greater than those in experiment
26,deepening in the mixed layer was dominant in
experiment23.If(1eepening is dominant,particularly in
a shallow mixed layer,upwelling occurs easily due to
variation of the density in the mixe(11ayer.This
upwelling has a significant i㎡luence on MLT cooling.
However,MI∫rcoolingtendsto be suppressed in areas
where the mixe(11ayer becomes thicker.Elsberlyε地」.
(1976)examinedthein且uenceoftheinitialthic㎞essof
the mixed layer and thermocline stabili七y for the same
atmospheric forcing.The amount of cooling was not
linearly related to the initial thickness of the mixed
Iayer,since the associated depth change(i.Cornillonθ渉
α」.(1987)reported that the greatest(iecrease of SST
occurred in slope waters north of the Gulf Stream
where the seasonal thermocline is the shallowest and
most compresse(1.Sakaidaαα」.(1998)examine(1the
role of the vertical oceanic stmcture in SST cooling.
The Oyashio profile is more suitable for SST cooling
than the Kuroshio profile because the mb【e(i layer is
thimer an(1the vertical gradient of sea temperatures in
the thermocIine is steeper.These two reports are
consistentwith the results ofexperiments22to27.
1~u of O.53is the same value in both the30m and
70m thickness of the mixed layer as in the50m
thickness of the mbζe(11ayer,so it is considered that
upwelling contributes to MLT cooling to the same
extent.It is then easily corぽirme(1in Figs.4,14(a),an(1
14(b)that the thickness of the mixe(11ayer is
determine(i by the magnitude of wind stresses,
regardless of the initial thickness of the mixed layer,
Since MLT cooling is related to the difference in the
amount ofinitial heat content in the mixed layer m(1er
the same wind stress,a difference in MLT cooling
occurre(l in experiments23and26.
MLT cooling in experiment29became somewhat
more significant than in experiment28because the
50 WadaA VoL52,No.2
entrainmenttemoftheheatcontentequa廿on(3-1)hasa greater effect on(lecreasing SST.Figures15(a)an(i
(b)depictthetime series ofMLTcooling,deepening of
the mixed Iayer,an(i variation of the depth at the
bottom of the thermocline layer at the丘xed point139。
E,220N in the great and small ve而cal gradients of sea
temperatures.The ldl profile in Fig.2was applied in
expehment28and Fig.15(a),while the lel profile was
applied in experiment29and Fig.15(b).Thedeepening ofthe mixed layer and shallowervariation of
the depth at the bottom of the thermocline layer in
experiment28were similar to those in experiment29.
A d迅erence in the vertical profile of sea temperatures
influencesMLTcooling
1~u is about O.50in the warmer temperature at the
bottom ofthe thermocline layer,butis aboutO.55in the
colder temperature at the bottom of the thermocline
layer.Adifference invertical seatemperatures causes a
di廷erence in the contribution of upwelling,because
entrainment rates change if the temperatures at the
bottom of the mixed layer change.This is why MLT
cooling differs between experiments28and29.
ヤ心ゆ
0
一〇.50
一黍.00
一1.50
一2.00
一2.50
一3.00
一3.50
0 6 嘆2 18 24 30 36
hour
42 48 54 60 66 72
m80.00
60.00
40.00i
20、00
0.00
一20.00
40.00
一60.00
i一←SST DEVIAマ10N(deg)
暑M屋X LAYE只 DEEF,ENING(m)
慨磯
Fig.14(a)The time series ofMLTcooHng(oC),deepening ofthe mtxed layer(m),and variation of
the depth at the bottom ofthe themlocline layer(m)at1390E and22。N from the initial
time to72hours integration in experiment23.The left a》ds indicates the temperature
measurement(。C),andthe righta⊇ds showsthemetermeasurement.
OCO.00
一〇.20
一〇.40
一〇.60
一〇.80
一1.00
一肇.20
一肇.40
司,60
一1.80
一2.00
τ・ー
■ヨー↑Il
11釘 ヨ
…
[『…E,韮
0 6 肇2 匪8 24 30
m50.OO
40.00
30.00
20.00
10.00
0.00
10.o軽
20。0α
チ マ DεVIAT10踵(deゆi
→ヨトーMIX LAYER
DεEPENiNG(m)
i一合一BOTTOMOF TH鉦RMOCU幾E 、 SHALしOWER(m)1
36 42 48 54 60 66 72
hour
30.od
40.OO
50.00
Fig,14(b)The time series ofMLTcooling(。C),deepening ofthe mlxed layer(m),and vahation of
the(1epth at the bottom ofthe the㎜ocline layer(m)at139。E and22。N丘om the initial
time to72hours integration in experiment26.The Ieft a}ds indicates the temperature
measurement(。C),an(l the righta滋s shows the metermeasurement。
2002 The Processes ofS訂CoolingbyTyphoon Passage and Case StudyofTyphoon Rexwith aMixed layer Ocean Model 51
。C
O.00
一〇、50
一1。00
一1.50
一2.00
一2。50
一3.00
ξ己FE
ξ葦=
歴
0612董82430 36 42 48 54 60 66 72
hour
m60.00
、α,,1 祠
1暑MIX風YER。.。。i DEEPENING(m)
2・.・d奮瓢認C鑛NE
S騒ALLOWεR(m)i
40.00
40、00
60.00
Fig。15(a)The time series ofMLTcooHng(。C),deepening ofthe mixed layer(m),and variation of
the depth at the bottom ofthe the㎜ocHne layer(m)at139。E and22。N from the initial
time to72hours integration in experiment28.The left aぬs indicates the temperature
measurement(℃),andthe righta⊇ds shows the metermeasurement.
OC
O.00
一〇.50
一1.00
一1、50
一2.00
一2.50
一3.00
1聖;}
E … 一 ■,
0 6 肇2 18 24 30 36 42 48 54 60 66
hour
OmO
O6
40.00
20.00
0、00
2αooi
十SST DEVIAτ韮ON(deg)
一E←M夏X LAYER DEEPEN【NG(m)
音BOYτOM OF THERMOCUNE S懸ALLOWER(m)
72
40.00
60.00
Fig.15(b)The time series ofMLT cooling(。C),deepening ofthe mixed layer(m)and variation of
the depth at the bottom of the the㎜ocline layer(m)at1390E and220N from the initial
time to72hours integration in experiment29.The Ieft a}ds indicates the temperature
measurement(。C)and the righta⊃ds shows themetermeasurement。
52 WadaA. Vo1.52,No.2
4.Physical and Thermodynamic Processes of M正T Coolin9
We described three stages ofMLTcooling in the
previous section.We noted the relationship between
MH’cooling and the amount of horizontal advection
each time at139。E,22。N in order to investigate the
role of some physical processes in detai1.Figure16
illustrates this relation.There are three stages be帥een
MI∫r cooling and the amount of horizontal advection.
The MLT initially decreases suddenly due to
entrainment.Negative advection is dominant in this
stage because the且xe(l point is Iocate(i on the right
side of the running typhoon.Next,the amount of
advection changes from negative to positive and the
ma}dmumMUrcoolingremains nearthe samevalue,一2.50C.Finally,the MI∫rgraduallydecreases similarlyto
near-inertial oscillation.
The first stage corresponds to the”inteminglell
stage(Elsberry6麺Z.,1976),where entrainment plays
an important role in increasing the thickness of the
mixe(11ayer.Entrainment is dominant particularly
before10hours integration.Cooler water has already
been produced by entrainment at the transition layer.
This water and the water below the thermocline are
transported to near the surface by upwelling.The
second stage corresponds to the(1evelopment of near-
inertialoscillation.Boththe MUrαndthethic㎞ess of
the血xed layer change periodically in this stage.The
second stage and the last stage demonstrate that the
contribution ofMLTadvectionto the total MLTcooling
is much less behind the typhoon
MLT cooling becomes greater as wind stress
becomes stronger and the translation of the typhoon
slows.In contrast,variation by upwelling becomes
weak and SST cooling is slight as the win(i stress
weakens an(1the translation speed of the typhoon
increases.Greatbatch(1983)suggested tha.t the
horizontal pressure gradient terms could be neglected
in momentum equationsfor”fast”or”large”storms.
WeconsiderherethatM工Tcoolingoccursmainlyby entrainment and upwelling.The entrainment rate is
deteminedbythewindstress,buoyancy,andve丘ical
current shear,while upwelling is affecte(1by the
variance of the mixe(11ayer thickness.The Froude
number was examined at the fixed point139。E,22。N
to investigate the ef6ect ofwind stress on MH’cooling
an(1deepening of the mixed layer.If the Frou(1e
number is high,the contribution of the wind stress is
sign迅cant or the mixed layFer is thimer.In this paper,
we confirm the contribution of the win(l stresses and
physical processes described in chapter4for MLT
cooling and mixe(l layer deepening.The Froude
numberis defined asfollows:
畜キ一【%毒酢
(9)
In(9),z6*is the伍ctional velocity an(1is calculate(1
&s the ra』tio of the win(1stress(τ)to the density(ρ)of
theMLT.Thethic㎞essofthembくedlayerishl andgis the gravitational acceleration.
0.01
0.005
0卜>.一〇.005
ヒ
一〇.01
一〇.0葉5
一〇.02
ヨ } 1 一「 i i … … 縛
+0-24h
、._ユニニ
1
薮 1『『魂 一一×24-48h
+48-72h騨 、 F
1…l
l
}
i l
一3 一2.5 一2 一1.5
0C
一1 一〇.5 0
Fig.16 The relationship between MLT cooling and the amount of MLT advection(。C)per
timestep(1200seconds))for a5m/s typhoon translation speed with different periods,
The horizontal axis represents the MLT cooling(oC),and the vertical a》ds shows the
amount ofMI』radvection(。C/timestep),一∂・▽T.砂is the m盈edlayer currents,Tis the
MIT,and▽isthegradientoperator.
2002 The Processes ofS訂CooHngbyTyphoon Passage and Case StudyofTyphoon Rexwith aMb【ed layer Ocean Model 53
0
一〇.5
一1
五 2 5 3
ー } 乞 一
一 一
(OD)智一8・ヒΣ
一3.5
一4
一4.5
粛××
謎G
。一
σ、8雌
.£米
痔誹
皿0
…ー畢………○…
0
0
0
1『藍米※×米×※一潔※1×※※一
潔
よL
i爾i・誉1:ll二瓢
0
一〇.5
1 5 2 にU
一 乳 一 之
一 『
(Q。協≦08↑一Σ
一3
0 0.0005 0.001 0,00で5 0.002 0.0025 0.003
Froude number
一3.5
霧 ガ ノ
日日日日日日鞠日図臼日醐
㌔ダ
1.士麺鋤
0 0.0005 0.OOマ 0.0015 G,002 0.0025 0、003
Froude number
0.0035
」0.004
Fig。17(a)The relationship between MLT cooHng(。C)and
the Froude number under different wind
stresses。The horizontal axis represents the
Froude number,and the vertical a}ds shows the
SST coolin9(。C).
Fig。18(a)The relationship between MLT cooHng(。C)and
the Froude number for different initiaI
thicknesses of the mixed layer.The horizontal
axis represents the Froude number and the
verticala⊇dsshowstheMLTcooling(。C).
140
120
0
0
0
0
0
8
6
4
ω器韮。峯』。>⑩も。x還
20
0
120
100
くず※×× 簗
来・諫業※一
、一椀難瓢凝 監 }
ロ ロ
= ※『3.5N50m
・0 3.ON50m -『「
0.001 0.0015 0.002 0.0025 0.003
Froude number
の 800⊆
x三
』 60>
で実署 40
20
0 0.00050 0
「ー【聖『『..甲‘Pーーー,-
II}t■=.■■=l11 1i 「 ,,ーーヨ■
171i
E6唱P■F
鳴
・◆需燈
◆ 、皿
福
、国、
♂日一
『
○
日
温日
◆
匪
◎.ー…PFP
◆
臼
←
一日σ日
◆
rーコー11 1 ,聖
、陵
m一Sm
m m
O O O
5 3 7
一 曹
S S
- / -
m m
5 に》 FD
◆
↓
0.0005 0.001 0.0015 0.002
Froロde number
0.0025 O.003
Fig.17(b)The relationship between the thickness of the
mbζed layer(m)and the Frou(le number un(1er
different wind stresses.The horizontal axis
represents the Froude number and the vertical
adsshowsthethic㎞essofthemixedlayer(m).
Fig.18(b)The relationship between the thickness of the
mixed layer(m)an(l the Frou(ie number for
different initial thicknesses of the mb【e(l layer.
The horizontal axis represents the Froude
number,and the vertical axis shows the
thic㎞essofthemixedlayer(m).
Figure17(a)illustrates the relationship between
the Froude number and MLT cooling under dhferent
win(1stresses,and Fig.17(b)shows the relationships
between the Froude number an(1the thickness of the
mb【ed layer under diぜerent wind stresses.We noted
that the Froude number has a larger value when the
MLT suddenly decreases,while the MLT remains
constant as the Froude number changes from the
ma虹mum to minimum values because the wind stress
is weakene(1by passage of the typhoon.At the stαge of
maximum thickness of the mixed layer,the Froude
number rapidlybecomes small because the appearance
ofthe typhoon eye weakens the wind stresses.
Figure18(a)depicts the relationship beなveen the
Froude mmber and MLT cooling under different
thic㎞esses ofthe mixed layer,and Fig.18(b)shows
the relationship between the Froude number and the
thickness of the mixe(11ayer under different mixe(1
1ayerthic㎞esses.ThemadmumFroudenumbersandmaぬmum thic㎞esses of the mixed layer in the three
cases are similar to those shown in Figs.17(a)and(b).
These factors indicate that the magnitude of MLT
cooling is not dete㎜ined by the丘ictional velocity,but
rather by the initial heat content of the mbくed layer.
Moreover,theFroudenumberinshallowerthic㎞esses
of the mixed Iayer can reach a maximum value most
rapi(11yamongthe three cases.
Figure19(a)shows the relationship between the
Froude number and MLT cooling under different
translation spee(1s,and Fig.19(b)shows the
relationship between the Froude number and the
thic㎞ess ofthe mixed layer un(ier dhferenttranslation
spee(ls.The Froude number for the10m/s translation
speed is the greatest of the three cases.In addition,the
Froude number for the slower translation speed can
reach the maximum value most rapidly of the three
54 WadaA VoL52,No.2
0
一〇.5
- FD 2
一 t 一
(O。協に編08↑一Σ
一2、5
一3
0
-一△・△、
.、・△
ぐ
亀△ △一△鋳粛
←5m/s.5・m]
、一●一3m/s_50m i
,一ム..!9醸s解50ml
0.0005 0.001 0.0015 0.002 0.OO25 0.003 0,0035 0.004
Froude number
Fig.19(a)The rela恒onship between MIT cooling(。C)and
the Froude number for different typhoon
translation speeds.The horizontal axis
represents the Froude number,and the vertical
a虹s shows the S凹coo1血g(。C).
120
100
0 0 0
8 6 4
の器⊆着置to澄モo×釜
20
0
lI
………
’1ヨ臼、.
巳騒、.、、
、.難馨紳衛・
…『1一・句 1
ミ 野1 臨
EE、1.-
漁..禽納毒
i
-4
瞬{ヨ
忽 ll戸訴癒㎜一
…一
i{多
鷺鷹「 圏 『・血・10m/s50m I - i l
lヨヨヨ
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
Froude number
Fig.19(b)The relationship between the thickness of the
mixe(11ayer(m)and the Froude number for
different typhoon translation speeds.The
horizontal a》ds represents the Frou(le number,
and the vertica1礎ds shows the thic㎞ess ofthe
mixed layer(m).
cases,and it is similar to the case under different
thic㎞esses ofthe mixedlayer.
MLT cooling and deepening ofthe n丘xed layer is
suddenly changed under different wind stresses and
the thicknesses of the mixed layer when the Froude
numberisgreater.However,this rule camotbe applied
to different translation speeds.It is thought that the
maximum thickness of the mixed layer is量ndepen(1ently determine(1by the wind stress and
translation speed of the typhoon.MLT cooling is
independent of the Froude number,but MH’cooling
has its own range of Froude numbers.The pattem of
Froude number variation versus MUf cooling is thus
uniquely determine(i under dhferent conditions,such
as the wind stress,the廿anslation speed of the typhoon
andtheinitialthic㎞essofthemixedlayer.
In conclusion,MI∫rcoolingis determinedbywind
stress,translation speed of the lyphoon,heat且uxes,the
initial thickness of the mixe(11ayer,and the vertical
gradients of sea temperatures in the themlocline layer.
However,the mechanism of MLT cooling is dHferent。
Themagni加deofwindstressdeteminesthethic㎞essofthemixedlayerirrespectiveoftheinitialthic㎞essof
the mixed Iayer an(l variations caused by upwelling。
The magnitude ofthe thic㎞ess ofthe mixed layer and
vahationofthedepthatthebo廿omofthethemoclinebyupwelling(雌ers amongtranslation speeds。
Entrainmentplays an importantpartin(1etemining the
thickness of the mixed layer.We must know the
con(1itions such as the frictional velocities,buoyancy
且uxes,and vertical current shears in the mixed layer in
order to detemine the entrainment rate.The buoyancy
is affected by surf&ce heat且uxes,so it is possible that
MLTcoolingoccurs dueto excessiveheatfluxes.
It is important that the initial vertical profiles of
seatemperaturesaf『ectMLTcooling.Both entrainment
andupwellingcontributeto MLTcoolinginthiscase。
5.Numerical Simulation ofTyphoon REX
5.1Typhoon Rex and Maritime Data Acquired by
Rハ1Kelfu Maru
Tヲphoon Rex appeared on August24,1998,south
ofOkinawa.There werefewertyphoons in the summer
of1998than in the average year.A warmer S訂,over
30。C spread.overthewestregionfrom140。E,whilethe
SSI’over the east region,from140。E,was somewhat
lower than that in the average year.The minimum
central pressure of Typhoon Rex suddenly descen(1e(1
from1002hPa onAugust25,1998to960hPa onAugust26.This typhoon moved trochoidally(Fig.20),and the
minimum central pressure rem2疽ned constant for about
a week.Typhoon Rex reache(1955hPa central pressure
and40m/s maximum win(1spee(1from OOOO(UTC)
August29to2100(UTC)August29,1998in JMA best-
track data.
Maritime observations by R/V Ke血Mam were
conducted behind Typhoon Rex from August24to
August31,1998(Fig.20).A rapid decrease ofthe SST,
inwhich the ma》dmum SSTcooling reache(1about3。C,
was observed around OOOO(UTC)August29,1998,
when the intensity of the typhoon became maximum
(Fig.21(a)).However,the win(1velocities in maritime
observationswere ffom6m/s to8m/s(Fig.21(b))and
were notsubstantial comparedwiththewindvelocityof
atypicaltyphoon.Maritime data also indicated thatthe
air temperature decreased rapidly and simultaneously
as the SST decreased.The air temperature had almost
the same value as the SST during the SST variation
from OOOO(UTC)August29to1800(UTC)August29,1998.Shayθ渉αZ.(2000)also repo1’te(i that the air
2002 The Processes ofSSTCoohngbyTyphoon Passage and Case StudyofTyphoon Rexwith aMixed layer Ocean Mode1 55
100。Ei110。E 120。E 唾30。Ei唾40。E 壌50。E 160。E 170。E 180。
500N
40。N
30。N
200N
壌00N
0。
欝灘熱、甕慧撫
一灘 一群… 霰 ・」’‘ K3
繋…獺…欝塑σ』〆
…灘 腱牛27
4奎一
〆
,禽〆
艦。
60卜Pa
82700960hPa
ノ
■‘』 50。N
40。N
30。N
200N
10。N
00100。E ”0。E 屡200E 130。E で40。E 1500E 1600E 曜700E 180。
Fig.20 The circles depict the JMA besレtrack data of Typhoon Rex(9804)from August24to
September6,1998.The shaded circles represent the posi廿ons of Typhoon Rex at OOOO
(UTC)。The date and the minimum pressure are also shown in this figure.The triangles
represent the observational track of Ke血Maru from August24to September1,1998.
The shaded tdangles indicate the positions of the ship at OOOO(UTC).The Knumber is
the day ofthe maritime observation in the region.
temperatures had almost the same values as the S訂,
based on observations by ocean buoy after the passage
of Hunricane Opa1.
5・21nitial and Boundary Conditions for the Numerical Simulation
In this section,we explain the initial and boundary
conditions for the numerical simula廿on of SST cooling
byTyphoon Rex.Itwas necessary to first determine the
initial conditions in the atmosphere and the ocean in
preparingforthe numerical simulation ofTyphoon Rex.
The model covers the area from100N to50。N and
from120。E to160。E.Each zonal and meridian grid
resolution is O.25degrees.The physical elements
consi(lere(l here to be the initial conditions of the
atmosphere are wind velocities,atmospheric pressures,
and air temperatures at10m heights near the surface.
The atmospheric pressure in afine grid is calculated by
Iinear interpolation from the coarse global objective
analysis{lata at the JMA(GANAL:1.250resolution at
Iatitude an(110ngitude).The wind velocities in a fine
gri(1were also primary calculated by linear
interpolation from the GANAL data.In ad(1ition,the
Rankin vortexes,which were calculated using the best-
track data in JMA,were composite(l on these GANAL
data.(We call this proce(lure”Typhoon Bogusing”).
Since GANAL data are usually update(1eve可six hours,
the atmospheric conditions maintain the value for sbく
hours,except that the Rankin voltexes are calculated
by linear interpolat三〇n every time step.The bulk
coefficients of the wind stresses are also modified.
Their values are change(1up to1.25times for a wind
speed between8m/s and25m/s and up to1.5times for
wind spee(1s over25m/s.This mo(1ification enables
SST cooling to be well simulated because of the
stronger wind stresses.The air temperatures recorded
by R/V Keifu Mam are assimilated around every
observatiomlstation of艮/VKeifuMaru.The dhference
in SST cooling between the numerical simulation and
the observation,without the effect of the heat nux,can
be(1iscusse(1using this assin五1ation.The a㎞ospheric
temperature convertingf『om the objective analysis data
to the observationa1(lat且acUlally contributes very liUlle
to SST cooling.No T立phoon Bogusing of the sea level
pressure was used in this experiment since a reduce(i
gravity appro》dmation is applie(1in this mode1,although
the effect of the bottom topography is important in
detemユiningthe ocean dynamics in a shallow area.
Levitus(1984)climatological data are used for the
sea temperatures and the salinities by linear
interpolation from120。E to160。E an(1from10。N to
50。N.In addition,TRMM/TMI one-day averaged data
56 Wad&A. Vol.52,No.2
。C
34
32
30
28
26
24
22
20
18
金temperature(。C〉
dew-Point temperature(。C)
一Sea Su㎡ace Temperature(。C〉
× Sea Levei Pressure(hPa)
{016
1014
1012
1010
1008
廃006
1004
1002
1000
hPa
嘩998/8/ 1998/8/ 1998/8/ 1998/8/ 1998/8/ 1998/8/ 1998/8/ 1998/8/ 遷998/8/ 匪998/8/ 1998/9/ 1998/9/
22 0=00 23 0:00 24 0=00 25 0=00 26 0=00 27 0:00 28 0:00 29 0=00 30 0=00 31 0=00 1 0=00 2 0=00
Fig.21(a)Maritime observation data acquired by R/V Keifu Maru.The air temperature(。C),the
dew point temperature(。C),an(l the S瑚「(oC)are shown on the left axis and the sea level
pressure(hPa)is shown on the right axis.The arrow indicates the maximum SST
decrease(luring the observations.
m/s
24
21
18
15
廓2
9
6
3
0
。一。曲
∴謡
コ
O皿800
『
『
『
ア
閲
π
『
○
OO
陽酵oQ
oo
8→←Wind speed(m/s)
o Wiハd directio為(degree)
360
315
270
225
180
135
90
45
1998/8/22
0=00
1998/8/24 1998/8/26 0=00 0:00
1998/8/28
0100
確998/8/30 1998/9/1
0:00 0:00
0
degree
Fig.21(b)Maritime observation data acquired by R/VKeifu Maru.The wind speed(m/s)is shown
on the left axis,and the wind direction(degrees),on the right axis.
2002 The Processes ofS訂CoolingbyTyphoon Passage and Case Study ofTyphoon Rexwith a Mb【ed layer Ocean Mode1 57
were used for the SST data.TRMM/TMI data covers
the area from38.1250N乙38.125。S and are at O.25。
resolution for both latitude and longitu(ie.The SST(1ata
from the previous day or the next daywere composited
on the grid where there was no data because of thick
clouds of severe storms or no obsen7ation.Levitus SST
data were used in grids that had no data for thr“e(iays.
These initial SST data correspond to the niaritime
observational data acquired by R/V Keifu Maru.
Ben(1er an(l Ginis(2000)applied』the NCEP daily
averaged SST and Levitus(1984)climatological data as
the initial conditions.
The thic㎞ess ofthe mixed layerwas setto30m
and that of the thermocline layer was set to170m for
the oceanic interior con(litions.Data of temperature
and salinity in the ocean,except f6r the sST,were
apPlied using Levitus climatological data.Water且uxes
due to evaporation were considere(1,but the
precipitation cou1(l be neglecte(1.
Following the above procedures,we performed
the spin-up Process to calculate the steady ocean
currents,so that the pressure gradient term balanced
the Coriolis term atthe initialtime.
△T(oC)is the deviation ofthe SSTover26。C△Z(m)is
the ver丘cal gri(l interva1,and H(m)is the depth over
260C.
Figures25(a),(b)and(c)(lepict the dist益bution
of the ocean heat content on August24,27,and31,
1998.The ocean heat content was the greatest in the
south at15。N,and was over20kcal/cm2from August
24to August.31,1998。Leipper and Volgenau(1972)
suggested that the ocean heat content over6kca1/cm2
is related to typhoon development.The ocean heat
contentwas maint3ined at over6kca1/cm2丘omAugust
24to August27,1998,(iuring Typhoon Rex.Figure26
shows the relationship between the ocean heat content
and the intensity of Typhoon Rex.During this period,
the mini単um central pressure of Typhoon Rex
suddenly descended from1002hPa on August25,1998,
to960hPa on August26.However,the ocean heat
content was less than6kca1/cm2after August27by
Typhoon Rex.Typhoon Rex maintained the intensity
but did not develop well because the ocean heat
content(lecrease(1(1ue to SST cooling.
5.3Results of the Numerical Simulation
The numerical simulation of Typhoon Rex was
performed from August24,1998,(Fig.22(a))to
August31,1998。The decrease of the SST cause(i by
Typhoon Rex began to appear after72holurs
integration on August27,1998,(Fig.22(b))in the
horizontal SST distribution.The decrease in the SST
became greater,particularly behind and to the right of
the typhoon track(Fig.22(c))onAugust31,1998.This
SST cooling was more noticeable馬hen the wind
stresses were stronger and the translation speeds were
lower.The distribution of the mode1-computed SST,
particularly the area of maximum SST cooling,was
similar to that of the three-day running mean
TRMM/TMISSTdata(Fig.23).
We compared the SSTvariationinthe modelwith
that ofthe S蟹observed by R/V Keifu Maru(Fig.24).
Except for the d置erence in the initial value of SST,the
tendencywas well simulated,particularlythe ma》dmum
SST decreases from1200(UTC)August28,1998,to
OOOO(UTC)August29。
We examined the relationship between SST
cooling and the intensity of Typhoon Rex.Leipper and
Volgenau(1972)suggested that the ocean heat content
could be defined as follows:
な
9・HC=Σρら△勉Z (9) h=O
whereρ(g/cm3)is the oceanic density,Cρ(1
ca1/g/。C)is the heat content with a constant pressure,
58 Wada A. Vol,52,No.2
59N
56N
55N
50N
27N
24N
2葉N
18N
15N
12N
SST Aug24by Mi×ed LGyer Oceqn ModeI
120E 125E 肇50E: 155E 肇40E 145E: 150ε 155E 筆60E
54
52
30
28
26
24
22
20
18
↑6
14
12
10
Fig。22(a)The initial S訂distribution on August24,1998.The shaded bar on the right side indicates
the range of the SST(。C).The contour intervals are2。C.The line represents the typhoon
trackfromAugust24to August31,1998.The typhoon mark is the position in the typhoon
center at OOOO(UTC).
39N
56N
55N
50N
27N
24N
21N
18N
15N
喋2N
SST Aug27by Mi×ed LQyer Oceqn ModeI
120E 125E 150E 155E 1尋OE 14・5E: 150E 155E 壕60ε
54
52
5Q
28
26
24
22
20
爆8
16
14
璽2
璽0
Fig.22(b)The mode1-computed S訂distribution on August27,1998.The shaded bar on the right
side indicates the range of the SST(。C).The contour intervals are2。C.The line
represents the typhoon track fromAugust24to August31,1998.The typhoon mark is the
pos圭t三〇n in the typhoon center at OOOO(UTC).
2002 The Processes ofS訂CoolingbyTyphoon Passage and Case Study ofTyphoon Rexwith a Mixed layer Ocean Model 59
59N
56N
55N
50N
27N
24N
21N
18N
15N
12N
SST Aug51 by Mi×ed Lqyer Oceqn Model
120E 霊25E 150E 155E 140ε 145E 150E 155E 160E
54
52
50
28
26
24
22
20
喋8
16
嘩4・
12
j o
Fig。22(c)Same as Fig.22(b)exceptforthe datel(August31,1998).
Fig.23 The left panel shows the distribution of three.days averaged SST acquired by
TRMM/TMI onAugust31.The rightpanel shows the distribution ofSSTdeviationsfrom
JMA climatological SST data averaged from1961to1990.The data were produced by a
TRISST(Ver.2.0)algorithm an(1supplied by the Earth Observ・ation Research Center,
National Space DevelopmentAgency ofJapan.
60 Wada A. Vol.52,No.2
32
31.5
3遷
30.5
O FD 9
3 q“ 2
2
(Q。)ΦΦ」boΦで
28.5
28
27,5 ㎜ 叩 P 門 『
㎜}㎝一一
「i {modei SST
K〉一〇bs SST(10m語)
27 ・ 1 1 ・ 一 ド n ・・ i 一一〃』
擁998/8/ 1998/8/ 唾998/8/ 1998/8/ 1998/8/ 1998/8/ 1998/8/ 1998/8/ 肇998/8/ 1998/9/
23α00 240100 250=00 260:00 270:00 280=00 290:00 300=00 3蒙0:00 1 0:00
Fig.24 Time series that shows the variation of the SST at the position of the maritime
observation.”Model S訂”indicates the model-computed SST.”Obs SST(10min)”is the
observation data acquired every10minutes by R/V Keifu Maru.The vertical axis shows
the sea surface temperature(。C).
ohc Aug24by Mi×ed Lqyer OceGn Model59N
56N
55N
50N
27N
24N
21N
18N
15N
喋2N
120E 125E
「合.角
蓉. 加
、○
150ε 155E 喋40E 歪ヰ5E: 150E 155E 160E
50
27
24
21
喋8
搭
12
9
6
5
Fig.25(a)The shade〔l portion and contours in〔licate the horizontal(listribution of the ocean heat
content(kcal/m2)on August24,1998.The shaded bar on the right side shows the range
of the ocean heat content(kcal/m2).The contour intervals are3(kca1/m2).The line
represents the typhoon trackfromAugust24to August31,1998.The typhoon mark is the
position in the typhoon center at OOOO(UTC).
2002 The Processes ofS解CoolingbyTyphoon Passage and Case Study ofTyphoon Rexwith a Mixed layer Ocean Model
ohc Aug 27by Mi×ed Lqyer Oceon ModeI
555522211蓬
幽6
φ
ヌΩ
120E 葉25E 嘩こ50E 嘩55E 140E: 窪45E: 摩50E 155E
Fig.25(b)Same as Fig.25(a)exceptfor the date;(August27).
160E
61
ohc Aug 31 by Mixed Lqyer Ocecn Model
59N
56N
53N
50N
27N
24N
21N 21
18N
窪5N
18
で20ε 125E で50E で55E で4Gε 璽45ε 雪50ε 155E マ60E
Fig.25(c)Same as Fig.25(a)exceptforthe datel(August31),
074摩852
3222摩璽1963
62 WadaA. Vol.52,No.2
lil
I4112!
086』弓20
→トOGeanHeatContent(o》er 26℃)←minimロmcentralpressure(hPa)
1010
1000
990
980
丁970 釦U o
960
950
940
930
1998/8/241998/8/251998/8/261998/8/27{998/8/28 1998/8/291998/8/30
0100 0:00 0=00 0:00 0:00 0:00 0:00 day
920
was adapted.Figure27also illustrates thatmodification
of the bulk coef且cients yielde(1a better result for SST
cooling.However,the effects of high waves an(1
whitecaps were neglected in this experiment.These
problemswillbe considered inthe future.
32 「㎜一31、5
31
Fig.26 Time series of the minimum pressure and the
ocean heat content over26。C by Leipper and
Volgenau(1972)at the position of the minimum
pressure of the tyl)hoon center.The verUcal axis
indicates the ocean heatcontent(kca1/m2).
30.5
ρ 30届史留29.5
で
29
28.5
28
27.5
一Eヨーbogus and bulk
-e-bogus
込一GANAL
1998/8 1998/8 1998/8 1998/8 1998/8 1998/8 1998/8 1998/8 1998/8 1998/9
6。The Ocean Response to Typhoon Rex
The ten(lency of SST cooling by Typhoon Rex is
well simulated comparedwiththe maritime SSTbyR/V
Ke血Mam shown in Fig.24.However,the magnitude
of maximum SST cooling by mmerical simulation is
smaller thξm that observed by R/V Ke血Mam.Figure
27shows the time series ofS釧「coolingfor the GANAL
wind stresses,the a(ldition of Typhoon Bogusing,an(l
Typhoon Bogusing with a mo(1ification of the bulk
coefficients.The weaker wind stresses in GANAL
reduce maximum SST cooling.The maximum SST
cooling obtaine(1by Typhoon Bogusing with the
modhication ofthe bulk coef且cients was the greatest of
the three cases,but the model camot perfectly
simulate the maンdmum SSTcoolingnotedbyR/VKe血
Mam. S訂cooling was observed by R/V Ke血Mam at
24。50N,134.750E on August29,1998.This SST cooling
appeared on August26where Typhoon Rex passed.
The maximum wind stresses on August26,1998,were
3.5N/m2in the case ofthe Typhoon Bogusing with the
mo(1mcation ofthe bulk coef且cients and2.2N/m2in the
case of the basic Typhoon Bogusing.Deepening of the
mixed layer was about10m,while the depth at the
bottom of the thermocline Iayer became about4m
shallower due to upwelling for the Typhoon Bogusing
with the modification of the bulk:coefficients.
Therefore,the win(l stresses may be insu丘icient for
reproducing maximum SST cooling.It has beenreported that there is a problem with bulk coef且cients
at high seas.Price(1981)examine(1the ef慮ect of bulk
coefficients for SST cooling and suggested the
possibility of an underestimation of about40%for S訂
cooling if the constant drag coefficient CD=1.5×10-3
3020
/α
Fig.27
/24 /25 /26 /27 /28 /29 /30 /31 /遷〇二〇〇
〇二〇〇 〇100 0:00 0:00 0:00 0100 0:00 0;00
Time series that indicates the variations of the
SST(。C).”GANAL”indicates that only GANAL
data were used for the atmospheric wind data,
”bogus”indicates that both GANAL data an(l the
Rankin vortex produced by J幽best track dat且
were used,”bogus and bulk”indicates that the
modhie(1bulk coe伍cients of Kon(10(1975),1.25
times from8to25m/s and1.5times over25m/s,
were used.
There is another problem with wind stresses.The
ma》dmum wind speed ofTyphoon Rex was not actually
observed伽一s伽but was determined by routineanalysis.1f the analysis of the Typhoon Bogusing ha(l
any errors compared with the初一s吻wind distribution,
the result ofthe simulationfor SSTcooling may include
such en℃rs.
Westatedinsection4thatitisimportantthatwind
stresses play a role in SST cooling.However,other
factors may affect SST cooling,such as the initial
thickness of the mixed layer,the vertical gradients of
sea temperatures,and heat fluxes.If the verticaI
gradients of sea temperatures at the area of ma}dmum
SSTcooling observed by R/VKe血Maruwere greater
than those in the Levitus(1984)climatological data,
SST cooling of the simulation may become greater.In
fact,the sea temperature around137。E and25。N was
cooler from June to July than in the average year,
accordingto the observationbyR/VRyFofuMaruσMA,
private communication).Bao8如1.(2000)envisaged the
relationship between the thickness of the mixed layer
an(l the intensity of typhoons using an atmosphere-
ocean wave coupled modeL The intensity of the
typhoon was suppressed when the mixed Iayer was
2002 The Processes ofS訂CoolingbyTyphoon Passage and Case StudyofTyphoon Rexwith aMb【edlayer Ocean ModeI 63
thin,while the intensity of a typhoon with a greater
thickness of the mb【ed layer was almost the same as
that in the uncoupled mode1.Therefore,the initial
thickness of the mixe(11ayer may have a significant
effect on SST cooling an(1the intensity of tyl)hoons.
7.Conclusions and Remarks
The ocean response to typhoons,particularly the
MLTvariation after the passage of a typhoon,was
investigate(1in this study using the mtxed layer ocean
mode1.The following meteorological or oceanic
conditions tended to be favorable for MLT cooling by
passage of a typhoon.
1)Strongeran(1more substantialwind forcing
2)Lowertranslation speed oftyphoons
3)Moreintensiveheatflux
4)Thimerinitialmixed layer
5)Greater temperature gradient in the
themocline layer
The results of1),2),4),an(15)are similar to the
results of Price(1981).However,we quantitatively
estimate MLT c601ing under different conditions.
Strong wind stresses determine the thickness of the
mixed layer regardless of its initial condition.Wind
stresses and typhoon translation spee(ls have a
significant effect on the entrainment and upwe11ing
processes.We can understand the process of MLT
coolingfromtherelationshipbetweenMLTcoolingand
the thickness of th¢mixed layer at each typhoon
translation speed.The entrainmentprocess is dominant
for MLT cooling mder different heat fluxes from the
sea to the atmosphere.The(1ecrease of the SST
changes due to the(1ifferent vertical profile of sea
temperatures under a constant win(1forcing,as
reflected in condition5).
We consider that MLT cooling is caused by
entrainment and upwelling.If the wind stresses are
strong and the translation speed of the typhoon is low,
a dynamic effect,such as entrainment an(1upwelling,
mainly contributes to a decrease in the MLT.Price
(1981)thought the contribution of heat nuxes to SST
cooling was smal1.One reasons for this is that his
entrainment rate was not include(i in the effect of
buoyancy.The thermodynamics ef£ect by heat fluxes
under a constant win(1stress and the translation speed
of a typhoon cannot be neglected because cooler water
is created at the transition layer by entrainment and
contributes to decrease MLT to some extent.MLT
cooling was about O.7。C greater and the maximum
(leepening of the mixed layer was10m deeper in
experiment20than under the condition of no heat flux
in experiment5.The vertical profiles of sea
temperatures are also important for MLT cooling。
Under these differing con(litions,SST cooling is
determinedbyentrainmentandupwelling. The rapid decrease of the SST obtained by R/V
Ke血Maru is reproducedwellbynumerical simulation
using the mixed layer ocean model.The Rankin vortex
was composite(i on the global analysis data to simulate
SST cooling by Typhoon Rex.It is necessary to
consi(1er the effect of the moving speed ofthe typhoon
on SST cooling.The decrease of SST cause(1by
Typhoon Rex corresponds to the peho(1when the wind
stresses were stronger and the translation spee(ls were
lower.However,since climatological data were used as
oceanic initial con(iitions,unrealistic vertical pro丘1es of
sea temperatures in the initial conditions may af醇ectthe
prediction of SST cooling in this numerical simulation.
Therefore,preparation of better qualitative initial
conditions is re(luire(1for a beUler pre(liction.
The mixed layer ocean model used in this paper
has been(ieveloped as a part of取【)hoon-ocean coupled
model in order to improve the forecast ofintensities of
typhoons.Further improvement of the mixed layer
ocean model will be expected to simulate the ocean
con(1ition more practical.One ofthe subjects we should
improve is an introduction of the effect of the bottom
topography in order to improve the pre(liction of
landing typhoon and the重yphoon that moves along the
coastal region,Another subject,which is easier to
practice than former one,is to make finely the
horizontal resolution of the mixed layer ocean mode1.
The丘nerhorizontalresolution ofthe mixedlayerocean
model embodies a S鋼「cooling by passage of typhoon
in a SST distr圭bution.However,we should also make
五nely the horセontヨ1resolution of typhoon model in the
typhoon-ocean coupled mo(lel because a(1etaile(i
distribution of the wind velocity on the surface also
embodies the SST cooling by typhoon.Besides,we
should reconfirm some parameterization metho(is in
planetary boundary layer such as the estimation of
momentum flux because of its great in旦uence for S訂
cooling.The mbζed layer ocean model will progress in
future for the purpose of the typhoon pre(liction with
the typhoon-ocean coupled model although there are
some subjects to improve for development ofthe雌xed
layeroceanmode1.
Ac㎞owledgement
I am grateful to two anonymous reviewers for
providing useful suggestions and comments.This
research is now carried out as”Research into
pre(1iction ofatyphoon using anumerical mode1”in the
Typhoon Department Meteorological ResearchInstitute,theJapan MeteorologicalAgency。Du血gthis
research,many recommendations and suggestions
64 WadaA Vo1.52,No.2
were received from Mr.ShoinYagi,the Director ofthe
Typhoon Department.We obtained extensive advice,
proposals about the mixed layer ocean mo(1el,and
information about source co(1es from Prof.1.Ginis at
the University of Rhode Island.We received many
suggestions from Prof.R.L。Elsbeny at the Naval Posし
Graduate School and Dr.C.Rowley in FNMOC at the
Naval Research L衰boratory.I thank Mr.Chikara Nara
for assistance with maritime observation data.The
observation datafromKe血Maruwere acquiredbythe
staff of Ke血Maru,including Takeo M&ehira,Chief
Observer、ITRISST(Ver.2.0)l was pro(iuced and
supplied by the Earth Obser▽a.tion Research Center,
National Space DevelopmentAgencyofJapan.
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66 WadaA. Vol.52,No.2
海洋混合層モデルによる台風通過時の海面水温低下の過程と台風Rexの事例調査
和田章義
様々な海洋内部の初期条件や大気境界条件が、台風に伴う海面水温低下に与える影響について調べた。強い風応力、
ゆっくりとした台風の移動、海洋から大気へ放出される過剰な熱フラックス、初期の混合層深度の設定そして海洋内部
の水温分布は台風通過に伴う海面水温低下に影響を与える。海面水温低下はエントレインメントと湧昇の効果が混合し
たプロセスで生じ、海面水温低下に対する慣性振動に代表される移流の影響は小さい。台風中心付近を通過する点にお
いて混合層の深まりと海面水温低下の関係を調べた結果、初期の混合層深度に関わらず風応力の強さと移動速度の違い
により混合層深度及び水温低下は決定される。また水温低下と湧昇、エントレインメントによる混合層の深まりの割合
は風応力の強さ、台風の移動速度の他、海洋の鉛直構造の違いに対しても密接な関連がある。一方で熱フラックスのみ
を変化させた場合、混合層の深まりに対する湧昇の効果は変化しないものの、熱フラックスが最大800W/m2に達する状
況では、熱フラックスを与えない場合に比べ、混合層水温は約0.7℃低下した。このようにエントレインメントによる混
合層下部における水温低下は混合層水温低下に影響を与える。
海洋気象観測船啓風丸によって観測された台風Rexによる約3℃の水温低下を理解するために、数値シミュレーション
を行った。モデルによって計算された水温変動は、海面水温の急激な低下や約3℃の海面水温低下といった観測の特徴
を良くとらえていた。モデルの計算結果から、この水温低下は台風による強い風応力と遅い移動速度に生じていたこと
を確認した。更にモデルにより計算された水温を使用して計算された海洋熱容量は台風強度の時間変化とよい関係を示
していた。