key factors in tropical cyclone prediction. Tropicalcyclone produces significant SST (sea surfacetemperature) changes 10-13. Monitoring changes in theSST under the storm center has shown the importancefor intensity prediction. The wind stress forcingproduces turbulent mixing and entrainment of coolerthermocline water into the mixed layer which leads todecrease of SST. This tropical cyclone - oceaninteraction makes the cyclone - ocean system onewith positive and negative feedbacks. A positivefeedback occurs during the genesis and developmentstages where the surface fluxes are enhanced byhigher wind speed and tropical cyclone can obtainnecessary heat flux from the ocean to intensify. Assoon as the tropical cyclone strengthens further,entrainment begins to play the role and SST decreaseswhich constitutes the negative feedback on theintensity of tropical cyclone.

In the earlier study the ocean response in terms ofcurrents and upper layer thickness, to moving cyclonealong the northwestward track, was examined using asimple reduced gravity model". The study also dealtwith the results of sensitivity experiments obtained bychanging cyclone parameters such as vortex size,

o

nI.

Jnd

Oceanic mixed layer variations during the movement of cyclone alongsymmetric tracks in the Indian Ocean

*A.A.Deo, D.W.Ganer &P.S.Salvekar

Indian Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune ~ 11 008, India1

*[E-mail: aad@tropmet.res.in]

Received 27 December 2004, revised 27 January 2006

The understanding of significant changes in the oceanic mixed layer is important for dynamical prediction of tropicalcyclone. Present study aims at examining the variations in the upper ocean parameters during the movement of tropicalcyclone in northern and southern Indian Ocean, by applying 1Y2 layer wind driven reduced gravity ocean model. Differentidealized tracks in the Bay of Bengal and their mirror images in southern hemisphere are considered, The model producedmixed layer depth, temperature and currents are compared for northern and southern hemispheric cyclone cases. Also theeffect of latitude for westward moving cyclones in both the hemisphere is investigated. For this, tracks in the Arabian Seaand their counter parts in the southern hemisphere are considered. The maximum cooling found in the wake of cyclone is ofthe order of 3 - 4°C, which is comparable with earlier studies and observations. This significant cooling can cause _weakening of the storm. This can be useful for prediction especially in the event of a cyclonic storm heading towards landand likely to make land fall. Moreover it is also seen that the mixed layer is cooled more on the right (left) of the storm trackin northern (southern) hemisphere. This cooling 'decreases for the storms translating along higher latitudes implying thesensitivity of the latitudinal location of the storm.

[Key Words: Numerical simulation, currents, Indian Ocean, cyclone, symmetric tracks, upwelling, Arabian Sea,Bay of Bengal, mixed layer]

Indian Journal of Marine SciencesVol. 35(2), June 2006, pp. 111-122

Introduction

Tropical cyclones are the most destructive amongseasonally recurring rapid onset natural hazards.About 80 to 100 tropical cyclones occur around theworld each year. The extensive coastal belt of India isvulnerable to these storms and 4 to 6 such stormsoriginate in the Bay of Bengal and the Arabian Seaevery year. Tropical cyclones which are characterisedby torrential rain, gales and storm surges, causeextensive damage to standing crops, heavy loss oflives and destruction of property.

Vaiious tropical cyclone - modeling studies in thepast dealt with the problems of convectiveparameterization, vortex movement, vortex flowinteraction". Also, several model studies have beenexercised for simulation of tropical cyclone,prediction of storm track and intensity't". The researchefforts to improve upon the prediction of cycloneintensity have centered on the storm's dynamicalinteractions with its environment. A good knowledgeof the ocean response to storm forcings is one of the

* Telephone : 91--020-25893600Fax: 91--020-25893825

112 INDIAN J. MAR. scr., VOL. 35, NO.2, JUNE 2006

vortex intensity, translation speed as well as changingthe model parameters such as initial conditions, gridlength, viscosity etc. Further, Deo et al.", studied indetail, the ocean impact of a tropical cyclone movingin different directions over Indian seas. Also thelatitudinal effect on the ocean response was examinedby considering westward tracks along differentlatitudes. In both the studies the tracks consideredwere in the northern hemisphere and there was notemperature equation in the model considered. Theobjective of the present paper is to understand, theoceanic impact of a tropical cyclone moving indifferent directions in northern Indian Ocean and itssymmetric counterpart in the southern hemisphere,using 1'12 layer wind driven reduced gravity oceanmodel with the inclusion of thermodynamic equation.The thermodynamics is included in the model asreported by Chang & Anthes 16. Moreover in contrastto earlier studies 14,15 the radius of the vortex in thepresent study is taken as 400 krn, which is close to themost of the real cyclones. In order to compare themodel results for the southern hemispheric tracks withthose for northern hemisphere, exact symmetric tracksin the northern and southern hemisphere areconsidered. The model derived currents, mixed layerdepth and temperature also are analysed here.

Materials and Methods

A simple in-house developed, 1'12 layer reduced. gravity ocean model over the tropical Indian Oceanregion (35°E-115°E, 300S-25°N) is used in this study.This model with the horizontal resolution of '12°x '12°,has one active layer overlying a deep motionlessinactive layer. The model equations are verticallyintegrated depth averaged momentum and continuityequations assuming no vertical shear in the horizontalfields. The formulation of the equations and thenumerical methods are fully described in BeheraSalvekar'",

The model equations in Cartesian co-ordinate are:u, + (VUlH)x + (VVIH)y-!V + (g'l2)(H2 )x

= AH ,\,2 (V)+ rxlPI ... (1)

Vt + (UVIH)x + (WIH)y +!V + (g '(2)(H 2 )y= AH ,\,2 (V) + ;/ PI ... (2)

Ht+Vx+Vy=we ••• (3)where V and V are zonal and meridional componentsof transport obtained from vertically integrating upperlayer velocity field,f is the Coriolis parameter (2Q sinrp).H is the upper layer thickness, s' = g (P2 - PI) I P2

IS the reduced gravity (0.02 ms -2). A H is the

horizontal eddy viscosity coefficient (1500 m 2 s -I)

and ''txz' and ''tyz' are the components of the windstress applied as a body force. We is a source termrepresenting entrainment from deep motionless layerinto the upper active layer. The source term used issame as that in Chang & Anthes";

We = m u*3Ig'H, H S:H min

=O,H;?Hmin

... (4)

where Hmin = 25 m, the efficiency parameter m = 2.5and u, is the frictional velocity derived from thesquare root of the wind stress magnitude. This term isincluded to prevent the surfacing of the interface (i.e.H = 0). The effect of this entrainment on the upperlayer .density, momentum and kinetic energy balancehas been neglected. It is assumed that the entrainedwater engulfed into the upper layer, has zero velocityand is instantaneously adjusted to the density PI' Themodel equations are solved numerically on a finitedifference mesh staggered in space (Arakawa C-grid)and integrated with respect to time (!1t =30 min)using leapfrog finite-difference scheme. An Eulerscheme is applied at every 49th step to eliminate thespurious growth that usually arises due to timesplitting in leapfrog scheme. Boundary conditions areno slip (V = V = 0) at land boundaries and modifiedradiation boundary condition" is applied at the openboundaries. This boundary condition allowsinformation to pass through the open boundary butdoes not allow to come into the model domain.

The thermodynamics is included to study thevariation in the temperature of the mixed layer duringthe passage of cyclone. The thermodynamic equationcontains the advection term, horizontal diffusion termand the effect of vertical mixing with bottom water.The cooling by evaporation and the effect of sensibleheat transfer are neglected, same as in Chang &A h 16 Th ..'nt es. e equation IS:

(JI'IJt =-V. lIT + AH ,\,2 T + (dTldt)m ... (5)

The effect of vertical mixing terms is incorporatedimplicitly so that the final temperature equation usesthe mixed layer depth before and after entrainmentand the temperature difference between the mixedlayer and the bottom layer

r'" = h o(To-T ~ + vl2a p r [(ht+J)2-ho2] + T b

where, To-Initial temperature in the mixed layer

DEO et al.: OCEANIC MIXED LAYER 113

.Tb-Initial bottom layer temperature

a-expansion modulus of water near T b=2xI 0-4 k-Iho-value of h at time t+ 1 before mixingPr-bottom layer densityv-Rate of density increase with increasing depthThe initial thermocline is 50 m deep and the

reduced gravity parameter g' as 0.02 mls2 resulted intoan initial gravity wave speed of 1 mls. The initialtemperature in the mixed and bottom layer are 29° Cand 22° C.

The model inputThe idealized symmetric cyclonic vortex similar to

that of a Rankine vortex is considered. The modelcyclone has tangential and radial wind componentsvarying with the radial distance r, as follows.

Tangential wind component:Vt (r) = Vm (rIRmax), O::;r ::;R,

= Vm (Rmax - rIRm~R), R < r < Rmax,Radial wind component:Vr(r) = 0.3 Vt (r),

where, Rmax = Radius of the storm = 400 kmVm = Maximum tangential wind = 20 mis, at a radialdistance R = 55 km.

The cyclonic wind stress derived from this is usedas input to the model. The model is integrated for theperiod of the storm duration (five days for all thecases considered here) from the state of rest with thetime step of 30 min.

Best track estimates of the Indian Ocean cyclones,both in the northern and southern hemisphere aredownloaded from the website of Joint Typhoon WarningCenter (www.npmoc.navy.mil/jtwc/ best jracks/) forthe years 1992-2002 and the tracks are plotted.Different idealized cyclonic tracks symmetric aboutthe equator in both the hemispheric Indian Ocean arechosen .such that the frequency of storms passingalong 'these tracks is quite high. Three tracks in theBay of Bengal are selected which are northwestward(Track A), northward (Track B) and westward (TrackC). Mirror images of these tracks (denoted by dashes)in the southern Indian Ocean are considered. (Fig. 1).The idealized cyclonic vortex as defined above, isallowed to travel along these tracks for 5 days in theseparate numerical experiments (Expts. 1-6), which isused as forcing to the model (Table 1). The upperocean response in terms of currents, temperature andmixed layer depth (MLD) during the passage ofcyclone, is examined. The latitudinal variation of the

15N

20NI~

ION

5N

EQIndian Ocean

5S P'~ _.B'I)'7

lOS s~l5S ·R'~20S1 ••

~ ~ ~ m m m ~ m ~ m ~Fig. I-Different symmetric storm tracks considered in thenumerical experiments.

Table 1- Numerical experiments carried out for the selectedtracks

Expt. Track Track Initial FinalNo Name Direction Position Position

1 Track A Northwestward 96E,8N 86E,18N

2 TrackB Northward 90E,6N 90E,16N

3 Track C Westward 92E,14N 82E,14N

4 Track A' Southwestward 96E,8S 86E,18S

5 Track B' Southward 90E,6S 90E, 16S6 Track C' Westward 92E,14S 82E,14S

7 TrackP Westward 72E,6N 62E,6N

8 Track Q Westward 72E, UN 62E,11N

9 TrackR Westward 72E,16N 62E,16N

Coriolis parameter (jJ) affects the atmospheric andoceanic motions on various scales. Geisler &Dickinson'" investigated the effect on the wakefollowing a hurricane along the westward track. Themodel equations used here have Coriolis term in it.Therefore model results will change with change oflatitude. Hence further experiments (Expts. 7-9) havebeen carried out to investigate the ocean mixed layerresponse to Indian Ocean cyclones moving westwardalong different latitudes, in the northern hemisphere.Three tracks along 6°N (Track P), 11ON (Track Q)and 16° N (Track R) are chosen. The same cyclonicvortex is moved along these tracks from nOE to Q2°Ein 5 days as model input. The model is integrated forcyclonic period and upper ocean parameters areinvestigated. The translation speed of cyclone in allthe experiments remains same (- 2.6 mls) except for

114 INDIAN J. MAR. scr., VOL. 35, NO.2, JUNE 2006

experiment no. 1 and 4. The details of these experi-ments are described in Table 1.

Results and DiscussionFigure 2 exhibits the model simulated upper layer

currents (a, b), divergence (c, d), mixed layer depthanomaly (MLDA) from the initial mixed layer depthof 50 m (e, f) and temperature change (g, h), from theinitial temperature of 29°C on the fourth day, for thetrack A in the left panel and for track A' in the rightpanel. The solid line drawn is storm track and the dotrepresents the position of the storm center. Thesurface circulation (Fig. 2 a, b) shows the divergenceof the flow near the storm center for tracks A and A'.The maximum magnitude of the flow is 0.69 m/s andis located to the right (left) of the storm track A (A').The bias is about 100 krn. This bias is explained bythe sense of rotation on either side of the track withrespect to time. The inertial forces turn the oceancurrents in the same (opposite) direction of windstress in the right (left) side of the track in thenorthern hemisphere hence the model field showsright bias in the northern hemispheric cyclonic tracks.For the southern hemisphere this is reversed basicallydue to change of sign off, the Coriolis parameter. Theinertial forces turn the ocean currents in the same(opposite) direction of wind stress in the left (right)side of the track and left bias occurs for the southernhemispheric storms. The maximum current lags thestorm center by about 500 krn. The region of

. upwelling is surrounded by the region ofdownwelling.

The divergence field (Fig. 2 c, d) shows positivevalues near the storm and then trails of negative andpositive values. Upwelling (down welling) in themixed layer depth field is associated with divergence(convergence). The positive MLD anomalies (Fig. 2e,f) are shaded which represent downwelling. Themaximum upwelling of 15 m occurs on the right (left)of the track A (A') at a distance of about 50 krn. It hasa lag of about 300 krn. In the wake of the cyclonethere are alternating regions of positive and negativevalues or oscillations in the model divergence field,MLDA and temperature field. These oscillations(waves) are associated with inertia - gravity wavesinduced by the storm and are both parallel andperpendicular to the storm track. The inertial wave inthe wake of the cyclone in the MLD field has awavelength of about 500 krn along the track direction.From the temperature field (Fig. 2g, h) it is seen thatmaximum cooling (negative temperature change) of

about 4°C occurs at right of the track on day 4, whichsuggests that the mixed layer on the right of the trackis cooled more than the left and there is right bias inthe temperature field. The isotherms exhibit. anoscillation with wavelength of - 650 krn. Theseresults are in agreement with the earlier studies of

h h . h . 16 20-22 The j . 1nort eJ!l enusp enc storms' etc. e mernagravity waves in the temperature field are incorrespondence with the number of waves in mixedlayer depth field, in the wake of the cyclone. The areaof temperature change is widely spread as comparedto that of mixed layer depth anomalies. The region ofinfluence under the cyclonic storm is more in thesouthern hemisphere as compared to that in thenorthern hemisphere due to wide-open ocean.However, there is no difference in the magnitude ofmodel fields for the track A and A'.

Figure 3 displays the model currents (a, b),divergence (c, d), MLDA (e, f) and temperaturechange (g, h) on the fourth day, for the tracks B (leftpanel) and B' (right panel). The surface circulation(Fig. 3a, b) shows the divergence of the flow near thestorm center which is also supported by thedivergence field (Fig. 3c, d). The maximum currentsof magnitude 0.65 m/s lie right (left) of the stormtrack B (B'). The MLDA (Fig. 3e, f) field also showsthe right bias and the maximum upwelling of 21 m isto the right (left) of the storm track B (B'). There islag of 400 krn between the storm iposition andmaximum upwelling. This is equivalent to 1.8 dayswith the prescribed cyclone speed. The region ofupwelling (negative MLDA) is surrounded by theregion of downwelling (positive MLDA). From thetemperature field (Fig. 3g, h) it is seen that. themaximum cooling of about 3°C occurs. right (left) ofthe track B (B ') suggesting the mixed layer cooling ismore to the right (left) of the track.

The model simulated currents. (Fig. 4a, b),divergence (Fig. 4c, d), mixed layer depth anomalies(Fig. 4e, f) and temperature change (Fig. 4g, h) on thefourth day, for the tracks C and C' are presented inFig. 4. The maximum currents (Fig. 4a, b) haveamplitude of 0.51 m/s but lie on the storm track forboth C and C'. Also the lag between the maximumcurrents and storm center is reduced as compared totrack A or A'. Divergence field (Fig. 4c, d) is exactlysymmetric about the storm track. The mixed layerdepth anomalies (Fig. 4e, f) show upwelled regionlying symmetrically about the tracks and no bias inthe field is seen for both the tracks C and C'. Themaximum upwelling of 18 m occurs right on the

DEO et al.: OCEANIC MIXpD LAYER 115

I•

••CI-6.0 Klzed layer depth anomaly

1II~(fL

•III

III

••._-- CI-2.0

=--"" ....--III~(h)

...III

III

I I••Ii IIIlI • • loe. lOll

Fig. 2-(a, b) Model currents with contours of speed (m/s), (c, d) divergence x 10 6 (s'), (e, t) mixed layer depth anomalies (m) and (g, h)temperature change (0 C) on fourth day for track A (left panel) and track A' (right panel). Solid line drawn is the storm track. The dotrepresents the position of the storm on the fourth day. Positive values for mixed layer depth anomalies and temperature change areshaded. Contour interval is mentioned at the top of the figure.

Track AINIl -----••(a) - U_. - -r--- CI-O.2~ I

I.

• ~. . . . . . . . . . .. . .. " ,...........'...... ...~... . .: : '>:

III

I

Dlvergence (x 10") CI-LO

Temperature Chan.AINIl ~ (g) ==::::::= - I

1111

Track A'Model Currents and speed CI=O.2

III~(b) :,•• • ..• ~I

... . . .: ::::::::;~~~~~ : :.-1 .... :.~ .... : ..: : ••• ! • • •.•.•.•• ••. •••••.•

III

1.1 ..•••.... ! •••••••.•

•• .........................

I

Dlvergence (x 10") CI=1.0

lIIi(d),tya ..

~a .

• ' (:::J <::l"" ., ('~4.I

P'4:;-"',.1II~ <:l~ '

CI-5.0

CI=2.0

Track 8'

116 INDIAN J. MAR. set, VOL. 35, NO.2, JUNE 2006

Track 8Model Currents and speed CI=0.2

. . : : : :: : : : : :::::::: : : :' .

: : :: : : : : : :: : : : : : : : : :~

\---.",.,---:1f'. : : :.: : : :.:..,..---;1811'--\-~---::J' ......•... 'J...-,...--i

:: :: :: : :1: ::::::.'r---]......... .. ..- "" ' .• • • • ~ •• <II •• •• •• • • •

::::.ts: :~~~~L ~~~~; :.... . ~ .... ~.... . -. . . . . . . . . .. .. .. .. .. .. " ~• •• • • • • • • •.•• c •.•.•.•.•.•.•.•• c

12M

EQ

-,-Divergence (x 10')

1811to

12M IS

6H125

,EQ .~ lIS

Mixed layer depth anomaly CI=5.0

12M

1811

(e) \--~

to

CI=1.0

1811

12M

to

70E 75E 80E 85E 80E 95£ 100£

Fig. 3-Same as in fig.2 except for the tracks Band B'

Model Currents and speed CI=0.2............. .....~

: : : : : : : : : ~: :: . : : : :~ ~

(b) : : : : : ~~~~~~~~~~~~~: :~IS :::::::::::::1:"':::::::· . . . . . . . .. .. .. .. .. .. .· . . . . . . . .. .. .. .. .. , .· . . . . . . . . .. .. .. .. , .· . . . . . . . . .. .. .. .. .. .125 •....•.•..••• . •••....

· . . . . . . . . . .. .... .. "" .....• • • • • • • • • • . •. c • •. •. • ••.•• •••••••••• c •.•. •• •.•.•••••· •.•.••.• I ..•.•.•. 1r ••••••••

to

lIS .

-,-Divergence x 10' CI=1.0

Mixed layer depth anomaly CI=5.0

lIS

to(f)

IS

125 .

Temperature Change CI=1.0

to(h)

IS·

125 .

lIS

IDOE

DEO et al.: OCEANIC MIXED LAYER 117

Track CYodel Currents and snead

:UI i ' =--= =--=' ==-= i

__ __ ___ CI=O,2

10

12111'..---"

••

-,-Divergence (x lOt) CI=1.0

:UI F ----- ==--=== I(c)

18111~

.1

Mixed layer depth encmaly241 i ===- ===- ==-= i

_ CI=5,O

Temperature Change Cl=2,O141 I, ,:::::===- ==-==--- :::::::==t

-,

Track C'Model Currents and speed Cl=O,2

(b) .. . . . . . . . . . . . . .

OSot •. • .• , ..•.. , .•.•....... " .•

... , " .

IlSi :::~::::•••• o. I' •.•. • ••

. •. •. • •. t • II •••.

IIS~ . . . . . ..•.•••••••

wi I-,-

Divergence (x lOt) Cl=1.0

IS

lIS

lIS

Wi I

llixed layer depth anomaly CI=5.0

IS(f)

lIS

lIS

wi I

Temperature Change CI=2.0

(h)as

128

183

•• ••• 1-801

Fig. 4-Same. as in fig.2 except for the tracks C and C'

118 INDIAN J. MAR. scr., VOL. 35, NO.2, JUNE 2006

Mixed layer depth anomalies

CI=5.0

20N

15N

20N

15N

10N

5N

EQ

55E 60E 65E 70E 75E 60E

Temperature changeTrack P

CI=1.0

Track Q

Track R

55E 60E 65E ,70E 75£' 80E

Fig. 5-(a, c, e) Mixed layer depth anomalies (m) in the left panel and (b, d, f) temperature change (C) in the right panel on fifth day forthe tracks P, Q and R. The dot represents the position of the storm on the fifth day. Solid line drawn is the storm track. Positive values areshaded.

DEO et aZ.: OCEANIC MIXED LAYER 119

Model Temp~rature change (CI=2.0)

Track P Track pI

5Nl (b)20N

EQ15N

5510N

5N 105

EQ 155

55 I I I I I I I 20560E 65E 70E 75E 80E 60E 65E 70E 75E 80E

Track Q Track QI-=====- =::l I

20N I (d)EQ

15N 55

10N105

5N15S

EQ

5S20SL

60£ 65E 70E 75E 80£ 60E 65E 70£ 75E 80E

Fig. 6-Model temperature change (0C) on fourth day for the tracks P, P', Q and Q'. The dot represents the position of the storm on thefourth day ..Solid line drawn is the storm track. Positive values are shaded.

track, which lags the storm center by 300 km. TheCoriolis parameter, f, does not change during thetranslation of the storm, hence the inertial forces actexactly equal along both the sides of storm track withrespect to the wind stress turning with time. Due tothis reason the bias remains absent. This is inagreement with the earlier study 15. The temperatureanomalies (Fig. 4g, h) reveal maximum cooling of2.5°C on the storm track but the second maximumoccurs on the right (left) of the track, hence, on thewhole the mixed layer is cooled more on the right(left) side for the track C {C').

The results from the experiments 7 to 9 aredepicted in the Figure 5. The MLDA field (Fig. Sa, c,e) shows no bias for the tracks P, Q and R, the tracksbeing parallel to the equator. The anomalies weakenfor tracks along higher latitudes. This is in agreementwith the earlier finding of northern hemispheric

tracks':'. However, the temperature anomalies (Fig.5b) show slight bias on the right of the track P. Thisbias vanishes for higher latitudes (Fig. 5d, f). Thecooling in the wake of the cyclone reduces for higherlatitudes. The magnitudes are 4.6 "C, 3.8 "C and 3°Cfor the tracks P, Q and R respectively, which impliesthat the amplitude of the inertial wave is reduced, inthe wake of cyclone for higher latitudes. Also it isvisualized that the frequency (wavelength) of inertialwaves in the wake of the cyclone increases(decreases) for higher latitudes. Similar results arereported in Chang & Anthes". These results fortemperature change and mixed layer depth anomalyare supported by theoretical investigation byGeisler23

, which indicated that the displacement of thethermocline in the wake of the cyclone is inverselyproportional to f 3 and wavelength is inverselyproportional to f alone. It is also seen that the lag

120 INDIAN J. MAR. scr., VOL. 35, NO.2, JUNE 2006

Model simulated magnitude of ocean currents (CI=O.2)

Storm track along eN20N~~--~~~~~~~~~----~(a)

15N

ION

5N

EQ

~~--------------~----~--~Storm track along 8N20N.-.---~~~~==~~~~----~

15N(c)

ION

5N

EQ

~L- __ ~ ~ ~

Storm track along tON~Nr-'---~~~~~~~~=-----~

5N

(e)15N

ION

EQ

eOE 851 701 751

Storm track along 7N

(b)

Storm track along 9N

(d)

Storm track along 11N

(f)

aOE 851 701 ?ril

Fig. 7-Magnitude of model currents (mJs) on third day for westward moving cyclones along 6°N, 7°N, 8°N, 9°N, lOoN and lloN. Thedot represents the position of the storm on the third day. Solid line drawn is the storm track.

DEO et al.: OCEANIC MIXED LAYER

between maximum upwelling (cooling) and stormcenter decreases for higher latitudes. The magnitudeof simulated currents, mixed layer depth, temperatureetc. remain same but in the opposite direction (resultsnot shown) for the tracks P' Q' and R' in the southernhemispheric Indian Ocean which are symmetric to thetracks P, Q and R (Fig.l).

In general the magnitude of the temperaturechanges obtained in different experiments above, forthe translating storm is comparable to the magnitudes.• d . I 24-26 H .roun III rea storm . owever, exact compansonscannot be made because of the sensitivity of thetemperature changes to the initial thermalstratification in the ocean.

Figure 6 shows the temperature change for 4th dayfor the tracks P, P', Q and Q'. The temperature fieldshows that trough and ridge axis is tilted along NW-SE direction for northern hemispheric tracks P, Q(Fig. 6a, c). Similarly, for southern hemispherictracks P', Q' the trough and ridge axes are tiltedalong NE-SW direction (Fig. 6b, d). Also, the coldanomalies in the wake of the cyclone, approach fromthe NE direction in the case of northern hemispherictracks (Fig. 6a, c) and from the SE direction forsouthern hemispheric cyclone (Fig. 6b, d). Whereasmixed layer depth field has trough axis exactly alongthe track in the wake of the cyclone (Fig. Sa, c, e).This shows the mixed layer is cooled more on theright (left) side of the track in northern (southern)hemisphere, but upwelling is exactly symmetric inthe wake of the cyclone for northern and southernhemispheric cyclones moving westward. The crosstrack oscillations in the temperature field areprominent as compared to the mixed layer depthfield.

The circulation field shows maximum currents onthe right (left) of the track P (P') but for tracks Q, Q',Rand R' the maximum lies on the track itself (Figurenot shown). This indicates that bias in the circulationfield vanishes somewhere between 6°N to l l'N. Inorder to get the latitude for unbiased circulation field,different experiments were performed wherein thesame cyclonic vortex is moved westward along thelatitudes 7°N, 8°N, 9°N and 10° N from 72° E to 62°Ein S days. The model currents show right bias for allthe days for the tracks along 7°N and 8°N. For thetracks along higher latitudes the bias vanishes slowly.Figure 7, shows the magnitude of the model currentson third day for the westward moving cyclone alongdifferent latitudes from 6°N to l I''N. It is seen that forthe track along lOONthe bias is completely absent.

121

To summarize, the simple wind driven reducedgravity model is used to study model upper layerresponses to northern and southern hemisphericmoving cyclone moving symmetrically about equator.Analysis of model results show that the model fieldshave right (left) bias for the tracks having northward(southward) component in the northern (southern)hemisphere. The basic reason for this is the inertialturning of the ocean currents is to the right in thenorthern hemisphere and to the left in the southernhemisphere.

The newly added temperature equation gives thecooling of the order of 4° C in the wake of thecyclone. It is comparable with earlier studies andobservations. The cooling of 3°C in the Arabian Seacyclone during S - 8 June 1998, is reported by PreasKumar et al.", Rao28 has also reported the SSTdecrease of 3- 4°C during cyclone of May 1979 in theBay of Bengal.

The cooling obtained by models depends on theinitial thermal stratification and mixing rates. Chang& Anthes" obtained the model cooling in the range of2-8°C, in different numerical experiments,considering thermal stratification of 10° C whileElsberry et at.27 estimated the cooling of I-3°C byassuming 2°C thermal stratification. Higher cooling of8°C is also in agreement with the cooling observed inGulf of Mexico24 and in the Atlantic'" in the wake ofhurricane. The different responses of the surfacetemperature indicate the need for accuratemeasurements of the thermal stratification before thestorm passage.

The temperature and mixed layer depth show thecooling and upwelling of the same magnitudes inassociation with divergence for the respective tracksin the northern and southern hemisphere. However,the region affected due to tropical storm in thesouthern hemisphere is more because of the openocean.

The bias in the mixed layer depth field vanishes forall the westward tracks in northern Indian Ocean andits symmetric counterpart in southern Indian Ocean.The upwelling of mixed layer occurs exactlysymmetric about the track whereas the cooling is notexactly symmetric and mixed layer is more cooled onthe right (left) for northern (southern) hemispherictrack. The circulation has bias for the case of lowerlatitudes and it vanishes for higher latitudes. Modelcurrents, anomalies in mixed layer and temperaturefield, become gradually weak for the tracks along

122 INDIAN 1. MAR. SCl., VOL. 35, NO.2, JUNE 2006

higher latitude. As well as for higher latitudes, thewavelength and amplitude of the inertial wave arefound to be reduced, in the wake of cyclone.

AcknowledgementAuthors are grateful to Dr. Brian Doty for the

GrADs .software, which is used for preparation of thediagram. The Joint Typhoon Warning Center (Hawaii,USA) is highly acknowledged for providing the besttrack data which was used for the selection of thestorm tracks. This work is carried out on SUN Ultra60 workstation, for which authors are thankful toDept. of Ocean Development, Delhi, India.

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