JOURNAL OF GEOPHYSICAL RESEARCH, YOLo 96, NO. C7, PAGES 12,667-12,683, JULY IS, 1991
The SeasonalCirculation of the Upper Oceanin the Bay of Bengal
JAMEST. POTEMRA,IMARK E. LUTHER,2AND JAMESJ. O'BRIEN
MesoscaleAir-Sea Interaction Group,Florida State University,Tallahassee
Analysis of the results of a multilayer, adiabatic, numerical model of the upper Indian Ocean, driven by climatological monthly mean winds, shows that the simulated currents in the northeastern Indian Ocean are in general agreement with available observations and interpretations. The main features of the ocean currents include large anticyclonic flow in the Bay of Bengal surface waters during the northern hemisphere winter. This gyre decays into eddies in spring and then transitions into a weaker, cyclonic gyre by late summer. The western recirculation region of this flow is an intensified western boundary current which changesdirection twice during the year. In the Andaman Sea, east of the Bay of Bengal, the oceanic flow changesdirection twice during the year; it is cyclonic during the spring and early summer and anticyclonic the rest of the year. Flow in the equatorial region shows the North Equatorial Current (NBC) flowing west during winter. Further south is the eastward flowing Equatorial Counter Current (ECC) and the westward flowing South Equatorial Current. In summer, the NEC switches direction, joins the ECC, and forms the Indian Monsoon Current. Investigation of the second layer of the model (the upper 450 m of the ocean) shows that flow during much of the year is baroclinic (strong vertical shear). Model layer thickness reveals coastal Kelvin waves propa!~ting along the coast, traveling the entire perimeter of the Andaman Sea and the Bay of Bengal. This wave excites westward propagating Rossby waves into the interior of the bay. Time series analysis of transport calculations yield significant peaks in the 20- to 3o-day range and 50- to ro-day range which are not likely directly forced by the applied wind stress.
INTRODUCTION few excellentatlases [see Duing, 1970;Wyrtki, 1971;Has- tenrath and Lamb, 1979;Cutler and Swallow, 1984].Al- The Indian Ocean is a particularly interesting, dynamically though these sourcescontribute significantlyto the under- rich area because of the changing wind patterns associated standingof dynamicsin the tropical Indian Ocean,data on with the Indian Monsoon. In spite of this, it is a relatively smallerscales, particularly in the Bay of Bengal,are some- poorly studied area. In addition, data coverage for the area is what limited. sparse. While some data sets do exist, they are predomi- The focus of this study therefore is to use a realistic, nantly on large space and time scales. Recent efforts, such as wind-drivenmodel to simulateocean currents in the region the monsoon experiment (MONEX) of the Global Atmo- of the Bay of Bengal,from 75°E to 111°Eand from 70Sto spheric Research Program (GARP), the First GARP Global 23°N. Similar studieshave beendone for other areasof the Experiment (FGGE), and the Indian Ocean Experiment Indian Ocean [Jensen, 1990] and using different models (INDEX), have substantially improved the availability of [Woodberryet al., 1989].A review of modelingefforts in the measured data in this region. Indian Oceanis given by Luther [1987].The results of the Individual studies have also contributed in recent years to model are comparedwith measureddata in an attempt to the understanding of the dynamics in the Indian Ocean. An understandthe dynamicsof the area.The modelused for this early effort by Wyrtki [1961] synthesized the available data study, developedat the Florida State University [Jensen, sets (at the time) of the properties of the Southeast Asian 1990],is a reducedgravity, three and one-halflayer model. waters. More recently, Legeckis [1987] demonstrated the Sincethe model has multiple layers, the subsurfacecur- appearance of a western boundary current in the Bay of rents can be studiedas well as the surface.The lowest layer Bengal using sea surface temperature data obtained from the is at rest, however, and only the results from the first advanced very high resolution radiometer (A VHRR) carried (surface)and second(subsurface) layers will be examined on board the NOAA 9 satellite. Data coverage from this here. satellite encompassesthe entire region of the Bay of Bengal Figure 1 showsthe typical large-scalecirculation pattern [McClain et al., 1985]. Rao et al. [1989] have generated associatedwith the winter and summermonsoons as com- mean monthly mixed layer depth, sea surface temperature, piled in Monatskartenfiir den IndischenOzean (Deutsches and surface current climatologies for the tropical Indian HydrographischesInstitut [1960], as depicted by Diiing Ocean. Molinari et al. [1960] analyzed three different sets of [1970]). The general surface circulation of the region in satellite-tracked drifting buoys and compiled a monthly northernwinter includeslarge anticyclonic flow in the Bay of climatology of surface currents in the tropical Indian Ocean. Bengal, a westward flowing North Equatorial Current Finally, oceanic data for the Indian Ocean is compiled in a (NEC), and eastward flowing Equatorial Counter Current (ECC) and a westward flowing South Equatorial Current I Now at STX, NASA Goddard Space Flight Center, Greenbelt, (SEC). The northern summer pattern is characterizedby M~land. counterrotatingeddies in the Bay of Bengal[Duing, 1970], 2Now at Department of Marine Science, University of South an eastwardflowing Indian MonsoonCurrent (IMC) and a Florida, St. Petersburg,. westwardflowing SEC. Copyright 1991 by the American Geophysical Union. The model reproducesthese flows, in generalagreement with the atlasespreviously discussed.In this presentation Paper number 91JCOI045. 0148-0227/91/9IJC-O 1 045$05.00 the model is first described. Next, a description of the 12.'667 12,668 PoTEMRA ET AL UPPER OcEAN CIRCULAnON IN BAY OF BENGAL
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Fig. 1. Large-scale circulation as observed during (a) the winter (northeast) monsoon, and (b) the summer (southwest) monsoon. (Deutsche.r Hydrographisches Institut [1960], courtesy of W. Diling).
climatology of the winds that force the model is given. work of Luther and O'Brien. This model, however, is Finally, the resultsof the modelare presented,first in terms composed of four isopycnal layers. The advantage of a of generalcirculation patternsand later in terms of oscilla- multilayered model is the ability of studying both surface and tions of certain features. subsurface currents. In addition, multiple layers provide greater accuracy in representing low vertical modes. 2. MODEL The layers of this model are assumed to have positive thickness everywhere at all times. In other words, the layers Isopycnic layered models have proven to be useful in the can not merge or surface, and the bottom topography is study of ocean dynamics. The work of Luther and O'Brien always in the lowest layer. The vertical modes for the Indian [1985] and Luther et al. [1985] demonstrates how a reduced Ocean, given by Gent et al. [1983], are used to select the gravity model can correctly simulate the currents of the initial layer thicknesses and densities. The four densities are western Indian Ocean. Further, Woodberry et al. [1989] chosen to be 1023.9, 1026.2, 1027.3,and 1027.9kg m -3. The show this model to be equally capable of reproducing the initial thicknesses of the layers are taken as 200 m for the top complex current system of the southern Indian Ocean. The layer, 250 m for the second layer, and 400 m for the third. success of these efforts gives rise to the numerical model For this model, horizontal eddy viscosity is based on the used for this investigation of currents in the Bay of Ben~. velocity field so that a thin layer with large velocities would The model employed for this study was developed at the experience more friction than a thick layer with equivalent Florida State University by Jensen [1990] and is based on the transport [Jensen, 1990]. The magnitude of the horizontal PoTEdA ET AL.: UPPEa OcEAN CIRCULADON IN BAY OF BENOAL 12,669
i-I w~=o Hl>Hmin ap.+ PJI--a" 9 ""~ (Pi - PI> a~ a. I-I For the bottom two layers this term is set to zero. The ~] effect of this entrainment on the upper layer momentum balance is neglected. Incorporating this entrainment velocity + (-"/,, - (1) only in the continuity equation represents turbulent entrain- Pi ment where entrained water into the upper layer has zero velocity and is warmed instantaneously to the upper layer density. For this particular study, however, this adjustment is rarely active. The value for H minis taken to be 60 m and the time constant 1"~ is 1.2 hours. j-1 The boundary conditions for this set of equations are given a." -g 2 (PJ- PI) ~ in two parts. First, for closed boundaries, i.e., land, the I-I as] no-slip condition is incorporated. At coastal points, there- fore, both components of the transport are set to zero. Next,
+ ( oj",'0' 0 " for open boundaries, a Sommerfield radiation condition is ~ i +F' (2) applied. The solution at the boundary is first separatedinto its PJ Pi vertical modes, and the radiation condition is applied to each where mode separately. The resulting current components for the boundary are computed as the sum of these vertical modes. Uj, Vj zonal and meridionalcomponents of vertically The only forcing is the applied wind stress. The wind integratedvolume componentsfor layerj; forcing is based on wind stress values taken from a climato- H II thicknessof layerj; logical monthly mean data set prepared by Hellerman and Pi density layer of j; Rosenstein [1983]. A pseudostress is formed by dividing P a atmosphericpressure; these data values by the product of an average drag coeffi- a radius of the Earth; cient and an air density. For this study a constant drag 41 longitude; coefficient of 0.0015 and an air density of 1.2 kg m-3 are 8 latitude; applied. A bicubic spline is then used to interpolate the data f Coriolis parameter; from their 2° resolution down to the model resolution. 9 gravitational acceleration; The domain of the model is from 25.1oS to 26.1~ and from 7J surface displacement; 34.SOEto 119.6°E. It should be noted that the model domain T tangentialstress due to vertical friction. is much larger than the domain of interest for this study. F. is the horizontalfriction for U transport,given by Results are investigated only in a small section of the total model domain. The open boundaries for the model are along A { HjV2 (~u .) _I 1 I a~ cos2 8 [Uf(l - 2 cos2 8) the 25°S parallel and along the 11~E meridian between Australia and Indonesia. Figure 2 shows the domain of the entire model, with a border outlining the region of interest for this study. The land values are given by the 200-m isobath. It is for this reason that shallow areas, which pose a significant imped- ance to flow, such as the channel between India and Sri Lanka, appear as land. Additionally, the two island chains, Nicobar and Andaman, appear as solid stretches of land. With these boundary conditions established, the equations are discretized, using a resolution of one tenth of a degree in +2sinl . (~)1} both horizontal directions. This was done, for the spatial 'H}~ (4) derivatives, using the Arakawa C grid, which is a mass- and 12,670 PoTEMRA ET AL.: UPPER OcEAN CIRCULAnON IN BAY OF BENO,
of November to February, the winds flow from the high pressure area over the Asian continent into the Bay of Bengal in a southwestward direction. These winds reach a maximum of about 6 m s -I off the east coast of India during the month of December. In the beginning stagesof the winter monsoon (November) the winds in the bay itself have a strong westward component down to approximately 7°N. Below 7°N, down to approximately 50S, the winds are very zonal toward the east. During winter, therefore, ~ repre- sents a line of cyclonic wind shear. This wind pattern moves south of the equator as the season progresses. This pattern is manifest as a region of large positive wind stress curl in early winter over the Bay of Bengal, down to about 5~. This region of positive wind stress curl moves south as a band between 10~ and the equator, bounded to Fig. 2. Model domain. The 200-m isobath is used for land the north and south by regions of negative wind stress curl, boundaries,and the openboundaries are labeled.The borderin the during the northern spring months. northern part of the domainindicates the particularregion investi. During March and April the winter monsoon begins the gatedin this study. transition to the summer monsoon. Anticyclonic winds de- velop over the bay and, in corUunction with the weak maxi- mum in the pressure field, bring flow with a southerly compo- energy-conserving scheme. The time integration was per- nent to the western and northern shores of the bay. During formed using an explicit leapfrog scheme with a forward March, flow from about 1000Ndown to 5~ is mainly toward the Euler scheme applied every 99 time steps to remove the west, while from the equator to about 1000Sit is toward the east. computational mode inherent to the leapfrog method. The This pattern breaks down with the full arrival of the summer Laplacian friction term is computed with a Dufort-Frankel monsoon in July. By May the winds are predominantly south- implicit scheme [O'Brien, 1986] for computational conve- westerly, originating in the southern hemisphereand sweeping nience. through the Bay of Bengal. At the peak of the summer The model is spun up from rest, with a time step of 20 min, monsoon in July, the maximum wind speedsare found over the by repeatedly applying the annual wind stress cycle. After 10 center of the bay and are of the order of 10 m s -I. years a quasi-periodic solution is obtained. The computer The summer wind stress curl is somewhat different than results are retained as fields of currents and layer thickness that seen in winter. The band of positive wind stress curl is every 6 days. much more narrow, extending from about 5°N to 15°N. In addition the patch of positive curl runs along the coast to the 3. WINDS extreme north of the bay rather than running in a zonal Monsoon regions are considered to be those that experi- direction. The maximum values appear off the coast of Sri ence the seasonal reversal in winds during the year. The Lanka. Indian Ocean, characterized by large season to season variations due to the Asian monsoons, is one such region. In the Bay of Bengal area the winds are southwesterly during 4. REGIONAL CIRCULATION the northern summer months and switch to northeasterly The large-scale circulation in the northern Indian Ocean during the months of the northern winter. The primary can be described in three parts. First, there is a large gyre driving force for this oscillation in wind patterns is the system which develops in the Bay of Bengal, the recircula- differential heating between land and water [Fein and tion portion of which is a western boundary current along the Stephens, 1987]. east coast of India [Legeckis, 1987]. The model simulates The wind patterns associated with the monsoon climaIe only wind-driven flow, but several factors could determine are compiled in the form of wind stress into a data set by the circulation in this region. It has been suggestedthat the Hellerman and Rosenstein [1983]. The model was forced by river systems of India contribute in a small way to the this climatological wind data set. This set was obtained by circulation in the Bay of Bengal [Somayajulu et al., 1987]. processing surface observations for approximately 100 years One large system is the Ganges, which empties into the (1870-1976) and calculating monthly norms and standard northern end of the bay. In addition, the severe weather deviations of the eastward and northward components of associated with the monsoon affects circulation in the form wind stress at standard anemometer height (10 m). of tide surges and fresh water input [Johns et al., 1985]. Figures 3a and 3b represent the wind conditions of Janu- Another component of the circulation in the northern ary and July (the contours shown are the pseudostress Indian Ocean is the flow into and around the Andaman Sea, values used to force the model). These are representative of bounded by Malaysia in the east and the Nicobar and the extreme conditions of the winter and summer monsoons, Andaman Island chains in the west. It could be expected that respectively. The months of April and October, shown in discrepancies between the model and measured results Figures 3c and 3d, demonstrate the transition period be- would be greatest in this region due to the geometry of the tween the monsoon seasons. A more complete picture of the model. The model cannot produce flow through the Malacca atmospheric conditions of the Indian Ocean can be found in Strait, since it is less deep than the 200-m depth contour that the atlas by Hastenrath and Lamb [1979]. the model uses as land (closed boundary). Data records During the northern hemisphere winter monsoon months indicate, however, that flow does travel from the South POrBMRA ET AL.: UPPER OCEAN CIRCULATION IN BAY OF BENGAL 12.671
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The results of the transport and Swallow, 1984]. Similarly, the two island chains are computations are presented as they apply. treated by the model as single areas of land, allowing ftow only between the two (through the Ten Degree Channel) and 4.1. The Bay of Bengal between Sumatra and Greater Nicobar Island (through the Great Channel). The model, therefore, treats the Andalnan Examining first the results in the Bay of Bengal reveals a Basin as more of an enclosed area than actually exists. large anticyclonic gyre centered at 15~, 86°E, fully estab- The final components of the ftow field in this region are the lished in February (see Plate la). This gyre is driven by the equatorial currents. These currents are the northern and northeasterly wind of the winter monsoon. The gyre direc- southern extremes of gyres centered near the equator [Mo- tion is consistent with the ship drift data of Cutler and linari et al., 1990]. The northernmost current is the North Swallow [1984] and the observations of Wyrtki [1961] and Equatorial Current, which flows toward the west during Tchernia [1980]. The gyre intensifies on the western side, northern winter, between SONand the equator [Piccard and along the coast of India, as it travels northward. The Emery, 1982]. Further south, from the equator to 80S,is the appearance of this western boundary current is evident in eastward flowing Equatorial Counter Current. During the satellite SST images of the area [Legeckis, 1987]. The buoy summer monsoon, the NEC reverses direction, joins the data of Molinari et al. [1990] reveal the gyre but do not show ECC, and forms the Indian Monsoon Current [Molinari et the western recirculation area (this may simply be due to the at., 1990]. In addition, an equatorial jet, explained by fact that a drifter buoy was not located in the relatively O'Brien and Hurlburt [1974], is formed during the monsoon narrow recirculation region). Although the flow is usually transition periods (spring and fall) [Wyrtki, 1973].This rapid, baroclinic, at this time of the year it is somewhat barotropic, eastward flowing current is centered about the equator. Fur- as evident by the similarity with the second layer results ther south is the South Equatorial Current. The SEC flows (Plate 2a). The center of the gyre appears at the thickest part from east to west all year, from about 15°Sup to the ECC. of the two model layers. The thickness is 460 m at the center of the circulation (14°N, 82°E). While it does not change direction during the year, it does In March (Plate Ib) the anticyclonic gyre is still evident in intensify in the northern summer months (May to September). the model results. The western boundary current is much In addition to these swface currents, there is also a subsur- more narrow, however. Along the coast, flow is northward face current, the Equatorial Undercurrent [Cane, 1980]. The but a few hundred kilometers offshore, and across the whole EUC is found east of ro"E during the northern winter. It is bay it is to the south. In addition, the upper layer thickness weaker than its counterparts in the Pacific and Atlantic oceans, along the coast is less than in February, corresponding to a and during the northern summer, when equatorial flow is to the rise in the thermocline. This upwelling region extends along east, it is not evident [Piccard and Emery, 1982]. the coast all the way around the bay. In this section these features are examined using the Flow in the lower layer (Plate 2b) shows the same narrow- results of the multilayered, wind driven model. Whenever ing of the gyre but also the formation of another. A cyclonic possible, comparisons with observed data are made. It circulation has developed in the lower layer directly to the should be noted, however, that the choice of an initial upper east of the first. It is centered at approximately 13°N, SrE. layer thickness affects the model-derived current speeds. The two gyres are joined at their southward branch, about For this investigation the initial upper layer thickness (based 300 kID from the coast of India. on Gent et al. [1983]) was taken as 200 m. Observations Two months later, in May (plate lc), this upper layer show a shallower mixed layer depth in this region [Rao et appears unchanged, with upwelling along the coast some- al., 1989] which would explain why the in situ data of what less than earlier in the year. In the second layer the Molinari et at. [1990] shows a higher current speed than flow has reversed direction to a cyclonic orientation. The those given by the model. The large-scale features previ- eastern gyre of the previous month has expanded westward ously mentioned are, in fact, well represented by the model. and overridden the anticyclonic gyre that was there. This Plate I shows the results of the model at six times during the motion is being fed strongly by the eastward flow around the year. The upper layer thickness, plotted using a color scale, southern tip of Sri Lanka. The flow in May is more ba- gives an indication of the upper 200 m of the ocean. The roclinic, therefore, with flow in the upper layer opposing current velocity is shown as vectors, with magnitude relative flow at deeper depths. to the arrow length and direction given by the arrow orien- The flow in the next month, June, as displayed in Plates ld tation. Speeds less than 0.010 m S-I are not displayed and and 2d, is even more baroclinic. The upper layer flow is speedsmore than 0.100 m s -I are truncated (to avoid making northward along both coastlines. At about IrN these two the figures too crowded). flows meet and turn south. A cyclonic circulation is thus Second interface topographies are shown in Plate 2. The established in the eastern portion of the bay with anticy- field in this case represents the distance from the surface to clonic flow directly to the west. This resembles the subsur- the interface between the second and the third layers of the face flow of the preceding month. The upwelling region along model. The velocity field is presented the same way as for the coast has propagated to the southern end of India, and it the upper layer. is followed by an area of downwelling; a region of deep The featUres of both the surface and subsurface circulation upper layer thickness (200 m) is evident off the coast of can be readily identified from these figures. To get a more Burma. quantitative idea of the flow, transports were computed at The cyclonic circulation flows around an area of shallow certain cross sections using the model data. Figure 4 shows thickness; the interface depth is at a minimum of 440 m at the the locations where the calculations of mass transport were center of the gyre. By August, this area has moved north and made. In the following sections, each part of the circulation hit the coast (plate 2e). The anticyclonic flow in the lower PoTEMRA ET AL.: UPPER OcEAN CIRCULATION IN BAY OF BENGAL 12.673 N N ~f mE 100 [ 110 E ~E ~f 100 [ 110 E ~ E 9J E 100 E 110 E 80 E 9J E 100 E 110 E Plate 1. Model resultsfor the surfacelayer, for (a) February,(b) March, (c) May, (4) June, (e) August, and U) November. The thickness of the upper layer is represented by the color scale (in meters), and it reftects dynamic topography. The currents in the surface layer are liven by the vectors. The major features of the circulation are the large circular ftow in the Bay of Bengal, flow into and around the Andaman Sea, and the equatorial currents. layer extendsover most of the bay. The circulation in the this large eddy is no more than a few centimeters per second, upper layer during August is not quite the sameas in the but this will develop into the large, bay-wide circulation of lower layer (plate Ie). Flow is cyclonic throughoutthe entire late winter (Plate la). In the southwest comer, a cyclonic bay. The region of downwelling has progressedwestward circulation has developed around a region of upwelling around the top of the bay. Also, a region of upwelling has centered at lOON,82°E. propagatednorthward around Sri Lanka. The upper layer Four stages of the ftow in the Bay of Bengal emerge. The thicknessreaches a minimum of 175m eastof the island. first is a large, anticyclonic gyre across the whole bay. This Toward the end of the year (Plates If and 1.1),there is appears throughout the winter months (December until weak ftow in both layers. The upper layer contains two March). This is followed by two, counterrotating ftows, rotating ftows. One, anticyclonic around a region of down- anticyclonic on the western side and cyclonic on the eastern 'velling; is in the northeasterncomer of the bay. Flow along side, producing northward currents along both coasts and 12,674 PoTEMRA ET AL.: UPPER OcEAN CIRCULATION IN BAY OF BENGAL ~f ~E tOO E 110 f .r ~f tOO f JIO! c f ! - ~E ~E IOOE IIOE MlE ~E IOOE IIOE Plate 2. Model results for the second layer, for (0) February, (b) March, (c) May, (d) June, (e) August, and southwardflow down the middle. This persistsuntil early positive transport indicates northward flow, and negative summer(April-June) whenthe anticyclonicregion dissipates transportreflects southwardflow. and the cyclonicflow extendsacross the wholebay. The The four stagesof the circulation are evident from these summermonths of July and Augustare characterizedby this time series.The bay-wide,anticyclonic circulation of winter large counterclockwiseflow. In the fall, two rotating flows is apparentfrom the positive transport throughST -1 (north- develop again, with southwardcurrents along both coasts ward flow) and the negativetransport at ST-2 (southward and northward flow in the center. flow). This occurs from December to March. Maximum This patterncan be seenmore quantitativelywith the time transport along the western boundary current is 3.5 Sv series plots of mass transports of Figure 5. Transport is during February in the upper layer. In the spring monthsof measured,using model data, at two transectson opposite April, May and Junetwo gyresare present,characterized by "'sidesof the bay (ST-l and ST-2 of Figure 4). In this case, positive transport through both ST-I and ST-2. Maximum PoTEMRA ET AL.: UPPER OcEAN CtRCULAnON IN BAY OF BENGAL 12.675 northward on the eastern side and negative on the western side. Maximum transport again is along the western bound- ary, and it reaches -1.20 Sv in late August. Finally, in the fall (September-November) the two-gyre system returns, this time with an anticyclonic circulation on the east side and cyclonic in the west. Transport reaches -2.0 Sv during early September in the western boundary current. The transport calculated in the second layer reveals the baroclinicity of the circulation. The second layer anticy- clonic gyre persists in December and January. Maximum transport occurs in the western boundary, reaching a value of 3.5 Sv in January. In February and March, while the upper layer still has southward flow along the eastern side of the bay, the lower layer begins to show northward (positive) ~ ~ l~ litE transport on both sides. This develops into cyclonic flow in April and May. In June, July, and August, flow is southward FII. 4. Transect locations. Mass transport throuib nine stations is computed usina model derived data. The first two stations (ST-) along both coasts, similar to the two gyre system of the and ST-2) are chosen to investipte Bow in the Bay of aenaai. Three upper layer in fall. Finally in September, October, and other stations (ST -3. ST-4. and ST-5) give an indication offtow in the November the flow is cyclonic throughout the whole bay. Andaman Sea. The fina1four transects (ST -6. ST- 7. ST-8. and ST-9) measure the equatorial ftow. 4.2 TheAndaman Sea transport at this time is 2 Sv in the westernboundary. The The flow in the AndamanSea is lesscomplicated than the other flow is very weak, with transportless than I Sv along flow in the Bay of Bengal. In March (Plate lb) flow in the the easterncoast. The eastern,weak cyclonic gyre expands upper layer enters the sea south of the Nicobar Islands, westward,drowning out the anticyclonic flow, and encom- cin:ulatescounterclockwise around the AndamanBasin and passesthe whole bay. In July and August transport is exits south of the AndamanIslands. This pattern continues DEC NaY OCT SEP RUG JUL MAY RPR FEB JRN 80E 90E LODE -4 -3 -2 -1 0 1 2 3 4 LAYER THICKNESS (M) Plate 3. Antisymmetric component of the height field at 3-N. The sloping contour indicates a wave traveling west .. (phase velocitY of 0.190 m s -I to the west and period of ~ days). Because of the silnal appearing in the antisymmetric plot ~nd the phase velocity. these waves are interpreted as Rossby waves. 12,676 PoTEMRA ET AL.: UPPER OcEAN CIRCULATION IN BAY OF BENGAL 1 '11"..." J'JI 1 ..." , , I ': rI Sf-2 if 0.5 0 -o.a -1 l I I I I I I I I I I I I I I JFMAMJJA80NOJF *»rnt 4 I '. ""81: 'A . 11- ':.. . ,. ., I I. '-'. ~ . 1.5 2 ~i ~ Q It 0.5 i 0 0 ~ .f .0.& ~ .. -3 ,...", , JFMAMJJASONDJF .1.6 JFMAMJJASOND aQfTH MONTH Fig. S. Time seriesof masstransport. Transport (in sverdrups)is calculatedusing the model velocity and height data. Hows through ST-I and ST-2 (seeFigure 4) are displayedas functions of days. Positive transport indicates northwardBow, while negativetransport showssouthward Bow. Transportfor the surfacelayer is above with the secondlayer below. Maximumtransports in the upperlayer, about 3.SSv, are seenin the westernboundary current during February. until July. Plates Ie, If, and la show that from July until relatively deep upper layer (i.e., deep thennocline), 210 m, spring (March), flow enters the basin from the north, circu- and flow is anticyclonic around it. lates clockwise, and exits in the south. In the lower layer, flow in February is anticyclonic around The results in the lower layer are somewhat different. a relatively thin region, 436 m. The southeast corner of the Flow changesdirection three times during the year instead of basin shows a thickening of the top two layers to about 450 once. In January, February, and March (plates 2a and 2b) m. By May this thick region has propagated around the flow is cyclonic. This switches in spring and early summer whole sea, increasing to 460 m. In May the maximum (April-July). August, September, and October show flow thickness is in the center of the basin, and flow is anticy- returning to a cyclonic orientation. Finally, in November clonic around it. A region of upwelling then follows in June, and December (plate 1.1)flow is anticyclonic. propagates up the coast, and moves to the center of the basin More interesting is the height field in this area. In Febru- by fall. ary a region of upwelling enters the Andaman Sea from the The calculations of mass transport in this area, made at south. The upper layer thickness is ISO m along the south- locations ST-3, ST-4, and ST-5 (see Figure 4) are shown in east coast of the basin. The anticyclonic circulation sur- Figure 6. Maximum transport along the east side of the sea, rounds a high value of upper layer thickness, 195 m. By in the upper layer, is 2 Sv northward in mid-May and 1.5 Sv March the upwelling region has propagated along the basin southward in early August. Transports in the lower layer coastline and extends well into the Bay of Bengal. The reach 2 Sv in May, but the flow is southward, and in upwelling pulse is followed in May by an area of down- February the flow is northward. welling. The center of the circUlation, now cyclonic, is ISOm Flow through the Great Channel (southern entry of the deep in the upper layer, and the downwelling signal along the Andaman Sea), the area of largest transport, peaks in Mayas coast reaches 195 m. well, reaching 3.5 Sv northward in the upper layer. Flow By June the upper layer is 215 m deep on the eastern side exiting the sea in the upper layer becomes 5 Sv in early of the sea, stretching northward along the coast. Finally, in August. The lower layer experiences northward transport August, the center of the Andaman Sea is marked by a though ST-5 in late February and early March (about 4.2 Sv). -.. ~";\1j ~~~f ~t~i IL --=<~ .1~~~ ... ~c~f'g.8 o~ . 0 .. li~~ 0[1~~ ~ - ~~~ (Sdn~afIS) - 0="- 1>" ~!~§: ."-.",, 'ii~-;~ 'g:goS~ ~ ao-~ - . I> ~ ~ 09 . oS~ 1 ,rs1,; ~I>~ ] 'b = 8. ~o~ ~ ~ GA.. ~.s o( ~ ]ti~,~ ~ Q, ... """...~' ~ 8.01> ... ~~:~ 2 ~ o~ ~ ~ 0( ,5 .a '; ~ --~~.c 2 ~t ~ ~ oS p . ~ ... .5{i)Q,~~.c Eo- f,Q oS ... ~DO-. ~~e8~ G; ..C'!oC'!~.. ~ ~'.s=~ : 0 0' 0 q q q q (SdntlOtl3ASI JJ40dSHWL j ':'°i ~l 'io.o ~ ~ c; I;' - i <...I>Oe - I>RoS O !~ ... oS,! 1>1I, C,,;,'5~ . ... o~ Ii; ~ ~ 0f ~ Q 8.li'f~ z !"1"!oS~ 0 !~.R~~ II) ~~!i~ 0(~ ~ 'fiI~ .~ oS ... ~~ ~~ l.t(I) '"i l,S ... 2 I: .. .. . N 0 ., 12,678 PoTENU ET AL.: UPPER OcEAN CIB.CULATION IN BAY OF BENGAL 15 ST-7 10 6 0 -5 JFMAMJJASONDJF Y)N1H 10 81-7 10 .I , MAMJJASONDJF ~ Fig. 7. Masstransport in the equatorialregion. Transport in sverdrupsis calculatedusing the modelvelocity and height data. Flows through ST-6, ST-7, ST-8, and ST-9 (seeFigure 4) are displayedas functions of days. Positive transportindicates eastward flow throughST-6, ST-7, and ST-8 and northwardflow into the basin through ST-9. A semiannualsignal is seenin both the upper layer (top) and the lower layer (bottom). Similar to other areas,the two layersare not in phase.Maximum transport at the equatoris seenin the upperlayer duringMay and in the lower layer during Februaryand May. The maximumsouthward flow in this layer, 4 Sv, is during to 75°E. The upper layer thickness on the eastern side has May and June. increased to 200 m. The counterclockwise gyre north of the equator has moved to 78°E. Flow north of 4°N is weakly to 4.3. Equatorial Currents the east while all other equatorial currents are to the west. In June the flow starts to follow the upper layer thickness The final components of flow in this region are the currents contour, moving toward the east but splitting at about 800E. in the equatorial region from 8°N down to JOg. During the Flow north of the equator is toward the east, while south of year, three distinct flows are generated in this area. In the equator it is more in a southeastward direction. Along February the upper layer shows (Plate 1a) currents flowing toward the west between Sri Lanka (6°N) and l°S. South of the equator the flow is impeded by the thickening of the layer l°S the flow is toward the east. The region centered at the and turns south. The upper layer thickness reaches a maxi- equator, from 3~ to 3°S, is thinner (ISO m) than the region mum along the coast (210 m) this month; however, the region just to the north and south (185 m). A region of downwelling of downwelling is beginning to decrease. By August, as is (upper layer thickness of about 195 m) can be seen emerging shown in Plate Ie an area of upwelling occurs at 7SOE.The from below Sumatra at 1010E. downwelling pulse moves towards the coast at this time as In the next month, March, the thin region at the equator well. Flow south of 4°S is to the west. North of 4°S, up to moves off the coast, toward the west, with counterclockwise 3°N, the flow is to the east. North of 3~, flow is again in the flow around it. This results in westward flow north of 3~ westward direction. Finally, in November, the equatorial and eastward flow from 3°N to the equator. South of the flow to the east strengthens while the westward flow in the equator the flow is still to the east. The area of large upper north weakens. layer thickness, which was off Sumatra in February, leaves The second layer results show a relatively strong equato- the coast at the equator propagating westward. By May rial jet established in February. Flow within 2° of the equator (Plate 1c), this signal has moved all the way across the region is rapid, toward the east. Flow north and south of this region PoTEMRA ET AL.: UPfta OcEAN Cl8CVLAnON IN BAY OF BENGAl. 12.679 . ST.I 2 0 -a .. .. JFUAUJJASOMOJ' a«»fTH I I VII. 7. (continued) is weakly to the west. The region of high velocity is marked the west, with a maximumof 5.5 Sv in January.The lower by a deep interface depth. In March the flow appears to layer, however, has net transport eastwardduring March, reverse at about 85°E. Further east, two regions of deep April, and May and again in October and November. The layer thickness (457 m) develop, symmetric about the equa- maximumduring this time is 2.5 Sv during April. During the Jor, with clockwise flow around the one north of the equator remainderof the year, transport is to the west. The maxi- and counterclockwise flow around the southern one. mum is 2.5 Sv during July. In May the two circulations have expanded across the The area further south, between 3~ and the equator region, producina strong westward flow at the equator and (ST-7). shows greater transport. In the upper layer, from eastward flow north and south of the equator. This continues in June, with an upwelling signal, coming from the south, March through June and October through December,the transport is to the east. Throughout the month of May it propagating to the north. At the equator the signal splits, with a large pulse traveling west and a smaller portion exceeds 10 Sv. The maximum westward flow, 4.5 Sv, continuing north. Later in the summer, in August, two appearsin August.At the samelocation in the secondlayer, circulations develop around areas of low depth (440 m), eastwardflow is foundduring January, February, and March counterclockwise in the north and clockwise in the south. and again in AulUst, September,and October. The largest There is strong eastward flow at the equator where the two eastwardtransport is in February, 9.5 Sv, and the largest systems meet. At the end of the year, in November, the flow westwardtransport, again9.5 Sv, is in May. is relatively weak, and at the equator it is toward the west. Finally, south of the equator, to 3OS(ST -8), transport in The calculations of transport in the equatorial region, the upper layer is shown to be westward during July, shown in Figure 7, show semiannual periods at all stations August,and September,and apin in February,in the upper except in the upper layer above 3~ which is annual. It also layer. In the lower layer, flow is to the west in April, May, becomes apparent that flow is out of phase in the two layers, June, and July, and aaain in November, December,and as is sometimes seen in the other regions. From 3~ to 6~ January.Maximum transport in the upperlayer is 5 Sv to the (ST -6) the Bow is baroclinic during most of the year. In the cast in November and .5 Sv to the west in late July. The upper layer the transport is toward the east from May second layer has somewhatgreater transport, with 8 Sv through October. It reaches a max.imum of 5.5 Sv in August. westwardin early Juneand 5 Sv in late August and againin During the rest of the year, November through April, it is to early February. 12,680 PonMRA ET AL.: UPPER OcEAN CIRCULATION IN BAY OF BENOAL 5. TEMPORAL VARIATIONS observed in the tropical Indian Ocean are forced by atmo- The time-varying signals of the model can be explored by spheric oscillations of similar frequency [Madden and means of various techniques. Many elements of the Indian Julian, 1972]. The model, however, reproduces these oscil- monsoon system are periodic, ranging from the diurnal lations despite being forced by monthly mean winds. The signal in rainfall to the annual shift in the winds [Krishna- fundamental period resolved in the wind forcing, therefore, murt; and Bhalme, 1976]. These oscillations in the atmo- is 60 days. spheric conditions create oscillations in the oceanic system. Effects on a seasonal scale also appear with the fonnation It is also true that oscillations in the ocean can bring about of waves at the equator and along the coast. An upwelling fluctuations in the atmosphere, although atmospheric condi- Kelvin wave, characterized by low upper layer thickness, tions are not incorporated in the model. appears in the model during early winter (Plate la). Travel- The most obvious signal associated with the monsoon ing east along the equator, it hits the coast and reflects back region is the annual reversal in currents associated with the as a downwelling, westward propagating Rossby wave. The changing monsoon winds. As was previously discussed, maximum amplitude of this wave is seen leaving the coast of currents in the Bay of Bengal, the Andaman Sea, and the Sumatra in early summer (Plate Id). Some of the energy of equatorial region change direction at least once, sometimes the Kelvin wave moves along the coast as two coastal Kelvin twice, during the year. Spectral analysis of the transport data waves, one northward and the other southward. This signal shows that in addition to these annual and semiannual can be found propagating the entire coastline around both signals there are other major frequencies inherent to the the Andaman Sea and the Bay of Bengal. major current systems as reproduced by the model. Two A time-longitude plot shows that as these Kelvin waves significant peaks in the spectra of the transport time series travel along the coast, they excite Rossby waves which appear in the 20- to 3O-day range and in the 50- to 6O-day travel across the basin. In Figure Sa, zonal velocity is plotted range. The northern currents in the upper layer, including since the configuration of the coast at 18~ is mainly in the the eastern and western coasts of the Bay of Bengal, flow in east-west direction. One significant feature of this picture is the Andaman Sea, and the North Equatorial Current, all the northwest/southeast sloping contour lines in the center of show peaks around 23 days. This same peak is seen at all the region, 87~ to 93°E. The tilt indicates a westward stations (Figure 4) in the lower layer. propagating feature. Calculation of the slope of this line, in The equatorial currents show a peak at a 3O-dayperiod in meters per second, yields the phase velocity of the westward the upper layer and a 26-day peak in the lower layer. signal at 0.035 m s-l. Gill [1982] calculates that Rossby Another strong signal is evident at a w-day period in the waves at lSON will propagate west with a phase velocity of equatorial region of both layers. This peak is found in the 0.047 m s -I. This suggests that the signal appearing in the northern points at a somewhat shorter period of 50 days. center of this plot is a Rossby wave that has been generated It has been suggestedthat oscillations at these frequencies on the east coast of the bay. PoTBMu ET AL.: UPPEa OcEAN CocUUTlON IN BAY OF BENGAL 12.681 ~'r_1~ is in the eastward direction. In February and March it reaches a maximum of 0.120 m s -1. In June the flow ::~W switches direction and remains westward until December. The current is slightly weaker at this time of the year, with OCT[ maximum velocity 0.080 m s -1 (westward). SEP R A plot of the upper layer height field, Figure 8b, shows similar features. The westward sloping lines are again evi- AUG \'; dence of the Rossby wave generated in the east coast. The JUL peaks in the contours in the eastern side of the bay show the coastal Kelvin waves which excite the Rossby waves. These JUN Kelvin waves bring upwelling from January until May fol- lowed by downwelling in June through October. The peaks MAY ~~~ of the excited Rossby waves can be seen in this fiIUre as they APR j propagate westward. Repeating this procedure for the second layer, at a lower MAR latitude, reveals a similar result (Figure 9). The coastline at FEB ltj~ this latitude is more north-south; therefore meridional veloc- ~ ity is used (the intrusion of the Andaman Islands accounts JAN for the gap in the contours in Figure 9 at 93°E). Maximum _"III\II\,""KI~I'4 velocity, again found in the western boundary current, is 82 E 84 E 86 E 88 E 90 E 92 E 94 E O.I~ m S-I. During January, February, and March this current flows to the north, in the lower layer, while in April Fia. 9. MeridiOllal velocity in the lower layer at I~. 1118 through August it is southward. The westward propagating c:ootours an from -0.160 m S-1 (IOUthward) to 0.160 m S-1 (northward) at an interval of 0.020 m s -1 (dashed contouR represent Rossby waves are evident. The phase is computed, from the southward velocity). The Andaman Islands block the sianal between slope of the lines, to be 0.050 m s -1. The theoretical value 9TH and 94OJ:..A westward propaptina Rossby wave, aencrated by a coastal Kelvin wave, appcus with phase speed of 0.047 m s -1 and [from Gill, 1982], is computed to be 0.054 m s -1. The phase speed is higher at this latitude than the previous case (phase period of 200 days as is evident in the slopina contoUR. The western intensification appears to change direction three times duriq the speed increases with decreasing latitude). year Oate May, mid-August, and late November). The results in the Andaman Sea (east of 92°E) show the current on the eastern side to be southward in February, March, and April and again in September, October, and The signal is lost on the western side of the bay as it November. During the rest of the year it is to the north. interacts with the western boundary current. This western More oscillations are seen in the equatorial waveguide. intensification appears in the contour plot between 85°E and This is a region a few degrees wide, depending on the 86°E. In the first half of the year, January to June, the flow trapping scale of a particular wave, centered at the equator. ~ 12,682 PoTEMRA ET AL.: UPPER OcEAN CIRCULATION IN BAY OF BENGAL Waves generated in this region become trapped owing to the Flow in the Andaman Sea is observed to clockwise in the effects of the Coriolis force. Three of the lower-frequency upper layer during most of the year. In spring and early waves (periods greater than 2 weeks) that propagate in this summer it is counterclockwise. In contrast, flow in the lower region are the eastward traveling Kelvin waves, the west- layer reverses four times during the year. Measured data in ward Rossby waves, and westward propagating mixed this region is not very abundant, but the atlases that do exist Rossby-gravity waves (Yanai waves). Plate 3 displays con- show weak drift currents in the north-south direction fed by tours, plotted using a color scale, of the antisymmetric the equatorial currents. The model treats this region as more component of the upper layer height field along 3°N. The of an enclosed area, so a circular flow can develop. Signifi- anti symmetric component is computed by subtracting the cant oscillations occur in this region at periods of 25 and 50 signal at 3°S from the signal at 3~. Signals symmetric about days in the surface layer and 20 and 36 days in the lower the equator are therefore eliminated. The antisymmetric layer. waves include even mode Rossby waves, which have rela- Finally, currents in the equatorial region are examined. tively low frequencies, and the higher-frequency Yanai The NEC, SEC, and ECC are all represented by the model. waves. Twice the time between minimum and maximum Also, an equatorial jet appears in the monsoon transition layer thickness indicates the wave period (found in Plate 3 to periods, consistent with observations. Spectral analysis shows be 240 days). This, along with a computed phase velocity of significant peaks at 30- and 6(}.day periods for currents in this 0.190 m s -1, is indicative of a Rossby wave. region. Rossby and Yanai waves are also observed propagating At the equator, the higher-frequency Yanai waves are westward in the equatorial wave guide. seen. Both Figure lOa and lOb (upper and lower model layers, respectively) show a westward propagating feature with phase speed of 0.300 m s -1 and period of about 30 days. AcklWwledgments. This work was supported by a NASA Train- eesbip in Physical Oceanography funded by Oceanic Processes, U sing these values in the linear dispersion relationship for NASA, by the Physical Oceanography Section of the Office of Naval equatorial waves, these can be identified as Yanai waves. Research and by NSF grant OCE-88ll3l6 from the Physical 0cean- These waves have been observed in all oceans, and they ography Section and the Climate Dynamics Section of NSF. Partial were first seen in the Indian Ocean by Luyten and Roemmich support was provided by the Florida State University through time granted on its Cyber 205 and Cray Y-MP supercomputers. The [1982]. Kindle and Thompson [1989] studied Yanai waves in authors express their appreciation to Alan C. Davis for his assis- detail using a one and one-half layer model of the Indian tance with computer computations and to Tommy Jensen for the use Ocean. of his model computations. REFERENCES 6. CONCLUSIONS Cane, M. A., On the dynamics of equatorial currents, With appli- The purpose of this study is to study the seasonal circu- cation to the Indian Ocean, Deep Sea Res., Part A, 27, 525-544, lation in the Bay of Bengal. Because of the scarcity of 1980. measured data in this region, a three and one-half layer Cutler, A. N., and J. C. Swallow, Swface currents of the Indian Ocean(to 25°S,l000E): Compiled from historicaldata archived by model, driven by climatological monthly mean winds, is the MeteorologicalOffice, Bracknell, UK, Rep. 187, 8 pp., 37 used to examine the dynamics of this region of the Indian charts,Inst. of Oceanogr.,WonnIey, England,1984. Ocean. This model accurately simulates the currents with DeutschesHydrographiscbes Institut, Monatskartenfilr den Indis- slight deviations from available observations. chenOuan, Publ. 2422,Hamburg, Germany, 1~. Flow in the Bay of Bengai is observed in the model results DQing, W., The Monsoon Regime of the Currents in the 1ndian Ocean,68 pp., East-WestCenter Press, Honolulu, 1970. to be strongly anticyclonic, in the surface layer, in the winter Fein, J. S., and P. L. Stephens (eds.), Monsoons, 632 pp., John months of December, January, February, and March. In Wiley, New York, 1987. spring and early summer, two circular flows develop, anti- Gent, P. R., K. O'Neill, and M. A. Cane, On the dynamical cyclonic on the western side of the bay and cyclonic on the formulation of the large-scalemomentum exchangebetween atmosphereand ocean,J. Mar. Res., 24, 105-112,1983. eastern side. In July and August the cyclonic circulation Gill, A. E., Atmosphere-Ocean Dynamics, 662 pp., Academic, San extends over the entire basin. Flow at this time is very weak, Diego, Calif., 1982. however. In the fall the two-gyre system forDls again with Hastenrath, S., Climate and Circulation of the Tropics, 455 pp., the appearance of an anticyclonic circulation in the eastern KIuwer Academic, Hingham, Mass., 1985. half of the bay. By winter this flow has moved across the Hastenrath, S., and P. J. Lamb, Climate Atlas of the Indian Ocean, vol. I, Surface Climate and Atmospheric Circulation, 19 pp., 97 bay, and the process repeats itself. The second layer of the charts, University of Wisconsin Press, Madison, 1979. model shows a similar pattern, but it occurs slightly out of Hellerman, S., and M. Rosenstein,Normal monthly wind stress phase with the surface layer. This circulation pattern is in over the world oceanwith error estimates,J. Phys. Oceanogr., general agreement with observations. Some assumptions are 13, 1093-1104,1983. Jensen,R. G., A numericalstudy of the seasonalvariability of the made because the scale of the model is more detailed than SomaliCurrent, Ph.D. dissertation,118 pp., Florida StateUniv., the measured data. Tallahassee,Feb., 1990. Oscillations in these currents are illustrated through spec- Jobns, B., A. D. Rao, S. K. Dube, and P. C. Sinha, Numerical tral analysis of transport data in this region. Significant peaks modelling of tide surge interaction in the Bay of Bengal, Philos. at 26- and 56-day periods are found at both the eastern and Trans. R. Soc. London, 313,507-535, 1985. Kindle, J. C., and D. J. Thompson,The 26-and 5o-dayoscillations western sides of the bay. In addition, the results of the model in the westernIndian Ocean:Model results, J. Geophys.Res., 94, height field show a coastal Kelvin wave, which originates at 4721-4736,1989. the equator, propagating around the entire western perime- Krisbnamurti, T. N., and H. N. 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O'Brien, pp. J. J. O'Brien, Mesoscale Air-Sea Interaction Group, Mail Stop 127-144,D. Reidel, Norwell, Mass., 1986. B-174, Love 012, Florida State University, Tallahassee, FL 32306. O'Brien, JamesJ., and H. E. Hurlburt, Equatorialjet in the Indian J. T. Potemra,STX, NASA GoddardSpace Flight Center,Code Ocean:Theory, Science,/84, 1075-1077,1974. 971,Greenbelt Road, Greenbelt, MD 20771. Piccard,G. L., and W. J. Emery,Descriptive Physical Oceanogra- phy, 249pp., Pergamon,New York, 1982. Rao, R. R., R. L. Molinari, and J. F. Festa, Evolution of the climatological near-surfacer themlal structure of the tropical (Received September 17. 1990; Indian Ocean, J. Geophys. Res., 94, 10,801-10,815, 1989. revised March 13. 1991; Somayajulu,Y. K., T. V. R. Murty, S. P. Kumar, andJ. S. Sastry, accepted March 18. 1991.)