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On the circulation in the Bay of Bengal during Northern spring inter-monsoon (March–April 1987)
Deep-Sea Research II 50 (2003) 855–865
On the circulation in the Bay of Bengal during Northern spring inter-monsoon (March–April 1987) M.T. Babu*, Y.V.B. Sarma, V.S.N. Murty, P. Vethamony National Institute of Oceanography, Dona Paula, Goa 403 004, India Received 3 March 1999; received in revised form 15 February 2001; accepted 15 February 2001
Abstract Temperature and salinity data collected from the Bay of Bengal 22 March–28 April 1987 (Northern spring intermonsoon) identified the seasonal anticyclonic gyre (ACG) at 161N, 861E characterized by warm (>241C) and lowsalinity (o34.2 PSU) water at 125 m depth. The western edge of the ACG was demarcated by a narrow, intense and meandering northward flow, termed the Western Bay of Bengal Current (WBBC) of the spring inter-monsoon period. Two cyclonic eddies (CE1 and CE2) were observed to the left of the WBBC; CE1 at 141N:82.51E and CE2 at 18.51N: 87.51E. These eddies were characterised by low temperature (B16.51C for CE1 and 141C for CE2) and relatively highsalinity water (>34.85 PSU) at their cores. The velocity of WBBC increased from 0.40 ms1 at 121N to 0.70 ms1 at 17.51N where it left the coast and turned eastward. The mean northward transport of the WBBC in the upper 200 m was 12 Sv (1 Sv=106 m3s1). These circulation features (the ACG, WBBC and cyclonic eddies) are well depicted in the maps of sea-surface height (SSH) topography derived from the residual SSHs of GEOSAT altimeter for the period 12 February–28 April 1987. The results show that the WBBC flowed against the northeasterly winds during February and that the current sets up almost three months ahead of the wind reversal. The ACG and in turn the WBBC intensified with the wind reversal in April when the wind stress curl attained a negative maximum. The SSH maps further indicate that formation of the ACG started in February and reached its maximum intensity during April with the coalescence of two anticyclonic cells. r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction It is well known that seasonally reversing monsoon winds affect the Bay of Bengal circulation significantly (Cutler and Swallow, 1984; Hastenrath and Greischar, 1989). The semi-enclosed nature of the bay and its proximity to the equator together with immense quantity of fresh water influx from Ganges and Brahmaputra rivers *Corresponding author. Fax: +91-832-223340. E-mail address: [email protected] (M.T. Babu).
contribute to the formation of a highly complex system of circulation in the bay. Despite all these complexities, very few oceanographic studies have been conducted in the bay and the available data are inadequate to describe the circulation and its variability. Hence, extensive oceanographic surveys were conducted in the Bay of Bengal during 1983–1991 as a part of the project, ‘‘Seasonal, annual and inter-annual variability of the North Indian Ocean’’. Utilizing the hydrographic data collected during post-southwest monsoon (October–November, 1983), southwest monsoon
0967-0645/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0645(02)00609-4
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M.T. Babu et al. / Deep-Sea Research II 50 (2003) 855–865
(June–August, 1984), and spring inter-monsoon (March–April, 1987 and 1991), the circulation characteristics were studied by Babu et al. (1991), Murty et al. (1992) Suryanarayana et al. (1993) and Shetye et al. (1993). The results have revealed the seasonality of the circulation, primarily driven by wind forcing. An anticyclonic circulation evolves during the spring inter-monsoon, which collapses with the intensification of the southwest monsoon in May. Subsequently, a large cyclonic gyre occupies the central, bay south of 151N during the southwest monsoon (June–September), continues until October–November, and drifts westward to the Indian coast during winter monsoon. The present investigation focuses on the circulation in the bay during Northern spring inter-monsoon (March–April) its evolution within the period using Geosat sea-surface height (SSH) topography. Several studies previously have reported that the circulation in the bay during spring inter-monsoon consists of a large anticyclonic gyre (ACG). Defant (1961) analysed the older ship drift climatology and concluded that an ACG filled the entire basin during spring. Also, the hydrographic data collected during the International Indian Ocean Expedition (IIOE) indicated an anticyclonic circulation with a northeastward flow in the western bay during March–April (La Fond and La Fond 1968; Duing, 1970; Wyrtki, 1971). The temperature field obtained from satellitebased advanced very high resolution radiometer (AVHRR) showed a meandering northward current in the western bay during spring intermonsoon period (Legeckis, 1987), indicating the possibility of an anticyclonic circulation in the interior bay during the same period. Drifting buoy trajectories indicated a large anticyclonic circulation confined between 101N and 201N (Molinari et al., 1990) during the same period. Shetye et al. (1993) analysed the Bay of Bengal circulation, computing the dynamic topography based on climatological temperature-salinity for distinct periods such as November–January, February– April, May–July, and August–October, and found that the anticyclonic circulation could be located only during February–April. It therefore would be worthwhile to look into the circulation in the bay
during spring inter-monsoon in order to analyse the generation and the evolution of the anticyclonic circulation in relation to wind variability.
2. Data and methodology Two hydrographic cruises were conducted in the region 121N–201 300 N in the Bay of Bengal onboard. The R.V. Gaveshani during the spring inter-monsoon (22 March–28 April, 1987). The temperature and salinity data from the upper 1000 m collected at 84 stations using Nansen sampling bottles have been utilised for this study. Considering the nature of mesoscale processes (Rossby radius of deformation B100 km), hydrographic stations were occupied 30–40 km apart in the western bay and 11 apart in the open sea (Fig. 1). In order to eliminate the uncertainties in the in situ temperature measurements, the Nansen bottles were fitted with two protected and one unprotected reversing thermometers (Ghola Precision, Germany; of temperature accuracy, 70.011C). The water samples were analysed onboard using Autosal (Model 8400 A, Guildline, Canada) and the conductivity ratios were converted into salinity (accuracy70.003 PSU) using the UNESCO Practical Salinity Scale (Anonymous, 1981). Temperature readings of the protected and unprotected thermometers were corrected using the calibration constants of each thermometer following the standard procedure (La Fond, 1951). The uncertainties in the temperature and salinity values were translated to an error of 0.017 dyn m in the surface dynamic height. This in turn led to a maximum error of 71.6 Sv (1 Sv=106 m3s1) at 201N and 276 Sv at 121N in the volume transports. The positioning of the vessel was determined using a MX1107 Satellite navigation system within 0.05 nm (approximately 92 m), corrected every 10 min. Therefore, the error in positioning the vessel was within 50–100 m, which had negligible effect on the volume transport estimates. We have considered 1000 db as reference level to compute the geostrophic currents and the associated volume transport between stations pairs along the hydrographic sections. Section E is not
M.T. Babu et al. / Deep-Sea Research II 50 (2003) 855–865
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Fig. 1. The study area with bottom topography and hydrographic stations. Depth contours are in metres.
considered for velocity computations as all stations are shallower than 200 m. For stations shallower than the reference level, the geostrophic currents are computed following Nowlin and Mc Lellan (1967). In this method, the dynamic height values of the adjacent deeper station (>1000 db) are appended below the depth of observation of the coastal station.
In addition to the hydrographic data we have also made use of SSH measured by GEOSAT altimeter during the repeat cycles 7–10 (17.05 days periodicity) between 12 February and 28 April, 1987. The processed GEOSAT altimeter residual SSH data were provided by Oregon State University, USA (Chelton et al., 1990). The collinear method was used to obtain the sea-surface
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Fig. 2. Distribution of: (a) surface temperature, (b) surface salinity, (c) temperature at 50 m, (d) salinity at 50 m, (e) temperature at 125 m, (f) salinity at 125 m, (g) temperature across 161N, and (h) salinity across 161N.
topography as explicit knowledge of geoid is not required in this method. The collinear algorithm involves the following steps: (i) preparation of corrected SSHs, (ii) removal of spikes, if any, from the corrected SSHs, (iii) interpolation of data at equal latitude spacing (0.11), (iv) removal of bias and tilt, (v) generation of a mean sea-surface from
all the repeat cycles, and (vi) subtract the mean profile from each pass to create sea-surface topography. The Geosat altimeter had a range measurement accuracy of 5 cm. The SSH topography was used to study the evolution of the circulation features prior to and during the period of observation. The Geosat repeat cycles 9 and 10
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Fig. 2 (continued).
(25 March–28 April, 1987) coincided well with the period of hydrographic observations (22 March– 28 April 1987) in this study.
3. Results 3.1. Thermohaline fields In the Bay of Bengal, sea-surface temperature (SST) reaches a minimum (26.01C) during winter
in January and a maximum (30.51C) during summer in May. The SST observed during the study period varied between 281C and 30.01C, with zonally aligned isotherms having higher temperatures in the central southern bay (Fig. 2a). The surface salinity exhibited meridionally aligned isohalines in the eastern parts, with relatively high saline waters off the western bay (Fig. 2b). In the Northern bay comparatively cold (o28.01C) and less saline (32.6 PSU) waters were present. At 50 m, the 27.01C isotherm and 33.8 PSU isohaline
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separated the cold, saline waters of the western bay from the warm less saline waters of the central bay (Figs. 2c and d). The thermohaline distribution was more distinct at 125 m depth, with warm (>241C) and relatively less saline (34.2 PSU) waters in the central and eastern regions (Figs. 2e and f). In contrast, two regions of cold and high saline waters were seen to the west and north: one located at 141N, 82.51E, the other at 18.51N, 87.51E. Figs. 2g and h represent the vertical distribution of temperature and salinity across a typical section along 161N. In the thermocline, the isotherms in the range 261–131C, with a maximum descent at 861E. Shoaling of the thermocline towards the coast was noticed west of 841E and east of 901E. The bowl shape of the thermocline is distinct in the upper 300 m, while it is limited to 150 m depth in the halocline. 3.2. Surface circulation from hydrographic data The dynamic topography at the surface with reference to 1000 db (Fig. 3a) shows significant gradient in the dynamic height represented by major features of the spring inter-monsoon circu-
lation. The most conspicuous feature is a large area of high dynamic topography (>2.0 dyn m) in the central bay manifested by the ACG centred at 161N, 861E. The regions of minimum dynamic height 1.55 and 1.45 dyn m represent the cores of two cyclonic eddies, the first one, CE1, in the south at 141N:82.51E; and the second, CE2, in the north 181N:881E. The 1.85 dyn m contour demarcates the region of northward flow in the western bay and the periphery of the ACG in the east. The northward flow in the west, termed as the Western Bay of Bengal Current (WBBC), is conspicuous with large horizontal gradients. The dynamic topography also shows that WBBC meanders at 141N, reaching the shelf north of 161N. Further, it continues northward as an intense coastal current up to 17.51N, where it leaves the coast to form a strong eastward jet between CE2 and ACG. Vertical structures of geostrophic currents across sections H–J are shown in Figs. 4a–c. The WBBC is evident with strong shear in the western part of the sections, extending up to 250 m depth. Across section J, the northward flow is noticeable, with two cores with a maximum current speed of 0.40 ms1 at 821E in the WBBC (Fig. 4a). Across section I, the WBBC meanders eastward to
Fig. 3. (a) Dynamic topography at sea surface w.r.t. 1000 db, (b) Geostrophic volume transport (Sv) between station pairs in the upper layer (0–200 m).
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Fig. 4. Vertical structure of geostrophic currents (ms1) across (a) section J, (b) section I, (c) section H, (d) section A and (e) section B. (positive values denote northward currents).
83–861E and the core velocity increases to 0.55 ms1 (Fig. 5b). The WBBC has a core velocity of 0.40 ms1 between 811 and 841E across section H (Fig. 5c). The contour of zero velocity at 861E demarcates the center of the ACG where maximum dynamic height is obtained. The return flow of the ACG could be seen as a broad southward flow (0.05–0.10 ms1) to the east of the zero contour. The northward current could be seen in the offshore with a speed of 0.30 ms1 across section A and with 0.70 ms1 across section B, 100 km away from the coast (Figs. 4d and e). The temperature and salinity distributions along sections B and C confirm the close proximity of the WBBC to the coast. Relatively strong vertical shear is observed in the WBBC near the coast along sections B (Fig. 4e) due to upward sloping of
temperature and salinity isopleths. Satyanarayana et al. (1991) also reported upward sloping of isopleths of chemical parameters such as dissolved oxygen, phosphate and ammonia along this section. An important observation that emerges from this analysis is the eastward deflection of the WBBC at two locations where cyclonic eddies CE1 and CE2 are encountered to its left, and subsequently the velocity field intensifies when the current is confined between the eddy and ACG. 3.3. Surface circulation from geosat altimeter For a better understanding of the time evolution of the circulation, Geosat Altimeter sea-surface topography (repeat cycles 7–10) was analysed. In order to cover the southern bay, the Geosat SSH
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Fig. 5. The sea-surface topography (cm) obtained from Geosat Altimeter residual heights for: (a) repeat cycle 7, (b) repeat cycle 8, (c) repeat cycle 9 and (d) repeat cycle 10. (17.05 day periodicity from 12 February–28 April, 1987).
values from south of 121N also was utilized. The Geosat SSH topography of repeat cycle 7 (the second half of February, 1987) revealed that the circulation is characterized by two anticyclonic circulation cells (positive SSH)—one in the north (NAC) centered at 161N, 861E, and the other in the south (SAC) at 101N, 851E (Fig. 5a)—with a meander in between. The core of NAC exhibits higher residual height at its center (30 cm) than the SAC. During repeat cycle 8, the meandering flow at 141N is seen as a cyclonic eddy (CE1 in Fig. 3a), while the anticyclonic cells retain in the interior (Fig. 5b). During repeat cycle 9, the size of NAC decreases, with the development of a cyclonic eddy in the northeastern bay (CE2 in Fig. 3a); and the SAC intensifies and moves westward (Fig. 5c). During repeat cycle 10, the spring inter-monsoon circulation is well established with the coalescence of the NAC and SAC to form a broad, meridionally elongated gyre extending from 61N to 171N
(Fig. 5d). This is seen as the ACG in the dynamic topography (Fig. 3a). Subsequently, the WBBC becomes an intense northward flow between 81 and 181N along the western edge of the ACG. Both CE1 and CE2 also intensify to higher negative magnitudes (20 to 30 cm) at their centers as seen in the dynamic topography (Fig. 3a). At 17.51N, the WBBC turns eastward and continues as a strong eastward jet at the northern edge of the ACG.
4. Discussion We have described the Northern spring intermonsoon circulation in the bay as evident from the dynamic topography and Geosat SSH maps. Both show identical circulation features such as the ACG, WBBC and cyclonic eddies. In order to study the dynamics of the WBBC and the gyre, we
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Fig. 5 (continued).
have computed the vertical component of windstress curl (curl t) based on the monthly mean climatological wind data at 11 11 grids (Hastenrath and Lamb, 1979). Weak northeasterly winds are present all over the bay during February. By March, the wind pattern along the western bay reverses, replacing the northeasterlies with south/ southwesterlies, forming a well-defined anticyclonic wind field by April. The curl t computed from this wind field shows negative values in the bay north of 8–101N during February–March, with intensification during April (Fig. 6), suggesting that the local curl t is favourable for driving an ACG in the interior. The Sverdrup transport estimate based on curl t suggested a southward transport of 5 Sv in the interior. When we close the gyre along the western parts, a poleward boundary current is obtained, which would be similar to the WBBC obtained from the dynamic topography and Geosat SSHs. One important point to be noted is that the WBBC flowed northward
against the northeasterly winds during February and the ACG is seen emerging as two weak anticyclonic cells. As the wind field intensified in April, the curl t attained a negative maximum (2.62 107 Nm3) and the two anticyclonic cells merged together to form the ACG. This final picture compares well with the dynamic topography and GEOSAT SSH of repeat cycle 10. These evidences clearly indicate that the wind-stress curl during the spring inter-monsoon is favourable for driving the ACG in the interior and that the WBBC sets up about three months ahead of wind reversal in the western bay. Further an analysis of the annual variation of curl t indicates that the negative curl t is conducive to drive the ACG only during the spring inter-monsoon period. The present estimate of the mean geostrophic volume transport (12 Sv) of the WBBC in the upper 200 m is within the range of earlier estimates from the hydrographic data (Fig. 3b). Shetye et al. (1993) reported 10 Sv transport during
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Fig. 6. Climatological monthly mean wind stress curl (107 Nm3) during February, March and April (Hastenrath and Lamb, 1979).
March–April 1991, and Sanilkumar et al. (1997) reported 16 Sv in the upper 500 m during May 1987. What could be the other factors that would contribute significantly to enhance the transport? Model studies suggested the presence of Rossby and Kelvin waves and their probable role in influencing the circulation in the Bay of Bengal (Potemra et al., 1991; Yu et al., 1991 and Mc Creary et al., 1996). Eigenheer and Quadfasel (2000) showed that in the bay, between 121 and 181N, the Sverdrup mode accounts for more than 60% of the observed variance in the SSH. This variability is mainly determined by the Rossby waves, trapped at the western boundary, related to Sverdrup circulation. It remains to be ascertained whether any part of the transport could be related to remote forcing from equatorial region by propagating Kelvin waves along the rim of the eastern bay. The TOPEX-Poseidon altimeter SSH anomaly for the period 1993–1997 suggests the presence of a high SSH along 121N between 821 and 841E and low SSH at 181N:871E during April (Prasanna Kumar et al., 2000); the high SSH coincides with the SAG and the low SSH with CE2 observed in our study. Further, it is interesting to
note that the results presented here have a close resemblance to the inferences of Yu et al. (1991), which are based only on remote forcing, in spite of differences in the location and intensity of the mesoscale features. This also emphasizes the hypothesis that remote forcing play’ a prominent role in driving the circulation in the bay. The effects of remote forcing and local wind forcing are to be substantiated using TOPEX/Poseidon altimeter data together with CTD data.
Acknowledgements The authors express their gratitude to Dr. Ehrlich Desa, Director, NIO, Dr. D. Panakala Rao and Dr. S. Prasanna Kumar for their keen interest in the Bay of Bengal studies. They are thankful to Prof. James G. Richman, Oregon State University, USA, for useful discussions and also for providing the Geosat altimeter residual height data for the Indian Ocean region. They acknowledge their gratitude to the reviewers for their constructive comments.
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References Anonymous, 1981. Back ground papers and supporting data on the practical salinity scale. UNESCO Technical Paper in Marine Science. Vol. 37, UNESCO, Paris, 144pp. Babu, M.T., Prasanna Kumar, S., Rao, D.P., 1991. A subsurface cyclonic eddy in the Bay of Bengal. Journal of Marine Research 49, 403–410. Chelton, D.B., Schlax, M.G., Witter, D.L., Richman, J.G., 1990. Geosat Altimeter observations of the surface circulation of the Southern Ocean. Journal of Geophysical Research 95, 17877–17903. Cutler, A.N., Swallow, J.C., 1984. Surface currents of the Indian Ocean (to 251S, 101E). Report No. 87. Institute of Oceanographic Sciences. Broadchill, UK. Defant, A., 1961. Water bodies and stationary current conditions at boundary surfaces. Physical Oceanography, Vol. 1. Pergamon. New York, pp. 451–475. Duing, W., 1970. The monsoon regime of currents in the Indian Ocean. University Press of Hawaii, Honolulu, USA, 68pp. Eigenheer, A., Quadfasel, D., 2000. Seasonal variability of the Bay of Bengal circulation inferred from TOPEX/Poseidon altimetry. Journal of Geophysical Research 105 (C2), 3243–3252. Hastenrath, S., Lamb, P.J., 1979. Climatic atlas of the Indian Ocean. Part I: Surface Climate and Atmospheric Circulation Charts 97. University of Wisconsin Press, Wisconsin, USA. Hastenrath, S., Greischar, L.L., 1989. Climatic Atlas of the Indian Ocean. Part III: Upper-Ocean Structure, Charts 247. University of Wisconsin Press, Wisconsin, USA. La Fond, E.C., 1951. Processing of Oceanpgraphic data. US Navy, Hydrographic Office, Washington, DC, USA. La Fond, E.C., La Fond, K.G., 1968. Studies of oceanic circulation in the Bay of Bengal. Vol. 1, Bulletin of National Institute of Science, India, pp. 169–183. Legeckis, R., 1987. Satellite observations of a western boundary current in the Bay of Bengal. Journal of Geophysical Research 92, 12974–12978. Mc Creary, J.P., Han, W., Shankar, D., Shetye, S.R., 1996. Dynamics of the East India Coastal Current, 2, Numerical solutions. Journal Geophysical Research 101 (C6), 13993–14010.
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Molinari, R.L., Oslon, D., Reverdin, G., 1990. Surface current distributions in the tropical Indian Ocean derived from compilations of surface buoy trajectories. Journal of Geophysical Research 95 (C5), 7217–7238. Murty, V.S.N., Sarma, Y.V.B., Rao, D.P., Murty, C.S., 1992. Water characteristics mixing and circulation in the Bay of Bengal during the southwest monsoon. Journal of Marine Research 50, 207–228. Nowlin Jr., W.D., McLellan, H.J., 1967. A characterisation of the Gulf of Mexico waters in winter. Journal of Marine Research 25, 29–59. Potemra, J.T., Luther, M.E., O’Brien, J.J., 1991. The seasonal circulation of the upper ocean in the Bay of Bengal. Journal of Geophysical Research 96 (C7), 12667–12683. Prasanna Kumar, S., Unnikrishnan, A.S., Muraleedharan, P.M., 2000. Signatures of Kelvin and Rossby wave propagation in the Northern Indian Ocean from TOPEX/ Poseidon altimeter. Proceedings of PORSEC, 2000, Vol. 1, 276–279. Sanilkumar, K.V., Kuruvilla, T.V., Jogendranath, D., Rao, R.R., 1997. Observations of the western boundary current of the Bay of Bengal from a hydrographic survey during March, 1993. Deep-Sea Research I 44, 135–145. Satyanarayana, D., Sahu, S.D., Panigrahy, P.K., Sarma, V.V., Suguna, C., 1991. Subsurface ammonium maxima in Northern Bay of Bengal. Marine Environmental Research 31, 123–136. Shetye, S.R., Gouveia, A.D., Shenoi, S.S.C., Sundar, D., Michael, G.S., Nampoothiri, G., 1993. The western boundary current of the seasonal subtropical gyre in the Bay of Bengal. Journal of Geophysical Research 98, 945–954. Suryanarayana, A., Murty, V.S.N., Rao, D.P., 1993. Hydrography and circulation of the Bay of Bengal during early winter 1983. Deep-Sea Research I 40, 205–217. Yu, L.L., O’Brien, J.J., Yang, .J., 1991. On the remote forcing of circulation in the Bay of Bengal. Journal of Geophysical Research. 96, 20449–20454. Wyrtki, K., 1971. Oceanographic Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, DC, USA.
On the circulation in the Bay of Bengal during Northern spring inter-monsoon (March–April 1987) M.T. Babu*, Y.V.B. Sarma, V.S.N. Murty, P. Vethamony National Institute of Oceanography, Dona Paula, Goa 403 004, India Received 3 March 1999; received in revised form 15 February 2001; accepted 15 February 2001
Abstract Temperature and salinity data collected from the Bay of Bengal 22 March–28 April 1987 (Northern spring intermonsoon) identified the seasonal anticyclonic gyre (ACG) at 161N, 861E characterized by warm (>241C) and lowsalinity (o34.2 PSU) water at 125 m depth. The western edge of the ACG was demarcated by a narrow, intense and meandering northward flow, termed the Western Bay of Bengal Current (WBBC) of the spring inter-monsoon period. Two cyclonic eddies (CE1 and CE2) were observed to the left of the WBBC; CE1 at 141N:82.51E and CE2 at 18.51N: 87.51E. These eddies were characterised by low temperature (B16.51C for CE1 and 141C for CE2) and relatively highsalinity water (>34.85 PSU) at their cores. The velocity of WBBC increased from 0.40 ms1 at 121N to 0.70 ms1 at 17.51N where it left the coast and turned eastward. The mean northward transport of the WBBC in the upper 200 m was 12 Sv (1 Sv=106 m3s1). These circulation features (the ACG, WBBC and cyclonic eddies) are well depicted in the maps of sea-surface height (SSH) topography derived from the residual SSHs of GEOSAT altimeter for the period 12 February–28 April 1987. The results show that the WBBC flowed against the northeasterly winds during February and that the current sets up almost three months ahead of the wind reversal. The ACG and in turn the WBBC intensified with the wind reversal in April when the wind stress curl attained a negative maximum. The SSH maps further indicate that formation of the ACG started in February and reached its maximum intensity during April with the coalescence of two anticyclonic cells. r 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction It is well known that seasonally reversing monsoon winds affect the Bay of Bengal circulation significantly (Cutler and Swallow, 1984; Hastenrath and Greischar, 1989). The semi-enclosed nature of the bay and its proximity to the equator together with immense quantity of fresh water influx from Ganges and Brahmaputra rivers *Corresponding author. Fax: +91-832-223340. E-mail address: [email protected] (M.T. Babu).
contribute to the formation of a highly complex system of circulation in the bay. Despite all these complexities, very few oceanographic studies have been conducted in the bay and the available data are inadequate to describe the circulation and its variability. Hence, extensive oceanographic surveys were conducted in the Bay of Bengal during 1983–1991 as a part of the project, ‘‘Seasonal, annual and inter-annual variability of the North Indian Ocean’’. Utilizing the hydrographic data collected during post-southwest monsoon (October–November, 1983), southwest monsoon
0967-0645/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0645(02)00609-4
856
M.T. Babu et al. / Deep-Sea Research II 50 (2003) 855–865
(June–August, 1984), and spring inter-monsoon (March–April, 1987 and 1991), the circulation characteristics were studied by Babu et al. (1991), Murty et al. (1992) Suryanarayana et al. (1993) and Shetye et al. (1993). The results have revealed the seasonality of the circulation, primarily driven by wind forcing. An anticyclonic circulation evolves during the spring inter-monsoon, which collapses with the intensification of the southwest monsoon in May. Subsequently, a large cyclonic gyre occupies the central, bay south of 151N during the southwest monsoon (June–September), continues until October–November, and drifts westward to the Indian coast during winter monsoon. The present investigation focuses on the circulation in the bay during Northern spring inter-monsoon (March–April) its evolution within the period using Geosat sea-surface height (SSH) topography. Several studies previously have reported that the circulation in the bay during spring inter-monsoon consists of a large anticyclonic gyre (ACG). Defant (1961) analysed the older ship drift climatology and concluded that an ACG filled the entire basin during spring. Also, the hydrographic data collected during the International Indian Ocean Expedition (IIOE) indicated an anticyclonic circulation with a northeastward flow in the western bay during March–April (La Fond and La Fond 1968; Duing, 1970; Wyrtki, 1971). The temperature field obtained from satellitebased advanced very high resolution radiometer (AVHRR) showed a meandering northward current in the western bay during spring intermonsoon period (Legeckis, 1987), indicating the possibility of an anticyclonic circulation in the interior bay during the same period. Drifting buoy trajectories indicated a large anticyclonic circulation confined between 101N and 201N (Molinari et al., 1990) during the same period. Shetye et al. (1993) analysed the Bay of Bengal circulation, computing the dynamic topography based on climatological temperature-salinity for distinct periods such as November–January, February– April, May–July, and August–October, and found that the anticyclonic circulation could be located only during February–April. It therefore would be worthwhile to look into the circulation in the bay
during spring inter-monsoon in order to analyse the generation and the evolution of the anticyclonic circulation in relation to wind variability.
2. Data and methodology Two hydrographic cruises were conducted in the region 121N–201 300 N in the Bay of Bengal onboard. The R.V. Gaveshani during the spring inter-monsoon (22 March–28 April, 1987). The temperature and salinity data from the upper 1000 m collected at 84 stations using Nansen sampling bottles have been utilised for this study. Considering the nature of mesoscale processes (Rossby radius of deformation B100 km), hydrographic stations were occupied 30–40 km apart in the western bay and 11 apart in the open sea (Fig. 1). In order to eliminate the uncertainties in the in situ temperature measurements, the Nansen bottles were fitted with two protected and one unprotected reversing thermometers (Ghola Precision, Germany; of temperature accuracy, 70.011C). The water samples were analysed onboard using Autosal (Model 8400 A, Guildline, Canada) and the conductivity ratios were converted into salinity (accuracy70.003 PSU) using the UNESCO Practical Salinity Scale (Anonymous, 1981). Temperature readings of the protected and unprotected thermometers were corrected using the calibration constants of each thermometer following the standard procedure (La Fond, 1951). The uncertainties in the temperature and salinity values were translated to an error of 0.017 dyn m in the surface dynamic height. This in turn led to a maximum error of 71.6 Sv (1 Sv=106 m3s1) at 201N and 276 Sv at 121N in the volume transports. The positioning of the vessel was determined using a MX1107 Satellite navigation system within 0.05 nm (approximately 92 m), corrected every 10 min. Therefore, the error in positioning the vessel was within 50–100 m, which had negligible effect on the volume transport estimates. We have considered 1000 db as reference level to compute the geostrophic currents and the associated volume transport between stations pairs along the hydrographic sections. Section E is not
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Fig. 1. The study area with bottom topography and hydrographic stations. Depth contours are in metres.
considered for velocity computations as all stations are shallower than 200 m. For stations shallower than the reference level, the geostrophic currents are computed following Nowlin and Mc Lellan (1967). In this method, the dynamic height values of the adjacent deeper station (>1000 db) are appended below the depth of observation of the coastal station.
In addition to the hydrographic data we have also made use of SSH measured by GEOSAT altimeter during the repeat cycles 7–10 (17.05 days periodicity) between 12 February and 28 April, 1987. The processed GEOSAT altimeter residual SSH data were provided by Oregon State University, USA (Chelton et al., 1990). The collinear method was used to obtain the sea-surface
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Fig. 2. Distribution of: (a) surface temperature, (b) surface salinity, (c) temperature at 50 m, (d) salinity at 50 m, (e) temperature at 125 m, (f) salinity at 125 m, (g) temperature across 161N, and (h) salinity across 161N.
topography as explicit knowledge of geoid is not required in this method. The collinear algorithm involves the following steps: (i) preparation of corrected SSHs, (ii) removal of spikes, if any, from the corrected SSHs, (iii) interpolation of data at equal latitude spacing (0.11), (iv) removal of bias and tilt, (v) generation of a mean sea-surface from
all the repeat cycles, and (vi) subtract the mean profile from each pass to create sea-surface topography. The Geosat altimeter had a range measurement accuracy of 5 cm. The SSH topography was used to study the evolution of the circulation features prior to and during the period of observation. The Geosat repeat cycles 9 and 10
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Fig. 2 (continued).
(25 March–28 April, 1987) coincided well with the period of hydrographic observations (22 March– 28 April 1987) in this study.
3. Results 3.1. Thermohaline fields In the Bay of Bengal, sea-surface temperature (SST) reaches a minimum (26.01C) during winter
in January and a maximum (30.51C) during summer in May. The SST observed during the study period varied between 281C and 30.01C, with zonally aligned isotherms having higher temperatures in the central southern bay (Fig. 2a). The surface salinity exhibited meridionally aligned isohalines in the eastern parts, with relatively high saline waters off the western bay (Fig. 2b). In the Northern bay comparatively cold (o28.01C) and less saline (32.6 PSU) waters were present. At 50 m, the 27.01C isotherm and 33.8 PSU isohaline
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separated the cold, saline waters of the western bay from the warm less saline waters of the central bay (Figs. 2c and d). The thermohaline distribution was more distinct at 125 m depth, with warm (>241C) and relatively less saline (34.2 PSU) waters in the central and eastern regions (Figs. 2e and f). In contrast, two regions of cold and high saline waters were seen to the west and north: one located at 141N, 82.51E, the other at 18.51N, 87.51E. Figs. 2g and h represent the vertical distribution of temperature and salinity across a typical section along 161N. In the thermocline, the isotherms in the range 261–131C, with a maximum descent at 861E. Shoaling of the thermocline towards the coast was noticed west of 841E and east of 901E. The bowl shape of the thermocline is distinct in the upper 300 m, while it is limited to 150 m depth in the halocline. 3.2. Surface circulation from hydrographic data The dynamic topography at the surface with reference to 1000 db (Fig. 3a) shows significant gradient in the dynamic height represented by major features of the spring inter-monsoon circu-
lation. The most conspicuous feature is a large area of high dynamic topography (>2.0 dyn m) in the central bay manifested by the ACG centred at 161N, 861E. The regions of minimum dynamic height 1.55 and 1.45 dyn m represent the cores of two cyclonic eddies, the first one, CE1, in the south at 141N:82.51E; and the second, CE2, in the north 181N:881E. The 1.85 dyn m contour demarcates the region of northward flow in the western bay and the periphery of the ACG in the east. The northward flow in the west, termed as the Western Bay of Bengal Current (WBBC), is conspicuous with large horizontal gradients. The dynamic topography also shows that WBBC meanders at 141N, reaching the shelf north of 161N. Further, it continues northward as an intense coastal current up to 17.51N, where it leaves the coast to form a strong eastward jet between CE2 and ACG. Vertical structures of geostrophic currents across sections H–J are shown in Figs. 4a–c. The WBBC is evident with strong shear in the western part of the sections, extending up to 250 m depth. Across section J, the northward flow is noticeable, with two cores with a maximum current speed of 0.40 ms1 at 821E in the WBBC (Fig. 4a). Across section I, the WBBC meanders eastward to
Fig. 3. (a) Dynamic topography at sea surface w.r.t. 1000 db, (b) Geostrophic volume transport (Sv) between station pairs in the upper layer (0–200 m).
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Fig. 4. Vertical structure of geostrophic currents (ms1) across (a) section J, (b) section I, (c) section H, (d) section A and (e) section B. (positive values denote northward currents).
83–861E and the core velocity increases to 0.55 ms1 (Fig. 5b). The WBBC has a core velocity of 0.40 ms1 between 811 and 841E across section H (Fig. 5c). The contour of zero velocity at 861E demarcates the center of the ACG where maximum dynamic height is obtained. The return flow of the ACG could be seen as a broad southward flow (0.05–0.10 ms1) to the east of the zero contour. The northward current could be seen in the offshore with a speed of 0.30 ms1 across section A and with 0.70 ms1 across section B, 100 km away from the coast (Figs. 4d and e). The temperature and salinity distributions along sections B and C confirm the close proximity of the WBBC to the coast. Relatively strong vertical shear is observed in the WBBC near the coast along sections B (Fig. 4e) due to upward sloping of
temperature and salinity isopleths. Satyanarayana et al. (1991) also reported upward sloping of isopleths of chemical parameters such as dissolved oxygen, phosphate and ammonia along this section. An important observation that emerges from this analysis is the eastward deflection of the WBBC at two locations where cyclonic eddies CE1 and CE2 are encountered to its left, and subsequently the velocity field intensifies when the current is confined between the eddy and ACG. 3.3. Surface circulation from geosat altimeter For a better understanding of the time evolution of the circulation, Geosat Altimeter sea-surface topography (repeat cycles 7–10) was analysed. In order to cover the southern bay, the Geosat SSH
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Fig. 5. The sea-surface topography (cm) obtained from Geosat Altimeter residual heights for: (a) repeat cycle 7, (b) repeat cycle 8, (c) repeat cycle 9 and (d) repeat cycle 10. (17.05 day periodicity from 12 February–28 April, 1987).
values from south of 121N also was utilized. The Geosat SSH topography of repeat cycle 7 (the second half of February, 1987) revealed that the circulation is characterized by two anticyclonic circulation cells (positive SSH)—one in the north (NAC) centered at 161N, 861E, and the other in the south (SAC) at 101N, 851E (Fig. 5a)—with a meander in between. The core of NAC exhibits higher residual height at its center (30 cm) than the SAC. During repeat cycle 8, the meandering flow at 141N is seen as a cyclonic eddy (CE1 in Fig. 3a), while the anticyclonic cells retain in the interior (Fig. 5b). During repeat cycle 9, the size of NAC decreases, with the development of a cyclonic eddy in the northeastern bay (CE2 in Fig. 3a); and the SAC intensifies and moves westward (Fig. 5c). During repeat cycle 10, the spring inter-monsoon circulation is well established with the coalescence of the NAC and SAC to form a broad, meridionally elongated gyre extending from 61N to 171N
(Fig. 5d). This is seen as the ACG in the dynamic topography (Fig. 3a). Subsequently, the WBBC becomes an intense northward flow between 81 and 181N along the western edge of the ACG. Both CE1 and CE2 also intensify to higher negative magnitudes (20 to 30 cm) at their centers as seen in the dynamic topography (Fig. 3a). At 17.51N, the WBBC turns eastward and continues as a strong eastward jet at the northern edge of the ACG.
4. Discussion We have described the Northern spring intermonsoon circulation in the bay as evident from the dynamic topography and Geosat SSH maps. Both show identical circulation features such as the ACG, WBBC and cyclonic eddies. In order to study the dynamics of the WBBC and the gyre, we
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Fig. 5 (continued).
have computed the vertical component of windstress curl (curl t) based on the monthly mean climatological wind data at 11 11 grids (Hastenrath and Lamb, 1979). Weak northeasterly winds are present all over the bay during February. By March, the wind pattern along the western bay reverses, replacing the northeasterlies with south/ southwesterlies, forming a well-defined anticyclonic wind field by April. The curl t computed from this wind field shows negative values in the bay north of 8–101N during February–March, with intensification during April (Fig. 6), suggesting that the local curl t is favourable for driving an ACG in the interior. The Sverdrup transport estimate based on curl t suggested a southward transport of 5 Sv in the interior. When we close the gyre along the western parts, a poleward boundary current is obtained, which would be similar to the WBBC obtained from the dynamic topography and Geosat SSHs. One important point to be noted is that the WBBC flowed northward
against the northeasterly winds during February and the ACG is seen emerging as two weak anticyclonic cells. As the wind field intensified in April, the curl t attained a negative maximum (2.62 107 Nm3) and the two anticyclonic cells merged together to form the ACG. This final picture compares well with the dynamic topography and GEOSAT SSH of repeat cycle 10. These evidences clearly indicate that the wind-stress curl during the spring inter-monsoon is favourable for driving the ACG in the interior and that the WBBC sets up about three months ahead of wind reversal in the western bay. Further an analysis of the annual variation of curl t indicates that the negative curl t is conducive to drive the ACG only during the spring inter-monsoon period. The present estimate of the mean geostrophic volume transport (12 Sv) of the WBBC in the upper 200 m is within the range of earlier estimates from the hydrographic data (Fig. 3b). Shetye et al. (1993) reported 10 Sv transport during
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Fig. 6. Climatological monthly mean wind stress curl (107 Nm3) during February, March and April (Hastenrath and Lamb, 1979).
March–April 1991, and Sanilkumar et al. (1997) reported 16 Sv in the upper 500 m during May 1987. What could be the other factors that would contribute significantly to enhance the transport? Model studies suggested the presence of Rossby and Kelvin waves and their probable role in influencing the circulation in the Bay of Bengal (Potemra et al., 1991; Yu et al., 1991 and Mc Creary et al., 1996). Eigenheer and Quadfasel (2000) showed that in the bay, between 121 and 181N, the Sverdrup mode accounts for more than 60% of the observed variance in the SSH. This variability is mainly determined by the Rossby waves, trapped at the western boundary, related to Sverdrup circulation. It remains to be ascertained whether any part of the transport could be related to remote forcing from equatorial region by propagating Kelvin waves along the rim of the eastern bay. The TOPEX-Poseidon altimeter SSH anomaly for the period 1993–1997 suggests the presence of a high SSH along 121N between 821 and 841E and low SSH at 181N:871E during April (Prasanna Kumar et al., 2000); the high SSH coincides with the SAG and the low SSH with CE2 observed in our study. Further, it is interesting to
note that the results presented here have a close resemblance to the inferences of Yu et al. (1991), which are based only on remote forcing, in spite of differences in the location and intensity of the mesoscale features. This also emphasizes the hypothesis that remote forcing play’ a prominent role in driving the circulation in the bay. The effects of remote forcing and local wind forcing are to be substantiated using TOPEX/Poseidon altimeter data together with CTD data.
Acknowledgements The authors express their gratitude to Dr. Ehrlich Desa, Director, NIO, Dr. D. Panakala Rao and Dr. S. Prasanna Kumar for their keen interest in the Bay of Bengal studies. They are thankful to Prof. James G. Richman, Oregon State University, USA, for useful discussions and also for providing the Geosat altimeter residual height data for the Indian Ocean region. They acknowledge their gratitude to the reviewers for their constructive comments.
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