Mean monthly wind driven climatological circulation model of the Bay of Bengal

A special feature of the Bay of Bengal circulation is its seasonal variation in response to the monsoonal winds. In the case of the Bay of Bengal, observationally very little is known about the large scale circulation. Theoretically, the problem of driving the circulation in the Bay of Bengal is more complex than that in other basins because of the presence of large quantities of fresh water discharge from Ganga-Brahmaputra-Meghna river systems, and also because the atmospheric driving forces even within a season are highly variable with frequent occurrences of tropical disturbances. Exploring the nature of the circulation in the Bay of Bengal is a problem of great importance in itself as well as for the critical role this region plays in the genesis of tropical disturbances which are the main source of large scale rainfall over the northern part of the Indian subcontinent. The surface circulation of the Bay of Bengal may, therefore, help in understanding the variation of rainfall over time scales ranging from the subseasonal to the interannual. Keeping this in view, an attempt was made towards the development of an oceanic climatological circulation model for the Bay of Bengal, which explains the seasonal variability of the currents. The model is fully non-linear and vertically integrated, with realistic basin geometry. The treatment of coastal boundaries involves a procedure leading to a realistic curvilinear representation of the western and eastern sides of the Bay of Bengal. This coastal representation has the advantage of taking into account the finer resolution in the shallow regions of the northern Bay. The model is forced by the monthly mean wind stress derived from 30 years (1950–79) of Comprehensive Oceanographic Atmospheric Data Sets (COADS). Special emphasis is given to the southern open boundary condition for the model. For this purpose, sensitivity experiments have been performed with six open boundary conditions and a comparative study of the results has been made. These sensitivity tests for the open boundary condition will help the development of a suitable coupled ocean-atmosphere model for this region. The model-generated main features are in general agreement with the known climatological circulation of the Bay of Bengal.     

Evolution of salinity field in the upper layers of the east central Arabian Sea and northern Bay of Bengal during summer monsoon experiments

The intra-seasonal variability observed in the salinity field of the upper layers at a few locations in the east central Arabian Sea and the northern Bay of Bengal during the summer monsoon seasons of 1977 and 1979 is documented with the aid of short time series (1–2 weeks) of salinity measurements made from USSR and Indian ships deployed during MONSOON-77 (1977) and MONEX-79 (1979) field experiments. In the Arabian Sea a typical subsurface maxima observed beneath the mixed layer base either disappeared or considerably weakened due to strong vertical mixing caused by the monsoonal forcing. In the northern Bay of Bengal the salinity variability in the top 30 m water column was rapid and appeared to be influenced by large amounts of fresh water from rain and probably from the major adjoining rivers. Some simple diagnostic calculations are presented to assess the relative importance of various processes which control the observed salinity variability.     

Hydrography and circulation in the northwestern Bay of Bengal during the retreat of southwest monsoon

The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling. The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10⁻³. The surface circulation was weak and led to a net transport of 2.0 × 10⁶m³.s⁻¹ directed towards northeast.     

Numerical Modelling of Storm Surge in the Head Bay of Bengal Using Location Specific Model

The head Bay of Bengal region, which covers part of Orissa and west Bengal in India as well as Bangladesh, is one of the most vulnerable regions of extreme sea levels associated with severe tropical cyclones which cause extensive damage. There has been extensive loss of life and property due to extreme events in this region. Shallow nature of the Bay, presence of Ganga-Brahmaputra-Meghna deltaic system and high tidal range are responsible for storm surges in this region. In view of this a location specific fine resolution numerical modelis developed for the simulation of storm surges. To represent mostof the islands and rivers in this region a 3km grid resolution is adopted. Several numerical experiments are carried out to compute the storm surges using the wind stress forcings representative of 1974, 1985, 1988, 1989, 1991, 1994 and 1999 cyclones, which crossed this region. The model computed surges are in good agreement with the available observations/estimates.     

Amino sugars in suspended particulate matter from the Bay of Bengal during the summer monsoon of 2001

Amino sugars (AS) are important constituents of organic matter. However, very little is known about their cycling in marine waters. In this research, we assessed the distribution and cycling of these compounds in waters of the Bay of Bengal. For this purpose, samples of suspended particu late matter (SPM) were collected from 8 depths (surface to 1000 m) at 6 locations during the 166th cruise of the ORV Sagar Kanya in the Bay of Bengal in July/August 2001. The SPM samples were analysed for particulate organic carbon (POC), particulate nitrogen (PN) and AS concentrations and composition. The AS varied between 0.4 and 17.5 nmol/l. Concentrations were high in the surface waters and generally decreased with increasing depth. AS concentration decreased from the south to north. AS accounted for 0.01 to 0.71% and 0.05 to 2.37% of POC and PN, respectively. Rapid decrease in AS-C% and AS-N% with depth indicates that these compounds were preferentially degraded relative to bulk POC and PN. The composition of AS suggests that glucosamine (GLU-N) and galactosamine (GAL-N) were present in the surface SPM samples, and their abundance decreased from surface downwards. Relatively, low values of GLU-N/GAL-N ratio indicate that the organic matter was mostly derived from the detritus of micro-organisms. Our data suggest that chitin, a polymer of the glucosamine produced by many marine organisms was not the major source of AS in the Bay. Rapid cycling of these compounds indicates their importance in the cycling of nitrogen in marine waters     

Role of biology in the air-sea carbon flux in the Bay of Bengal and Arabian Sea

A physical-biological-chemical model (PBCM) is used for investigating the seasonal cycle of air-sea carbon flux and for assessing the effect of the biological processes on seasonal time scale in the Arabian Sea (AS) and Bay of Bengal (BoB), where the surface waters are subjected to contrasting physical conditions. The formulation of PBCM is given in Swathi et al (2000), and evaluation of several ammonium-inhibited nitrate uptake models is given in Sharada et al (2005). The PBCM is here first evaluated against JGOFS data on surface pCO₂ in AS, Bay of Bengal Process Studies (BoBPS) data on column integrated primary productivity in BoB, and WOCE Il data on dissolved inorganic carbon (DIC) and alkalinity (ALK) in the upper 500 meters at 9°N in AS and at 10°N in BoB in September–October. There is good qualitative agreement with local quantitative discrepancies. The net effect of biological processes on air-sea carbon flux on seasonal time scale is determined with an auxiliary computational experiment, called the abiotic run, in which the biological processes are turned off. The difference between the biotic run and abiotic run is interpreted as the net effect of biological processes on the seasonal variability of chemical variables. The net biological effect on air-sea carbon flux is found to be highest in southwest monsoon season in the northwest AS, where strong upwelling drives intense new production. The biological effect is larger in AS than in BoB, as seasonal upwelling and mixing are strong in AS, especially in the northeast, while coastal upwelling and mixing are weak in BoB.     

Fluxes of material in the Arabian Sea and Bay of Bengal — Sediment trap studies

In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986 to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production and transport, however, differ in both the regions. In the Arabian Sea, average annual fluxes are over 50gm^-2y^-1 in the western Arabian Sea and less than 27gm^-2 y^-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central Bay of Bengal (over 50gm^-2y^-1) and are least in the southern part of the Bay (37gm^-2y^-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns in the surface waters as a result of increased freshwater and nutrient input from rivers. Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents.     

On the observed synoptic variability in the thermal structure of the upper northern Bay of Bengal during MONEX-79

The short-term variability observed in the near surface meteorological parameters and in the vertical thermal structure of the upper layers of the northern Bay of Bengal during a weak monsoonal regime is examined with the aid of time series measurements. The variability of the mixed layer depth is interpreted in terms of forced mixing caused by the surface wind stress and free mixing by buoyancy flux, Ekman pumping controlled by the curl of the surface wind stress, convergence associated with a clockwise gyral circulation and stratification caused by freshwater discharges from rivers. The daily-averaged current vectors in the upper layers indicate the presence of clockwise gyral circulation in the polygon area.     

Variations of Surface Air Temperature Over the Land Areas in and Around the Bay of Bengal

The inter-annual variation and linear trends of the surface air temperature in the regions in and around the Bay of Bengal have been studied using the time series data of monthly and annual mean temperature for 20–40 years period within 1951–1990. The study area extends from Pusma Camp of Nepal in the north and Kuala Lumpur of Malaysia in the south and between 80--100 ° E. The annual variation of temperature has also been studied using the mean monthly temperature for the variable time frames 1961–1975, 1976–1990 and 1961–1990. The trend of temperature has been analyzed using linear regression technique with the data from 1961–1990, which showed that the warming trend is dominant over the study areas except for a few stations. It has been found that Nepal shows predominant warming trends. Bangladesh and the adjacent areas of India and the northern part of Bay of Bengal adjacent to the Bangladesh coast have shown strong warming trends of the annual temperature with maximum at Dhaka (0.037 °C/year). The near equatorial zone, i.e., southern India, Sri Lanka and part of Thailand and Malaysia (Kuala Lumpur) shows warming trends in the annual mean temperature with strong warming at Pamban and Anuradhapura (around 0.04 °C/year). The cooling trends have been observed at a few stations including Port Blair, Yangoon and Cuttack. Further analysis shows the presence of prominent ENSO scale of variations with time period 4–7 years and 2–3 years for almost all the stations. The decadal mode with T >7 years is present in some data series. The results of the variations of temperature with respect to the Southern Oscillation Index (SOI) show that SOI has some negative correlation with temperature for most of the stations except those in the extreme northeast. It has been found that positive anomaly of temperature has been observed for El Niño events and negative anomaly for the La Nina events.