High saline waters in Bay of Bengal

Bay of Bengal is well known for less saline waters in the surface layer of northern Indian Ocean. High saline waters of the Bay are considered as an influx from the Arabian Sea within a depth range of 200 to 900 m. Some of the recent observations in the western Bay of Bengal have shown salinity values higher than those reported earlier (35-2 × 10⁻³). Such values are explained on the basis of regional climatology suggesting their local formation on the shallow continental shelf during pre-monsoon months and their subsequent distribution along the coast.     

孟加拉湾晚第四纪的碳酸盐溶解旋回

孟加拉湾由于陆源物质大量输入产生稀释效应,使碳酸盐含量表现为冰期时减小、间冰期时增大的“大西洋型”旋回。通过对该区四支活塞岩芯的有孔虫溶解指数、浮游有孔虫沉积通量、CaCO₃ 沉积通量和 >16 0 μm粗组分沉积通量的分析发现,研究区碳酸盐溶解作用强烈,表现出冰期减弱而间冰期增强的总趋势,且溶解作用滞后于浮游有孔虫壳的氧同位素旋回,尤以氧同位素 4、5期的高溶解度情况与印度洋和太平洋氧同位素 5期晚期至 4期为CaCO₃溶解高峰一致。因此,孟加拉湾地区的CaCO₃ 旋回是大西洋型稀释作用与太平洋型溶解作用两者叠加的产物.     

An experimental study of radiative fluxes in the south Bay of Bengal during BOBMEX 1999

Time series measurements of radiative fluxes were made onboard INS Sagardhwani (SD) in the south Bay of Bengal near DS3 (13‡N and 87‡E) during the BOBMEX field experiment. An inter-comparison experiment conducted at DS3 showed that the radiative fluxes measured by Kipp and Zonen, Albedo meter and net Pyrgeometer onboard SD and by Eppley radiometers onboard ORV Sagar Kanya (SK) are well matched. It may be mentioned that the measurements showed consistency and good agreement between SD and SK ships, even though no Gimbal mounting was used for radiation instruments onboard SD. The main aim of the experiment was collection of high quality radiation data during the monsoon period, which can give an insight into the nature of the ocean-atmosphere coupling. The data on the four radiative fluxes collected on SD are averaged at 5 minute intervals and then hourly and daily averages have been computed. The hourly shortwave albedo and the atmospheric transmission factor are also computed and the variation of albedo in relation to the solar altitude and the transmissivity factor (TF) are studied. The mean albedo over the south Bay of Bengal under clear, partly cloudy and overcast skies are found to be 0.05, 0.07 and 0.2 respectively.     

Controls of dimethyl sulphide in the Bay of Bengal during BOBMEX-Pilot cruise 1998

The air-sea exchange is one of the main mechanisms maintaining the abundances of trace gases in the atmosphere. Some of these, such as carbon dioxide and dimethyl sulphide (DMS), will have a bearing on the atmospheric heat budget. While the former facilitates the trapping of radiation (greenhouse effect) the latter works in the opposite direction through reflectance of radiation back into space by sulphate aerosols that form from oxidation of DMS in atmosphere. Here we report on the first measurements made on DMS in the Bay of Bengal and the factors regulating its abundance in seawater. Phytoplankton alone does not seem to control the extent of DMS concentrations. We find that changes in salinity could effectively regulate the extent of DMSP production by marine phytoplankton. In addition, we provide the first ever evidence to the occurrence of DMS precursor, DMSP, in marine aerosols collected in the boundary layer. This suggests that the marine aerosol transport of DMSP will supplement DMS gaseous evasion in maintaining the atmospheric non-sea salt sulphur budget.     

A study on the structure of the convective atmosphere over the Bay of Bengal during BOBMEX-99

Convective activity is one of the major processes in the atmosphere influencing the local and large-scale weather in the tropics. The latent heat released by the cumulus cloud is known to drive monsoon circulation, which on the other hand supplies the moisture that maintains the cumulus clouds. An investigation is carried out on the convective structure of the atmosphere during active and suppressed periods of convection using data sets obtained from the Bay of Bengal and Monsoon Experiment (BOBMEX). The cumulus convection though being a small-scale phenomenon, still influences its embedding environment by interaction through various scales. This study shows the variation in the kinematic and convective parameters during the transition from suppressed to active periods of convection. Convergence in the lower levels and strong upward vertical velocity, significant during active convection are associated with the formation of monsoon depressions. The apparent heat source due to latent heat release and the vertical transport of the eddy heat by cumulus convection, and the apparent moisture sink due to net condensation and vertical divergence of the eddy transport of moisture, are estimated through residuals of the thermodynamic equation and examined in relation to monsoon activity during BOBMEX.     

Distribution of particulate organic carbon in the central Bay of Bengal

Particulate organic carbon (POC) was measured for 77 water samples collected over a 3000 m water column along 88° E in the central Bay of Bengal. The POC values varied from 80 to 895 μg per litre at the surface and 171 to 261 μg per litre at 2000 m. The POC decreased with increasing water depth at all the stations. Deep water concentrations of POC were higher than those reported from other oceanic waters. Distribution of POC was not influenced by water masses. The POC was not significantly correlated with chlorophylla.     

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.     

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.     

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.     

On the impossibility of the co-existence of monsoons and severe storm surges in the Bay of Bengal

The problem of storm surges is introduced briefly. The utility of passive microwave observations to study this phenomenon is pointed out. The reasons for the nonoccurrence of severe surges in the Bay of Bengal, during monsoon regimes, is discussed in this paper. It was demonstrated that the predominant reason for lack of severe surges over the Bay of Bengal coast is due to the absence of weak wind shear during monsoon seasons.Atmospheric Environment Service, Ice Center, Environment Canada, 373 Sussex Drive, Ottawa, Ontario, Canada K1A OH3.     

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.     

May 2003 Disaster in Sri Lanka and Cyclone 01-B in the Bay of Bengal

Heavy rainfall deluged South-Western Sri Lanka between the 11th and19th of May 2003 and led to its worst disaster in 50 years. Floods andlandslides claimed 260 lives. The World Meteorological Organizationcited it as evidence for the increase of anomalous climatic extremes inrecent years. Here, a meteorological analysis is presented of this disasteras part of a sustained effort to undertake meteorological applications fordisaster management. There were intense low-level westerlies over SriLanka related to cyclone 01-B that made its way across the Bay of Bengalat least 700 km away. The southeastwardly traverse of the cyclone wasstalled for a few days by anomalous north-westerly geostrophic windsver South Asia. Here, it is argued that orographic rainfall induced bythis stalled cyclone and seasonal inter-tropical convergence zone cloudbands over Sri Lanka led to the deluge. The trajectory of the cyclone wasremote from Sri Lanka and this led to no cyclone hazard warnings beingissued. No cyclones have made landfall in Sri Lanka in May in the last100 years. This study shows that one must exercise vigilance not only inthe path of the cyclone but also remotely due to the modulation of thecyclone by other atmospheric phenomenon and topographic features.This flood may have been predicted with contemporary local area weatherprediction models and this example points to the need for the developmentof local area weather prediction models as part of disaster warning systems.This study also demonstrates the use of meteorological diagnostics forpost-event analysis of hydrometeorology of disaster events.     

Abundance and Ecology of Endemic Asterorotalia trispinosa from the Western Bay of Bengal: Implications for its Application as a Paleomonsoon Proxy

A total of 97 surface sediment samples were collected from the continental shelf and slope of the western Bay of Bengal to assess the potential application of Asterorotalia trispinosa as a paleomonsoon proxy. The relative abundance of living as well as total (living and dead) A. trispinosa ranges from being absent to a maximum of ~31%. The highest abundance of A. trispinosa is close to the outfall region of the Ganga-Brahmaputra Rivers and decreases away from the river mouths. Contrary to that, A. trispinosa is rare in front of both the Cauvery and Pennar river outfall regions, in the central part of the western Bay of Bengal. The living A. trispinosa abundance is the lowest in between two major river systems, viz. Ganga-Brahmaputra-Mahanadi and Krishna-Godavari. The relative abundance of both the total and living A. trispinosa is strongly positively correlated with ambient seawater temperature, and negatively correlated with %Corg and salinity. Based on the spatial distribution, we conclude that A. trispinosa is stenohaline in nature, rather than euryhaline, and further that the increased relative abundance of A. trispinosa indicates warmer and only marginally hyposaline environment. Even though the ecological preference of A. trispinosa suggests it as a potential paleomonsoon proxy, the restricted distribution implies limited application.     

孟加拉湾深海记录中的等深流活动特征及其环境意义初探

实地观测和对沉积记录的研究表明，孟加拉湾存在大的等深流活动。通过采自孟加拉深海东部和中部的3支活塞岩心的学积学研究表明，那些氧化色彩强烈，钼质生物壳体溶蚀严重，不存在粒序层理的粉砂质薄层实际上是等沉积作用的产物，源自两极的等深流是全球温盐循环系统的关键组分，它们的过去150ka的历史中曾经多次得到强化，不仅对研究区的环境变化产生重要影响，而且可能作为纽带，加强了低纬地区与极区在气候意义上的遥相关，等深流的强化与冰川旋顺之间似乎不存在密切关联，氧同位素第3，4，5期内均可发现明显的强化记录，关于强化的控制机制有待深入探讨。     

孟加拉湾若开褶皱带晚新生代构造特征初步研究

孟加拉湾若开褶皱带位于印度-缅甸山脉西部山前,由NNW—SSE向带状分布的多排背斜构成,其构造特征研究仍然十分薄弱。本文通过钻井资料和二维地震反射剖面精细构造解析,尝试分析若开褶皱带晚新生代构造特征,重点关注若开褶皱带的滑脱层发育特征及背斜几何学及运动学特征,结果表明若开褶皱带发育多个滑脱层:①底部滑脱层,位于约6.5s(双程走时)处;②中部滑脱层,层位存在变化,可能位于第四系底部或上中新统下方约2.5s处。在区域挤压作用下,若开褶皱带发育与底部滑脱层和中部滑脱层相关的滑脱褶皱,构造变形主要受控于底部滑脱层,而中部滑脱层影响了局部构造变形。生长地层记录显示若开褶皱带构造变形自东往西迁移,变形前缘形成于第四纪。基于构造分析结果提出了若开褶皱带褶皱变形的两种运动学端元模型:模型1中不发育中部滑脱层,滑脱褶皱发育于底部滑脱层之上;模型2中发育中部滑脱层,滑脱褶皱发育于中部滑脱层和底部滑脱层之上,形成上、下两套构造层。若开褶皱带背斜几何学和运动学特征受下伏滑脱层控制,背斜在走向上叠置、分叉可能暗示着背斜下伏滑脱层在走向上发生了改变。流体超压可能是影响若开褶皱带构造变形的重要控制因素。     

Seasonal Variability of Cyclone Heat Potential in the Bay of Bengal

Monthly maps of cyclone heat potential (CHP) in the Bay of Bengalhave been prepared by using Levitus climatological data set. Seasonal variability ofCHP in the Bay of Bengal has been studied using the CTD data sets collected duringfive cruises during the period, 1993–1996. High value (>30 kcal/cm²) of CHP coincided with anticyclonic gyre (ACG) and the low value of CHP (16 kcal/cm²) coincided with thecyclonic gyre (CG). This emphasizes the importance of gyres in the distribution ofCHP, which play an important role in the intensification of cyclones/depressions.CHP is >14 kcal/cm² over Andaman Sea, southern and Central Bay of Bengal where the generation and movement of cyclones take place during post south west monsoon season (October–November). A depression formed on 07.11.95 at 11°N; 91°E and intensified into a cyclonic storm by 8th November evening and crossed Orissa Coast on 9th November 1995. A few days before its formation, the value of CHP at the origin of thiscyclone was about 20 kcal/cm². To understand the exact role of CHP in theformation and intensification of cyclones/depressions over Bay of Bengal, more intense and systematic data sets are essential.     

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     

孟加拉湾若开海域晚新生代构造变形及其对油气的控制作用

利用钻井、二维和三维地震资料,剖析了孟加拉湾若开海域晚新生代的构造变形特征,探讨了构造变形对油气的控制作用。区域深度地质剖面揭示,研究区南部仅发育底部滑脱层(深度10 km),而北部则发育底部滑脱层(深度12 km)和中部滑脱层(深度4 km);受滑脱层的控制,研究区南部仅发育一套构造层,而北部则发育变形不协调的上、下两套构造层;南部背斜的南北向延伸距离、波长及背斜间隔距离均明显大于北部。通过北部局部构造精细解析表明,研究区北部变形相对较复杂,上构造层主要发育近南北向的背斜和次级张扭性右旋走滑断层,二者形成时间分别为晚第四纪和晚第四纪末。若开海域晚新生代的构造变形对圈闭形成、油气运聚和保存条件具有重要的控制作用。指出研究区南部平缓褶皱带构造—岩性复合圈闭具备形成大油气田的条件,是下一步油气勘探的重要目标。     

Geochemistry of core sediments from the southeastern Bay of Bengal:Inferences on weathering and early diagenetic changes

A sediment core(ABP24/05),collected at a water depth of 3520 m from the southeastern Bay of Bengal was studied to determine the change in chemical weathering during the last glacial to deglacial periods and the factors of sedimentary environment which controlled earliest diagenetic changes in the sediment after its deposition.High ratios of K/Rb,Ti/Al and Zr/Rb during~45 to~18 cal kyr B.P.in the core sediments may be attributed to the stronger physical erosion and turbidity currents activity during this period.This might have brought a higher quantity of unaltered minerals to the study area.Low ratios of K/Rb,Zr/Rb,and Ti/Al and increase of SiO₂/TiO₂,Rb/Al and Cs/Al from~18 cal kyr B.P.to present may be indicating an increase in the rate of chemical weathering during this period.The time of increased chemical weathering in the study area is consistent with deglaciation warming in the tropical Indian Ocean and strengthening of river runoff into the Andaman Sea.Climate change during the interglacial period by increased solar insolation thereby strengthened the summer monsoon which might have led to intensified chemical weathering in the source region since~18 cal kyr B.P.The low organic carbon(OC),high Mn/Al,Fe/Al and the Mn-oxides minerals precipitation indicate prevailing of oxic conditions during~11 cal kyr B.P.in the core sediments,which is contradictory to suboxic conditions developed in the deep ocean sediments in the western Bay of Bengal and the equatorial Indian Ocean.The low terrigenous influx and export of less OC to the bottom sediments might have created a favorable condition for the formation of Mn-oxides in the study area during Holocene.