Bay Of Bengal Waters Research Articles

Spatio-temporal variation of mesozooplankton in the northern coastal waters of Bay of Bengal

Mesozooplankton (MSP) are significant primary and secondary consumers in aquatic ecosystems that influence biogeochemical cycles and are considered important bioindicators of ecosystem functioning. Data on the spatio-temporal variability of the MSP population from the estuarine and coastal waters of northwestern Bay of Bengal (BoB) is scarce. The present study is the first detailed investigation on the spatio-temporal dynamics of MSP along the estuarine and coastal waters of northwestern BoB from two contrasting seasons over a period of three years based on univariate and multivariate statistical analyses. The crucial drivers for spatio-temporal variability in the MSP community were water temperature, salinity, dissolved oxygen (DO) concentration, nutrient (nitrate, phosphate, silicate) concentrations, and Chlorophyll (Chl) a concentration. The highest mean MSP abundance was obtained during summer monsoon (SM). The results of this study state that there is a significant seasonal difference in the hydrographical parameters that change the community dynamics of MSP inhabiting the estuarine and coastal waters of BoB. The MSP species assemblages during winter monsoon (WM) were governed by high salinity and DO levels while those during SM were associated with high water temperature, nutrients, and Chl a concentration. A clear seasonal shift from the gelatinous community in WM to the crustacean community in SM was documented. A total of 172 MSP species were identified during WM, and 200 MSP species during SM, with 37 species being exclusively detected during WM and 65 species being exclusively recorded during SM. The most abundant species during WM were Chrysaora caliparea, Cyanea nozakii, Sagitta bipunctata, Pleurobrachia pileus, and Acromitus flagellatus to name a few and those during SM consisted of Undinula vulgaris, Acartia (Odontacartia) spinicauda, Oithona simplex, Oithona similis, and Oncaea venusta. Copepoda was the most dominant taxa during all sampling seasons, while ichthyoplankton and meroplankton fauna were low during WM and comparatively higher during SM, indicating the top-down control of their population by gelatinous taxa. Biodiversity indices reflected well to excellent diversity patterns of MSP. In the current scenario of global warming and climate change, our research highlights the need for comprehensive long-term monitoring of estuarine and coastal waters of western BoB, which harbour a rich marine faunal diversity, to identify the bioindicators of seasonal variability among major MSP groups.

Distribution and biogeochemical perspectives of nutrients in the Eastern Equatorial Indian Ocean

The seasonal reversal of ocean circulation associated with seasonal change in the direction of prevailing winds and the occurrence of several anomalous events in the Eastern Equatorial Indian Ocean (EEIO) make this region dynamic and complex in terms of its biogeochemical characteristics. Two multidisciplinary cruises were conducted to measure nutrients and associated physicochemical parameters across the water column (up to 1000 m) of the EEIO during boreal summer and winter monsoons to understand the distribution of nutrients and their spatio-temporal variability from a biogeochemical perspective. The seasonality in the thermohaline structure of the region is indistinct except for surface salinity drop during summer monsoon due to more precipitation on-site and in adjoining areas. Low concentrations of chlorophyll at the surface and in the deep chlorophyll maxima represent the oligotrophic nature of this region. Surface water was found nutrient-depleted (0.03–0.4 µM Nitrate, 0.02–0.13 µM Phosphate). The maxima of vertical profiles of nitrate and phosphate were recorded at a shallower depth (150–200 m) when compared to its maxima in usual oceanic conditions, but a silicate maximum was recorded in deeper water. In the surface and upper mixed layer paucity of nutrients resulted in low N:P and N:Si ratios. Therefore, nitrogen limitation is evident. The overall ratio of N:P yielded a mean value of 15.33 and matched with the representative literature value for the Indian Ocean. The minimum oxygen values (<50 µM) in the deep water (150–200 m) indicated a hypoxic condition. No signature of denitrification and a moderate nitrate deficit were observed in deep waters. The negative values of Nitrate anomaly (N-tracer) at 50–100 m depth were attributed to a Watermass influenced by denitrification. The prevailing oligotrophic condition caused limited synthesis of organic matter and subsequently little decomposition in deep water. The maxima in the apparent oxygen utilization (AOU) profile are confined to 150 to 200 m depth and represent the most active zone for regeneration that is limited to shallow depth. Regenerated nutrients reached maxima at shallower depth and primarily control material cycling in this region. Supply of nitrate to the surface water based on the preformed values of prevailing water mass was primarily by Bay of Bengal water. According to the findings of this study, preformed nitrate concentrations between 100 and 200 metres below the surface were found very low, indicating that Indonesian Through Flow (ITF) has little impact on the distribution of nutrients in this area.

Characteristics of a Seasonal Front in the Southern Bay of Bengal: Dynamics, Mixing, and Water Mass Transport

Abstract The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea. Significance Statement Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns.

Performance of SMAP and SMOS Salinity Products under Tropical Cyclones in the Bay of Bengal

To compare the accuracy of satellite salinity data of level-3 Soil Moisture Active Passive V4.0 (SSMAP) and debiased v5 CATDS level-3 Soil Moisture and Ocean Salinity (SSMOS) before and after tropical cyclones (TCs) in the Bay of Bengal (BoB), this study used the sea surface salinity of Argo (SArgo) to assess SSMAP and SSMOS before and after the passage of 10 TCs from 2015 to 2019. The results indicate that the SSMAP and SSMOS agreed well with SArgo before and after 10 TCs. It can be seen that the correlation between SSMAP and SArgo (before TCs: SSMAP = 0.95SArgo + 1.52, R2 = 0.83; after TCs: SSMAP = 0.87SArgo + 4.34, R2 = 0.79) was obviously higher than that of SSMOS and SArgo (before TCs: SSMOS = 0.68SArgo + 10.38, R2 = 0.62; after TCs: SSMOS = 0.88SArgo + 3.98, R2 = 0.58). The root mean square error (RMSE) was also significantly higher between SSMOS and SArgo (before TCs: 0.84 psu; after TCs: 0.78 psu) than between SSMAP and SArgo (before TCs: 0.58 psu; after TCs: 0.47 psu). In addition, this study compared SSMAP and SSMOS during two TCs that swept in nearshore and offshore waters, and the results show good agreement between SSMAP and SArgo in the nearshore and offshore waters of BoB. In the BoB, both SSMAP and SSMOS can retrieve sea surface salinity well, and SSMAP is overall better than SSMOS, but the SMOS salinity product can fill the gap of SMAP from 2010 to 2015.

The equatorial Indian Ocean upper water-column structure influenced by cross-basinal water exchange over the last ~40000 years

The central equatorial Indian Ocean (CEIO) is influenced by both, summer and winter monsoon and is known for cross basin exchange of water between the Arabian Sea (AS) and the Bay of Bengal (BoB). Here, we present planktic foraminiferal assemblage record from a sediment core (SSD-044/GC-01, 5°N; 77°E) from the CEIO, which is in the pathway of this cross-basin water exchange, in order to reconstruct changes in upper water column structure during the last 43 ka, at millennial year timescales. Our data displays an increase in ratio of mixed layer eutrophic to mixed layer oligotrophic foraminifer species from MIS (Marine Isotope Stage) 3 to MIS 2 which further decreased sharply during the last deglaciation and remained low thereafter. This trend matches well with the sedimentary Corg content which is interpreted as a productivity signal, in line with other studies from this region. Furthermore, the relative abundance of thermocline foraminifer species displayed highest values during MIS 3, a decrease through the MIS 2, a sharp decrease through the early deglaciation and an increase thereafter into the Holocene. Our records suggest that the increased productivity at this site seems to be fed by nutrient injection from nutricline to the photic zone which manifests in a corresponding increase in thermocline foraminifera species. The nutrient injection to the photic zone maybe a result of increased vertical/convective mixing due to increased flow of cooler/high-salinity BoB water probably caused by decreased freshening of the BoB during glacials. Furthermore, our data suggests a warm, stratified upper water column and low surface nutrient (oligotrophic) conditions during the last deglaciation and the Holocene.

Enhanced Double-Diffusive Salt Flux from the High-Salinity Core of Arabian Sea Origin Waters to the Bay of Bengal

AbstractThe inflow of high-saline water from the Arabian Sea (AS) into the Bay of Bengal (BoB) and its subsequent mixing with the relatively fresh BoB water is vital for the north Indian Ocean salt budget. During June–September, the Summer Monsoon Current carries high-salinity water from the AS to the BoB. A time series of microstructure and hydrographic data collected from 4 to 14 July 2016 in the southern BoB (8°N, 89°E) showed the presence of a subsurface (60–150 m) high-salinity core. The high-salinity core was composed of relatively warm and saline AS water overlying the relatively cold and fresh BoB water. The lower part of the high-salinity core showed double-diffusive salt fingering instability. Salt fingering staircases with varying thickness (up to 10 m) in the temperature and salinity profiles were also observed at the base of a high-salinity core at approximately 75–150-m depth. The average downward diapycnal salt flux out of the high-salinity core due to the effect of salt fingering was 2.8 × 10−7 kg m−2 s−1, approximately one order of magnitude higher than the flux if salt fingering was neglected.

Predicting the rapid intensification and dynamics of pre-monsoon extremely severe cyclonic storm ‘Fani’ (2019) over the Bay of Bengal in a 12-km global model

The present study investigates into genesis, intensification, dynamical behavior and the prediction of Extremely Severe Cyclonic Storm (ESCS) ‘Fani' over the Bay of Bengal (BoB) in the National Centre for Medium-Range Weather Forecasting (NCMRWF) global numerical weather prediction (NWP) modeling system ‘NCUM.’ The global model is adopted from the UK Met office's Unified Model (UM). The ESCS ‘Fani' formed from 26 April–04 May 2019, over the warm waters of BoB. In the criterion of landfalling tropical cyclones (TCs), which crossed the Orissa coast, it was the most intense cyclonic storm during pre-monsoon season since 1965. The TC exhibited a markedly different characteristic having genesis very near to the equator (near 2.7°N and 88.7°E) and possessing one of the longest tracks (~3030-km) over the BoB region. The NCUM global model was operationally run during the occurrence of the TC. The diagnosis of the TC's genesis and rapid intensification (RI) in the model is carried out using various metrics such as Genesis potential parameter (GPP), winds at 850-hPa, vertically integrated moisture flux, potential vorticity (PV) at isentropic level 315 K, Severe Weather Threat (SWEAT) index and daily averaged latent heat flux, etc. It is found that the early genesis, structure, RI, and movement of the TC were well captured by the model in advance. The Model predicted TC tracks for a total of seven initial conditions (from 27 April to 03 May 2019, at 24-h interval) are found closely co-related with the observed best track of the TC provided by India Meteorological Department (IMD). The lower values of Direct Position Errors (DPEs), along-track errors (ATEs), and cross-track errors (CTEs) indicate the skillful prediction of the TC by the model. The structure, amplitude, and location (SAL) simulated precipitation of the TC over the BoB and along the central-east coast of India are in agreement with the observed systems. Our detailed diagnostic analysis suggests that the TC formation, development, and intensification was mostly controlled by the warmer BoB sea surface temperatures.

Primary productivity connects hilsa fishery in the Bay of Bengal

Tropical hilsa shad (Tenualosa ilisha) contributes significantly to the society and economy of Bangladesh, India and Myanmar, but little is known about their habitats across the life cycle and their relationship with environmental drivers. This study describes spatial and temporal variability of productivity in the Bay of Bengal (BoB) relating to hilsa fishery. Decadal data on net primary productivity, nutrients (i.e. nitrate, phosphate and silicate) and zooplankton were collected from Aqua MODIS, world ocean database and COPEPOD respectively with spatial resolution 1°×1°. Moreover, monthly abundance of phytoplankton, hilsa catch and long-term catch dynamics were analyzed to determine the associations between variables. The present study was extended over 3.568 million km2 area, of which 0.131–0.213 million km2 area characterized as the most productive with net primary production of >2,000 mg C/m2/day, 0.373–0.861 million km2 area as moderately productive with 500–2,000 mg C/m2/day, and 2.517–3.040 million km2 area as the least productive with <500 mg C/m2/day which were consistent with field verification data. In case of nutrients, the Ganges-Brahmaputra-Meghna (GBM) delta was rich in nitrate and phosphate than that of the Ayeyarwady delta, while silicate concentration persisted high all over the northern BoB including the deltas. A peak abundance of phytoplankton was observed in GBM delta during the months of August-November, when ~80% of total hilsa are harvested in Bangladesh annually. Variations in seasonal productivity linked with nutrients and phytoplankton abundance are important factors for predicting hilsa habitat and their migration patterns in the deltaic regions and shelf waters of BoB. These results can be useful in forecasting potential responses of the hilsa in BoB ecosystem to changing global ocean productivity.

No nitrogen fixation in the Bay of Bengal?

Abstract. The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which – for some, yet unconstrained, reason – are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key in understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N2) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one-quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB's nitrogen input, suggesting a potential role of N2 fixation in those waters. Here, we present a survey of N2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N2 fixers comparable to other oxygen minimum zone (OMZ) regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N2 fixers present throughout the water column (samples collected between 10 and 560 m water depth). While similar communities of N2 fixers were shown to actively fix N2 in other OMZs, N2 fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ. Consistent with this, no N2 fixation signal was visible in δ15N signatures. We suggest that the absence of N2 fixation may be a consequence of a micronutrient limitation or of an O2 sensitivity of the OMZ diazotrophs in the BoB. Exploring how the onset of N2 fixation by cyanobacteria compared to non-phototrophic N2 fixers would impact on OMZ O2 concentrations, a simple model exercise was carried out. We observed that both photic-zone-based and OMZ-based N2 fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which can exhaust remaining O2 traces in the BoB.

A Model for the Vertical Chlorophyll-a Distribution in the Bay of Bengal Using Remote Sensing Data

Deep chlorophyll maxima (DCM) layer is the concentrated patch of phytoplankton, which occurs as a layer at a depth extending few meters vertically and spanning many kilometers horizontally. These highly productive regions of the ocean are signatures of critical physical, chemical, and biological processes in the aquatic environment. This study reports and characterizes a DCM layer observed in the Bay of Bengal during three different cruises conducted in 2015, 2016, and 2018. The relationships of the defining parameters for the DCM like the depth and maximum chlorophyll concentration are modeled with respect to the biogeochemical parameters and are validated for the region. A new approach using the density for the estimation of the vertical chlorophyll profile was formulated. The DCM characteristics and covariation with respect to the other parameters are, thus, formulated and the factors controlling the DCM in this region are characterized. The model was applied to satellite remote sensing data to derive the vertical profiles of chlorophyll layer in the Bay of Bengal waters. The results were discussed in the context of understanding of the underlying biological and physical processes as well as subsurface phytoplankton biomass and the carbon cycle in the region.

Influence of the coastal circulation and water-mass characteristics in structuring the zooplankton community of the eastern Arabian Sea

Understanding the forcing mechanisms that determine the coastal ecosystem dynamics has always been a challenging topic of research. The Arabian Sea (AS) coastal ecosystem in the western half of the northern Indian Ocean is influenced by the seasonally reversing atmospheric forcing and structured by distinct water masses along its depth. Although AS differs greatly from its eastern counter-half, Bay of Bengal (BoB) in the thermohaline characteristics, the inter-basin exchanges of the water masses between these ocean basins add complexity to its hydrography. Hence, the present study was attempted to understand the influence of the intrusion of the BoB water in structuring the zooplankton community of the coastal AS. In the mixed layer depth, a conspicuous spatial heterogeneity was noticed in the distribution and abundance of the zooplankton community contributed by the cumulative influence of the higher phytoplankton biomass and the advection of population through BoB water intrusion regulated by the coastal circulation in the southern part. The study also assessed the interaction of the zooplankton community with the specific water masses defined by distinct thermohaline characteristics along the vertical profile of the AS coastal waters. Three distinct water masses were identified within the 200 m depth of AS each categorized by discrete salinity and density features. The distinct groups of the dominant taxon, Copepoda identified through redundancy analysis demonstrate the species-specific preferences of the community to different water mass characteristics in the AS. This study addressing how hydrographic characteristics shape the macroecology of zooplankton in the coastal AS, will add new insight in understanding the dynamic ecosystems of the northern Indian Ocean.

Classification of algal bloom species from remote sensing data using an extreme gradient boosted decision tree model

ABSTRACTCoastal and open ocean regions throughout the world are now subject to an array of toxic, harmful, or more intense algal blooms with an increasing trend of incidence over large geographical areas due to anthropogenic factors such as pollution and climate shifts. To date, detection capabilities of causative species based on remote sensing data are greatly limited because of the difficulties in interpreting the composite reflectance signal from different water features and types. In the present study, an accurate and reliable method is developed to automatically detect the onset of blooms and correctly classify the bloom species in Arabian Sea and Bay of Bengal waters using remote sensing data. A data-driven approach using machine learning algorithm is devised based on reflectance spectral signatures and tested on several MODIS-Aqua (Moderate Resolution Imaging Spectroradiometer) data for classifying the dominant water categories, including clear ocean waters devoid of sediments and algal blooms, sediment-laden coastal waters, and three major algal blooms, Trichodesmium erythraeum, Noctiluca scintillans and Cochlodinium polykrikoides. An extreme gradient boosted decision tree (XGBoost) model is chosen to improve the prediction accuracy by prevention of overfitting, which increases the scalability of the model on several unseen test data. This model was trained using 1.5 million samples and resulted in a classification accuracy of over 98%. When the results were validated using forty thousand random samples from the known blooms, an overall accuracy more than 96.8% was achieved. The applicability of the trained XGBoost model was further verified using MODIS-Aqua images, and it showed promise for successful detection and identification of well-documented blooms. The use of spectral information to classify algal blooms makes this method more robust and easily adaptable to different ocean colour sensors with a scope to accommodate other major algal blooms.

Implications of stratification on the heterotrophic plankton community (micro- & meso zooplankton) in the South Eastern Arabian Sea

The response of the plankton community (micro- herein after MZP and mesozooplankton) to the seasonal intrusion of Bay of Bengal (BoB) water into the Arabian Sea from November (Initial Phase-IP) to January (Peak Phase-PP) and resultant stratification was tracked in the present study. During the winter monsoon (November – February) as part of seasonally reversing monsoonal circulation, low saline BoB waters intrude into the Arabian Sea which initiates stratification. The peak phase of salinity stratification in the Eastern Arabian Sea is in January during which a thick barrier layer develops in the region. The study showed that during the initial phase of the BoB water intrusion, fluctuation was observed in the MZP and mesozooplankton community with noticeable coast-oceanic variations while such variation was not found during the peak phase when the entire SEAS was influenced by the intruded low saline BoB waters. This variation in the interaction between MZP and mesozooplankton during the initial phase can be attributed to the seasonal transition which was more pronounced in the offshore during the initial phase. A statistically significant correlation was observed between barrier layer thickness (BLT), an indicator of stratification with plankton diversity. The insignificant coast-oceanic variations (homogeneity) in planktonic components during the peak phase can be attributed to higher abundance of grazers. Among the MZP community, the ciliates (CTS) formed the predominant group and its contribution varied between 50% & 59% during the initial and peak phase respectively with the highest dissimilarity observed along the Trivandrum transect. The study points to the fact that interactions between trophic levels during the seasonal transition phase when stratification sets in, is subtle which is evident as coast-oceanic variation, while in established stratification the interaction between plankton at different trophic level, gets stabilized due to balance between the Bottom-up and Top-down controls.

Submarine groundwater discharge derived strontium from the Bengal Basin traced in Bay of Bengal water samples

Evaluating the submarine groundwater discharge (SGD) derived strontium (Sr) flux from the Bengal Basin to the Bay of Bengal (BoB) and determining its isotopic composition is crucial for understanding the marine Sr isotopic evolution over time. Measurements of spatially and temporally distributed water samples collected from the BoB show radiogenic 87Sr/86Sr, high Sr, calcium (Ca) concentrations and high salinity in samples collected dominantly from 100–120 m depth, which can be explained only by the contribution of saline groundwater from the Bengal Basin. These results provide a direct evidence of the SGD-Sr flux to the BoB. This SGD-Sr flux is however, spatially heterogeneous and using conservative hydrological estimates of the SGD flux to the BoB, we suggest a SGD Sr flux of 13.5–40.5 × 105 mol/yr to the BoB. Mass balance calculations using Sr concentrations and 87Sr/86Sr suggest up to 7% contribution of SGD to the 100–120 m BoB water samples. The identification of SGD at 100–120 m depth also provides an explanation for the anomalous variations in barium (Ba) concentrations and the δ18O-salinity relationship in intermediate depths of the BoB.

Yttrium and rare earth element partitioning in seawaters from the Bay of Bengal

AbstractThe dissolved yttrium (Y) and rare earth element (REE) concentrations of seawater samples collected along a north‐south hydrological transect within the Bay of Bengal (BoB) have been analyzed to estimate contributions of the Ganges and Brahmaputra (G‐B) river inputs to the dissolved REE distribution of the Northern Indian Ocean. Surface water masses of the BoB are characterized by Y/Ho ratios (84) intermediate between the G‐B river suspended sediment (41) and water mass from the South Indian Ocean (93). Covariation of MREE (middle REE, Sm) and LREE (light REE, La) concentrations suggests that the dissolved REEs in surface waters (upper 100 m depth) of the BoB (Sm/La = 0.21) appear to derive mainly from the freshwater discharge of the G‐B river system. In contrast, values obtained in the intermediate and deep waters (Sm/La = 0.14) suggest a mixing of dissolved REEs deriving from the release of G‐B river suspended particles (Sm/La = 0.16) and the contribution of Antarctic Bottom Water (AABW) (Sm/La = 0.12). Consequently, we propose that MREE/MREE* ratios in the BoB waters could be an accurate proxy to trace lithogenic inputs from the G‐B river system. The dissolved and particle remineralization Nd fluxes from G‐B river system are calculated to constitute about 9% and 4% of the global dissolved river discharge and “boundary inputs” flux. Our estimation indicates that the massive G‐B river system inputs could greatly alter the dissolved REEs distribution in the BoB and contribute to the dissolved REEs budget in the ocean.

Evidence for the existence of Persian Gulf Water and Red Sea Water in the Bay of Bengal

The high-salinity water masses that originate in the North Indian Ocean are Arabian Sea High-Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW). Among them, only ASHSW has been shown to exist in the Bay of Bengal. We use CTD data from recent cruises to show that PGW and RSW also exist in the bay. The presence of RSW is marked by a deviation of the salinity vertical profile from a fitted curve at depths ranging from 500 to 1000 m; this deviation, though small (of the order of similar to 0.005 psu and therefore comparable to the CTD accuracy of 0.003 psu), is an order of magnitude larger than the similar to 0.0003 psu fluctuations associated with the background turbulence or instrument noise in this depth regime, allowing us to infer the existence of RSW throughout the bay. PGW is marked by the presence of a salinity maximum at 200-450 m; in the southwestern bay, PGW can be distinguished from the salinity maximum due to ASHSW because of the intervening Arabian Sea Salinity Minimum. This salinity minimum and the maximum associated with ASHSW disappear east and north of the south-central bay (85A degrees E, 8A degrees N) owing to mixing between the fresher surface waters that are native to the bay (Bay of Bengal Water or BBW) with the high-salinity ASHSW. Hence, ASHSW is not seen as a distinct water mass in the northern and eastern bay and the maximum salinity over most of the bay is associated with PGW. The surface water over most of the bay is therefore a mixture of ASHSW and the low-salinity BBW. As a corollary, we can also infer that the weak oxygen peak seen within the oxygen-minimum zone in the bay at a depth of 250-400 m is associated with PGW. The hydrographic data also show that these three high-salinity water masses are advected into the bay by the Summer Monsoon Current, which is seen to be a deep current extending to 1000 m. These deep currents extend into the northern bay as well, providing a mechanism for spreading ASHSW, PGW, and RSW throughout the bay.

Adrift Upon a Salinity-Stratified Sea: A View of Upper-Ocean Processes in the Bay of Bengal During the Southwest Monsoon

The structure and variability of upper-ocean properties in the Bay of Bengal (BoB) modulate air-sea interactions, which profoundly influence the pattern and intensity of monsoonal precipitation across the Indian subcontinent. In turn, the bay receives a massive amount of freshwater through river input at its boundaries and from heavy local rainfall, leading to a salinity-stratified surface ocean and shallow mixed layers. Small-scale oceanographic processes that drive variability in near-surface BoB waters complicate the tight coupling between ocean and atmosphere implicit in this seasonal feedback. Unraveling these ocean dynamics and their impact on air-sea interactions is critical to improving the forecasting of intraseasonal variability in the southwest monsoon. To that end, we deployed a wave-powered, rapidly profiling system capable of measuring the structure and variability of the upper 100 m of the BoB. The evolution of upper-ocean structure along the trajectory of the instrument's roughly two-week drift, along with direct estimates of vertical fluxes of salt and heat, permit assessment of the contributions of various phenomena to temporal and spatial variability in the surface mixed layer depth. Further, these observations suggest that the particular ``barrier-layer'' stratification found in the BoB may decrease the influence of the wind on mixing processes in the interior, thus isolating the upper ocean from the interior below, and tightening its coupling to the atmosphere above.

Development of a High-Resolution Multiscale Modeling and Prediction System for Bay of Bengal, Part I: Climatology-Based Simulations

A high-resolution (10 km × 10 km) multiscale ocean modeling system was developed for short-term (1 - 2 weeks) ocean state hindcasting/forecasting in the Bay of Bengal (BOB) region. This paper is Part I of a two-part series of studies. The Regional Ocean Modeling System (ROMS) was implemented and initialized with Levitus 1/4° climatological fields for short-term forecasting. The results from these climatology-based model simulations for three representative months (February, June and October) in three different seasons (winter, summer and autumn) are discussed herein. This high-resolution model implementation simulates most of the observed dominant circulation features. The multiscale features during February include an anticyclonic basin-scalegyre with a strong western boundary current (WBC) in the western basin, the formation of several shallow mesoscale eddies in the head of the Bay and a cyclonic sub-basin-scale Myanmar Gyre in the northeast. During June, no well-defined boundary current is simulated along the Indian coast; instead, alternating cyclonic and anticyclonic eddies appear along the east coast with cross-basin eastward flow to support a deep cyclonic Andaman Gyre. In October, a basin-scale cyclonic gyre with a continuous well-defined East India Coastal Current (EICC), weak inflow from the Malacca Strait to the Andaman Sea and advection of BOB water into the Arabian Sea via the Palk Strait are simulated well by the model. A number of mesoscale eddies appear on the eastern half of the basin during October. Physical pattern of simulated eddies and transports across selected sections are validated against available drifter climatology, ARGO data and previous observations. Application of this system to synoptic short-term predictions for October 2008 will be presented in Part II.

Remote Sensing Observations of Ocean Surface Chlorophyll and Temperature with the Impact of Cyclones and Depressions over the Bay of Bengal Water

ABSTRACTMODIS-Aqua derived eight-day composite chlorophyll concentration data analyzed to study the impact of cyclones and depressions on the Bay of Bengal productivity. A total of 15 cyclonic storms and depressions picked up from the India Meteorological Department datasets. MODIS-Aqua data analyzed during October 2002 to December 2009. There was observation of depressions (D), deep depressions (DD), cyclonic storms (CS), severe cyclonic storms (SCS), and very severe cyclonic storms (VSCS) with categories of intensities (“T” numbers 1.5–6) and wind speeds (25–108 knots). The chlorophyll concentration observed to be high (1.0–5.0 mg/m3) with the impact in the coastal and offshore waters. Quickscat scatterometer data showed high wind speed (about 10 meters/second). Sea surface temperature (SST) observed to be decreased (roughly 2°C) with effect of cyclones. The cyclone numbers, intensity, and chlorophyll concentration has been observed to be increasing from 2002 to 2009, with observation of VSCS “Sidr” during 2007. The study would be interesting to link carbon flux/sequestration, marine food chain, and harnessing fishery resources in a postcyclone period.

Evaluation of the influence of monsoon climatology on dispersal and sequestration of continental flux over the southeastern Arabian Sea

Associated with the coupled land–ocean heating–cooling, seasonal reversal of monsoon winds regulates hydrography, precipitation, upwelling, fluvial influx and a host of biogeochemical processes. We evaluate the role of the monsoon winds, fluvial discharge and currents on continental flux dynamics from advection magnitude of clays over an anomalous high precipitation regime of the southeastern Arabian Sea. Coupled with the intense rains and a high fluvial discharge, we archived an elevated content of detrital clays (>41mgl−1) during the southwest monsoon over the <40m water depth. The deeper waters (>40m), however, had much reduced detrital clays (<6mgl−1) year round. Therefore, two distinct environments (high and low detrital clays) prevailed over the shelf. Derived from the variations in the clays in the local riverine discharge and in the seawater loads, we found a dominant role of the southwest monsoon winds and that of the winter and the southwest monsoon (summer) coastal currents over the dispersal and sequestration processes. Over the inner shelf, an alongshore (equatorward) advection and merging of several, small, fluvial plumes by the southwest monsoon winds had sequestered most of the local fluvial discharge over the shallow region only. The outer-shelf received detritus mostly as a result of (a) the high salinity Arabian Sea Water during the southwest monsoon, (b) through the Bay of Bengal Water and (c) by way of aeolian supply from the Arabia and the Somalia.