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The distribution of fish larvae in estuaries is crucial for comprehending the functioning of these ecosystems and their role as nursery grounds for marine fish species. Data regarding larval fish assemblages in the coastal waters of Bangladesh are currently scarce. This study examines the spatial and seasonal dynamics of larval fish communities in these waters using a two-year sampling effort with a bongo net. This study investigates the diversity, abundance, and distribution of larval fishes at four sites (Bakkhali River Estuary, Moheshkhalipara, Naf River Estuary, and Rezu Khal Estuary) on the southeastern coast of Bangladesh. A total of 10,387 fish larvae representing 25 families were identified. Clupeidae (42.9%), Engraulidae (35.5%), Ambassidae (10.1%), Myctophidae (2.1%), and Gobiidae (1.9%) were the five most dominant families based on the larval catch composition. Marked seasonal differences in the larval fish abundance were observed. However, these differences were not observed when comparing the four sampled sites. Additionally, the correlation between environmental variables and the larval abundance indicated that salinity negatively influences (p < 0.001) the larval fish abundance. This is the first comprehensive investigation of larval fish assemblages in the coastal waters of Bangladesh. The findings of this study enhance our understanding of the nursery habitat requirements for the early development stages of both migratory and resident species in subtropical coastal waters, providing valuable information for potential applications in management and conservation efforts.

Microplastic pollution has a significant threat to marine ecosystems, yet its impact on spineless cuttlefish (Sepiella inermis) remains under-researched. This study aims to address this gap by analysing microplastic contamination in Sepiella inermis from the North-East Bay of Bengal. This species is widely consumed and transported globally as food, thus holding significant health concerns. A total of 40 adult female cuttlefish were collected from two sampling sites (18°36′31.35″N 87°48′10.63″E and 15°43′35.37″N 88°12′07.01″E) in the Bay of Bengal. Tissue samples from tentacles, gut, and nidamental glands were analysed for microplastic content, alongside sediment and surface water samples. Parameters such as microplastic abundance, size, shape, and colour were recorded. The average abundance of microplastic particles was measured at 2.003 particles per gram in tentacle tissue, 2.31 particles per gram in gut tissue, and 0.99 particles per gram in nidamental gland tissue. The gut tissue exhibited the highest abundance of microplastics per gram. Chemical characterization using FT-IR and confocal Raman spectroscopy identified 11 types of microplastic polymers. Of the 11 types of plastic polymers identified, PVC was the most prevalent, accounting for 17.64 % of the microplastics found across all tissues. PVC microplastics can cause significant harm to marine life and human health by accumulating in the food chain and releasing harmful chemicals like phthalates, which can lead to endocrine disruption. ABS, PET, PP, PE, and PA microplastic polymers are highly persistent in environment, leading to long-term pollution in oceans. When ingested by marine organisms, they can disrupt entire ecosystems. In humans, the accumulation of these microplastics can impair the immune system and contribute to chronic diseases. The Pollution Load Index (PLI) was calculated for each tissue type, revealing that gut tissue is more prone to microplastic pollution compared to the nidamental gland and tentacles. The average PLI per gram of gut tissue was 2.26, which was significantly higher than 1, indicating substantial pollution. This research highlights the urgent need for comprehensive strategies to mitigate microplastic pollution, given the potential health risks associated with the consumption of contaminated marine species.

Introduction: Assimilating all available observations in numerical models may lead to deterioration of the analysis. Ensemble Forecast Sensitivity to Observations (EFSO) is a method that helps to identify all such observations which benefit the analyses. EFSO has never been tested in an ocean data assimilation system because of a lack of robust formulation of a squared norm against which beneficiality of observations can be estimated. Methods: Here, we explore the efficacy of EFSO in the ocean data assimilation system that comprises the ocean model, Regional Ocean Modeling System (ROMS), coupled to the assimilation system Local Ensemble Transform Kalman Filter (LETKF), collectively called LETKF-ROMS, in the Bay of Bengal by envisaging a novel squared norm. The Bay of Bengal is known for its higher stratification and shallow mixed layer depth. In view of baroclinicity representing the stratification of the ocean, we use the modulus of the baroclinic vector as the squared norm to evaluate forecast errors in EFSO. Results: Using this approach, we identify beneficial observations. Assimilating only the beneficial observations greatly improves the ocean state. We also show that the improvements are more pronounced in the head of the Bay of Bengal where stratification is much higher compared to the rest of the basin. Discussion: Though this approach doesn't degrade the ocean state in other regions of the Indian Ocean, a universal squared norm is needed that can be extended beyond the Bay of Bengal basin. [ABSTRACT FROM AUTHOR]

The distribution of fish larvae in estuaries is crucial for comprehending the functioning of these ecosystems and their role as nursery grounds for marine fish species. Data regarding larval fish assemblages in the coastal waters of Bangladesh are currently scarce. This study examines the spatial and seasonal dynamics of larval fish communities in these waters using a two-year sampling effort with a bongo net. This study investigates the diversity, abundance, and distribution of larval fishes at four sites (Bakkhali River Estuary, Moheshkhalipara, Naf River Estuary, and Rezu Khal Estuary) on the southeastern coast of Bangladesh. A total of 10,387 fish larvae representing 25 families were identified. Clupeidae (42.9%), Engraulidae (35.5%), Ambassidae (10.1%), Myctophidae (2.1%), and Gobiidae (1.9%) were the five most dominant families based on the larval catch composition. Marked seasonal differences in the larval fish abundance were observed. However, these differences were not observed when comparing the four sampled sites. Additionally, the correlation between environmental variables and the larval abundance indicated that salinity negatively influences (p < 0.001) the larval fish abundance. This is the first comprehensive investigation of larval fish assemblages in the coastal waters of Bangladesh. The findings of this study enhance our understanding of the nursery habitat requirements for the early development stages of both migratory and resident species in subtropical coastal waters, providing valuable information for potential applications in management and conservation efforts. [ABSTRACT FROM AUTHOR]

Paradigms and conceptualisations are essential tools used in international relations literature. Such conceptualisations, however, continue to be heavily dependent on Western epistemology and strategically oriented. Such theoretical models and projections fail to adequately conceptualise the Indo-Pacific region. The neglect of local connectivity and subregions within the Indo-Pacific world has resulted in a relative understudy of the Bay of Bengal zone, a crucial subregion within the Indo-Pacific. The essay argues for a more holistic approach by amalgamating present strategic concerns with indigenous versions of regional projections developed historically. Only by grasping the significance and relevance of past build-ups, it would be possible to understand the Bay of Bengal zone and its effectiveness in creating a vibrant and dynamic Indo-Pacific region. [ABSTRACT FROM AUTHOR]

Continuous understanding of the ongoing ocean acidification (OA) is essential for predicting the future impact of OA on marine ecosystems. Here we report the results of open ocean time-series measurements (19 cruises) of seawater pH in total hydrogen ion scale (pHT) and associated parameters in the Arabian Sea (AS) and the Bay of Bengal (BoB). During southwest monsoon (SWM), the pHT within the 30 to 100 m water column shows the maximum difference between the two basins with BoB pHT being lower (up to ~0.39 units) than AS which could be due to freshwater influx from rivers, mixed layer dynamics, and cold-core eddies. However, during Spring inter-monsoon (SIM), the pHT of BoB follows the trend of AS. A contrasting finding is that the lowest pHT occurs at 350 to 500 m in the BoB while it is ~1000 m in the AS. The pHT within the 150 to 1500 m layer of these two basins shows lower values by 0.03 (±0.02) in the BoB as compared to the AS. The possible reasons for the low pHT within the BoB oxygen minimum zone (OMZ) could be due to intrusion of western Pacific water in the BoB, freshwater influx from rivers, variations in OMZ of the two basins, higher temperature (~2°C) within the OMZ of the AS, and denitrification in the AS. The pHT in both the basins (500 to 1000 m) is lower than in the North Atlantic and higher than in the North Pacific waters; however, the pHT in the 200 to 500 m is lower in the BoB than in all these basins. This study highlights the under-saturation of calcium carbonate at very shallow depths (~ 100 m) in the BoB, indicating that the plankton in the BoB are facing a major risk from OA compared to the AS and need further investigation. [ABSTRACT FROM AUTHOR]

We construct freshwater balance in the Bay of Bengal (BoB) within a control volume (CV) bounded by 1,018 kg/m3 isopycnal surface using observations and ocean reanalysis during 2011–2015. Freshwater in CV is maximum in October–November due to monsoonal rain and river inflows, and minimum in April–May. Water lighter than 1,018 kg/m3 is not transported out of BoB, implying that freshwater lost from CV is mixed away entirely within the basin. From freshwater budget, we infer moderate diapycnal mixing rates (∼0.8 × 10−5 m2/s) in boreal spring and summer; in winter (December–January), the rate of freshwater loss to subsurface ocean is 0.015 m/day, corresponding to a median turbulent diffusivity of 4.2 × 10−5 m2/s, with standard error of 25%. We show that enhanced winter mixing across the shallow pycnocline is due to reduced shortwave radiation and subseasonal episodes of surface buoyancy loss when cool, dry northeast monsoon winds blow over BoB. Plain Language Summary: The freshwater from monsoonal rivers and rain discharged into the Bay of Bengal (BoB) forms a shallow (<10 m deep) salinity‐stratified layer which has implications for the regional air‐sea interaction. There is very limited understanding of the mixing of the freshwater owing to the absence of near‐surface turbulence measurements in BoB. It is important to understand the disappearance of the shallow fresh layer in the Bay due to it's linkages to the regional hydrological cycle and the near‐surface stratification. In this study, we infer the seasonality of basin‐scale mixing beneath the surface layer of BoB from a freshwater volume balance using a combination of observations and ocean analysis. We find that the highest rates of freshwater mixing (mean and median values of about 5.5 × 10−5 m2/s and 4.2 × 10−5 m2/s) occur during winter (December–January), mainly driven by enhanced surface buoyancy loss thereby reducing the freshwater content at a mean rate of 0.015 m/day. This study has implications for improvement of ocean and climate models, which generally have too‐high mixing rates and poor representation of the salinity‐stratified near‐surface layer in BoB. Key Points: Low‐salinity water from monsoon rain and rivers covers nearly 80% of the northern Bay of Bengal's surface in October–November, and nearly vanishes by April–MayThe freshest (salinity <30 pss) and lightest (density <1,018 kg/m3) water is not transported across the southern boundary, indicating modification or storage within the bayFreshwater balance shows that most of the rain and river water is mixed across the 1,018 kg/m3 isopycnal surface in winter during episodes of enhanced surface buoyancy loss [ABSTRACT FROM AUTHOR]

Fukuyoa species are potential producers of polyether neurotoxins that are toxic to aquatic biota and humans. A strain of Fukuyoa (CSIRCSMCRI 004) was isolated from India's Bay of Bengal coast. The clonal culture was identified using light and scanning electron microscopy and molecular phylogenetic analysis of the LSU rRNA gene and ITS (ITS1-5.8S gene-ITS2) sequences. Its toxicity was assessed using a 48 h Artemia bioassay. The morphological features observed included globular shape, descending cingular displacement, broad pentagonal 1′ plate, and large second apical plate (2′). The identity of the strain was confirmed to be F. paulensis based on the LSU rRNA gene and ITS sequences phylogenies. Strain CSIRCSMCRI 004 shared an identical sequence with the type strain of F. paulensis and other strains reported from different locations, based on the LSU rRNA gene and ITS sequences phylogenies. Exposure of brine shrimp larvae to its cellular lysate resulted in lethality (48 h LC50 = 0.256 mg ml−1) and behavioral abnormalities. The current study provides the first evidence of F. paulensis in the Bay of Bengal. It also reveals that CSIRCSMCRI 004 is toxic to susceptible biota like the brine shrimp and could produce ciguatera-associated toxins. The findings of this study highlight the need for sampling and screening of ciguatera-related species in the Bay of Bengal region to understand the associated public and environmental health risks. [ABSTRACT FROM AUTHOR]

Saltmarsh land-cover (SLC) ecosystems, composed of unvegetated mudflats, saltmarshes, mangroves, and/or seagrass communities, are vulnerable to climate-induced impacts, such as sea level rise. Extracting a seamless and consistent waterline from satellite imagery is a major challenge because of environmental factors, such as turbidity, water depth and multiple types of underwater vegetation cover that introduce noise in the extraction of information. Hence, a water index, derived from multi-temporal Landsat 8 (OLI) data, acquired under different tides is proposed for mapping land-water across SLC wetlands by tracking waterlines. This provided inundation maps and defined eco-zones to specify south-eastern Bangladesh wetland composition. The NDWI_1 (McFeeters’s water index) applied to 42 OLI images and derived land-water difference maps generated inundation gradient maps with an overall classification accuracy of 87.8%. The simple intersection and union of region-of-interests extracted from the tide heights above the mean low-water springs enabled the mapping of four categories of wetland composition based on hydroperiods: a) irregularly inundated (II), regularly inundated (RI), irregularly exposed (IE; high floodplain), and subtidal (river bed and deep water sea). For all of the three study sites, mangrove, seagrass, non-mangrove and agriculture were all prominent on the IE eco-zone, while only saltmarsh was dominant on the II eco-zone. These maps of SLC wetland will enrich previous concepts of eco-zonation models that include salinity, erosion, accretion and rate of sea level rise as factors, suggesting that inundation extent and tidal phase complexities should be considered in the remote sensing of SLC composition for improved models of SLC vegetation response to climate change. [ABSTRACT FROM AUTHOR]

This study examines the seasonal and intraseasonal modulation of near-inertial wind power associated with fluctuations in unidirectional wind speed in the Bay of Bengal (BoB). For that purpose, we use concurrent measurements of high-resolution in situ near-surface current and wind speed from six moorings in the BoB. It is found that the annual mean of near-inertial wind power in the BoB shows roughly similar magnitude (0.25–0.35 mW m−2) at all the mooring locations. However, in response to the seasonal evolution of monsoonal wind forcing, near-inertial wind power shows significant annual variability, with a maximum during summer (~ 0.4–0.5 mW m−2) and fall (~ 0.3–0.4 mW m−2) and a minimum during winter (~ 0.1 mW m−2) and spring (~ 0.2 mW m−2). In addition, it is also found that modulation of near-inertial wind power due to summer monsoon intraseasonal oscillation (MISO), such as its magnitude, reaches as large as ~ 1 mW m−2 at the mooring in the northern BoB during phases 3–4 of MISO. Using a high vertical resolution of current profile data, the near-inertial kinetic energy (NIKE) budget in the mixed layer in the northern BoB shows good temporal correspondence with the magnitude of the rate of change of NIKE and near-inertial wind power, with a maximum magnitude of the rate of change of NIKE lags the wind power by 24 hr. The NIKE budget also indicates that a significant portion of near-inertial wind power dissipates in the mixed layer and rarely energises the depth regime underneath the mixed layer. [ABSTRACT FROM AUTHOR]

The fundamental ocean properties are vital in predicting the earth's climate. Such predictions are essential for human life in various terrestrial applications. Therefore, to forecast the parametric variations in the Bay of Bengal with less prediction error, a Heuristic Neural Network (HNN) is proposed by optimizing the solutions of Long Short-Term Memory (LSTM) using swarm intelligence by Repeated Iterative Technique (RIT). The existing algorithms have proved better accuracy only during the sea surface prediction of temperature. The proposed HNN model predicts dominant parameters of the Bay of Bengal for its horizontal and vertical variations from 10 to 2000 m depth. The efficiency of the proposed HNN model is verified by comparing the results with recent prediction algorithms in fusion with LSTM, from which the outcome reveals that the proposed HNN model obtains 98.9% for temperature prediction, 99% for pressure, 98.8% for salinity, and 98.1% for density prediction. The results prove overall good accuracy of forecast compared to the existing prediction techniques. [ABSTRACT FROM AUTHOR]

This study investigates phytoplankton functional group variations in the western Bay of Bengal (WBoB) during the Spring Intermonsoon. Samples were collected from four cross-shore transects: Mahanadi (MN), Vamsadhara (VD), Godavari (GD), and Krishna (KS). East India Coastal Current and warm gyre influenced the southern transects (KS, GD), VD was experiencing moderate upwelling and MN was characterized by low salinity and oligotrophic conditions due to freshwater input. In response to hydrography, phytoplankton biomass and functional types differed within and between the transects. Chlorophyll-a (Chl-a) was spatially high in VD and low in MN. The subsurface Chlorophyll-a maxima (SSCM) was prominent and shallow in the MN and VD, compared to the southern transects. Total diagnostic pigments concentration was high in VD, followed by GD, KS and MN. Phytoplankton functional groups and each groups contribution to Chl-a was calculated through CHEmical Taxonomy (CHEMTAX). Diatoms and cyanophytes were the dominant functional types in the surface layers. Progressive shift from diatoms in the nearshore region to cyanophytes in the offshore was observed. The low saline and low-nutrient conditions were conducible for the growth of cyanophytes, while nutrient-rich optimum light layer of SSCM and upper layer of VD were favorable for diatoms. Cryptophytes contribution to Chl-a was higher in southern transects compared to the north. Prymnesiophytes and prasinophytes were high in the subsurface and deep layers could be due to their adaptions to light and nutrients. The present study highlights the significance of physical processes associated hydrography in structuring the phytoplankton functional types. [ABSTRACT FROM AUTHOR]

The tephra layers in multiple sediment cores from the offshore region of the Mahanadi basin in the northern Bay of Bengal were investigated for possible volcanic sources. The glass shards from those tephra layers were studied for size distribution, texture, and elemental geochemistry to establish chronostratigraphic markers for regional and global Quaternary correlation. The textural features of fine-grained (silty) volcanic glasses suggest the distal source of these tephra deposits. Major element composition with elevated SiO2 contents ranging between 75%–76% and dominance of K2O (> 4.5%) over CaO (< 0.9%) suggest ashes have originated from siliceous rhyolitic melts, similar to the petrographic composition of tephra from the Toba volcano. The bulk trace element compositions of the same glass shards were comparable with those reported in the youngest Toba tephra reported elsewhere. Likewise, the LREE-dominated chondrite normalized REE profiles of tephra from the Mahanadi basin closely resemble the characteristic REE patterns in Toba ash from other parts of the Indian Ocean and thus confirmed the contribution of the youngest Toba super-eruption for this ash layers. [ABSTRACT FROM AUTHOR]

As a postgraduate student at the Department of History, University of Hyderabad, India, I was introduced to Michael Pearson's scholarship on the Indian Ocean and related studies thereon. While teaching the course "The World of the Indian Ocean," Professor Rila Mukherjee emphasized Michael Pearson as one of the first to systematically conceptualize the interconnected maritime space of the Indian Ocean, and particularly, in her own work, the Bay of Bengal. Pearson's many works shaped my research ideas and provided frameworks for my future research interests. In this paper, I will focus on the significant historiographical and conceptual interventions of Michael Pearson and how they shaped the development of my research on magh predation in the northern Bay of Bengal. [ABSTRACT FROM AUTHOR]

The distribution of dissolved oxygen (DO) affects the survival of marine organisms. In order to explore the temporal and spatial variation of sea surface DO and oxycline in the Oxygen Minimum Zone (OMZ) of Bay of Bengal (BoB), we investigated the vertical DO distribution from 0 to 200 m in the central BoB using Bio-Argo and remote sensing data from 2013 to 2017. The results showed that the salinity stratification caused by the input of a large amount of runoff and rainfall induced the significant change of the DO in the central BoB, and DO decreased significantly at ~40 m, and dropped to less than 20 μmol·kg-1 within 200 m. The sea surface DO was negatively correlated with the sea surface temperature (r=-0.75). The oxycline depth was significantly correlated with the 23 isotherm and sea level anoma ℃ ly (r=0.93 and 0.81). The surface DO concentration was mainly affected by the surface temperature, and the oxycline depth was mainly controlled by the mesoscale eddies in the OMZ of the BoB. [ABSTRACT FROM AUTHOR]

Marine Heat Waves (MHWs) can cause significant distress to marine environment and modulate air-sea interaction, which in turn can have economic and societal impacts. This study aims to identify surface and subsurface MHWs in the Bay of Bengal using available sea-surface temperature data and buoy observations spanning four decades. The results show significant increase in the number, frequency, duration, and intensity of surface MHWs in recent years. To better understand the relationship between MHW occurrences and different phases of two climate phenomena, namely, the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation, multiple extreme MHW event years between 2008 and 2018 are analyzed. The findings show that the surface MHWs become more prominent during El Niño/positive IOD events due to weakened winds, increased net heat flux input towards the ocean, increased stratification and warming tendency through vertical processes in the presence of inversion. This, in turn, affects the surface biological productivity in the region. Additionally, surface MHWs were also found to be driven by surface currents and eddies. On the other hand, subsurface MHWs develop during negative IOD/La Niña conditions due to the deepening of the thermocline, which is forced by coastally trapped downwelling Kelvin waves and reflected downwelling Rossby waves, and is followed by substantial freshening of deeper layers. This study provides a deeper understanding of the causes of surface and subsurface MHWs in the Bay of Bengal, which is a crucial basin in influencing monsoon and cyclonic events affecting its surrounding landmasses. [ABSTRACT FROM AUTHOR]

A new conceptual framework based on satellite data, including chlorophyll (CHL), sea surface temperature (SST) fronts, relative winds, current vectors, Ekman transport, and eddies, has been developed to identify potential fishing zones (PFZ) in the Bay of Bengal (BoB). The framework aims to provide persistent forecasts, even under cloudy conditions, based on feature propagation. The validation of the PFZ was carried out using fish catch data collected by the Fishery Survey of India (FSI) between 2016 and 2018. Hooking rates (HR) from longlines and catch per unit effort (CPUE) from trawl nets were used to analyse the data points in hook rate categories (1.0–3.0 and > 3.0) and CPUE categories (50–100 kg and > 100 kg) and interpret them with the PFZ maps. The analysis showed that the high fish catch locations were consistent with persisting features in the BoB, such as high chlorophyll patches, SST fronts, and cyclonic eddies. The high fish catch locations based on hook rate and high CPUE were found to be collocated with the high chlorophyll persisting features and thermal gradients in the BoB. The regression analysis shows that availability of the food (CHL) had the strongest correlation with fish catch, followed by the comfort condition (fronts and eddies). [ABSTRACT FROM AUTHOR]

Thunderstorms are a weather phenomenon characterized by lightning, thunder, and precipitation, and in Bangladesh, they are a common occurrence with the potential to cause significant damage to life and property. The objective of this study is to examine the sensitivity of the horizontal grid resolution of the WRF-ARW model for simulating thunderstorms in Bangladesh. Two cases from August 2022 were examined. The model was integrated over a 60-h period, with hourly outputs collected from August 16, 0000 UTC, to August 19, 1200 UTC, and two experiments were conducted using horizontal resolutions of 9 and 3 km. The results were validated using observational and ERA-5 reanalysis data. The findings showed that the 9-km resolution had better spatial agreement with ERA-5 for CAPE, although 3 km overestimated the maximum value compared to ERA-5. Thus, increasing the grid resolution will require more processing resources but will not always benefit the model's efficiency for the given model setting. Higher grid resolution generally leads to better spatial accuracy because it captures more fine-scale features and processes in the atmosphere. Especially at coarse grid resolutions, fine-scale topography may be poorly represented or even completely smoothed out. However, the choice of physical parameterization schemes in the WRF model can affect its performance at different resolutions. Some parameterization schemes may perform better at coarser resolutions, while others may be more suitable for finer resolutions. [ABSTRACT FROM AUTHOR]

Amino acids are the more labile compounds of organic matter in aquatic and terrestrial ecosystems. The variability of particulate amino acids in coastal waters depends on several biological (in situ production, grazing by heterotrophs) and physical (river discharge, terrestrial runoff, precipitation) processes. Coastal water samples of the Bay of Bengal were collected at three fixed locations from surface to 100m depth to investigate seasonal variability, transformation, and degradation of amino acids and their sources in Bay of Bengal coastal waters. Various biogeochemical parameters were collected along with particulate amino acids to understand their impact on amino acid composition distribution and variability. Particulate amino acids vary from 0.21 to 7.1µ mol/L, and the highest concentrations are during the non-monsoon period (4.3 µ mol/L) than the monsoon period (2.5 µ mol/L). During the non-monsoon period, amino acids show a significant linear positive relation with phytoplankton biomass (Chl-a) and suspended matter in the monsoon period. The relations indicate that amino acids during the non-monsoon were contributed from the in situ production and during the monsoon from terrestrial inputs. The ratio of amino acids to hexosamines (AAs/HAs) and the ratios of glucosamine (Glc-NH2) to galactosamine (Gal-NH2) were on average 15.5 and 5.2 in non-monsoon and during monsoon; they were 3.3 and 2.1, indicating that during the monsoon period, the amino acids were more of aged and which could be from resuspension of soil or refractory organic matter, which could enter the coastal waters because of precipitation and in non-monsoon, the higher ratios suggesting that in situ biological processes contributed these amino acids. This study indicated that during the monsoon period, physical processes like precipitation, terrestrial runoff, sediment resuspension, and non-monsoon period biological processes like in situ production significantly contribute organic carbon to the coastal waters of the Bay of Bengal. [ABSTRACT FROM AUTHOR]

Abstract Tropical cyclones (TCs) over the Bay of Bengal (BOB) can interact with the South Branch Trough (SBT) as they move northward and potentially amplify Rossby waves. This study evaluates the features of Rossby waves and their downstream impact on rainfall in South China. Results indicate that TC‐SBT interactions primarily occur in May and October‐November (Oct‐Nov), with probabilities of 59% and 53% respectively. Notably, the Rossby wave train associated with BOB TCs is more pronounced during Oct‐Nov due to the stronger subtropical westerly jet, in contrast to May. The downstream atmospheric response results in positive (negative) rainfall anomalies over South China in May (Oct‐Nov), particularly on the day following the maximum interaction day. Previous researches concerning TC‐extratropical flow interaction mainly focus on other basins where TCs move to higher latitudes, this study provides fresh insights into Rossby waves related to TC‐SBT interactions over the southern Tibetan Plateau.

Abstract This study focuses on the environmental conditions of the Mahanadi Estuary near Paradeep Harbor and the adjacent sea. Data collected from May 2013 to April 2020 from 32 GPS fixed stations was analyzed to assess the water quality in different zones (estuarine, mixed zone, mixed zone south, and mixed zone north) of study area. Parameters such as pH, SST, TSS, nitrite, phosphate, silicate, TOC, chlorophyll, fecal coliform, and heavy metals were used to estimate the Water Quality Index (WQI) for each zone. The study found a deterioration (> 30%) in the overall water quality of the Mahanadi Estuary from 2013 to 2020, potentially attributed to river inflows, port activities, and industrial outflows in to the coastal ecosystem. Seasonal variations in temperature, salinity, turbidity, nitrite, nitrate, and ammonia were observed. The water quality showed a deteriorating trend in estuarine, mixed zone, mixed zone south, and mixed zone north. Based on the water quality indices, the ecosystem shows moderate levels of stress. The degraded water quality highlights the need for a targeted mitigation plan to reduce external pressures and enhance the overall ecosystem quality. Graphical Abstract

Sea Surface Temperature (SST) is of great importance to study several major phenomena due to ocean interactions with other earth systems. Previous studies on SST based on statistical inference methods were less accurate for longer prediction lengths. A considerable number of studies in recent years involve machine learning for SST modeling. These models were able to mitigate this problem to some length by modeling SST patterns and trends. Sequence analysis by decomposition is used for SST forecasting in several studies. Ensemble Empirical Mode Decomposition (EEMD) has been proven in previous studies as a useful method for this. The application of EEMD in spatiotemporal modeling has been introduced as Multidimensional EEMD (MEEMD). The aim of this study is to employ fast MEEMD methods to decompose the SST spatiotemporal dataset and apply a Convolutional Long Short-Term Memory (ConvLSTM)-based model to model and forecast SST. The results show that the fast MEEMD method is capable of enhancing spatiotemporal SST modeling compared to the Linear Inverse Model (LIM) and ConvLSTM model without decomposition. The model was further validated by making predictions from April to May 2023 and comparing them to original SST values. There was a high consistency between predicted and real SST values.

This study analyses particulate organic carbon (POC) and particulate nitrogen (PN) export from Indian monsoonal rivers to the north Indian Ocean. Indian monsoonal rivers export approximately 1.2 Tg yr−1 (1Tg = 1012 g) of POC and 0.14 Tg yr−1 of PN, with about two-thirds entering the Bay of Bengal (0.8 and 0.1 Tg yr−1, respectively) and the remaining reaches to the Arabian Sea (0.4 and 0.04 Tg yr−1, respectively). Remarkably, just four rivers from northwest India's black soil-dominated regions contribute about half of the total POC and PN exports (0.64 and 0.06 Tg yr−1, respectively). This is due to substantial erosion in these catchments, resulting in suspended matter concentrations averaging 596 ± 252 mg L−1, significantly higher than catchments dominated by red sandy, red loamy and alluvial soils (54 ± 56 mg l−1). In contrast, rivers originating from catchments with heavy precipitation, a tropical wet climate, red loamy soils (with peaty and marshy characteristics), rich tropical wet evergreen and moist deciduous forests, and higher soil organic carbon content yield more POC and PN (1704 ± 383 kgC km−2 yr−1 and 261 ± 56 kgN km−2 yr−1, respectively) than the other rivers of India (951 ± 508 kgC km−2 yr−1 and 120 ± 57 kgN km−2 yr−1, respectively). These findings stress that the export flux and yield of POC and PN from the Indian monsoonal rivers are primarily influenced by the interplay of hydrological, lithological, environmental, and climatic conditions within the catchment, rather than river size. Moreover, this study highlights the significant impact of incorporating POC data from medium-sized rivers worldwide, as it reveals that yield is independent of river size. This calls for a re-evaluation of global POC export fluxes, taking into account hydrological, lithological, environmental, and climatic factors. [ABSTRACT FROM AUTHOR]

Forecasting tropical cyclones with climate and physical variability and observed cyclonic disturbances has been developed over the years for all the ocean basins successfully and is still one of the priorities for disaster risk reduction policymaking. This study attempts to forecast seasonal cyclonic disturbances and severe cyclonic storms over the Bay of Bengal, where about 80% of the tropical cyclones of the North Indian Ocean are formed. We have used three time-series models, namely, the seasonal autoregressive integrated moving average with exogenous variables (SARIMAX) model, artificial neural network-nonlinear autoregressive with exogenous variables (ANN-NARX) model, and the hybrid model. The basic purpose of considering three different models is to improve the forecasting accuracy of tropical cyclones. We have shown that the intensification rate of the severe cyclonic storms over the Bay of Bengal has been significant and increasing over the years. Results show that the ANN-NARX model with sea surface temperature and near-surface wind speed as predictors is the best performance model for long-term forecasting of cyclonic disturbances. Hence, the distribution of cyclonic disturbances is non-linear. The correlations between observed and predicted occurrences are 0.80 and 0.85 for cyclonic disturbances and severe cyclonic storms, respectively, corroborating, by and large, the forecasting accuracies of some previous studies. The forecasting of cyclonic disturbances indicates that they will vary from 5 to 13 annually and there will be, on average, one severe cyclonic storm per year. The likelihood of occurrence of severe cyclonic storms is most significant in the post-monsoon season. This forecast till 2050 would help the scientific community and policymakers significantly for applications and good disaster risk governance. [ABSTRACT FROM AUTHOR]

This study aims to provide insights into the growth pattern and conditions of Atropus atropos in the Bay of Bengal, Bangladesh. The lack of information on this species in the region has made it crucial to carry out such research for better understanding about its growth and health in this particular habitat. A total 100 individuals were collected from fishers catch during November 2021 to October 2022 by using traditional fishing gears such as trawl and gill nets (mesh size: 2-3 cm). Data showed that TL was found to be between 15 and 21 cm and the BW remained as 46 and 136 g. All LWRs had r2 values below 0.945 and were significant (P<0.001). The estimated allometric coefficient (b) suggested the combined sexes showed isometric growth (b~3). The statistical analysis conducted in this study revealed a highly significant LLR (p<0.001) with an r2 value exceeding 0.944. The form factor (a3.0) was found as 0.0155, while the size at first sexual maturity (Lm) was determined as 12.36 cm TL. The natural mortality (Mw) was calculated as 1.57 y-1. A major connection between KR and BW for this species, indicating that KR was an exceptionally significant in revealing the health of A. atropos. Thereafter, a high condition rating (KF, KA, and KR) indicates that the species is in short and deep shape. This study found that the habitat was in a state of equilibrium and the mean WR indicated that there was no significant deviation from the expected value of 100 for A. atropos. Consequently, information gathered from the present study would be impactful for managing this fishery in the Bay of Bengal, Bangladesh, along with the surroundings. [ABSTRACT FROM AUTHOR]

Mass redistribution in the Earth system induce variations of the Earth's gravity field. Now, the time-varying gravity models from the Gravity Recovery and Climate Experiment (GRACE) mission can only estimate the large-scale gravity changes, so the high-resolution marine gravity trend (MGT) model is urgently required to detect small-scale Earth's mass migration. The sea level change is a significant response to marine gravity field change. Here, we propose to estimate the high-resolution MGT using the sea level trend (SLT). Firstly, the SLT model caused by marine mass change (MMC) on 5′ × 5′ grids covering the Bay of Bengal (BOB) is established based on multisatellite altimetry data and EN4 quality-controlled ocean data, named BOB_MMC_SLT. Then, the marine mass trend (MMT) is calculated using the BOB_MMC_SLT. The spherical harmonic function (SHF) method is applied to estimate MGT using the MMT, and this MGT model on 5′ × 5′ grids, named BOB_SHF_MGT, is used to study marine gravity changes and their associated geophysical processes. The results show that, the MGT mean of BOB_SHF_MGT is about 0.14 μGal yr−1, which indicates that marine gravity in BOB is rising. The earthquakes mainly occur in the southeastern BOB where MGT is obviously rising, which may be related to the increased density of the Burma Plate due to the subduction of the India Plate and the Australia Plate. BOB_SHF_MGT shows that the marine gravity rise rate is increasing from the 85°E ridge to Andaman–Nicobar ridge, with a maximum at the location where the India Plate subducts to the Burma Plate. The MGT model based on altimetry data constructed by SHF method is important for the study of small-scale mass migration near the subduction boundaries. [ABSTRACT FROM AUTHOR]

The role of oceans in regulating the global climate is significantly affected by the spatial and seasonal variations in the carbon dioxide (CO2) gas exchange process at the marine-atmosphere interface. Regulation of the partial pressure distribution at the sea surface depends on physical forcings, chemical processes, and biological interactions in the marine environment. While the global ocean acts as a net CO2 sink, fluctuations in these marine system characteristics on a local or regional scale can potentially impact the universal trend. Being a sparsely sampled oceanic region, the key factors modulating the carbon system of BoB are yet to be understood. This study attempts to analyse the relative significance of the parameters such as sea surface temperature (SST), sea surface salinity (SSS), and Chl-a, as well as monsoon river influx in defining the CO2 partial pressure (pCO2) distribution in BoB. Results revealed a comparatively weak correlation of seawater carbon dioxide partial pressures (pCO2_sw) with Chl-a, with a maximum R-value of -0.7 in post-monsoon. In contrast, strong correlations are observed in the case of SST and SSS. SST showed a high correlation in both pre- (R = 0.99) and post (R = 0.98) monsoon seasons, whereas a moderate to strong correlation was obtained for SSS in pre- (R = 0.77) and post (R = 0.94) monsoon, respectively. The analysis showed a relatively strong influence of physico-chemical factors over the spatial variation of partial pressures in BoB. [ABSTRACT FROM AUTHOR]

During a survey on monogenean parasites of clupeiform fishes of Visakhapatnam coast, Bay of Bengal, apart from typical mazocraeids, 6 species of nonmazocraeids, including two new species were encountered. These belonged to 5 genera distributed over 4 families: Family Gastrocotylidae: Engraulicola thrissocles from Thryssa mystax, T. setirostris, T. malabarica; Engraulicola longisomum n.sp. from Thryssa mystax, T. setirostris, T. malabarica; Pellonicola elongatus from Ilisha filigera; family Megamicrocotylidae: Megamicrocotyle chirocentrus from Chirocentrus dorab; family Microcotylidae: Microcotyle clupei n.sp. from Thryssa mystax and T. setirostris; family Diclidophoridae: Keralina opisthopterus from Opisthopterus tardoore. Engraulicola longisomum n. sp. differs from E. thrissocles in the larger body size, in the relatively smaller size of haptor and also in the number of clamps and alignment of reproductive organs. Microcotyle clupei n.sp. the first species of the genus to be reported from clupeiform fishes differs from other species of the genus, mainly in the presence of a circlet of spines on the oral sucker. Descriptions are provided for all the species collected. Megamicrocotyle johnstoni is synonymized with M. chirocentrus. [ABSTRACT FROM AUTHOR]

Marine environments wherein long-term microbial oxygen consumption exceeds oxygen replenishment can be associated with oxygen minimum zones (OMZ). The Bay of Bengal OMZ (BOB-OMZ) is one of the most intense OMZs globally. To assess the contribution of bacterial oxygen consumption to oxygen loss in BOB-OMZ, we measured bacterial production (BP), temperature, salinity, and dissolved oxygen (DO) in the whole water column. We then compared the estimated bacterial oxygen demand (BOD) with diapycnal oxygen supply (DOS) at depths of 50-200 m in the southern BOB in January 2020. The average BP was 3.53 ± 3.15 µmol C m-3 h-1 in the upper 200 m of four stations, which was lower than those reported in other tropical waters. The vertical distribution of BP differed between the open ocean and nearshore areas. In the open ocean, temperature and DO were the most important predictors for BP in the whole water column. In the nearshore areas, when DO increased sharply from the suboxic state, extremely high BP occurred at 200 m. The average estimated BOD/DOS could reach up to 153% at depths of 50-200 m, indicating advection and anticyclonic eddies probably are important DO replenishment pathways in the BOB. [ABSTRACT FROM AUTHOR]

India has consistently demarcated the Bay of Bengal as a security space in standard realist terms which in turn leads to its conversion as a zone of security. The article probes two questions in this context. First, what drives India's insecurity and its imperative to control the Bay of Bengal? Apart from the traditional defence imperative, India pursues an aspirational model of security that is linked to its regional prominence and growing maritime footprint. Second, what are India's security responses in the Bay of Bengal? While realist security theories are often helpful in explaining India's security behaviour, there are several episodes when India's responses are also suboptimal on a realist scale where it does not completely resort to either defensive or offensive realism to attain security. Beyond defending threats and accumulating power, India's security engagements are additionally motivated by the concerns of autonomy, leadership and pre-eminence in this maritime space. [ABSTRACT FROM AUTHOR]

The energetics in the Bay of Bengal (BoB) region is influenced by primary and secondary mesoscale ocean processes, such as boundary currents, gyres, and eddies. Using the turbulent kinetic energy budget equation, the fluxes and their role in BoB dynamics are studied. A regional domain with a horizontal resolution of 0.1 ∘ is considered in the Indian Ocean, which includes the BoB region. The analysis of flow energetics reveals differences in eddy kinetic energy (K), shear production flux (P), buoyancy flux (B), and viscous dissipation (ε) terms. Our results reveal the presence of an inverse energy-cascade mechanism in the southern Bay of Bengal, wherein energy flows back into mean flow structures from the turbulent eddies. Further analysis shows that buoyancy flux, B, is the major source of turbulence production in the north and central BoB regions. These results define the mechanisms by which energy is produced and transferred in different regions in the Bay of Bengal. [ABSTRACT FROM AUTHOR]

The discharge of freshwater from major rivers into the northern Bay of Bengal (BoB) increases dramatically during the summer monsoon season, reaching a peak in August–September, and there is a corresponding increase in the vertical salinity gradient in the upper ocean. Here we study the impact of seasonally evolving near‐surface salinity stratification on the response of ocean mixed layer temperature (MLT) to Summer Monsoon Intraseasonal Oscillations (MISO), using accurate surface fluxes and high vertical resolution (∼2 m) hydrographic measurements from a mooring in the northern BoB (17.8°N, 89.5°E) during June–September 2019. Prominent MLT warming and cooling with a range of 1.5°C is observed between suppressed (clear skies, calm winds) and active (cloudy, windy) phases of MISO convection. However, the intraseasonal MLT response to the active phase of a late‐season MISO event is minimal compared to MISO events in early summer. We infer this is primarily due to the much smaller contribution from oceanic vertical processes (∼6 Wm−2) in late summer 2019, compared to their role in early summer (−15 to −55 Wm−2). During the active phase of the MISO event of late summer 2019, the combined effect of reduced entrainment and weak vertical temperature gradients associated with a barrier layer inhibits near‐surface cooling. Conversely, the near‐surface salinity stratification and the barrier layer are weak during MISO events in the early summer of 2019—these hydrographic conditions lead to enhanced MLT cooling in response to MISO, apparently through a freer turbulent exchange of cool thermocline water with the surface layer. Plain Language Summary: It is important to accurately represent the upper ocean and air‐sea interaction processes that modulate sea surface temperature variations associated with Summer Monsoon Intraseasonal Oscillation (MISO) in coupled models used for monsoon forecasting. The freshwater input to the Bay of Bengal (BoB) from continental rivers increases in May and reaches its peak in August–September during the summer monsoon. Hence, near‐surface salinity stratification is not uniform throughout the summer, and this can affect the relative contribution of different processes in the mixed‐layer heat budgets of the late‐season versus early season MISO events. The present study examines the difference in mixed‐layer temperature response to MISO with respect to seasonally evolving near‐surface salinity stratification in the northern BoB using accurate surface fluxes and high‐vertical resolution temperature, salinity, and current measurements (∼2 m) from a mooring in the northern BoB (17.8°N, 89.5°E). Our analysis reveals that cooling due to vertical processes in the ocean is much higher during early season MISO events than late‐season event, primarily due to strong near‐surface salinity stratification and a strong barrier layer that develops later in the monsoon season. Key Points: The impact of seasonally evolving near‐surface salinity stratification on the ocean mixed layer temperature response to MISO is examinedThe ML heat storage is positive during the suppressed and negative during the active phases of MISOThe cooling due to vertical processes in the ocean is much higher during early season MISO events than the late‐season event [ABSTRACT FROM AUTHOR]

The present study reports the cloudy grouper, Epinephelus erythrurus for the first time from off Odisha coast. The species was confirmed by using morphometric analyses and COI gene sequencing. Hence the study updates the fish species diversity along the Odisha coast by adding another Epinephelus species to the family Epinephelidae faunal composition. [ABSTRACT FROM AUTHOR]

This study focuses on the environmental conditions of the Mahanadi Estuary near Paradeep Harbor and the adjacent sea. Data collected from May 2013 to April 2020 from 32 GPS fixed stations was analyzed to assess the water quality in different zones (estuarine, mixed zone, mixed zone south, and mixed zone north) of study area. Parameters such as pH, SST, TSS, nitrite, phosphate, silicate, TOC, chlorophyll, fecal coliform, and heavy metals were used to estimate the Water Quality Index (WQI) for each zone. The study found a deterioration (> 30%) in the overall water quality of the Mahanadi Estuary from 2013 to 2020, potentially attributed to river inflows, port activities, and industrial outflows in to the coastal ecosystem. Seasonal variations in temperature, salinity, turbidity, nitrite, nitrate, and ammonia were observed. The water quality showed a deteriorating trend in estuarine, mixed zone, mixed zone south, and mixed zone north. Based on the water quality indices, the ecosystem shows moderate levels of stress. The degraded water quality highlights the need for a targeted mitigation plan to reduce external pressures and enhance the overall ecosystem quality. [ABSTRACT FROM AUTHOR]

The Arabian Sea (AS) and the Bay of Bengal (BoB) are the major part of the Indian Ocean where cyclonic activities prevail each year, resulting in extreme precipitation events, particularly during the pre-monsoon season. Despite the significance of cyclones in Nepal, no studies have investigated their impact on the isotopic composition of atmospheric water vapour (d18 Ov, dDv, and d-excessv). Here, we present the results of continuous measurements of the isotopic composition of atmospheric water vapour at Kathmandu from 7 May to 7 June 2021 during two pre-monsoon cyclone events, namely cyclone Tauktae formed over the Arabian Sea, and cyclone Yaas formed over the Bay of Bengal. We observed a significant depletion of d18 Ov and dDv during both cyclone events compared to before and after the cyclone events which was attributed to changes in moisture sources (local vs. marine) as inferred from backward moisture trajectories. The outgoing longwave radiation (OLR) and regional precipitation during cyclone events together with the observed correlation between vertical velocity and d18 Ov showed high moisture convergence and heavy convection at and around the measurement site which caused unusually depleted d18 Ov during that period. Moisture convergence and convection were stronger during cyclone Yaas which resulted in higher (lower) d-excessv (d18 Ov), compared to Tauktae, possibly due to strong downdrafts during the cyclone-related rain events which can transport vapour with higher (lower) d-excessv (d18 Ov) toward the surface. Our study reveals that tropical cyclones that originated from the BoB and the AS modulate isotopic signals of near-surface atmospheric water vapour considerably in Nepal. Hence caution should be made while interpreting the isotopic variability during the non-monsoon season and the effect of cyclones on the isotopic composition of precipitation and atmospheric water vapour. Our results shed light on key processes governing the isotopic composition of atmospheric water vapour at Kathmandu and may have implications for the paleoclimate reconstruction of tropical cyclone activity. [ABSTRACT FROM AUTHOR]

Northward tropical cyclones over the Bay of Bengal (BoB TCs) often interact with atmospheric circulation, transporting large amounts of water vapor to the Tibetan Plateau (TP), causing extreme precipitation. The BoB surrounded by land on three sides and the complex topography of the TP bring challenges to implementing numerical simulation in these regions. However, the scarcity of data in the two areas makes it necessary to find a technological process to perform practicable numerical simulations on the BoB TC and its induced extreme precipitation to carry out further research. In this study, the WRF 3.9.1 is used to perform many simulation experiments on a northward BoB TC Rashmi (2008) from 24 to 27 October 2008 associated with a record-breaking extreme precipitation on the TP, indicating that the selection of the simulation region, the source of initial-boundary conditions, and the cumulus convection schemes are three important factors influencing the results. We examined and compared the simulation of Rashmi with 10 experiments that were generated by combining The Final Operational Global Analysis (FNL) reanalysis data and the European Centre for Medium-Range Weather Forecasting 5(th) generation reanalysis (ERA5) data as initial-boundary conditions with five cumulus convection schemes. Most of the experiments can predict Rashmi and precipitation in the TP to a certain degree, but present different characteristics. Compared with FNL, the ERA5 performs well regarding Rashmi's intensity and thermal structure but overestimates Rashmi's moving speed. For the extreme precipitation in the TP, experiments suffice to reproduce the heavy rainfall (>25 mm/day) in the TP, with TS and ETS scores above 0.3 and most HSS scores greater than 0.4. The optimal experiments of three stations with extreme precipitation deviated from the actual precipitation by less than 15%. The ERA5 TDK scheme is recommended as the optimal solution for balancing the simulation of Rashmi and its extreme precipitation in the TP. [ABSTRACT FROM AUTHOR]

The eddy diffusivity in the ocean pycnocline K N = γ ε / N 2 is analyzed based on field measurements of the turbulent kinetic energy (TKE) dissipation rate ε z and buoyancy frequency N z profiles and an assumed mixing efficiency of γ = 0.2 . The microstructure measurements were taken between 2013 and 2019 in various regions of the Bay of Bengal (BoB) as well as the central Gulf Stream (GS) and deep waters of the Southern California Bight (SCB). The space–time variability of K N z , t observed in the pycnocline of the southeastern BoB is likely related to internal-wave generated turbulence, identified by its following of MacKinnon–Gregg scaling (MacKinnon and Gregg in J Phys Oceanogr 33(7):1476–1492, 2003). The probability distribution function of diffusivity CDF K N could be well fitted by generalized extreme value distribution (GEVD) for all regions of BoB and other oceans. Mixing rates in the upper pycnocline of BoB during extended southwestern monsoon period tend to be larger in the southern parts of the Bay compared to central and northern parts. Statistics of K N in the GS and SCB waters appear to be similar to those in the southern BoB with a characteristic median value ~ 2 × 10 - 6 m2/s, suggesting relatively low intensity of vertical mixing therein compared to the canonical pycnocline diffusivity of K N ≈ 10 - 5 m2/s. Article Highlights: Internal wave instabilities appear to be a dominant mechanism of generating energetic mixing events in the Bay of Bengal (BoB). Mixing rates (diffusivities) in the upper pycnocline tends to be larger in the southern BoB than in the northern part of the Bay. Generalized extreme value distribution approximates well the probability functions of the diffusivity in all tested regions of BoB as well as in the other oceans. [ABSTRACT FROM AUTHOR]

IntroductionAssimilating all available observations in numerical models may lead to deterioration of the analysis. Ensemble Forecast Sensitivity to Observations (EFSO) is a method that helps to identify all such observations which benefit the analyses. EFSO has never been tested in an ocean data assimilation system because of a lack of robust formulation of a squared norm against which beneficiality of observations can be estimated.MethodsHere, we explore the efficacy of EFSO in the ocean data assimilation system that comprises the ocean model, Regional Ocean Modeling System (ROMS), coupled to the assimilation system Local Ensemble Transform Kalman Filter (LETKF), collectively called LETKF- ROMS, in the Bay of Bengal by envisaging a novel squared norm. The Bay of Bengal is known for its higher stratification and shallow mixed layer depth. In view of baroclinicity representing the stratification of the ocean, we use the modulus of the baroclinic vector as the squared norm to evaluate forecast errors in EFSO.ResultsUsing this approach, we identify beneficial observations. Assimilating only the beneficial observations greatly improves the ocean state. We also show that the improvements are more pronounced in the head of the Bay of Bengal where stratification is much higher compared to the rest of the basin.DiscussionThough this approach doesn’t degrade the ocean state in other regions of the Indian Ocean, a universal squared norm is needed that can be extended beyond the Bay of Bengal basin.

Abstract We construct freshwater balance in the Bay of Bengal (BoB) within a control volume (CV) bounded by 1,018 kg/m3 isopycnal surface using observations and ocean reanalysis during 2011–2015. Freshwater in CV is maximum in October–November due to monsoonal rain and river inflows, and minimum in April–May. Water lighter than 1,018 kg/m3 is not transported out of BoB, implying that freshwater lost from CV is mixed away entirely within the basin. From freshwater budget, we infer moderate diapycnal mixing rates (∼0.8 × 10−5 m2/s) in boreal spring and summer; in winter (December–January), the rate of freshwater loss to subsurface ocean is 0.015 m/day, corresponding to a median turbulent diffusivity of 4.2 × 10−5 m2/s, with standard error of 25%. We show that enhanced winter mixing across the shallow pycnocline is due to reduced shortwave radiation and subseasonal episodes of surface buoyancy loss when cool, dry northeast monsoon winds blow over BoB.

Two phytoplankton blooms triggered by the tropical cyclone (TC) Hibaru were studied in the Bay of Bengal. Hibaru occurred in southeastern Sri Lanka in January 2005. After the passage of Hibaru, two strong phytoplankton blooms appeared in the study area (3.5° N-6° N, 83.5° E-88.5° E). In this study, the dynamic mechanisms were investigated with remote sensing, multisource reanalysis products and Argo float data. The first bloom on January 19 to 20 was induced by upwelling with the upper cyclonic eddy and mixed entrainment caused by Hibaru, where the maximum chlorophyll a (Chl-a) concentration was 0.235 mg•m–3. Sea surface cooling and heavy rainfall also occurred. The second bloom from January 27 to 28 was triggered by the interaction of the upper cyclonic eddy and submarine anticyclonic eddy after the passage of Hibaru, where the maximum of the Chl-a concentration was 0.124 mg•m–3. With the submarine anticyclonic eddy and weakened barrier layer thickness (BLT), the subsurface horizontally converged chlorophyll and nutrient water was uplifted with upwelling. This study contributes to the assessment of the ecological impact of ocean eddies during the passage of TC in the Bay of Bengal.