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1

Response of the Bay of Bengal to super cyclone Amphan examined using synergistic satellite and in-situ observations

Chacko Neethu, Jayaram Chiranjivi

Oceanologia

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2022

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No. 64 (1)

131--144

EN

Tropical cyclone Amphan is the first super cyclone that happened in the north Indian Ocean in the last 20 years. In this work, multi-platform datasets were used to investigate the responses of the upper ocean to cyclone Amphan. The most striking response was the cold wake left by the cyclone spanning the entire Bay of Bengal with an amplitude up to ∼4°C. Satellite salinity observations revealed that the maximum increase in surface salinity was ∼1.5 PSU on the right side of the track of Amphan. Surface circulation was also observed to be modulated with the passage of a cyclone with a rightward bias in the change in its speed and direction. The currents observed from a moored buoy showed strong inertial oscillations. Argo observations showed that changes induced by the cyclone occurred up to 150 m depth of the cyclone and ocean heat content in the upper 150 m depth decreased due to the passage of the cyclone. There was an enhancement of surface chlorophyll concentration (∼1.5 mg/m3) after the passage of the cyclone, which was centred along the track of the cyclone where the winds were the highest. Mixed layer heat and salinity budget analysis showed that the sea surface cooling and increase in salinity was primarily driven by vertical mixing processes, though horizontal advection contributed meagrely. This study also brings forward the fact that regional differences exist in the responses of the ocean to the forcing of cyclones.

2

Inconsistent response of biophysical characteristics in the western Bay of Bengal associated with positive Indian Ocean dipole

Seelanki Vivek, Nigam Tanuja, Pant Vimlesh

Oceanologia

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2022

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No. 64 (4)

595--614

EN

The Bay of Bengal (BoB) is known to have high primary productivity at its western margin during the Indian summer monsoon season (June–September). This higher coastal productivity is mainly caused due to the near-surface nutrient availability maintained by the local coastal upwelling process. The surface winds in the Indian Ocean significantly vary during El-Niño/La-Niña and Indian Ocean dipole (IOD). The current study examines the sea surface temperature (SST) and Chlorophyll-a (Chl-a) anomalies in the western BoB for the period of 18 years (2000 to 2017), using a coupled regional ocean biophysical model. All considered positive IOD (pIOD) years show discrete behavior of biophysical features in the western BoB. The co-occurrence years of pIOD and El-Niño modes are associated with contrast biophysical anomalies. In the analyzed pIOD events, the years 2006 and 2012 show an enhancement in the Chl-a anomalies whereas, the other two years (2015 and 2017) experience Chl-a decrement. The western BoB was anomalously warmer during the 2015 and 2017 pIOD years compared to the other two pIOD years (2006, 2012). This inconsistent response of biophysical features associated with pIOD years is investigated in terms of local surface flux (momentum, heat, and freshwater) changes over the western BoB. The combined impact of local surface flux changes during the individual years remains the major contributing factor affecting the upper-ocean stratification. Ultimately, the stratification changes are responsible for the observed inconsistent response of biophysical features by significantly altering the upper-ocean mixing, upwelling, and nutrient availability in the western BoB.

3

Spatial Distribution and Diversity of Marine Zooplankton Adjacent to the St. Martin's Island, Bangladesh

Alam Md. Jobaer, Kamal A. S. M. Maksud, Ahmed Md. Kawser, Khondker Moniruzzaman, Fayyaz Rafid

Journal of Ecological Engineering

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2022

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Vol. 23, nr 10

154--163

EN

Bangladesh, a country that lies on the north shores of the Bay of Bengal, has been blessed with a plethora of natural resources. St. Martin’s Island, the only coral-bearing island in Bangladesh, is undoubtedly regarded as one of the most prized possessions. It is rich in biodiversity, but truly scientific information on it is lacking. In the present investigation, 14 stations adjacent to St. Martin’s Island within the Bay of Bengal were sampled for determining the composition, spatial distribution, species richness, and diversity of pelagic zooplankton. Samples were collected in the pre-monsoon hot season. From the community of zooplankton, 34 species were reported. Most of the species belonged to copepods. Besides, Polychaeta and Cirripedia were also present in the community. The most significant species belonged to Oithona, Canthocalanus, Balanus, Euterpina, and Microsetella. Total zooplankton standing crop varied from 45,000–125,000 ind/m3 and the highest number of species (8) were observed at station 7. Strong variability in the distribution of species was observed in the studied stations. The number of genera collected at each station varied from 4–8. Data on species richness (Df) varied from 1.30–3.04 and that of the Shannon-Wiener Index (H) from 1.33–1.93. The species composition of zooplankton and the species richness and the diversity index of the population was comparable to the other studies carried out in the Central and Western Bay of Bengal.

4

Role of riverine inputs, low saline plume advection and mesoscale physical processes in structuring the Chlorophyll a distribution in the western Bay of Bengal during Fall Inter Monsoon

Loganathan Jagadeesan, Narasimha Rao Darapu, Joseph Ignatious, Parambil AswinDev Meleth, Rachuri Vivek, Swarnaprava Behera, Kalathil Balachandran Kizhakkepat

Oceanologia

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2021

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No. 63 (4)

403--419

EN

This study delineates the role of small and medium river inputs, Low Saline Plume Advection (LSPA) and eddies in hydrography alteration and Chlorophyll a (Chl. a) in the Western Bay of Bengal. Samples were collected across five transects viz: Hooghly (HO), Mahanadi (MN), Rushikulya (RK), Visakhapatnam (VSKP) and Godavari (GD) during Fall Intermonsoon. Each transect consists of 7 or 8 locations from inshore to offshore. LSPA propagates southward concordance with the East India Coastal Current (EICC) and its southward flow strengthened by a cold-core eddy. LSPA results in the intermittent low salinity in the cross-shore section of HO, MN and RK. Upper layer Chl. a is 2-3 folds higher in inshore and in LSP-influenced locations than in its adjacent stations. The present study identified Double Chlorophyll a Maxima (DoCM) in LSPA-influenced slope regions of MN and RK. DoCM is less known in the BoB. DoCM has both the Surface Chl. a Maxima (SCM) and Subsurface Chl. a Maxima (SSCM). SSCM layer is relatively shallow and intense in slope and offshore regions of MN and RK due to their closeness with cold-core eddy. The present study highlights that freshwater discharge from small and medium rivers impacts hydrobiology around 10-50 km from the shore depends on the magnitude of river influx. LSPA is away from the local inputs and impacts hydrobiology (>500 km) along the path. EICC and eddies together regulated the direction of LSPA. Existing eddies nature alters vertical hydrobiology in slope and offshore regions.

5

Microplankton size structure induced by a warm-core eddy in the western Bay of Bengal : Role of Trichodesmium abundance

Chinnadurai Karnan, Retnamma Jyothibabu, Nagarathinam Arunpandi, Subramanian Pandiyarajan Rethinam, Singaram Parthasarathi, Shoba Santhikrishnan

Oceanologia

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2021

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No. 63 (3)

283--300

EN

Mesoscale warm-core eddies are common in the Bay of Bengal (BoB), and this study in the western BoB during Pre-Southwest Monsoon (April 2015) presents how a prolonged warm-core core eddy could modify the microplankton biomass and size structure. To investigate this, field sampling and laboratory analyses were augmented with satellite data sets of sea surface temperature (SST), winds, mean sea level anomaly (MSLA), geostrophic currents and chlorophyll-a. High SST with positive MSLA (≥ 20 cm) and a clockwise circulation, represented the occurrence of a large warm-core eddy in the western BoB. Time series data evidenced that it was originated in the mid of March and persistent there till early June, which in turn caused a decrease in the surface nutrients and chlorophyll-a. The abundance and biomass of microplankton were negligible in the warm-core eddy region. FlowCAM data showed a significant decrease in the autotrophic microplankton parameters in the warm-core eddy (av. 13 ± 9 ind. L−1 and 0.1 ± 0.04 µgC L−1, respectively) as compared to the surrounding locations (av. 227 ± 143 ind. L−1 and 0.8 ± 0.5 µgC L−1, respectively). Low nutrients level in the warm core eddy region favoured high abundance of needle-shaped phytoplankton cells dominated by Trichodesmium cells. As a result, the size of micro-autotrophs in the warm-core eddy was larger (av. 91,760 ± 12,902 µm3 ind.−1) than its outside (av. 50,115 ± 21,578 µm3 ind.−1). This is a deviation from our belief that the oligotrophy decreases the phytoplankton size. We showed here that the above understanding might not be infallible in warm-core eddies in the northern Indian Ocean due to its inducing effect on the Trichodesmium abundance.

6

Historical occurrences of marine microalgal blooms in Indian peninsula : Probable causes and implications

Oyeku Oyeshina Gideon, Mandal Subir Kumar

Oceanologia

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2021

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No. 63 (1)

51--70

EN

The Indian marine environment supports employment for over 200 million people, including revenue of nearly $7 billion per annum. However, ecological goods and services of the shallow coast and the marine environment of the Indian peninsula are being affected by recurrent blooms of microalgae. One hundred and six published literature, starting from the first report in 1908 to 2017, were reviewed to investigate the historical occurrences of marine microalgal blooms (MMBs) around the Indian peninsula. 154 MMBs comprising 24 genera and 7 classes were reported during the study period. Noctiluca (dinophyceae) and Trichodesmium (cyanophyceae) bloom contributed 34.4% and 31.8% of total blooms. PCA revealed that high sea surface temperature (SST) and salinity were significant driving forces for Trichodesmium blooms formation, while high nutrients (NO3-N, PO4-P, and SiO4-Si) and low salinity triggered prymnesiophyceae, raphidophyceae, bacillariophyceae and most of the dinophyceae blooms. Noctiluca blooms were linked with both eutrophication and the abundance of prey organisms. HABs were generally dinophyceae dominated and were associated with mass mortality of aquatic fauna, human intoxication, paralytic, and ciguatera shellfish poisoning and even death. Increasing SST and anthropogenic influences around the Indian peninsula could increase the occurrences of MMBs (including HABs) and the number of causative taxa. Proper safety measures such as routine monitoring of phycotoxin levels in the environment and local seafood are required to be put in place in other to protect the health of the public.

7

A study on loops and eddies identified from the trajectories of drifters in the North Indian Ocean

Dora Shrikant, Aparna Sreekumar G.

Oceanologia

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2021

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No. 63 (4)

516--530

EN

We identify loops and eddies from the trajectories of the drifters in the North Indian Ocean (NIO) from October 1985 to March 2019. We use the geometric identification method to identify loops and eddies and compare them with the loops identified from loopers provided by Lumpkin (2016). In NIO, the number of loops estimated from loopers is less than the number of loops and eddies identified by the geometric identification method. A total of 761 loops are identified, of which 346 are eddies, whereas the loops identified from loopers are only 149. Larger radii loops and eddies are observed in the western and central Bay of Bengal (BoB) and the southwestern part of the Arabian Sea (AS). Temporal variation of loops and eddies shows a peak during April–May in the AS and September–October in the BoB. In the BoB, the temporal variation of cyclonic eddies matches with the variation in chlorophyll.

8

New insights into the structural and tectonic settings of the Bay of Bengal using high resolution earth gravity model data

Narayan Satya, Kumar Ujjawal, Pal Sanjit Kumar, Sahoo Soumyashree Debasis

Acta Geophysica

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2021

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Vol. 69, no. 6

2011--2033

EN

The residual anomaly of GECO model-derived gravity data has been enhanced using first vertical derivative, tilt derivative and balanced horizontal derivative for delineation of structural and tectonic features over the Bay of Bengal (BOB). Entire BOB basin is classified in eastern, central and western basins for analysis of the delineated lineaments to understand the tectonic setting. It is observed that major lineament trends in the western and central basins are N–S, NE–SW, and in the eastern basin N–S, NNE–SSW, while prominent lineament trend for the entire BOB basin is in N–S direction. The statistical analysis of the lineament attributes, viz. lineament density, circular standard deviation, circular variance for the lineaments, reveals that the crust under the central basin has experienced the maximum tectonic disturbances followed by the western and the eastern basins. Euler depth solutions for structural index (SI) zero estimate the possible source depths of the lineaments as (i) 3.0–9.0 km (possibly top sedimentary layer), (ii) 9.0–18.0 km (possibly intermediate sedimentary layer), (iii) 18.0–24.0 km (possibly upper crustal layer), (iv) 24.0–33.0 km (possibly lower crustal layer) and (v) 33.0–64.0 km (possibly associated with Lithosphere). Power spectral analysis and 2D forward modelling indicate that sediment thickness varies from 2.0 to 17.0 km; crustal thickness varies from 4.0 to 16.0 km, and Moho varies from 10 to 34 km depth over the BOB. The present study confirms that the origin of the 85°E ridge is due to the sagging of ridge crust into the mantle lithosphere, whereas the NER has a hotspot origin.

9

Seasonal and annual variability in chlorophyll-a in the shelf region of the Northern Bay of Bengal using MODIS-Aqua data

Abdur Rouf Muhammad, Antu Al-Hasan, Noor Imran

Oceanological and Hydrobiological Studies

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2020

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Vol. 49, No. 4

398--407

EN

Chlorophyll-a (Chl-a) concentration is an important issue in ocean ecosystem management and research. This study investigates seasonal and annual variability in Chl-a and its relationship with sea surface temperature (SST) and river discharge in the shelf region of the Northern Bay of Bengal (BoB), as well as validates satellite data against in-situ data. Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite data on Chl-a concentration and SST from 2002–2018 were used in this study. River discharge data were obtained from the Bangladesh Water Development Board (BWDB). The annual Chl-a concentration ranged from 2.08 to 2.94 mg m−3, with an average of 2.43 ± 0.24 mg m−3. The Chl-a concentration was found higher (2.21 ± 0.56 mg m−3) during the northeast monsoon (October–February) and lower (1.81 ± 1.14 mg m−3) during the pre-monsoon season (March–May). The study revealed a declining trend in Chl-a concentration from 2002 to 2018, and the rate of change was −0.0183 mg m−3 year−1. Chl-a concentration showed a weak inverse relationship with SST, both annually and seasonally, especially in the pre-monsoon season. River discharge masked the effect of SST on Chl-a variability during the southwest and northeast monsoon. A reasonable correlation (r = 0.78) was found between the MODIS-Aqua data and in-situ Chl-a observations.

10

Investigating the role of air-sea forcing on the variability of hydrography, circulation, and mixed layer depth in the Arabian Sea and Bay of Bengal

Srivastava A., Dwivedi S., Mishra A. K.

Oceanologia

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2018

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No. 60 (2)

169--186

EN

An effort is made to understand and quantify the influence of near surface zonal and meridional winds, incoming shortwave radiation, and freshwater flux air-sea forcings on the seasonal variability of the hydrography, circulation, and mixed layer depth of the Arabian Sea (AS) and Bay of Bengal (BoB). Sensitivity experiments using an ocean general circulation model are carried out for this purpose in the Indian ocean around 65°-95°E, 5°-22°N during 1998-2014 (17 years). In the absence of near surface wind forcing, the sea surface temperature of the region greatly increases in all the seasons, whereas, in the absence of incoming shortwave radiation forcing, exactly opposite is the case. The sea surface salinity of the AS and BoB decreases in the absence of wind and shortwave radiation forcings, whereas, in the northern BoB it increases in the absence of freshwater flux forcing. The sub-surface changes in the stratification of temperature and salinity are also investigated. The influence of the air-sea forcings on the mixed layer depth of the region is found to be highly seasonally dependent. The effect of air-sea forcings on the seasonal variability of the upper ocean vertical stability is studied using the vertical shear of the horizontal velocity, buoyancy frequency, and energy required for mixing as quantifiers. The near surface wind forcing has highest contribution in changing the surface circulation of the region.

11

Estimation of diatom and dinoflagellate cell volumes from surface waters of the Northern Indian Ocean

Chitari R. R., Anil A. C.

Oceanologia

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2017

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No. 59 (3)

389--395

EN

Phytoplankton samples collected from the Northern Indian Ocean (Bay of Bengal, northern Arabian Sea, and Dona Paula Bay Goa, west coast of India), were utilized to quantify changes in cell size, cell volume and carbon per cell of diatoms and dinoflagellates. The dataset from the Bay of Bengal also provides inter- and intra-annual variations (April 2008 to March 2010). The variations in cell size and volume were large in regions influenced by the riverine influx or terrigenous inputs. An interregional comparison of commonly available forms (8 species) points out that cell volumes are highest in the North Atlantic and lowest in the Mediterranean. The information provided will be useful in estimation of carbon biomass and biogeochemical studies.

12

Chlorophyll distribution by oceanic model and satellite data in the Bay of Bengal and Andaman Sea

Suwannathatsa S., Wongwises P.

Oceanological and Hydrobiological Studies

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2013

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Vol. 42, No. 2

132-138

EN

An oceanic model and satellite data are used to evaluate the seasonal distribution of chlorophyll-a (Chl-a) in the Bay of Bengal (BoB) and Andaman Sea. Satellite data show high Chl-a concentrations because high Chl-a concentrations reduce CO2 and increase O2 at the sea surface, indicating fish abundance in the ocean. Sample collection alone cannot provide an accurate overview of Chl-a concentration over an entire region. The satellite data concerning Chl-a concentration, phytoplankton absorption coefficient, and Sea Surface Temperature (SST) are from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) project and from the Moderate Resolution Imaging Spectroradiometer (MODIS). The oceanic model is created to give the surface circulation as a result. The research finds that the simulation is in agreement with SST, Chl-a concentration, and phytoplankton absorption coefficients obtained from satellites. The conclusion is that the oceanic model can be used to implicitly explain the seasonal distribution of Chl-a in the Bay of Bengal and Andaman sea.