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To commemorate the contributions made by Indian Naval Ship (INS) Kistna during the first International Indian Ocean Expedition programme a scientific cruise was planned in the Bay of Bengal during November 2017 onboard INS Sagardhwani. The objective was to study the thermohaline properties along 10 °N in the Bay of Bengal and understand the sound channel characteristics in the region. A fresh water cap of ~20 m thickness was noticed in the top layers of the Bay of Bengal occupying the region extending from 83 °E - 93 °E. A prominent anti-cyclonic eddy was noticed around 10 °N centered between 90 °E and 93 °E, a negative sea level (west of 90 °E) co-existed with a positive sea level anomaly to its east along the observational tracks along 10 °N. The climatological temperature and salinity sections also showed the same features as observed along the transect. The study also sheds light on the deep sound channel characteristics along 10 °N. [ABSTRACT FROM AUTHOR]

In the present study, we have used TRMM multi-satellite precipitation analysis (TMPA) 3B42 and global precipitation mission (GPM): IMERG “precipitation Cal” products to identify and quantify the asymmetry and distribution of rainfall in tropical cyclones formed over Bay of Bengal (BoB). For the period 2010-2013, TMPA products have been made use of and for the period 2014-2017, GPM-IMERG products are used to study the aforementioned features. Overall, 17 cyclones covering 75 events/days ranging from depression to very severe cyclonic stage of the system have been analysed. Our analysis revealed some interesting features on asymmetry, direction of maximum rain-receiving zones, relation of T-number with maximum and total rainfall in rain-receiving areas. The study reveals the direction of rainfall zone is mostly towards west and southwest directions of the storm centre in BoB. This point gains importance in the context of earlier reports, where it is mentioned that the direction of maximum rainfall is in east. The study also infers that the maximum rainfall and total rainfall need not be dependent on the intensity (indicated by T-number). We also attempted to classify the rain-receiving zones, based on both the rainfall-rate window and area covered in each window of rainfall, and a criterion has been proposed. The distribution of rainfall is classified, as sharply falling, slowly falling and nearly constant. [ABSTRACT FROM AUTHOR]

An ecosystem disruptive bloom of red Noctiluca scintillans (hereafter Noctiluca) was observed in coastal waters of the north-western Bay of Bengal during April 2014. Based on the principle of phytoplankton group/species specific remote sensing reflectance (R), a technique of detecting green Noctiluca and diatom was developed earlier using R at 443, 488, and 531 nm of Moderate Imaging Spectroradiometer-Aqua (MODIS). This was appropriately modified to detect bloom of red Noctiluca in coastal waters of the Bay of Bengal. Additional R data at longer wavelengths viz. 667 and 678 nm were included in the existing algorithm, and the spectral shapes were accounted to detect the bloom of red Noctiluca. The classification scheme discriminates red Noctiluca from the green form of the same species and diatom. Phytoplankton group/species products were generated using the modified approach and validated with the reported events of red and green Noctiluca blooms in the Indian coastal waters. The present study also highlights two specific results based on MODIS retrieved time-series phytoplankton group/species image analysis: first, the observation of coexistence of diatom, red, and green Noctiluca in coastal waters of the north-western Bay of Bengal, and the second, phytoplankton community shift resulting in red/green Noctiluca proliferation following diatom. [ABSTRACT FROM AUTHOR]

Tropical oceanic regions are frequently prone to deep convections. Hence, it is very essential to understand the features of convection with the help of oceanic and atmospheric variables such as sea surface temperature (SST), outgoing longwave radiation (OLR), rainfall, relative humidity, columnar water vapour (CWV) etc. and the linkage among them. In our present study, we have divided the Bay of Bengal (BoB) into ten different subregions (SR) and have attempted to study the connection between the above-stated variables during convective and non-convective events in the southwest monsoon (SWM) season (June to September) for the period 1998-2010. The monthly behaviour of SST/OLR decreased by 0.5 °C/14 W/m from May to June and increased by 0.1 °C/7 W/m from September to October. Among the ten SRs, SR 5 and SR 10 are observed to be coldest and warmest, respectively, based on the SST variations. Intra-seasonal oscillations of the above-mentioned variables show the influences of quasi-biweekly oscillations (QBWO) and Madden-Julian oscillations (MJO). As the threshold values for SST, OLR and rainfall were already reported, we have drawn our attention to deduce a threshold value for water vapour in lower level troposphere (water vapour density (WVD) at 850 mb) which highly influences the convection. In arriving at a threshold of low-level water vapour, we have analysed the convective and non-convective events of each central 1 × 1° grid in all the SRs for the period from 1998 to 2010, along with water vapour scale height. Our analysis inferred that the low-level water vapour density at 850 mb varied above 12 g/mduring convective days and below 12 g/mduring non-convective days. We noticed that the variability in water vapour density is more in non-convective days than in convective days over BoB. The results of the study may be useful to understand the water vapour dynamics with SST, OLR and rainfall. [ABSTRACT FROM AUTHOR]

The head Bay region bordering the Bay of Bengal is highly vulnerable to tropical cyclones. Catastrophic risks from storm surge and associated inundation are quite high due to high population density in coastal areas, socio-economic conditions, and shallow bathymetry. It features the world's largest deltaic system comprising of 'Sunderbans' bordered by West Bengal and Bangladesh. In a geomorphologic sense, the head Bay region is a low-lying belt comprising several barrier islands and river drainage systems, numerous tidal creeks, and mud flats having a high risk for widespread inundation. In addition, the high tidal range together with low-lying topography leads to high risk and vulnerability from storm surge inundation. During May 2009, a severe cyclonic storm Aila struck West Bengal causing enormous destruction to life and property along coastal belts of West Bengal and Bangladesh. It was the strongest pre-monsoon cyclone in the past two decades that had landfall in West Bengal. This work reports on a numerical study for hypothetical storm surge and associated inundation from Aila using the ADCIRC model. The study covers a comprehensive qualitative analysis on water level elevation and onshore inundation for West Bengal and Bangladesh regions. The estimated peak storm surge was about 4 m in the Sunderban region that propagated into all major riverine systems, inundating the river banks as well the inland areas. Numerical simulations indicate an average inland penetration distance of 350 m with a maximum of 600 m at various coastal locations in West Bengal and Bangladesh. The study emphasizes the need and importance of inundation modeling system required for emergency preparedness and disaster management. [ABSTRACT FROM AUTHOR]

A comprehensive analysis on seasonal distribution of phytoplankton community structure and their interaction with environmental variables was carried out in two local water types (type 1 < 30 m isobath and Type 2 > 30 m isobath) at a coastal site in north-western Bay of Bengal. Phytoplankton community was represented by 211 taxa (146 marine, 37 fresh, 2 brackish, 20 marine–fresh, and 6 marine–brackish–fresh) belonging to seven major groups including 45 potential bloom forming and 22 potential toxin producing species. The seasonal variability depicted enrichment of phytoplankton during pre-monsoon in both water types. Total phytoplankton abundance pattern observed with inter-annual shift during monsoon and post-monsoon period at both water types. In both water types, diatom predominance was observed in terms of species richness and abundance comprising of centric (82 sp.) and pennate (58 sp.) forms. Pennate diatoms, Thalassiothrix longissima and Skeletonema costatum preponderated in both the water types. The diatom abundance was higher in type 1 in comparison to type 2. In general, SiO4 found to fuel growth of the dominant phytoplankton group, diatom in both the water types despite comparative lower concentration of other macronutrients in type 2. [ABSTRACT FROM AUTHOR]

The past decade has witnessed an increased intensity of cyclones in the Bay of Bengal region. With higher winds spread over a larger area, the associated risk and coastal vulnerability have increased with wider destructive potential from high waves, storm surges, and associated coastal inundation. The very severe cyclones that made landfall over the Bay of Bengal in the past decade had strong winds in their outer cores, unlike those cyclones that made landfall in previous years. The original parametric wind formulation performs well for more compact cyclones, but at a radial distance far away from the cyclone centre, the winds are under-estimated. Hence, there is a need to revisit and modify this formula for practical applications, and this study attempts to provide a better representation of the overall radial distance in the wind field envelope. The study postulates a 3/5-power law fitted to the original wind formulae, which provides a reasonably good estimate for the surface wind field. The recent very severe cyclones that developed over the Bay of Bengal provided an excellent test-bed to verify this hypothesis, which is supported by validation from six in-situ buoys. The modified wind formula used with a coupled hydrodynamic model (ADCIRC + SWAN) simulated the storm surge and wave characteristics associated with a recent very severe cyclonic storm 'Hudhud' that made landfall in Andhra, located on the east coast of India in 2014. The study also investigated the dependence of coastal geomorphic features and beach slope on the variability of wave-induced setup. Computed significant wave height and storm surge show an excellent match with wave-rider buoy and tide gauge observations. [ABSTRACT FROM AUTHOR]

The article examines the temporal variation of phytoplaknton assemblage on marine and estuarine ecosystems and the implication of cyclone Phailin in the Bay of Bengal. Topics discussed include the phytoplankton community structure and water quality in the Rushikuyla estuary, higher total abundance of phytoplankton cells during post-Phailin and factors contributing to the decline of species diversity during post-Phailin.

East coast of India is characterized by low to medium topography and an extensive network of major estuaries, bays, mangrove creeks, rivers and tidal inlets that permit inland flooding during tropical cyclones. A coupled wave + surge hydrodynamic modeling system (ADCIRC + SWAN) is implemented to simulate storm surge, still water level elevation and wave induced setup associated with ‘ Phailin ’, a very severe cyclonic storm that made landfall in the Odisha State, east coast of India, during October, 2013. The coupled model provides a realistic description on the dynamic interaction of tides, wind, waves and currents, which is critical for operational needs. The study assesses the role of wave-induced setup on the net water level elevation using time varying wave radiation stress that is dynamically updated in the coupled model run. Numerical experiments are carried out for both storm surge alone and coupled mode versions. Dependent upon complex bathymetry and coastal geometry, inclusion of wave-induced setup in coupled runs results in an additional 23–36% increase of peak surge relative to an uncoupled, surge-tide simulation. The significant wave height from coupled model also shows an excellent match with observed wave heights from a wave-rider buoy located off the Odisha coast. The comparison of surge residuals between model and observation also exhibits a good match. The study highlights the importance of having a coupled wave-hydrodynamic model for operational needs in the north Indian Ocean. [ABSTRACT FROM AUTHOR]