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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

2021

No. 63 (4)

403--419

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.

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

2021

No. 63 (3)

283--300

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.

The response of cyclonic eddies to typhoons based on satellite remote sensing data for 2001-2014 from the South China Sea

Yu Fangjie, Yang Qiongqiong, Chen Ge, Li Qiuxiang

Oceanologia

2019

No. 61 (2)

265--275

Eddies are known to be affected by typhoons, and in recent years, the general three-dimensional structure, as well as features of the spatial and temporal distributions of eddies have been determined. However, the type of eddy that is most likely to be affected by a typhoon remains unclear. In this paper, quantitative and qualitative methods were used to study the eddies that are most sensitive to upper-ocean tropical cyclones (TCs) from the perspective of eddy characteristics, and the quantitative results showed that not all eddies were enhanced under the influence of typhoons. Enhancement of the eddy amplitude (Amp), radius (Rad), area (A), or eddy kinetic energy (EKE) accounted for 92.3% of the total eddy within the radius of the typhoon. Qualitative analyses showed the following: First, eddies located on different sides of the typhoon tracks were differently affected, as eddies on the left side were more intensely affected by the typhoon than eddies on the right side, and second, eddies with short lifespans or small radii were more susceptible to the TCs.