Current Science

Rashmi Sharma ¹, Smitha Ratheesh ¹, Sujit Basu ¹

Affiliations
1 Oceanic Sciences Division, Atmospheric and Oceanic Sciences Group, Space Applications Centre, Ahmedabad 380 015, IN

Abstract

A statistical emulator technique, namely polynomial chaos, has been used to estimate two time-dependent biological parameters of a coupled physical-biological model of the Indian Ocean. This has been achieved by minimizing a distance function representing misfit between model simulated and satellite-derived surface chlorophyll. First, the parameters have been assumed to be constant in time and optimized values have been found by minimizing a time-averaged distance function. Since no significant improvement in model simulation has been found using a fixed set of optimum parameters, minimization has been carried out daily, assuming the parameters to be time-dependent. Emulation with this set of parameters has led to a significant improvement in the simulated surface chlorophyll. Smoothing of the parameters with singular spectrum analysis has caused less noisy simulations, at the cost of increasing the model data misfit. Time-varying parameters have been found to be more suitable for the hindcast of daily averaged chlorophyll both in the Arabian Sea and the Bay of Bengal.

Keywords

Coupled Physical–Biological Model, Distance Function, Polynomial Chaos, Surface Chlorophyll.

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Smitha Ratheesh ¹, Rashmi Sharma ¹, K. V. S. R. Prasad ², Neeraj Agarwal ¹, Rashmi Sharma ¹, V. S. R. Prasad ²

Affiliations
1 Space Applications Centre, Oceanic Sciences Division, Ahmedabad 380 058, IN
2 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam 530 003, IN

Abstract

Any Change In Precipitation, Evaporation And River Discharge, By Virtue Of Its Impact On The Distribution Of Ocean Salinity, Leaves Its Inevitable Signature On The Freshwater Content (fwc) In The Oceans. In This Study, Synergistic Use Of Satellite Data And Numerical Ocean Circulation Model Is Explored To Examine The Seasonality Of Fwc Of The Upper 30 M Water Column Of The Bay Of Bengal (bob). For This Purpose, First The Sea Surface Salinity (sss) From Aquarius Is Assimilated Into A Model Of The Indian Ocean. Strength Of Assimilation Is Judged By Comparing Simulated Sss With Satellite And Argo Datasets. An Overall Improvement Of 39% Is Observed In Sss Over Free Run Of The Model Without Data Assimilation. Next, The Focus Is Shifted To The Spatial And Temporal Variability Of Fwc Of The Upper 30 M Of Bob In Relation To The Different Components Of Freshwater Forcing. A Delay Of Three Months In The Peak Of Fwc Is Observed With Respect To The Peak Of Net Freshwater Influx For Bob As A Whole. However, The Nature Of The Response Of Fwc To The Total Freshwater Input Forcing In The Major River-dominated Regions Of Bob Is Different From That For The Whole Bob. The Relative Role Of River Influx In Controlling Fwc In These Regions Is Well Brought Out In The Study. For The Ganga–brahmaputra Region, River Run-off Is Observed To Be A Crucial Parameter In Regulating Fwc, Whereas For Both Irrawaddy River Region And Central Bob, Precipitation Dominates The Response. The Response Of Salinity In The Uppermost Part Of The Northern Bob To The Total Freshwater Input Is Much More Rapid Than In The Other Regions.

Keywords

Freshwater Content, Sea Surface Salinity, Seasonal Variability, Upper Ocean Region.

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Neeraj Agarwal ¹, Rashmi Sharma ¹, Pradeep Thapliyal ¹, Rishi Gangwar ¹, Prateek Kumar ¹, Raj Kumar ¹

Affiliations
1 Earth, Ocean, Atmosphere and Planetary Sciences Area, Space Applications Centre, Indian Space Research Organisation, Ahmedabad 380 015, IN

Abstract

The study presents assessment and potential oceanographic applications of sea-surface temperature (SST), ocean net shortwave radiation (SWR) and chlorophyll concentration (CC) observations obtained from various geostationary platforms. SST and SWR from imager on-board Indian National Satellite (INSAT- 3D) and CC from Global Ocean Color Imager (GOCI) on-board communication ocean and meteorological satellite (COMS) have been used in the analysis. Relative advantages of high temporal resolution obtained from the geostationary platform compared to polar orbiting platforms are demonstrated. Comparison of INSAT-3D SST with observations gives a correlation of 0.85 and RMSE of 0.81 K. These platforms definitely provide a highly reliable source of continuous observations, which is useful in monitoring dynamic oceanic features such as thermal fronts, chlorophyll blooms, air–sea exchange fluxes, etc. on diurnal to daily timescales.

Keywords

Chlorophyll Concentration, Geostationary Satellites, INSAT-3D, Sea-surface Temperature, Shortwave Radiation.

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Smitha Ratheesh ¹, Aditya Chaudhary ¹, Neeraj Agarwal ¹, Rashmi Sharma ¹

Affiliations
1 Oceanic Sciences Division, Space Applications Centre, ISRO, Ahmedabad 380 015, IN

Abstract

Role of ocean dynamics on vertical velocity of Ekman pumping (VVE) is analysed using simulations from very high resolution Ocean General Circulation Model (OGCM) configured for the Bay of Bengal (BoB). For this purpose, OGCM is forced with SCATSAT-1 scatterometer wind fields for 2017. Three mechanisms which modify VVE in the ocean are addressed in this study; the first results from the influence of sea surface temperature (SST) on wind field, and the other two arise from the influence of ocean surface currents (OSCs) on the wind field. Analysis for different length scales ranging from mesoscale to sub-mesoscale is also carried out. The results suggest a significant role of ocean dynamics on VVE, especially over submesoscale range (spatial scales of the order of 2– 10 km). Relative vorticity of OSC-induced Ekman pumping is found to be quite high (~3 m/day) at 2 km length scale, especially along the periphery of mesoscale eddies and along the filament structures. Impact of SST on VVE is least amongst the three factors and is observed to be significant only up to the length scales of 30 km. For length scales less than 10 km, relative vorticity-induced Ekman pumping increases drastically and the total Ekman pumping vertical velocity is predominantly controlled by the relative vorticity of OSC-induced Ekman pumping only.

Keywords

Ekman Pumping, Ocean Dynamics, Scatterometers, Vertical Velocity, Wind Field.

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Rashmi Sharma ¹

Affiliations
1 Oceanic Sciences Division, Space Applications Centre, ISRO, Ahmedabad 380 015, IN

Abstract

Comprehensive Remote Sensing is a series of nine volumes comprising allinclusive discussions across various disciplines of the earth system in the context of remote sensing. The editor-inchief, Shunlin Liang, has done a commendable job in bringing together more than 100 authors to contribute nearly 120 chapters in these series. Volume 8 of the series is specifically dedicated to the oceans. This volume comprises 11 chapters covering various aspects of ocean remote sensing.

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Surisetty V. V. Arun Kumar ¹, Rakesh Kumar Luhar ², Rashmi Sharma ¹, Raj Kumar ¹

Affiliations
1 Earth, Ocean, Atmosphere, Planetary Sciences and Applications Area, Ahmedabad 380 015, IN
2 Mechanical Engineering Systems Area, Space Applications Centre (ISRO), Ahmedabad 380 015,, IN

Abstract

Lagrangian drifters are analogues of particles that are relevant to flow-field characterization and therefore they represent realistic surface currents compared to Eulerian techniques. The use of global navigation satellite system (GNSS) in such drifters with Differential Global Positioning System mode at high frequency (5–10 Hz) sampling and post-processing kinematic results in position estimates with centimeter-level accuracy. In the complex nearshore zone, deploying expensive instruments is a risk due to greater chances of loss. To avoid this, two drifters have been designed and developed using a low-cost Emlid Reach® GNSS receivers, antennae and ‘off-the-shelf’ PVC components to measure the surface currents. The dimensions of the drifters were optimally chosen to minimize the wind and wave impacts and to increase the subsurface current drag. An analysis of relative position and velocity errors from stationary observations indicates that the drifter can resolve motion accurately with minimal errors of ±1 cm and ±2 cm/s respectively. These drifters were used to measure surf zone currents at the RK Beach, Visakhapatnam during May 2018 and to successfully identify dangerous rip current zones. This study presents the design, development aspects, error analysis and testing of GNSS drifters. Although these drifters are primarily developed to measure the rip current velocities and trajectories in the nearshore zone, they can also be operated in any marine environment like rivers, lakes, estuaries, etc. without change in the design. An extensive study using a fleet of such drifters is required to understand the complex physical processes in the marine environment.

Keywords

Drifter, Error Estimation, Rip Currents, Relative Position and Velocity.

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Surisetty V. V. Arun Kumar ¹, Ch. Venkateswarlu ², B. Sivaiah ², K. V. S. R. Prasad ², Rashmi Sharma ¹, Raj Kumar ¹

Affiliations
1 Earth, Ocean, Atmosphere, Planetary Sciences and Applications Area, Space Applications Centre (ISRO), Ahmedabad 380 015, IN
2 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam 530 004, IN

Abstract

Determining nearshore bathymetry by traditional surveying methods is a challenging task as it involves huge costs and efforts. Most of the coastal shallowwater zones worldwide either remain unmapped or not updated. Bathymetry estimations from optical satellite imageries have been increasingly implemented as an alternative tool for traditional bathymetry surveys. In this study, we examine the usefulness of freely available, five-day revisit and relatively highresolution Multi Spectral Instruments (MSI) on-board Sentinel-2A and 2B twin satellites. A process workflow has been developed which automatically incorporates a robust atmospheric correction through ACOLITE software and multi-scene compositing of several scenes to improve the reliability and no data gaps. Two study sites in India are explored owing to their variability in submarine morphology. High-resolution bathymetry maps are generated through a log-ratio transform model calibrated with minimal in situ data from the jet ski soundings. The satellite-derived bathymetry obtained has an overall bias of –0.01 and 0.02 m, and ischolar_main mean square error of 1.09 and 0.93 m respectively, at two study sites up to 15 m depth. The consistency in bathymetry retrieval indicates a potential for automated application for the benefit of operational and scientific studies. These high-resolution maps capture small-scale nearshore features like sandbars and rip channels, which are of prime importance for coastal and beach managers.

Keywords

Optical Remote Sensing, Multispectral Imagers, Nearshore Bathymetry Maps, Rip Channel, Twin Satellites.

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Suchandra A. Bhowmick ¹, Neeraj Agarwal ¹, Rashmi Sharma ¹, R. Sundar ², R. Venkatesan ², C. Anoopa Prasad ¹, K. N. Navaneeth ¹

Affiliations
1 Space Applications Centre, Indian Space Research Organization, Ahmedabad 380 015, IN
2 National Institute of Ocean Technology, MOES, Chennai 600 100, IN

Abstract

Amphan, a category-5 tropical cyclone, originated over Bay of Bengal (BoB) and had a landfall in West Bengal, India on 20 May, causing havoc in the region. In this study, in-situ buoy and various satellite measurements are used to analyse the ocean condition before and after the storm, primarily from the air–sea interaction perspective. Widespread anomalous warming was observed in BoB before the event, due to high net surface insolation received by the ocean. The warm SST anomalies in the central BoB were coincident with anti-cyclonic warm core eddies, implying availability of higher oceanic heat content. Observations from BD13 buoy, close to the cyclone track showed heating of the overlying atmosphere due to this ocean warming. Strong surface cooling was observed after passage of the cyclone due to wind induced upper-ocean mixing that is stimulated by low stratification in BoB.

Keywords

Air–Sea Interaction, Oceanic Conditions, Satellite and In situ Observations, Tropical Cyclones.

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