Current Science

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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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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