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Sound speed variation in the coastal waters off Cochin and signature of subsurface maxima

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Abstract

We investigated the vertical and horizontal variation of the sound speed using the Thermodynamic Equation Of Seawater-2010 (TEOS-10) and monthly CTD observations at Cochin estuary and adjacent coastal waters (inlet to 20 km offshore) during 2018. The results showed that the sound speed varied from 1500 m/s in the Cochin inlet to 1549 m/s offshore. The seasonally changing river influx to the shelf resulted in a spatially varying salinity plume (0–35 psu) towards offshore, contributing significantly to the sound speed at near-surface waters. Such variation of saline stratification altered the sound speed up to ± 20 m/s in the Cochin inlet and adjacent coastal waters. We observed a subsurface-maximum zone of sound speed (1540 m/s) in June (monsoon season) due to the combined effect of upwelling and river discharge. This subsurface maximum formed only at the starting stage of upwelling, which disappears when the intensity of stratification decreases due to the mixing of upwelled water with the salinity plume. Correlation analysis showed that temperature was the actively prompting parameter in determining the sound speed at off Cochin waters during pre-monsoon (with a maximum of 1549 m/s recorded in April), while the combined effect of temperature and salinity influenced during monsoon and post-monsoon.

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Data availability

The data that support the findings of this study are available from the corresponding author, Muraleedharan K.R., upon reasonable request.

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Acknowledgements

The study was carried out under the project GAP3168 funded by the Naval Research Board (NRB) and the authors are grateful to NRB. The authors are thankful to the Director, NIO, Goa, and Scientist in Charge, NIO, Kochi, for providing the facilities throughout the paperwork writing. The field survey throughout this study was supported by the GAP3157 project under NRB and the authors are thankful to the staff who are working on project GAP3157.

Funding

The study was funded by the Naval Research Board (NRB) and CSIR-National institute of oceanography.

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Authors and Affiliations

  1. CSIR-National Institute of Oceanography, Regional Centre-Kochi, Kerala, India

    Abdul Azeez S , Revichandran C , Muraleedharan K. R , Sebin John, Seena G , Ravikumar C. Nair &  Manju K. G

  2. Research Scholar, Bharathidasan University, Tiruchirappalli, Tamil Nadu, India

    Sebin John & Seena G

  3. India Meteorological Department (IMD), New Delhi, India

    Sebin John

Authors
  1. Abdul Azeez S
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  2. Revichandran C
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  3. Muraleedharan K. R
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  5. Seena G
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  6. Ravikumar C. Nair
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Contributions

Abdul Azeez S managed the field survey, data analysis, and writing of the manuscript. C Revichandran administered the project, and was involved in the discussion and draft preparation. Muraleedharan K R identified the problem, field survey, analysis, and interpretation of the data. Muraleedharan K R, Abdul Azeez S, Seena G, Sebin John, Ravikumar C Nair, and Manju K.G were involved in the field survey and data collection. Abdul Azeez S, Sebin John, and Ravikumar C Nair prepared the figures. The first draft of the manuscript was written by Abdul Azeez S, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Muraleedharan K. R .

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This article does not contain any studies with human participants or animals performed by any of the authors.

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The authors declare no competing interests.

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Responsible Editor: Bruno Castelle

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Abdul Azeez S, Revichandran C, Muraleedharan K. R et al. Sound speed variation in the coastal waters off Cochin and signature of subsurface maxima. Ocean Dynamics 71, 923–933 (2021). https://doi.org/10.1007/s10236-021-01472-x

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  • DOI: https://doi.org/10.1007/s10236-021-01472-x

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