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Upper layer characteristics of the South Eastern Arabian Sea associated with an unusual low saline pool during fag end of southwest monsoon

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Abstract

Observational evidence on an anomalous low saline pool due to excess rainfall and runoff during September 2020 in the South Eastern Arabian Sea (SEAS) is explained based on in-situ and satellite/reanalysis datasets. The study shows the presence of a low saline pool in the coast to offshore section between 10 and 14°N, which is associated with weak winds (3–5 m/s) and surface currents (0.1–0.2 m/s), compared to the long-term average. The pool is observed with strong vertical temperature gradient of 4°C and salinity gradient of 3.5 psu in the upper 20 m column. Cyclonic eddy in the central SEAS influences the distribution of low saline pool which suppress the coastal upwelling in the region. However, active upwelling is observed (surface temperature <26°C) in the southern transects (8–9°N) associated with strong wind (>6 m/s) and surface current (>0.2 m/s) which is also supported with weak rainfall and runoff. The high phytoplankton biomass in the nutrient-rich active upwelling region supports high zooplankton biomass. Contrastingly, the low saline pool region experienced low nitrate and phosphate concentration resulting into less chlorophyll concentration and that might be a plausible reason for the low zooplankton biomass.

Research highlights

  • An unusual low saline pool was observed in the mixed layer of SEAS during fag end of SW monsoon 2020.

  • Anomalous rainfall and runoff associated with weak wind, currents lead to the formation of the low saline pool.

  • Cyclonic eddy observed in the region defined the flow within and the overall pattern of the pool region.

  • The low saline pool inhibits coastal upwelling to the subsurface in the central SEAS.

  • The low saline pool resulted in a stratified water column in the upper layer leading to reduced phytoplankton concentration and zooplankton biomass.

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Acknowledgements

Authors are grateful to the Secretary, Ministry of Earth Sciences, India and the Director, CMLRE for their encouragement and support. The authors are also grateful to the Vessel Management Cell and all the fellow participants and crew members onboard FORV Sagar Sampada (Cr-399) for their support in data collection. The authors wish to acknowledge the use of satellite and reanalysis datasets obtained from https://marine.copernicus.eu, https://cds.climate.copernicus.eu and http://apdrc.soest.hawaii.edu. This is CMLRE contribution no. 149.

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Authors and Affiliations

  1. Centre for Marine Living Resources and Ecology, Ministry of Earth Sciences, Kochi, 682 508, India

    S Vishnu Narayanan Nampoothiri, Muhammad Shafeeque, V S Arun, B R Smitha, Kusum Komal Karati, K Rasheed, C R Asha Devi & Hashim Manjebrayakath

  2. School of Marine Sciences, Cochin University of Science and Technology, Kochi, 682 016, India

    S Vishnu Narayanan Nampoothiri & V S Arun

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  1. S Vishnu Narayanan Nampoothiri
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  2. Muhammad Shafeeque
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  3. V S Arun
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  5. Kusum Komal Karati
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  8. Hashim Manjebrayakath
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Contributions

VNNS: Data collection, planned, analysed and wrote the manuscript. MS and KKK: Data collection, analysis and writing. AVS: Analysis and plotting. SBR and RK: Conceptualization and writing. ACR and HM: Overall supervision and scrutiny of draft manuscript.

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Correspondence to S Vishnu Narayanan Nampoothiri.

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Communicated by C Gnanaseelan

Supplementary materials pertaining to this article are available on the Journal of Earth System Science website (http://www.ias.ac.in/Journals/Journal_of_Earth_System_Science).

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Vishnu Narayanan Nampoothiri, S., Shafeeque, M., Arun, V.S. et al. Upper layer characteristics of the South Eastern Arabian Sea associated with an unusual low saline pool during fag end of southwest monsoon. J Earth Syst Sci 131, 129 (2022). https://doi.org/10.1007/s12040-022-01886-1

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  • DOI: https://doi.org/10.1007/s12040-022-01886-1

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