EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Importance of wind variations and intersecting waveguides near Sri Lanka for the intraseasonal sea level variability along the west coast of India

Iyyappan Suresh1, Jerome Vialard2, Matthieu Lengaigne2, Takeshi Izumo2, and Muraleedharan Pillathu Moolayil1
Iyyappan Suresh et al.
  • 1CSIR-National Institute of Oceanography, Physical Oceanography, Dona Paula, India (
  • 2LOCEAN-IPSL, Sorbonne Université (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, Paris, France

Remote wind forcing plays a strong role in the Northern Indian Ocean, where oceanic anomalies can travel long distances within the coastal waveguide. Previous studies for instance emphasized that remote equatorial forcing is the main driver of the sea level and currents intraseasonal variability along the west coast of India (WCI). Until now, the main pathway for this connection between the equatorial and coastal waveguides was thought to occur in the eastern equatorial Indian Ocean, through coastal Kelvin waves that propagate around the Bay of Bengal rim and then around Sri Lanka to the WCI. Using a linear, continuously stratified ocean model, the present study demonstrates that two other mechanisms in fact dominate. First, the equatorial waveguide also intersects the coastal waveguide at the southern tip of India and Sri Lanka, creating a direct connection between the equator and WCI. Rossby waves reflected from the eastern equatorial Indian Ocean boundary indeed have a sufficiently wide meridional scale to induce a pressure signal at the Sri Lankan coast, which eventually propagates to the WCI as a coastal Kelvin wave. Second, local wind variations in the vicinity of Sri Lanka generate strong intraseasonal signals, which also propagate to the WCI along the same path. Sensitivity experiments indicate that these two new mechanisms (direct equatorial connection and local wind variations near Sri Lanka) dominate the WCI intraseasonal sea level variability, with the “classical” pathway around the Bay of Bengal only coming next. Other contributions (Bay of Bengal forcing, local WCI forcing) are much weaker.

We further show that the direct connection between the equatorial waveguide and WCI is negligible at seasonal timescale, but not at interannual timescales where it contributes to the occurrence of anoxic events. By providing an improved understanding of the mechanisms that control the WCI thermocline and oxycline variability, our results could have socio-economic implications for regional fisheries and ecosystems.

How to cite: Suresh, I., Vialard, J., Lengaigne, M., Izumo, T., and Pillathu Moolayil, M.: Importance of wind variations and intersecting waveguides near Sri Lanka for the intraseasonal sea level variability along the west coast of India , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19919,, 2020


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displays version 3 – uploaded on 05 May 2020
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  • CC1: Comment on EGU2020-19919, Joanne Williams, 13 May 2020

    Hi Suresh, thanks for this presentation. Does this link only apply to the oxygenation of the Arabian Sea, or would it also be relevant to the Bay of Bengal oxygenation?

    • AC1: Reply to CC1, Jérôme Vialard, 13 May 2020

         Hello Joanne,

      Well, the results above are  mostly relevant for the WEST coast of India, that is downstream of the direct equatorial connection and influence of winds in the vicinity of Sri Lanka. AT the intraseasonal timescale, the Bay of Bengal sea level (and thus thermocline and oxycline) variability is almost entirely due to signals coming from the equatorial region through the "classical" pathway for the wind driven part. But there is also a strong variability associated with eddies at the intraseasonal timescale, that makes the wind-driven signals less relevant than along the west coast of India, where eddies have a weaker amplitude.

      I hope it answers your comment.

    • AC2: Reply to CC1, Suresh I., 13 May 2020

      Yes, this new link is between the equatorial waveguide and the west coast of India, through coastal Kelvin waves.  These coastal Kelvin waves radiate away Rossby waves as they propagate poleward and can thus influence the thermocline/oxycline depths in the interior Arabian Sea (mainly in the southeastern Arabian Sea).

      This new link does not contribute to the Bay of Bengal.  And Jerome has already explained the Bay of Bengal processes in his reply. I would just add the interior forcing in the Bay of Bengal that may have influence on the thermocline/oxycline varations there.

      • CC2: Reply to AC2, Joanne Williams, 14 May 2020

        Thank-you both, that makes sense. Best of luck with your research, Jo

displays version 2 – uploaded on 05 May 2020, no comments
I included the author affiliations in the first (title) slide.
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