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Ocean Science An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/os-2017-83
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
10 Oct 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).
Estimation of oceanic sub-surface mixing under a severe cyclonic storm using a coupled atmosphere-ocean-wave model
Kumar Ravi Prakash, Tanuja Nigam, and Vimlesh Pant Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi-110016
Abstract. A coupled atmosphere-ocean-wave model used to examine mixing in the upper oceanic layers under the influence of a very severe cyclonic storm Phailin over the Bay of Bengal (BoB) during 10–14 October 2013. Model simulations highlight prominent role of cyclone induced near-inertial oscillations in sub-surface mixing up to the thermocline depth. The inertial mixing introduced by the cyclone played central role in deepening of thermocline and mixed layer depth by 40 m and 15 m, respectively. A detailed analysis of inertial oscillation kinetic energy generation, propagation, and dissipation was carried out at a location in northwestern BoB. The peak magnitude of kinetic energy in baroclinic and barotropic currents found to be 1.2 m2 s−2 and 0.3 × 10−2 m2 s−2, respectively. The power spectrum analysis suggested a dominant frequency operative in sub-surface mixing was associated with near-inertial oscillations. The peak strength of 0.84 m2 s−1 in zonal baroclinic current found at 14 m depth. The baroclinic kinetic energy remain higher (> 0.03 m2 s−2) during 11–12 October and decreased rapidly thereafter. The wave-number rotary spectra identified the downward propagation, from surface up to the thermocline, of energy generated by inertial oscillations. A quantitative analysis of shear generated by the near-inertial baroclinic current showed higher shear generation at 40–80 m depth during peak surface winds. Analysis highlights that greater mixing within the mixed layer take place where the eddy kinetic diffusivity was high (> 6 × 10−11 m2 s−1). The turbulent kinetic energy dissipation rate increased from 4 × 10−14 to 2.5 × 10−13 W kg−1 on approaching the thermocline that dampened mixing process further downward into the thermocline layer.

Citation: Prakash, K. R., Nigam, T., and Pant, V.: Estimation of oceanic sub-surface mixing under a severe cyclonic storm using a coupled atmosphere-ocean-wave model, Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-83, in review, 2017.
Kumar Ravi Prakash et al.
Kumar Ravi Prakash et al.
Kumar Ravi Prakash et al.

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Short summary
Parameters at the sea surface are determined by the air-sea fluxes of heat, salt, and momentum. Surface wind speed drives the oceanic surface circulation and mixing of temperature and salinity up to certain depth (mixed layer depth) from the sea surface. In this study, we examined oceanic mixing process using numerical models under strong cyclonic winds. Results highlight important role of inertial oscillations in sub-surface mixing.
Parameters at the sea surface are determined by the air-sea fluxes of heat, salt, and momentum....
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