Upper oceanic response to tropical cyclone Phailin in the Bay of Bengal using a coupled atmosphere-ocean model
A numerical simulation of very severe cyclonic storm ‘Phailin’, which originated in southeastern Bay of Bengal (BoB) and propagated northwestward during 10–15 October 2013, was carried out using a coupled atmosphere-ocean model. A Model Coupling Toolkit (MCT) was used to make exchanges of fluxes consistent between the atmospheric model ‘Weather Research and Forecasting’ (WRF) and ocean circulation model ‘Regional Ocean Modelling System’ (ROMS) components of the ‘Coupled Ocean-Atmosphere-Wave-Sediment Transport’ (COAWST) modelling system. The track and intensity of tropical cyclone (TC) Phailin simulated by the WRF component of the coupled model agrees well with the best-track estimates reported by the India Meteorological Department (IMD). Ocean model component (ROMS) was configured over the BoB domain; it utilized the wind stress and net surface heat fluxes from the WRF model to investigate upper oceanic response to the passage of TC Phailin. The coupled model shows pronounced sea surface cooling (2–2.5 °C) and an increase in sea surface salinity (SSS) (2–3 psu) after 06 GMT on 12 October 2013 over the northwestern BoB. Signature of this surface cooling was also observed in satellite data and buoy measurements. The oceanic mixed layer heat budget analysis reveals relative roles of different oceanic processes in controlling the mixed layer temperature over the region of observed cooling. The heat budget highlighted major contributions from horizontal advection and vertical entrainment processes in governing the mixed layer cooling (up to −0.1 °C h−1) and, thereby, reduction in sea surface temperature (SST) in the northwestern BoB during 11–12 October 2013. During the post-cyclone period, the net heat flux at surface regained its diurnal variations with a noontime peak that provided a warming tendency up to 0.05 °C h−1 in the mixed layer. Clear signatures of TC-induced upwelling are seen in vertical velocity (about 2.5 × 10−3 m s−1), rise in isotherms and isohalines along 85–88° E longitudes in the northwestern BoB. The study demonstrates that a coupled atmosphere-ocean model (WRF + ROMS) serves as a useful tool to investigate oceanic response to the passage of cyclones.
KeywordsTropical cyclone Phailin Bay of Bengal Upwelling WRF ROMS COAWST model
The ocean observation programme of the National Institute of Ocean Technology (NIOT), Chennai, is gratefully acknowledged for the deployment and maintenance of OMNI buoy. OMNI buoy BD09 and BD10 data were acquired from the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad. ECCO2 is a contribution to the NASA Modeling, Analysis, and Prediction (MAP) programme. KRP acknowledges UGC-CSIR for his fellowship support. The present study benefitted from the funding supports under the HOOFS programme of INCOIS, Hyderabad (ESSO, Ministry of Earth Sciences, Govt. of India), and the Ocean Mixing and Monsoon (OMM) programme of the National Monsoon Mission, Govt. of India. The High Performance Computing (HPC) facility provided by IIT Delhi and the Department of Science and Technology (DST), Govt. of India, are thankfully acknowledged. The authors thank the two anonymous reviewers for their constructive suggestions that helped to improve the manuscript. Graphics were generated in this manuscript using Ferret and NCL.
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