J. Earth Syst. Sci. (2020) 129:194 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-020-01457-2 (0123456789().,-volV)(0123456789().,-volV) On the dynamics of cyclogenesis, rapid intensiBcation and recurvature of the very severe cyclonic storm, Ockhi 1, 2 3,4 5 6 SDSANAP *, M MOHAPATRA ,MMALI ,PPRIYA and D VARAPRASAD 1India Meteorological Department, Ministry of Earth Sciences, ODce of the Climate Research and Services, Shivajinagar, Pune 411 005, India. 2India Meteorological Department, Ministry of Earth Sciences, ODce of the Director General of Meteorology, Mausam Bhavan, Lodi Road, New Delhi 110 003, India. 3Centre for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, USA. 4Present address: Andhra Pradesh Disaster Management Authority, Kunchanapalli, Andhra Pradesh, India. 5Centre for Climate Change Research (CCCR), Indian Institute of Tropical Meteorology, Ministry of Earth Sciences, Pashan, Pune 411 008, India. 6National Remote Sensing Centre (NRSC), Hyderabad 500 037, India. *Corresponding author. e-mail: [email protected] MS received 17 January 2020; revised 22 April 2020; accepted 10 June 2020 A very severe cyclonic storm (VSCS), ‘Ockhi’ started its journey from southwest Bay of Bengal (BoB) over Comorin area (7.5°N–77.5°E) as a low-pressure area on 28 November, 2017. Initially it moved north-westwards across Lakshadweep Islands without hitting Tamil Nadu or Kerala coast, then moved north-eastward over the Arabian Sea and dissipated upon hitting south Gujarat coast on 06 December, 2017. Rapid intensiBcation (RI), clockwise recurvature and a longer track were the major features of the VSCS, Ockhi. In the present study, an attempt has been made to understand the mechanism of cyclo- genesis, RI and recurvature of the Ockhi using satellite and reanalysis datasets. Initially, role of easterly waves (EW) and Madden Jullian Oscillations (MJO) on the cyclogenesis of tropical cyclone Ockhi is addressed. Our analysis suggests that the EW (MJO) played a seminal (insignificant) role in precondi- tioning the atmosphere for the cyclogenesis of the Ockhi. Our detailed analysis using various oceanic parameters indicate that, the passage of the cyclonic storm over the regions of high thermal energy, especially warmer ocean mean temperature (OMT) at 100 m depth, was instrumental in its rapid intensiBcation. Further, we addressed the recurving feature of the VSCS Ockhi using steering Cow analysis. It is found that strong north-eastward steering winds, embedded in subtropical westerlies with deep southward extent, favoured the recurving of the Ockhi towards north-eastward by suppressing the conventional westward (north-westward) track movement. Keywords. Tropical cyclone; easterly waves; rapid intensiBcation; recurvature; upper ocean thermal energy; tropical dynamics. Supplementary materials pertaining to this article are available on the Journal of Earth Science Website (http://www.ias.ac.in/ Journals/Journal˙of˙Earth˙System˙Science). 194 Page 2 of 13 J. Earth Syst. Sci. (2020) 129:194 1. Introduction Higher values of TCHP, depth of 26°C isotherm, ocean mean temperatures at different depths Although there has been substantial improvement maintain the warmer SSTs without significantly in tropical cyclone (TC) track forecasts, the fore- reducing the temperature due to TC induced cast of the TC intensity is still a grey area of cooling near convective areas of the TC. research due to the complexity of the processes Sometimes, TCs are formed due to the intensi- involved (DeMaria et al. 2005). The prediction of Bcation of wavelike disturbances (Riehl 1945; the track and intensity of the TCs are very Burpee 1975). These tropical depressions (TD), important in order to provide sufBcient lead time which propagate with a periodicity of 3–7 days and to disaster managers for planning, decision-making wavelength of 2000–2500 km (Burpee 1972), are and resource disposition (Mattocks and Forbes called easterly waves (EWs). Over warm oceanic 2008). TC intensiBcation is caused by various waters, these disturbances often intensify by the complex processes such as upper ocean interaction, latent heat of evaporation and condensation. Over TC dynamics and atmospheric circulation (Gray warmer SSTs ([26°C), the troughs in EWs deep- 1979; Holliday and Thompson 1979; Merrill 1988; ens and lead to a tropical storm and sometimes Emanuel 1999; Emanuel et al. 2004; Lin et al. 2008; tropical cyclones (Burpee 1975;Juryet al. 1991; Mainelli et al. 2008; Pun et al. 2011). Another Berry and Thorncroft 2005; Ross and Krishna- challenge in TC forecasting is its rapid intensiB- murthy 2007). The occurrences of EWs are gen- cation. The potential role of the sea surface height erally more frequent over the tropical Atlantic anomalies (SSHA) and upper ocean thermal energy and the western PaciBc Ocean. Easterly waves are (UOTE) in rapid intensiBcation (RI) is investi- observed over Indian Ocean region as quasi-sta- gated by several researchers (Shay et al. 2000; Ali tionary as well as travelling modes (Saha 2009). et al. 2007, 2013a, b; Mainelli et al. 2008; Lin et al. Formation of the EW induced synoptic systems 2013). With the introduction of satellite altimeters, over Bay of Bengal (BoB) and associated heavy it has been possible to operationally monitor the rainfall events over southeast peninsular India is UOTE over the global TC ocean basins (Pun et al. dominant during El-Nin˜oregimesascomparedto 2007; Lin et al. 2008). Lin et al. (2012) used SSHA La-Nin˜a (Sanap et al. 2018). Using satellite ima- obtained from satellite altimetry to determine the gery, Balachandran et al. (1998)identiBed the ocean heat content. Previous studies have noted inverted V-type EWs over North Indian Ocean that TC heat potential (TCHP), which represents (NIO). The travelling EWs generally develop over the ocean heat contained in waters warmer than the western PaciBc Ocean and arrive over the 26°C, is a useful predictor for TC intensity (Shay Indian Ocean region by crossing the southeast et al. 2000; Goni and Trinanes 2003; Ali et al. 2007; Asian peninsula. Lin et al. 2008, 2012; Mainelli et al. 2008; Goni A table of classiBcation of the TCs based on et al. 2009). Various high intensity TCs in different maximum sustained wind and pressure deBcit at basins have been observed to rapidly intensify over the centre of TC is provided in supplementary the regions of high TCHP. TCHP is computed material (source: IMD). The VSCS, ‘Ockhi’ initi- globally from altimeter-derived vertical tempera- ated as a low-pressure area over the southwest BoB ture proBles. Unlike SST, it is a measure of the on 28 November, 2017, intensiBed rapidly, moved subsurface ocean thermal structure, which is north-westwards at Brst over the Arabian Sea, then strongly correlated with sea surface height and recurved north-eastwards and dissipated upon plays an important role in the intensiBcation hitting Gujarat coast on 06 December, 2017. The of tropical cyclones, provided that atmospheric best track of the cyclone, Ockhi is plotted in conditions are also favourable. Bgure 1. According to the IMD report, Ockhi was Mathematically, one of the rarest of its kind due to the following Z reasons: 26 TCHP ¼ Cp qðÞT À 26 dz; ð1Þ • It was the fourth cyclonic storm developed over 0 Comorin Sea (south of Kerala and Tamil Nadu where Cp is the speciBc heat capacity at constant and west of Sri Lanka). However, cyclone Ockhi pressure, q is the density of water and T is the did not hit Tamil Nadu and Kerala coast, but temperature (°C) of layer dz. moved across Lakshadweep Islands. J. Earth Syst. Sci. (2020) 129:194 Page 3 of 13 194 co-ordination with Global Precipitation Measure- ment (GPM) mission and the Precipitation Pro- cessing System (PPS). These data are available at 3 hourly intervals with 0.1° latitude–longitude grids. High resolution (4 km) daily outgoing long wave radiation (OLR) data, measured from INSAT-3D geostationary satellite is obtained from Indian Space Research Organisation’s (ISRO) meteorological and oceanographic satellite data archival centre (MOSDAC, https://www.mosdac. gov.in/). The daily data of TCHP, subsurface Figure 1. Best track of the very severe cyclonic storm, Ockhi. temperature and ocean mean temperature (OMT) The best track data is obtained from the Regional Specialised at different depths are obtained from the National Meteorological Centre (RSMC), Tropical Cyclones, New Remote Sensing Centre (NRSC), Hyderabad, Delhi. India. These oceanic parameters are utilised in the present study at 0.25° 9 0.25° resolution. Kindly refer Sasamal et al. (2018) for more details about • It rapidly intensiBed at the genesis stage. It this dataset (https://www.nrsc.gov.in/nices). The intensiBed from deep depression into a cyclonic methodology of estimation of OMT is given in storm over Comorin area within six hours. Jagadeesh et al. (2015). The TC track and • Ockhi had a clockwise recurving track. The T-number details are obtained from Regional track length of the cyclone was 2538 km. Specialised Meteorological Centre (RSMC), TC, • The life period of the cyclone was 162 hrs (6 days India Meteorological Department (IMD), New and 18 hrs) against a long period average of 4.7 Delhi, (http://rsmcnewdelhi.imd.gov.in). RSMC, days for VSCS over NIO. TC, New Delhi is the nodal agency for issuing In the present study, an attempt has been made to tropical weather outlooks and TC advisories for the understand the role of EWs, oceanic conditions, beneBt of the countries in the World Meteorologi- and large-scale atmospheric Cow pattern on cyclo- cal Organisation, Economic and Social Commission genesis, RI and recurvature of the Ockhi using for Asia and the PaciBc (WMO/ESCAP) Panel satellite and ERA5 reanalysis datasets. Datasets region bordering the Bay of Bengal and the Ara- utilised in the present work are described in bian Sea, namely, Bangladesh, Maldives, Myan- section 2. Section 3 discusses the significant out- mar, Oman, Pakistan, Sri Lanka and Thailand.