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 , rapid intensiBcation and recurvature of the very severe cyclonic , 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 , 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 Islands without hitting or coast, then moved north-eastward over the Arabian Sea and dissipated upon hitting south 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 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. ; 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 (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 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 ). 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, , Myan- section 2. Section 3 discusses the significant out- mar, Oman, Pakistan, Sri Lanka and Thailand. come from analysis and section 4 summarises the results with conclusions. 3. Results

2. Dataset It is documented that the major atmospheric parameters responsible for cyclogenesis are mid- The dynamical Belds associated with Ockhi are tropospheric (500 hPa) relative humidity (RH), examined using European Centre for Medium total precipitable water (TPW), 850 hPa relative Range Weather Forecasting (ECMWF) Bfth gen- vorticity, 200 hPa divergence and vertical wind eration reanalysis (ERA-5, Hersbach and Dee shear (VWS) (Gray 1979). In addition, convective 2016) dataset. This data is available at hourly available potential energy (CAPE) and convective interval with 0.3° grid resolution. Past studies have inhibition (CIN) is also analysed. Figure 2 depicts shown that European reanalysis dataset represents the time evolution of anomalies of above-men- many observed features of the equatorial waves tioned parameters for the period 27 November to (Kiladis et al. 2009; Schreck et al. 2011). Tropical 03 December, 2017. The evolution of the RH (%, is identiBed using an Integrated Bgure 2a–g) and TPW (kg mÀ2, Bgure 2h–n) shows Multi-satellitE Retrievals for Global precipitation positive anomalies in the range of 30–50, which measurement (IMERG) derived daily gridded data indicate the build-up of the sufBcient moisture of precipitation (HuAman et al. 2015). This is which can aid for intensiBcation of the synoptic maintained by Goddard Earth Sciences Data and system with time. In addition, enhancement of Information Services Centre (GES DISC), in lower level relative vorticity (SÀ1, Bgure 2o–u) and 194 Page 4 of 13 J. Earth Syst. Sci. (2020) 129:194

(a) (h) (o) (v) (cc) (jj) (qq)

(b) (i) (p) (w) (dd) (kk) (rr)

(c) (j) (q) (x) (ee) (ll) (ss)

(d) (k) (r) (y) (ff) (mm) (tt)

(e) (l) (s) (z) (gg) (nn) (uu)

(f) (m) (t) (aa) (hh) (oo) (vv)

(g) (n) (u) (bb) (ii) (pp) (ww)

Figure 2. Time evolution of the anomalies of various atmospheric Belds for the period 27 November, 2017 to 03 December, 2017 (a–g) mid-tropospheric relative humidity (%), (h–n) total precipitable water (kg mÀ2), (o–u) 850 hPa relative vorticity (SÀ1), (v–bb) 200 hPa divergence (SÀ1), (cc–ii) vertical (msÀ1), (jj–pp) convective available potential energy (J/kg), and (qq–ww) convective inhibition(J/kg). All the Belds are obtained from ERA5 reanalysis at 0.3° 9 0.3° resolution. upper level divergence (SÀ1, Bgure 2v–bb) indicate meridional component of the wind for 26 November enhancement of low-level convergence and upper 2017 to 10 December, 2017 (Bgure 3). The loca- level divergence over the region, favouring the tions of trough and ridge in easterly Cow are the cyclogenesis. Higher values of CAPE (CIN) during places where winds are switching from southerlies cyclogenesis (away from cyclogenesis) over the (positive anomalies) to northerlies (negative region is observed (see Bgure 2jj–pp and qq–ww). It anomalies). The westward propagation of the is also noted that the SST (see Bgure 8a) oA wave-like disturbances is clearly seen from Bgure 3. India–Sri Lanka coast over southeast Arabian Sea Northerly meridional wind, subsidence, divergence was [ 29°C which is also favourable for cyclogen- and fair weather are the general characteristics esis of the Ockhi. The atmospheric circulation ahead of the approaching easterly wave trough and features which were instrumental in building up southerly meridional winds, rising motion, con- the atmosphere conducive for cyclogenesis is vergence and active weather are the characteristics explored in the next sub-section. over and behind the easterly wave trough. Figure 3 clearly indicates that the region of trough (ridge) is associated with rainfall (fair weather) activity. The 3.1 Cyclogenesis: Role of easterly wave results further corroborate with the conceptual understanding regarding weather associated with Geetha and Balachandran (2014) documented the trough (ridge) in easterlies mentioned above. Time usefulness of 700 hPa meridional wind to identify evolution of the streamline and relative vorticity at the easterly wave activity over Indian Ocean 700 hPa level for the period 27 November to 01 region during northeast monsoon season. In order December, 2017 is shown in Bgure 4. The system- to investigate the EW activity over Indian Ocean atic trough-ridge-trough (wave) pattern is seen region during cyclogenesis of Ockhi, we plotted from western PaciBc Ocean to southwest BoB. The Hovmoller€ diagram of rainfall and 700 hPa time evolution of the trough in easterly located at J. Earth Syst. Sci. (2020) 129:194 Page 5 of 13 194

(a)

(b)

Figure 3. Time longitudinal diagram of (a) Global Precipitation Measurement (GPM) satellite estimated rainfall anomaly (mm dayÀ1) and (b) meridional wind anomaly (msÀ1) for the period 26 November, 2017 to 10 December, 2017. The Belds are latitudinally averaged for the region 0°–10°N. southwest BoB and associated relative vorticity Belds, TC related signal (e.g., rainfall) can be clearly show formation and westward movement of minimized by multiplying a Gaussian weighting the synoptic system over the region. This further function (see equation 2; purple curve in Bgure 5) corroborates with the IMD Ockhi cyclone report to daily rainfall anomalies (Aiyyer and Molinari (2017), which states that under the inCuence of the 2008; Schreck et al. 2011). trough in easterly, a fresh low-pressure area Here Gaussian weighting function is deBned as: (LOPAR) was formed over southwest BoB and 8 9 adjoining area of south Sri Lanka, and the equa- <> => ÀrxðÞ; y 2 torial Indian Ocean at 0300Z of 28 November, hi wxðÞ¼; y 1 À exp 2 ; ð2Þ :> À1=2 ;> 2017. 2 RðÞ2ln 2 To clearly bring out and conBrm the role of EWs on cyclogenesis of Ockhi, here we adopted the where r(x, y) is the distance from a given point to method proposed by Schreck et al. (2011) to iden- the TC centre and R is the radius at half maximum tify the role of convectively coupled waves in of the weighing function. To remove the eAect of cyclogenesis. Generally Bltering techniques are TC, rainfall at each location is multiplied by the used to isolate the signal of convectively coupled weight from equation (2). At zero radius of the TC, waves from various atmospheric Belds. However, all of the rainfall above climatology is removed, Schreck et al. (2011) demonstrated that TC sig- and the removal fraction decreases with rad- nificantly alters rainfall anomalies associated with ius following the Gaussian function. Rainfall at equatorial waves and MJO. They proposed that radius R (here we used R = 500 following Schreck before applying Bltering to various atmospheric et al. 2011) are half-associated with TC and 194 Page 6 of 13 J. Earth Syst. Sci. (2020) 129:194

Figure 4. Time evolution of the relative vorticity (SÀ1, shaded) and streamlines at 700 hPa for the period 27 November, 2017 to 01 December, 2017. These parameters are derived using horizontal and meridional components of the wind Belds from ERA5 reanalysis.

half-associated with the environment, including equatorial waves. This removal procedure can be applied to all TC positions, regardless of the basin or intensity (Schreck et al. 2011). Main purpose of this method is to minimise the TC related rainfall and to identify the other contributing factors, especially equatorial waves, in the cyclogenesis. Figure 5 shows an example from 0000 UTC rainfall of 30 November, 2017 to illustrate the method to remove the TC related signals. It shows latitudinal cross-section of rainfall through the centre of cyclone Ockhi along 75° E longitude. The storm is associated with the intense rainfall of Figure 5. Latitudinal section along 75°E of total rainfall about 178 mm dayÀ1 (black curve) at the core (black), with tropical cyclone (TC) removed (red), climatol- ogy (green) and TC weighing function (purple) for 0000Z of 30 region of Ockhi. By contrast, the climatological November, 2017. The GPM satellite derived rainfall data at rainfall rate (green curve) at the core region of 1 0.1° 9 0.1° grid resolution is utilised for generating this plot. Ockhi is \40 mm dayÀ . The red curve in Bgure 5 J. Earth Syst. Sci. (2020) 129:194 Page 7 of 13 194 shows the resulting rainfall rate after removing TC Band pass Blter, corresponding to EW and MJO, related rainfall. After the removal of TC related is applied to TC removed rainfall anomalies to rainfall, the rainfall rate of 178 mm dayÀ1 in Ockhi study their role in cyclogenesis of the Ockhi. Role core region is reduced to the climatological value of of the equatorial waves in tropical cyclogenesis is around 45–50 mm dayÀ1 (red curve). The anoma- generally identiBed when the Bltered rainfall lies at large distances (e.g., [1000 km) from the anomalies exceed a threshold value at the genesis storm are virtually unchanged because they are location. Schreck et al. (2011) discussed that the completely attributed to the large-scale environ- value 0 mm dayÀ1 is too small threshold because mental Cow including equatorial waves. Before unrelated noise can produce a positive anomaly applying Bltering technique to isolate the wave and threshold of 6 mm dayÀ1 is too large as two related signal from rainfall Beld, removal of TC third of the would not have precursors. associated rainfall is important, as storm-related Between these extremes, they found a reliable signal can contaminate wave associated signal in range of thresholds from 2 to 4 mm dayÀ1. Fol- the vicinity of storm. Figure 6(a) shows spatial lowing them, here we used 4 mm dayÀ1 as a map of the total rainfall, while the rainfall unre- threshold for attributing equatorial waves to lated to TC Ockhi after applying the weighting tropical cyclogenesis. Figure 7 shows a Hovmoller€ function for 30 November, 2017 is shown in diagram of the rainfall averaged over 0–10° N for Bgure 6(b). It can be seen from Bgure 6(b) that the the period 21 November to 7 December, 2017 which rainfall in the core region of TC has been signifi- includes the period of formation of the Ockhi. The cantly reduced. The TC unrelated rainfall unBltered rainfall is shaded and MJO and EW (Bgure 6b) is subtracted from original data wave Bltered anomalies are contoured at +4 mm (Bgure 6a) to produce TC related rainfall which is dayÀ1. Analysis shows that MJO related convec- shown in Bgure 6(c). tion (Bgure 7a) was conBned to maritime continent of the tropical Indian Ocean (eastern Indian (a) Ocean; 93°–102° E), while it was absent over the region of the cyclogenesis of the Ockhi. The actual position of the MJO activity for the above-men- tioned period is further validated from the phase diagram of the MJO obtained from the Bureau of Meteorology, Australia (see Bgure S1). It is seen from Bgure S1 that MJO activity was weak over Indian Ocean region during the cyclonogenesis (b) period of Ockhi. The result clearly indicates that the MJO has insignificant role in cyclogenesis of the Ockhi. On the other hand, rainfall anomaly contours ([ 4 mm/day) associated with the west- ward propagating EW coincides with the region of cyclogenesis of Ockhi. Analysis conBrms that the convectively coupled equatorial EW significantly contributed in cyclogenesis of the Ockhi. (c) 3.2 Rapid intensiBcation: Role of SST and upper ocean thermal energy

As mentioned before, Ockhi rapidly intensiBed from DD to CS in 6 hours, on 30 November, 2017. While moving west-northwestward, Ockhi further intensiBed into a Severe Cyclonic Storm (SCS) over Lakshadweep area at 0000Z of 1 December, 2017 and Very Severe Cyclonic Storm (VSCS) over Figure 6. (a) Total rainfall (mm), (b) modiBed rainfall rates following the TC removal, and (c) TC related rainfall for 30 southeast Arabian Sea to the west of Lakshadweep November, 2017. The GPM satellite derived rainfall data at on 0900Z of 1 December, 2017. The spatial distri- 0.1° 9 0.1° grid resolution is utilised for the analysis. bution of SST, TCHP, OMT of TCHP (the mean 194 Page 8 of 13 J. Earth Syst. Sci. (2020) 129:194

(a)

(b)

Figure 7. Hovmoller€ diagram of the actual rainfall (shaded) and Bltered wave related anomalies for (a) Madden Julien Oscillations (MJO) and (b) Easterly waves (EWs). Filtered anomalies with rainfall values exceeding 4 mm dayÀ1 are contoured. The GPM satellite derived rainfall data at 0.1° 9 0.1° grid resolution is used for Bltering. temperature of the water till 26°C isotherm), OMT clearly indicate the movement of Ockhi from the at different depths on 30 November, 2017 are shown area of comparatively lower ocean thermal energy in Bgures 8 and 9. The SST, TCHP and OMT of to the region of higher ocean thermal energy TCHP show the values [29°C, 70 kJ cmÀ2 and (Bgures 8 and 9). Most importantly, the region of 28°C, respectively, over the region of its intensiB- high OMT of 100 m layer ([26°C) and the VSCS cation from SCS to VSCS (Bgure 8). Subsurface stage of the cyclone coincide exactly, indicating temperature indicates the persistence of the 26°C that OMT of 100 m layer inCuenced the intensity of isotherm till 100 m depth (see Bgure 9). A similar the cyclone. spatial distribution can be seen for the SST, OMT The closer relationship between the OMT of 100 of 50 m depth and TCHP. Interestingly, the anal- m depth and the cyclone intensity is clearer from ysis shows that the region of high SST (Bgure 8a) Bgure 10, where the values of the SST, TCHP and coincides with the region of RI of cyclone Ockhi. OMT at 50, 100 and 150 m depth along the cyclone This indicates that abnormally warm SST over track with respective T-numbers are shown. It is Comorin region aided for the rapid intensiBcation noted that even though slight reduction in SST on of cyclone Ockhi. Hourly variations of the atmo- 2 December, 2017, the UOTE parameters show spheric variables (mid-tropospheric RH, 850 hPa increase in their values. It is intriguing to note from relative vorticity, 200 hPa divergence, vertical the Bgure that the OMT of 100 m along the track of wind shear, total precipitable water) from 22 to the Ockhi (cc = 0.8) coincides better with inten- 0300 UTC is analysed (see Bgure S2) to study the sity change as compared to SST and TCHP (please role of atmosphere on rapid intensiBcation. It is see Bgure 10). Hence, it can be concluded that the understood from the analysis that, there is no path of the Ockhi over the region of anomalously substantial variation in these parameters during high SST and subsurface temperatures provided the rapid intensiBcation. Therefore, it can be con- the additional energy for its rapid intensiBcation. cluded that the warmer SST over the region played Track of the cyclone Ockhi depicts the reduction of an important role in rapid intensiBcation of the its intensity after crossing 15°N. UOTE parameters Ockhi. The region of intensiBcation and mainte- depicted in Bgures 8 and 9 clearly indicate that the nance of its intensity from SCS to VSCS category substantial reduction in subsurface temperature J. Earth Syst. Sci. (2020) 129:194 Page 9 of 13 194

(a) (a)

(b) (b)

(c) (c)

Figure 8. (a) (°C), (b) Ocean mean Figure 9. Ocean mean temperature (°C) at (a)50m,(b) 100 temperature (°C), and tropical cyclone heat potential (KJ m and 150 m depth for 30 November, 2017. Data for these cmÀ2) for 30 November, 2017. Upper ocean thermal energy oceanic parameters are obtained from NRSC, Hyderabad, (UOTE) data is obtained from National Remote Sensing India. Centre (NRSC), Hyderabad, India. TC (Guard 1977). Generally, it is found that over the region (especially OMT100) resulted in recurving tracks are closely associated with retreat dropping the intensity of the Ockhi. of a subtropical ridge and arrival of a subtropical trough. The winds north of a TC are more impor- 3.3 Recurvature: Role of mid-latitude westerlies tant as far as its recurvature is concerned. For example, if a trough is located west-northwest of The prediction of the recurving track of a TC is the TC, there is a high probability for recurvature very difBcult and challenging to the forecasters of TC (IMD 1996). The cyclonic disturbances over (Holland and Wang 1995; Thu and Krishnamurti the NIO, mainly move along west-northwest 1992), as it involves further complex processes than direction, while some cyclonic disturbances recurve non-recurving one (Zhang and Tao 2015). The from an initial north-westward direction to the recurvature of a TC could be inCuenced by number northward direction (often after reaching 15°–20° of factors such as steering Cow (Kasahara 1959), N latitude) and Bnally towards north-eastward subtropical high (Evans and Allan 1992; Chen et al. direction (IMD 1996). 2009), trough in mid-latitude westerlies, monsoon As discussed, TCs move under the inCuence of systems (George and Gray 1977), vorticity advec- external as well as internal forces. While external tion (Elsberry 1990), zonal winds in the middle and forces refer to the eAect of large-scale atmospheric upper troposphere, etc. The inCuence of the winds processes on the movement of the TC (through north of a TC is generally dependent on the posi- steering Cow), internal forces arise from the storm tion of the mid-latitudinal trough relative to the itself and by interacting with steering Cow 194 Page 10 of 13 J. Earth Syst. Sci. (2020) 129:194

Figure 10. Time variation of sea surface temperature (°C, red curve), ocean mean temperature at 50 m (°C, green curve), 100 m (°C, blue curve), 150 m (°C, purple curve) depths, tropical cyclone heat potential (KJ cmÀ2, black curve) and T-number (light blue curve) for 30 November to 05 December, 2017. All Belds are averaged for 2° 9 2° box centering the of the cyclone. Intensity of the cyclone is mentioned in upper x-axis of the Bgure.

(a) (f)

(b) (g)

(c) (h)

(d) (i)

(e) (j)

Figure 11. Evolution of the outgoing longwave radiation (OLR, shaded WmÀ2) and streamline indicating the steering Cow (wind data vertically averaged from 850 to 300 hPa level) for the period 28 November 2017 to 07 December, 2017. OLR data at 4 km resolution is obtained from the INSAT-3D geostationary satellite. The wind data is from ERA5 reanalysis at 0.30° 9 0.30° grid resolution. J. Earth Syst. Sci. (2020) 129:194 Page 11 of 13 194

(Mohanty 1994). Steering level is the atmospheric systems over NIO, we further examined the role of level at which speed and direction of the sur- easterly waves in preconditioning the atmosphere rounding wind best correlate with those of the for cyclogenesis. For this purpose, we applied band cyclone. Here we examine the TC steering Cow by pass Blter, corresponding to EW and MJO, on averaging the wind Belds between 850 and 300 hPa rainfall by removing the TC signal from it. It is level following Wang et al. (2013). Anomalous found that, under the inCuence of the trough in steering Cow pattern (streamlines) and OLR for easterly a low-pressure area was formed over the period 28 November to 07 December, 2017 is southwest BoB and adjoining area of south Sri shown in Bgure 11. One can note the initiation of Lanka, and the equatorial Indian Ocean at 0300Z southward intrusion of mid-latitude westerlies over of 28 November, 2017. Our diagnostic analysis Arabian Sea from 29 November, 2017, while it is using Hovmoller€ diagram, streamline and 700 hPa more prominent and well-structured from 02 relative vorticity further conBrms the role of EW December, 2017 onwards. In other words, strong activity on cyclogenesis of Ockhi. north-eastward steering Cow with deep southward Various studies have demonstrated that ocean extent was seen during mature phase of the Ockhi. heat content in waters warmer than 26°C, is a This steering Cow pattern favoured the recurving useful predictor for TC intensity. Various high of the Ockhi and was instrumental in suppressing intensity TCs in different basins have been its conventional westward (north-westward) track observed to rapidly intensify over the regions of movement. In accordance with the prominent high TCHP. The region of rapid intensiBcation of intrusion of subtropical westerlies, one can also the Ockhi from SCS to VSCS indicate the higher see an eastward shift in upper level anticyclonic than threshold values of SST ([ 29°C), TCHP circulation and associated ridge pattern. ([ 70 KJ cmÀ2) and OMT of TCHP ([28°C). This In addition to the Ockhi, a well-marked low- implies that path of the cyclone over the regions of pressure area depicted by lower OLR (Bgure 11e) anomalously high SST, subsurface temperatures, was also seen over southeast BoB on 02 December, TCHP and OMT with depth of the 26°C isotherm 2017 and subsequently concentrated into depres- up to 100 m favoured the rapid intensiBcation of sion (IMD 2017). Analysis indicates (Bgure 11e–j) cyclone Ockhi. OMT of 100 m layer has inCuenced that under the inCuence of anomalous anti-cyclonic the intensity of the cyclone more than any of the steering Cow over eastern Indian Ocean region, the other parameters. synoptic system initially moved west-northwest- The synoptic systems over the NIO generally ward and then recurved to northeastward. move along west-northwest direction, while a few recurve from an initial north-westward direction 4. Summary and conclusions to a northward direction, often after reaching 15°–20°N latitude and Bnally towards north-east- The VSCS, Ockhi was initiated as a low-pressure ward direction (IMD 1996). After its initial north- area over the southwest BoB, intensiBed rapidly, westward movement, Ockhi recurved north- moved north-westwards initially, then recurved eastwards and hit the Gujarat coast. In order to north-eastwards and dissipated upon hitting understand the role of the upper atmospheric Cow Gujarat coast, northern part of the west coast of on the recurvature of the Ockhi, we examined India. It was one of the rarest of its kind and steering Cow (by averaging winds from 850 to 300 claimed more than 400 lives. This study examined hPa), streamlines and OLR anomalies for the per- the role of the EWs, UOTE and southward intru- iod 28 November to 7 December, 2017. It is noted sion of the upper level sub-tropical westerlies on that eastward movement, with pronounced south- the cyclogenesis, intensiBcation and recurvature of ward intrusion, of the mid-latitude upper level Ockhi using high resolution ERA5 reanalysis and westerly trough led to recurvature of the VSCS satellite datasets. Initially, we investigated the Ockhi. time evolution of atmospheric parameters, such as Though the track and intensity forecast of the lower level relative vorticity, upper level diver- cyclones over NIO has been improved in recent gence, midlevel relative humidity and vertical wind decades (Mohapatra et al. 2013a, b), studies on shear during the life cycle of Ockhi. It is found that rapid intensiBcation of the cyclone over NIO seek atmospheric conditions were highly favourable for further attention. Availability of the real time the cyclogenesis of Ockhi. As EWs often lead to observations of the UOTE over NIO is indispens- formation and intensiBcation of the synoptic able for determining the RI of the cyclones. Though 194 Page 12 of 13 J. Earth Syst. Sci. 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Corresponding editor: KAVIRAJAN RAJENDRAN