Interdecadal variability of the pre-monsoon cyclone characteristics over the Bay of Bengal

The low-pressure systems intensified to cyclones prior to the onset of the Indian summer monsoon season over the North Indian Ocean are referred to as pre-monsoon season (PMS) cyclones. Climate change is amplifying the pre-monsoon cyclone landscape, fostering more frequent and intense storms with altered tracks, resulting in heightened risks for coastal communities and economies. This study investigates the interdecadal variations in tropical cyclone and key large-scale atmospheric parameters that influence the characteristics of cyclones, including track, frequency over the Bay of Bengal (BoB) during the pre-monsoon season from 60 years of data. The large-scale atmospheric parameters are analyzed by calculating climatological anomalies. It is noted that the frequency of cyclones making landfall over the eastern Indian coastal landmass has increased in the recent decade compared to the past five decades. Compared to the past fifty years, the percentage frequency has increased in the recent ten years by 50%. At the low level, stronger easterlies are dominant and upper-level jet streams shift to lower latitudes, indicating that the path of cyclones has shifted from the north (N)-northeast (NE) to the northwest (NW), i.e. towards the east Indian coastal landmass, over the recent decade. In contrast to the previous five decades, an unusual low-pressure region has emerged over the NW India and Pakistan regions, creating a favorable path for cyclones moving towards the Indian region in recent decade. Cyclones have been more intense in the recent decade than they were in the previous five decades, according to the rise in low- and mid-level specific humidity and temperature over the BoB.


Introduction
Concern over global warming, which is a result of human-induced climate change and has a significant impact on extreme weather events, has grown substantially in recent decades.However, scientific evidence suggests that tropical cyclone (TC) intensification and changing behavior over different ocean basins are influenced by climate change.TCs primarily occur during the pre-monsoon (March to May) and post-monsoon (October to November) seasons over the North Indian Ocean (NIO), which includes the Bay of Bengal (BoB) and Arabian Sea (AS).The BoB is approximately four times more climatologically active than the AS, and the NIO makes up 7% of the globally distributed TCs (Dube et al 1997).According to earlier research, TC frequency in the BoB for May has significantly increased (Singh et al 2000).Additionally, they found that cyclonic disturbances that form in the pre-monsoon season have a substantial probability of progressing to a severe cyclonic stage.In recent years, Amphan was the first super cyclonic storm (SuCS) in the twenty-first century over the BoB, following the Odisha SuCS of 1999 and the first pre-monsoon season SuCS after 1991 (IMD 2020).Fani was the most intense cyclonic storm (CS) that crossed the Odisha coast in the pre-monsoon season during the satellite era (1965( onwards) after 1982( (IMD 2019)).The prerequisites for TC genesis and intensification include a sea surface temperature (SST) of at least 26.5 • C (79.7oF) to a depth of about 50 m (164 feet), the Coriolis effect, large mid-level tropospheric relative humidity (RH), large low-level relative vorticity (RV), intense convective activity, weak vertical shear of horizontal wind, ocean heat content, and large-scale circulations (Palmen 1948, Gray 1968, DeMaria et al 2001, Ramesh Kumar and Sankar 2010, Girishkumar and Ravichandran 2012, Mohapatra et al 2015, Li et al 2019, Singh and Roxy 2022).
Large-scale spatial and temporal variability in the genesis of TCs, track characteristics, and aspects of landfall are essential scientific issues that need to be addressed (Tyagi et al 2010, Mohapatra et al 2014).According to Sugi et al (2014), global warming is changing the frequency of TCs in the Indian Ocean.Based on their analysis, the reduction in strength of the upward flow of mass in the tropics is associated with a decrease in the frequency of TCs.Vertical wind shear (VWS), ocean heat content, coupling strength, environmental moisture, inner core dynamics, cloud microphysics, large-scale interactions, and the prevailing thermodynamic state all play a role in the change in TC intensity due to their multi-scale nonlinear interactions (Pattnaik and Krishnamurti 2007, Pattnaik et al 2011, Kutty and Gohil 2017, Rai and Pattnaik 2018, Baisya et al 2020, Munsi et al 2021, Vishwakarma et al 2022).Therefore, it is quite difficult to determine intensity accurately.Dai et al (2023) reported that the synoptic component plays a beneficial role in the formation of TCs over NIO when vertical zonal wind shear anomalies fall between −6 and 6 m s −1 and total vertical zonal wind shear falls between −12 and −3 m s −1 .Geetha and Balachandran (2014) have investigated the decadal changes of the cyclonic disturbances over the NIO basin for the past five decades , which comprise depressions and all TC classifications.They found that the translational speed (TS) of cyclonic disturbances over the BoB ranged from 6.4 to 9.6 km h −1 over the first four decades , and over the recent decade (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), this has increased to 17.6 km h −1 .Nath et al (2016) have also studied decadal variations of ocean heat content and TC activity over the BoB during the 1955-2013 periods.According to Rajeevan et al (2013), epochal variation in the storm's atmospheric VWS anomalies and oceanic heat content contributed to more violent TCs occurring during the most recent epoch (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) than during the earlier epoch (1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992).Priya et al (2022) examined the TC characteristics across the NIO basins, comprising CS and above in terms of stormy days (SD).They reported that from SD1 to SD5, deep wind shear over BoB is comparatively higher during the pre-monsoon (5-15 m s −1 ) compared to the post-monsoon (5-8 m s −1 ).This could be one of the reasons why there were fewer TCs during the pre-monsoon season over the BoB.Additionally, based on climatology, an increasing number of TCs are developing curvature patterns in the direction of the Bangladesh and Myanmar regions, preventing them from reaching the Indian subcontinent for the pre-monsoon season over the BoB.This is because a significant (positive) southerly deep shear anomaly over India's eastern coast prevents storms from advancing towards India and causes them to recurve into Bangladesh and Myanmar.The comparison of pre-monsoon season BoB landfalls reveals that TCs are making relatively more landfalls across the Indian mainland than pre-monsoon season AS.The geopotential height (GPH) anomaly that was negative over the Indian continent during pre-monsoon season over BoB may have had an impact on this pattern.Regarding the trajectory, intensification cycle, and rainfall, Vishwakarma and Pattnaik (2022) investigated the typical characteristics of the TCs.They also examined the important precipitation efficiency mechanisms of two pre-monsoon season TCs, Fani and Yaas, to understand how rainfall is regulated. Teir findings imply that even comparatively weaker TCs can cause catastrophic rainfall and destruction.During the pre-monsoon season, TCs took the northward track and made landfall across the Bangladesh and Myanmar coasts (Mondal et al 2021).The changing status of TCs over the NIO have been studied by Deshpande et al (2021).They found that in two epochs (1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019), the post-monsoon season TC tracks for AS and BoB seems to be the same, while change is seen for pre-monsoon season TCs over both the basins.
The characteristics of TC activity in the BoB, such as strength, frequency, genesis location, trajectory, and average longevity, are influenced by the El Niño-Southern oscillation (ENSO) and Indian Ocean Dipole (IOD) (Mahala et al 2015, Bhardwaj et al 2019, Albert et al 2021).According to the results of their study (Mahala et al 2015), the frequency of TCs is at its peak during La Nia years, years with an IOD (-ve), and times when both of these conditions overlap.Albert et al (2021) reported that, compared to the El Niño period, the genesis locations migrate westward and towards the equator during La Niña.Additionally, during the pre-monsoon season in La Niña years, the wind streamlines at 200 hPa are moving in a westerly direction, which is favorable for the development of cyclones over BoB.La Niña's pre-monsoon season is marked by higher RH600, RV850, and lower outgoing longwave radiation (OLR500) values, which favor the development of intense cyclones.When the Madden-Julian oscillation was in its convective phase, TC genesis predominately took place (Krishnamohan et al 2012).Huangfu et al (2022a) have studied the roles of synoptic-scale waves (SSWs) and intraseasonal oscillations (ISOs) in the onset of the South China Sea (SCS) summer monsoon.Based on the analysis of eddy kinetic energy, they found that compared to the two (10-20 d and 30-60 d) ISOs components, the SSW (2-10 day period) over the SCS key region play a significant role in maintaining the SCS summer monsoon onset.The onset of the SCS summer monsoon can be triggered by a tropical cyclone (TC) (Huangfu et al 2017).A favorable environment for the development of SSWs is provided by the significant easterly VWS, the accelerated upward motion, and the abundant moisture supply in the SCS and northwest Pacific regions (Huangfu et al 2022b).The western North Pacific (WNP) and neighboring countries had increased TC tracks between 2011 and 2021 because of SSWs' tendency to move northward.As opposed to this, westward-moving TCs were more likely to be favored by westward-propagating SSWs prior to the early 2010s (Huangfu et al 2023).Intense typhoons (ITYs) over WNP, which maintain high intensity prior to landfall, and TCs undergoing rapid intensification (RI) over the SCS and landfalling in SC, with a higher maximum intensity and location of maximum intensity closer to the coast of SC, are related to the increase in maximum landfall intensity between 2012 and 2018 (Liu and Chan 2020).The Pacific Decadal oscillation and large-scale atmospheric favorable conditions of vertical velocity, RV, RH, and warm SST anomalies are linked to an increase in the WNP TC frequency between 2011 and 2020 (Wang and Wang 2023).Over East Asia and the western Pacific, the spatial mode of circulation anomalies has shifted westward due to surface warming reacceleration in offshore China since 2011 (Tang et al 2020).Favorable (unfavorable) large-scale conditions caused by Atlantic Multidecadal oscillation positive (negative) phases tend to increase (decrease) TC lifetime over the extra tropics, resulting in more (fewer) opportunities for TCs to be accelerated to a high translation speed by the midlatitude westerly steering flow (Guo et al 2023).
It is evident from the aforementioned discussions that a very limited number of comprehensive investigations have been conducted to examine the interdecadal variations in the characteristics of TCs as well as key large-scale atmospheric parameters that influence the characteristics during the intense phase of the TCs, i.e.Frequency, track, intensity, wind, GPH, specific humidity (SPH), and temperature across the BoB during the pre-monsoon season from the 60 year datasets.Understanding the TC characteristics and their important variations in the BoB basins during the pre-monsoon season is important in the context of climate change.Furthermore, no research has been done on the BoB basin's links to important TC parameters, specifically its frequency, track, and intensity categories, on an interdecadal basis during the pre-monsoon season.This article has made an effort to address these gaps by providing new insights that will improve our understanding.The following sections have been introduced to the manuscript: data and methodology (section 2), results and discussion (section 3), and conclusions (section 4).

Data and methodology
The whole 60 years  of TC datasets are taken into account to examine variations in the key parameters (track, frequency, and intensity) during pre-monsoon season over the BoB basins.In addition, to further analyze the interdecadal variabilities of TCs, 60 years of wind, GPH, SPH, and temperature data are taken into consideration.The large-scale atmospheric parameters are analyzed by calculating climatological anomalies.Climatological anomalies, or deviations from the mean, are created by subtracting the climatological normal from observed data.Climatological normals are monthly averages calculated over at least 30 consecutive years .From the International Best Track Archive for Climate Stewardship (IBTrACS 2010(IBTrACS , 2018) ) and the India Meteorological Department (India Meteorological Department (IMD) 2022a, 2022b), a collective total of 45 cases for BoB basins are taken into consideration to conduct an analysis.The tracks are classified into three groups: N, NW, and NE, based on 70% of a TC's lifespan track (from origin to landfall).In the case of the BoB basin, the demarked areas that can be found between 88 and 92 degrees East ( o E) are used as a point of reference for north (N) directions (figure 1).This N-designated area's left (right) side is designated as the NW (NE) direction.The letters N, NW, and NE are used to denote TCs crossing Bangladesh (BGD), India (IND), and Myanmar (MMR), respectively.

Results and discussion
In this section, we discuss the interdecadal variations seen in the track, frequency, and intensity (category) of TC across the BoB basin during the pre-monsoon season from 1963 to 2022.In addition, key large-scale atmospheric parameters that influence the characteristics of cyclones are also examined in this section.

Large-scale environmental conditions
The significant role of large-scale atmospheric parameters, such as the wind, mid-wind shear (difference between 500 and 850 hpa), GPH, SPH, and temperature over the BoB basin, during pre-monsoon season, is examined in the context of interdecadal variability of the TCs characteristics and behavior for 1963-2022.This exercise aims to elucidate the typical large-scale signatures over the region to bring new insight into the prevalent synoptic-scale conditions over the Indian subcontinent and the regions that are neighboring it.
The large-scale atmospheric parameters are analyzed by calculating climatological anomalies.Climatological anomalies, or deviations from the mean, are created by subtracting the climatological normal from observed data.Climatological normals are monthly averages calculated over at least 30 consecutive years (1991-2020) (figures 4-12).In these figures, the left-side color bar represents wind speed (figures 4-6), mid-wind shear (figure 7), GPH (figure 8), SPH (figures 9 and 11), temperature (figures 10 and 12), and the right-side color bar represents the category (intensity) of TCs.This study has been carried out to give accurate anomalous signatures of the large-scale features.-2012, (c) 1993-2002, (d) 1983-1992, (e) 1973-1982, (f) 1963-1972.The left-side colorbar represents wind speed, and the right-side colorbar represents the category (intensity) of TCs.

Wind
The strength and movement of the TCs are impacted by the wind's speed and direction at the low-level and mid-level of the atmosphere, respectively (George and Gray 1976).It is noted that easterlies are found to be stronger At mid-level in the atmosphere, the wind's direction affects the movement and steering of TCs.This is so that the storm's track and intensity are affected by how the storm is being pushed by the mid-level winds.Because of the influence of 500-300 hPa steering flows, more than half of the TCs over BoB that occurred from 1990 to 2019 recurved from their original path (Akter and Tsuboki 2021).It is found that interaction between westerlies and easterlies at east Indian coastal belts (5-6 m s −1 ) at mid-level during the pre-monsoon season over the BoB basin in the recent decade (figure 5(a)) created a barrier for storms headed towards BGD and MMR while allowing them to take a diversion towards the eastern Indian coastal landmass.On the contrary, there is no such type of interaction (only westerlies) has been seen (2-5 m s −1 ) in the past five decades, suggesting that a greater number of TCs are going toward BGD and MMR regions (figures 5(b)-(f)).The effect of upper-level wind on TCs is discussed in the following section.
The path and speed of TCs are also affected by the upper-level jet stream (Choi and Moon 2012, Yu et al 2017, Finocchio and Doyle 2019, Do and Kim 2021).The jet streams are narrow, fast-moving wind bands that are found in the high atmosphere and have the power to drive storms.It is observed that the shifting of the upper-level jet stream to the lower latitude (9-10 m s −1 ) during the pre-monsoon season over the BoB basin in the recent decade (figure 6(a)) implicates that the path of TCs changes from the N-NE to the NW direction, i.e. towards the Indian region.But such a type of signature has not been found in the past five decades, giving the idea that the majority of TCs are going in the N and NE directions, i.e.BGD and MMR (figures 6(b)-(f)).Overall, it is concluded that easterlies are stronger at low-level, interaction of easterlies and westerlies at east Indian coastal belts at mid-level, and shifting of upper-level jet-stream to the lower latitude of the atmosphere in the recent decade for BoB in the pre-monsoon season gives a clear signature that the path of the TCs has shifted from N-NE to NW direction, i.e. towards east Indian coastal landmass, compared to the past five decades.

Mid-wind shear
Mid-wind shear (MWS) is the term used to describe the vector difference in wind speed and direction between two levels (500-850 hPa) and also plays a crucial role in the development and intensity of TCs.Easterly wind shear is significantly more favorable than westerly shear.So, MWS can either hinder or enhance the development and intensification of TCs (Nolan and McGauley 2012).It is interesting to note that for BoB during the pre-monsoon season, MWS has increased (4-5 m s −1 ) over BoB, BGD, and MMR regions in the recent decade (2013-2022) (figure 7(a)), has acted as a resisting force for further growth of TCs towards BGD and MMR and facilitated a recurve path towards east Indian coastal landmass.But comparatively, it is seen that, during the past five decades , MWS was observed to be relatively weaker (1-4 m s −1 ) over the same regions, attributed to the fact that there was no obstruction for the growth of TCs towards BGD and MMR regions (figures 7(b)-(f)).Overall, it is concluded that MWS might be one of the contributing factors for TCs facilitating a recurve path towards the Indian region in the recent decade during pre-monsoon season over BoB.

GPH
GPH measures the height of a pressure level above sea level.For the period covering 1963-2022, the GPH anomaly has been computed for pre-monsoon season over the BoB (figure 8).The TC path trajectories are quite relevant to the subject of our discussion.It is revealed that, there is an anticlockwise circulation of wind   , this type of signature has not been seen, which means low-pressure zones are found over the entire N-NE India, BGD, and MMR regions, inferring that the majority of TCs are moving in the N and NE directions, i.e.BGD and MMR (figures 8(b)-(f)).Overall, it is concluded that the shifting of the low-pressure zone to the NW India and Pakistan regions in the recent decade created a favorable path for the TCs to come towards the East Indian coastal landmass in BoB during pre-monsoon season compared to the past five decades.However, for BoB, some additional parameters may have had a significant impact on regulating the TC trajectory, including the position of the trough/ridge axis, deep shear, large-scale steering current, SST, and the size of the storm.Thus, rather than TC intensification, the GPH anomaly has been investigated in connection with its trajectory.2003-2012, (c) 1993-2002, (d) 1983-1992, (e) 1973-1982, (f) 1963-1972.The left-side colorbar represents specific humidity, and the right-side colorbar represents the category (intensity) of TCs.2003-2012, (c) 1993-2002, (d) 1983-1992, (e) 1973-1982, (f) 1963-1972.The left-side colorbar represents temperature, and the right-side colorbar represents the category (intensity) of TCs.

Conclusions
Overall, a decrease in the frequency of TCs (20) that have occurred in the most recent three decades  as compared to the previous three decades , where there were 25, over the BoB basin for pre-monsoon season.However, there was a sequential decadal reduction in the number of TCs (11, 8, and 6) in the past three decades  in comparison to (4, 8, and 8) in the recent three decades .It is interesting to find that, compared to the previous five decades , the most TCs (4) are moving in the NW direction and the least TCs (1) are moving in the NE direction for the BoB basin during pre-monsoon season in the most recent decade (2013-2022), which indicates the path of TCs has shifted from N-NE to NW directions.Hence, we have noted that a greater number of TCs are crossing the East Indian coastal landmass in the recent decade and a smaller number of TCs are crossing the BGD and MMR regions for BoB in pre-monsoon season compared to the past five decades.The percentage frequency has increased to 50% for IND, whereas it has reduced to 37.5% for BGD and 12.5% for MMR in the last decade.Further, it is found that more intensified storms (SCS category and above) have formed over BoB for pre-monsoon season in the recent decade.
Easterlies are stronger at low-level, interaction of easterlies and westerlies at east Indian coastal belts at mid-level, and shifting of upper-level jet stream to the lower latitude of the atmosphere in the recent decade (2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022) for BoB in the pre-monsoon season gives a clear signature that the path of the TCs has shifted from N-NE to NW direction, i.e. towards east Indian coastal landmass, compared to past five decades .Mid-wind shear has increased (4-5 m s −1 ) over BoB, BGD, and MMR regions in the recent decade (2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022), has acted as a resisting force for further growth of TCs towards BGD and MMR and facilitated a recurve path towards east Indian coastal landmass compared to the past five decades  for pre-monsoon season.We have found that for BoB in pre-monsoon season, an anomalous low-pressure zone (0 to −4 m) has prevailed over the NW India and Pakistan regions, which creates a favorable path for the TCs to head towards the Indian subcontinent; as a result, most TCs have been heading NW over the recent decade (2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022).It is also noted that TCs are more intense in the recent decade (2013-2022) compared to the past five decades  due to the rise in low-and mid-level SPH and temperature over BoB during pre-monsoon season.
The ERA-5 data are aimed to present the synoptic-scale (large-scale) features and have a horizontal resolution of 25 km.Due to its poorer resolution, the cyclone intensity in these 60 years of ERA-5 data is noticeably underestimated, implying that storms were weaker than those recorded (IBTrACS/IMD); thus, these datasets do not clearly show vortex-scale circulation.As a result, large-scale circulation (steering) flow from ERA data will have less vortex-scale circulation, particularly at higher levels (500 hPa), and large-scale flow is less contaminated.An accurate representation of the dominant large-scale conditions in the area can be found in the composite anomaly plots of the large circulation for steering flow at 500 hPa of wind and GPH.Additionally, a reasonable representation of the faraway large-scale flow from the storm zone would be the composite anomaly of 500 hPa GPH and the wind of 45 cyclones over 60 years (from long-term climatology).This would have just a minimal amount of contamination from the vortex-scale circulations contained within these data.Here, showing the faraway large-scale flow pattern is more important than focusing on the anomaly in the storm vortex zone.However, this minor data limitation is accepted by us.The IMD data from 1963 to 2022 has been considered for our analysis, in addition, several other studies have carried out using this data including data prior to satellite era i.e. 1980 (Nath et al 2016, Mondal et al 2022, Priya et al 2022).Therefore, the data prior to 1980 might have some limitations.However, results of this study is robust and can have immediate implications for forecasting agencies as well as other organizations, particularly in the coastal states of east India, where variations in the TC's characteristics can be used for planning and policymaking to create an infrastructure and society that are more resilient.
(f)).Similarly, the TC tracks in N, NW, and NE (2, 2, 4) for[1973][1974][1975][1976][1977][1978][1979][1980][1981][1982] indicate that the NE direction is the most dominant for most of the TC tracks, followed by the N and NW directions (figure 1(e)).Likewise, the TC tracks in N, NW, and NE (3, 2, 1) for 1983-1992 implicate that most of the TC tracks are in N, followed by NW and NE directions (figure1(d)).For the years 1993-2002, the TC tracks in the N, NW, and NE (3, 0, 0) show that most TCs move in the N direction, followed by the NE and NW (figure 1(c)).For 2003-2012, it is noted that the tracks of TCs are in the N, NW, and NE (3, 2, 3), referring to the fact that most of the TC tracks are in the N and NE, followed by the NW direction (figure1(b)).Contradictorily, the tracks of TCs were found to be N, NW, and NE (3, 4, 1) for the period 2013-2022, inferring that TCs have a strong tendency to move towards NW, followed by N and NE directions (figure1(a)).It is interesting to see that, compared to the previous five decades, the most TCs (4) are moving in the NW direction and the least TCs (1)
(3-4 m s −1 ) at low levels during the pre-monsoon season over the BoB basin in the recent decade (2013-2022) (figure 4(a)), suggesting that the path of the TCs has shifted from the N-NE to the NW direction, i.e. towards the east Indian coastal landmass, and more intense TCs are formed.But comparatively, it is seen that, during the past five decades (1963-2012), easterlies have been observed to be relatively weaker (1-3 m s −1 ), which indicates the maximum number of TCs are going in the N and NE directions, i.e. towards BGD and MMR regions (figures 4(b)-(f)).The impact of mid-level wind on TCs is discussed in the following section.
3.2.4.SPH and temperature SPH measures the amount of water vapor in the air.The mean SPH and temperature anomalies at low-levels (up to ∼3 km) of the atmosphere are presented for pre-monsoon season over the BoB basin for the 1963-2022 period in figures 9 and 10, respectively.It is found that in the recent decade (2013-2022), SPH and temperature anomalies have a strong positive anomaly (0.0004-0.0008 kg kg −1 and 0.5-1 oC), suggesting that pumping of more amount of moisture into the atmosphere through deep convection over BoB during pre-monsoon season (figures 9(a) and 10(a)).As a result, more intense TCs have formed in the recent decade.But in the past five decades (1963-2012), SPH and temperature anomalies have been relatively weaker (−0.0004 to 0.0004 kg kg −1 and 0-0.5 • C), inferring that TCs are comparatively less intense (figures 9(b)-(f) and 10(b)-(f)).A weak negative temperature anomaly (−0.5 • C) has been found throughout the Indian region, followed by a strong positive anomaly (1 • C) over the eastern coast of India.These findings generally indicate that the intensification process is ongoing in the oceanic region (figures 10(a)-(f)).The mean SPH and temperature anomaly at mid-level (∼4-9 km) of the atmosphere are presented for pre-monsoon season over the BoB basin for the 1963-2022 period in figures 11 and 12, respectively.It is

Table 2 .
Frequency (percentage frequency) of tropical cyclones (TCs) crossing different countries surrounding the Bay of Bengal (BoB) in pre-monsoon season during 1963-2022.