Perspective Chapter: The Coastal Migration of the Locations of Tropical Cyclone Rapid Intensification over the North Indian Ocean

Abstract

The migration of maximum intensity poleward is triggering a shift in the rapid intensification (RI) locations of tropical cyclones (TC) towards the coast of ocean-rim countries. The study investigates changes in the distribution of locations of RI during the pre-monsoon and post-monsoon seasons in recent warming climate scenarios over the North Indian Ocean (NIO) basin. Over the Bay of Bengal (BOB), the percentage of annual RI TC frequency exhibits a stable or slightly decreasing trend (20–100%), contrasting with a notable surge (50–100%) over the Arabian Sea (AS) in recent years. The distribution of RI TC location gradient is meridional during the pre-monsoon season and is confined zonally below 15°N during the post-monsoon season over BOB. The corresponding locations over AS are confined between 10°N–15°N and 12°N–17°N latitudinal regions. An inverse relation between the simultaneous rise in SST and RH550 is evident during the pre-monsoon season, while the relation fails during the post-monsoon season over BOB. While sea surface temperature and mid-tropospheric relative humidity play a crucial role in RI, the observed changes in tropospheric vertical wind shear patterns and upper-level divergence alignment in current climate conditions are identified as influential factors shaping the distribution of RI location over BOB and AS.

1. Introduction

The North Indian Ocean (NIO), encompassing the Bay of Bengal (BOB) and the Arabian Sea (AS), associated with hugely populated coastal regions are no stranger to the formidable forces of tropical cyclones (TCs). Unlike other ocean basins, NIO TCs exhibits a bimodal nature in TC genesis, i.e., the primary peak is observed during post-monsoon (October, November, and December) and the secondary peak is seen during pre-monsoon season (March, April, May). These natural phenomena have tremendous potential to wreak havoc on coastal regions. One intriguing aspect of these cyclones is their ability to undergo rapid intensification (RI), a phenomenon characterized by a swift increase in wind speeds ≤30knots in 24 hours [1].

Song and Klotzbach [2] reported a significant poleward shift in the annual mean maximum intensity latitude post-1980. Nevertheless, the migration rate is nonuniform on decadal timescales, having a significant increasing trend over the West-North Pacific basin. This observed change is primarily linked to the sea surface temperature (SST) variation over the basin. Studies such as [3, 4, 5] indicated that the twenty-first century TCs will most probably form within a broader range of latitudes than those of the past 3 million years as there will be an increasing mid-latitude TC genesis favorability and the average migration rate of TC away from tropics at present is nearly a degree of latitude per decade. The variation in regional Hadley circulation is also found to be impacting the displacement of TC genesis and intensification [6]. Despite several studies focusing on the poleward migration of TC genesis and intensification characteristics, the regional shift in the intensification feature of TC still remains a puzzle, especially the zonal migration of RI locations of TCs.

Studies focusing on the global distribution of TCs, and their annual frequency and intensity have already been carried out by many researchers (e.g., [7, 8, 9, 10]). However, studies addressing the propagation of rapid intensification locations are negligible for the NIO region. In the current climate scenario, the weakening of the summer monsoon circulation has resulted in less southward ocean heat transport [11]. The decrease in heat transport has led to the accumulation of heat in the NIO region with an increase in the high-intensity TCs and the consequent RI of these systems. As the majority of the Southeast Asian people live around the NIO coastal region, any long-term change in TC climatology would greatly affect these lives and the economy [12].

The prediction of the maximum intensity of TCs is still a major challenge for most of the operational agencies [13, 14], and the prediction gets worse if the TC suffers RI in its lifetime. As the NIO basin is a comparatively small basin and has a high coastal population, the threat increases by many folds when the TC encounters RI near the coast. Climatologically, the annual TC frequency is decreasing, and the number of highly intensified systems is increasing over the basin. Furthermore, the RI TC frequency is also found to be rising in recent years [15].

Surprisingly, the intense TC activity over AS has drawn the attention of the research community in changing climate scenarios. Though the annual frequency over BOB is higher than AS in post-monsoon season, the rapidly intensifying TC frequency over AS is comparable to that of BOB, confirming that AS is more vulnerable to rapidly intensifying systems in present climate conditions. Numerous studies have shown that the annual TC frequency over AS has been rising in recent years and suggested many possible reasons for the rise such as increased SST, decreased wind shear, and anthropogenic pollutants over the basin [9, 16, 17, 18]. The matter of concern is that most of these systems are attaining high-intensity levels in current climate conditions and any shift in their RI location, whether towards the Arabian Peninsula or Indian coast would result in higher socio-economic loss to the countries.

Similarly, the BOB coastal areas experience a significant number of highly intensified landfalling systems every year. India is among the top ten countries in the world in terms of absolute losses from disasters between 1998 and 2017, totalling an estimated 79.5 billion dollars [19, 20]. One unfortunate instance of a landfall concerning rapidly intensifying TCs in the BOB is the 1999 Odisha Super Cyclone. In this case, the system escalated from a severe cyclonic storm (SCS) with a maximum sustained wind (MSW) of at least 48 knots on October 27th to reach an intensity of MSW ≥ 120knots by October 28th, by experiencing RI, i.e., from a tropical storm to category 5 storm in Saffir–Simpson hurricane wind scale (SSHWS) in just 24 hrs. The event was classified as a Super Cyclonic Storm (SuCS) according to the cyclone classification by maximum sustained wind speed and pressure deficit adopted by the India Meteorological Department [20, 21]. The system underwent RI when the system was closer to the coast and due to a certain degree of uncertainty in initial track and intensity, the larger size of the storm, and the lack of public awareness back then, the storm caused a huge death toll, enormous destruction and loss of properties. Nonetheless, during TC Phailin (2013) and Hudhud (2014), India was highlighted internationally due to its continuous disaster management improvements [22] and its achievements in saving lives.

The propagation of any of the maximum intensity location or RI location towards the coastal regions is closely linked to the potential destruction by their associated heavy rainfall [23], storm surges, strong wind, and flash floods resulting in inundation of low-lying coastal areas. The coastal propagation of the zone of RI attainment of NIO TCs is a complex interplay of atmospheric conditions, coastal topography, and the dynamics of the storm itself. The NIO basin has encountered an elevated SST, especially during the pre-monsoon and post-monsoon seasons in present climate conditions. Beyond surface temperatures, the NIO exhibits a high ocean heat content, allowing cyclones to draw energy from a deep ocean reservoir. The availability of this extensive heat content contributes significantly to sustaining RI [24]. Vertical wind shear (VWS), the change in wind speed and direction with altitude, plays a pivotal role in cyclone development. Wind shear is one of the important factors as it allows the storm to maintain a well-organized structure [25]. A moist atmosphere is essential for the latent heat release crucial to the intensification process.

Since NIO is an active breeding ground for highly intensified storms, understanding the above-said climatic factors becomes paramount. The RI of North Indian TCs serves as a reminder of nature’s immense power and the need for continued efforts in understanding its long-term behavioral changes and adapting to these dynamic forces. Recognizing the implications of this propagation is essential for coastal communities, policymakers, and disaster management authorities to develop resilient infrastructure, implement effective evacuation plans, and mitigate the potential impact of these natural disasters. As climate change continues to influence cyclonic patterns, ongoing research and advancements in the climatology of TCs are essential for improving our ability to predict and respond to these dynamic coastal threats. The current chapter investigates the coastal migration pattern of potential zones for the RI of TCs formed over NIO and subsequent environmental factors influencing the concerned movement of TC locations over the BOB and AS basin by considering the pre-monsoon and post-monsoon seasons separately.

2. Data and methodology

The present study is carried out over the NIO region, i.e., 50°–100°E and 5°–30°N domain (Figure 1) for the period 1991–2022. To carry out the present study, the TC position and intensity were considered from the India Meteorological Department (IMD) best track datasets available at www.rmcchennaieatlas.tn.nic.in. The adopted methodology for the generation of the best track data is elaborately discussed in IMD technical note version 2.0/2011 (IMD, 2011). Singh et al. [26] observed that the IMD best track data is reliable enough to be used for studies focusing on TC activity. The tropical systems formed over NIO are categorized into 7 categories, i.e., Depression if MSW is 17–27knots, deep depression (DD) if the MSW is 28–33knots, cyclonic storm (CS) if MSW is between 34 and 47 knots, severe cyclonic storms (SCS) if MSW is 48–63knots, very severe cyclonic storm (VSCS) if it has MSW between 64–89knots, extremely severe cyclonic storm (ESCS) if MSW is within the range 90–119knots and super cyclonic storm (SuCS) if the MSW is higher than 120knots (www.rsmcnewdelhi.imd.gov.in, [9]). Further, the study considers storms with intensity ≥34 knots only. The RI locations in this chapter are considered to be the latitude/longitude positions of the TCs when the system began to show signatures of the rapid increase in the wind intensity, i.e., thereafter the MSW intensity increased by 30 knots in 24 hours.

Figure 1.

Schematic representation of the North Indian Ocean (NIO) region and associated coastal countries.

The atmospheric parameters such as SST, u and v winds at 1000-200hpa, divergence at 200hpa, and mid-tropospheric relative humidity at 550hPa (RH550) are collected from European Center for Medium Range Weather Forecasts reanalysis 5 (ERA5) hourly data on pressure levels with 0.25° horizontal resolution datasets (cds.climate.copernicus.eu). Malakar et al. [27] analyzed different re-analysis datasets to study the evolution of TCs Over NIO and found that ERA5 gives the best estimate of TC state among all considered reanalysis datasets and hence the study considers ERA5 data for analysis. The deep-layer wind shear (DWS) is computed as the magnitude of the vector wind difference between 200 and 850hPa.

3. Studied gradient of TC activity

3.1 RI TC climatology

The long-term changes in both the annual TC frequencies and rapidly intensifying TCs, as shown in Figure 2a and b, depict a slightly decreasing trend over BOB and an increasing trend over AS during 1991–2022. Similarly, there is a consistent decline in the percentage of the 5-year running mean of RI TCs that occurred relative to the annual total TC frequencies over the BOB. In contrast, there is a noticeable upward trend in the corresponding percentage for the AS. Here the frequencies include the systems during both the pre-monsoon and post-monsoon seasons. Several studies have shown that the number of global TC suffering RI has increased in recent decades [15, 28]. On the contrary to the global TC trend, the 5-year running mean of the percentage of RI TCs over BOB has decreased from ~80% in 1995 to ~10% during 2022. Over the AS, the corresponding percentage has increased to ~100% during the current decade. Subsequently, the general conception of a rising trend in the RI scenarios over NIO could be attributed to the rise over AS.

Figure 2.

Time series of annual total TC frequency (blue bar), RI TC frequency (green bar), linear trend for annual frequency (blue lines), and RI TCs (green lines) over (a) BOB and (b) AS. The percentage and 5-year running mean of RI TCs over (c) BOB and (d) AS. Here all the analysis of the annual trends includes pre-monsoon and post-monsoon season systems only.

3.2 Gradient of RI locations

Regardless of the near decrease (increase) in the overall frequency of TCs (RI TCs) across the NIO, areas with high landfall TC frequencies exhibit significant coastal vulnerability. Additionally, the migration of locations where these systems undergo RI doubles the threat to the coastal areas. Figure 3 indicates a shift in the RI locations of TCs formed over BOB towards the southern coastal regions of India, i.e., towards Andra Pradesh (15.91°N, 79.74°E) and Tamil Nadu (11.12°N, 78.65°E) coast during both pre-monsoon and post-monsoon season. It is observed that the region of 85°–90°E and 10°–15°N has suffered the greatest number of RI systems during both seasons. During pre-monsoon season, although the frequency of TCs that underwent RI is less (~9 only) during 1991–2022, nearly 70% of systems experienced RI in the latitude region of 10°N to 15°N (Figure 3). During the post-monsoon season of the early and mid-90s, the RI positions were nearly parallelly ~5° away from the coast of India. However, with increasing warming conditions over NIO, the corresponding positions are found to be distributed in an arch manner over BOB, mostly confined to regions below 15°N latitudes increasing vulnerability for the lower coastal regions of India as well as the Gulf of Thailand. As it’s not only the landfall of these systems with sudden enhanced maximum intensity but also the heavy rainfall and wind of these events that cause destruction to the nearby areas. The location of maximum frequency remaining unchanged as that of pre-monsoon season, the environmental condition over the oceanic region needs to be investigated to understand the reason behind the suitability for RI over this particular grid. It is to be noted that the distribution of the TC RI region is longitudinally distributed during pre-monsoon and nearly latitudinal during the post-monsoon season over BOB (Figure 3).

Figure 3.

The gradient of locations of RI during 1991–2022 for (a) pre-monsoon and (b) post-monsoon over BOB. The color bar represents the years of RI TC formation over the basin.

Figure 4 illustrates the shift in the location where tropical systems formed over AS experienced RI. The figure shows that the regions bound between the ~12°N to 17°N are prone to RI throughout the study period. Singh et al. [18] showed that due to a change in wind pattern over the AS basin, the IAA regions or the Arabian Peninsula coast (19.49°N, 47.44°E) and Gujarat (22.67°N, 71.57°E) regions are more prone to TC landfall, i.e., the TCs either travels westwards towards the IAA or recurves to make landfall over Gujarat. The AS coastal regions exhibit similar susceptibility to RI TC cases. During post-monsoon season, the region of RI mostly lies parallel to the coast of IAA and has been migrating towards the coastal areas during the present decade with increase in the frequency, representing the presence of favorable environmental conditions over the region to support the RI of the storm generated over the basin. It enhances the possibility of a continuous rise of threats to both the Indian and Arabian coastal countries.

Figure 4.

The gradient of locations of RI during 1991–2022 during (a) pre-monsoon and (b) post-monsoon over AS. The color bar represents the years of RI TC formation over the basin.

The trend for change in latitude and longitude of RI experiencing regions during the pre-monsoon season is not significant due to the small sample size or lesser number of systems during the season (Figure 5a and c). The quadrennium variation of latitudinal and longitudinal variation over BOB during pre-monsoon season indicates a decrease in latitude (inter-quartile range 8.5°N–11°N) and longitude positions (inter-quartile range between ~86°E and 87°E) of RI over the basin for the period 2019–2022, representing the increase in landfall of RI systems over southern coastal regions in recent years. During post-monsoon season, a broader range of latitude attainment is observed by RI TCs post-2007 (Figure 5b). Moreover, the mean longitude is found to be decreasing during the period 2007–2018, from a mean of ~92°E to 81°E with an interquartile range of 75°E–87°E and then an insignificant rise post-2019 (Figure 5d). The decreased mean RI latitude (Figure 5b), broad range of latitude, decrease in mean RI longitude (Figure 5d), and comparatively narrow range of longitude during the quadrennials post-2007 increased the exposure of lower regions of India to TC destruction.

Figure 5.

South-west migration of the latitude (a, b) and longitude (c, d) of RI TCs during pre-monsoon and post-monsoon seasons over BOB respectively. The left panel represents the pre-monsoon season, and the right panel represents the post-monsoon season. The red line represents the mean values of the latitudes and longitudes.

Nevertheless, the increase in RI locations post-2019 represents an increased threat over eastern coastal states of the BOB basin, i.e., Myanmar (21.91°N, 95.95°E) and the Gulf of Thailand (Figure 5b, and d). It can be seen from Figure 5 that the zero or lower RI activity during any quadrennium of the pre-monsoon season encourages enhanced RI activity over BOB during the corresponding post-monsoon season and vice versa. The possible explanation for the enhanced RI activity during one season suppressing RI activity during the other for any particular quadrennium could be attributed to the unutilized heat or thermal energy during the pre-monsoon season being harnessed by TCs during the post-monsoon season, leading to RI events and vice-versa. However, the dynamics behind the observed interconnected TC RI activity over BOB needs detailed investigations and is not covered in this chapter. The number of RI TC occurrences over AS is lower, resulting in similar insignificant variations for both seasons (figures not shown for brevity). Nonetheless, an increase in the interquartile range of latitude (~16°N–17.8°N) and longitude (~67°E–72°E) post-2019 shows an increased threat of RI TCs formed over AS to western Indian states during post-monsoon season.

4. Variation of environmental factors

The TC intensification process is related to the internal dynamics of the system and the internal dynamics depend on the environmental factors prevailing throughout the TC activity [29, 30, 31]. Under favorable environmental conditions, TCs can continuously intensify until they approach their maximum intensity, and unless any adverse environmental conditions are experienced, the likelihood of TC weakening during the intensification phase is rare. Any long-term change in TC activity is correlated to the change in climatic conditions over the region. Several parameters such as SST, mid-tropospheric relative humidity (RH550), vertical wind shear (VWS), and upper-level divergence are found to modulate the TC intensity [32, 33]. The section below discusses the change in the environmental factors in recent climate conditions and its effects on the propagation of RI locations of TC over the NIO region.

4.1 Sea surface temperature

Emanuel [34] represented TCs as a Carnot heat engine that draws energy from the underlying ocean as entropy flux and loses energy due to surface friction. TCs intensify when the energy production is greater than the dissipation. Therefore, the SST plays a crucial factor in TC intensification [35]. Figure 6 represents the area averaged SST (°C) anomaly over 4° latitude and longitude regions over BOB and AS. The tick marks and labels of Figure 6a and b represent average SST anomaly over 80°–83°E/3–6°N, 80°–83°E/7–10°N, 80°–83°E/11–14°N, 80°–83°E/15–18°N, 80°–83°E/19–21.5°N and then the longitude regions increased to 84–87°E for the latitude ranges defined between 3°N–21.5°N and so on with the same set of latitude ranges. Similar divisions are made for AS for longitudes 50–74°E and latitudes 3–23°N. Since the SST gradient over NIO is not very sharp, averaging the data in chunks helps detect any frequency variability for the analysis period.

Figure 6.

Average SST anomaly (°C) variation during pre-monsoon (a, c) over BOB and AS and during post-monsoon seasons (b, d) over BOB and AS respectively from 1991 to 2022. Here, the anomalies are normalized by the means for the analysis period.

It is observed that the average SST over BOB during pre-monsoon ranges between 27.5–31°C, during post-monsoon between 27 and 29.6°C, and over AS the ranges are 23–30.5°C and 24.5–29.7°C respectively. During the analysis, it is observed that TC’s rapid intensification is presumably related to the positive SST anomaly for any year over the basin. However, the notably elevated SST anomaly since 2015 does not appear to be conducive for a high rate of RI activity for both seasons in the BOB. For example, the years such as 1998, 2005, 2010, and 2016 during pre-monsoon and years 1997, 2002, 2009, and 2015 during the post-monsoon season where the rise in SST anomaly is abnormally high (Figure 6a and b), supported zero RI TC activity. As discussed in Section 3.3, for most of the years where high RI TC activity during pre-monsoon suppresses the activity during post-monsoon and vice-versa, the SST anomaly is found to be comparatively higher during that particular season than the other. For example, the quadrennium 1995–1997, and 1999–2002 during pre-monsoon season show zero or negligible RI TC frequency whereas a noticeable rise in activity during post-monsoon season could be seen. Similarly, during 2003–2006, where pre-monsoon season experienced higher RI TCs suppressed the activity during the post-monsoon season over BOB (Figure 5).

And, in contrast to the Carnot engine theory, the annual TC frequency over the basin is found to be decreasing despite rising SST over BOB post 2015. The role of SST in intensification depends upon the initial intensity of the storm, size, and other environmental factors to favor further intensification of the system. Similarly, over AS the higher TC activity during pre-monsoon and post-monsoon is supported by higher SST anomaly, contrary to BOB. It is to be noted that the SST over AS was comparatively colder than BOB till 2002 and is rising in the current warming climate supporting a higher number of RI TCs. Most of the AS regions have an average SST range of 27°C to 30.5°C during the pre-monsoon season and regions eastward of 62°E for all considered latitudinal bands have an average SST of 27.5–29.5°C post-2011 during post-monsoon season. Several literatures suggested that the SST range of 27.5–29°C is conducive to TC intensification [9, 36, 37]. Xu and Wang [35] stated that 99.8 and 84.4% of intensifying TC cases occurred over SSTs greater than 26 and 28°C in the WNP, compared with 88.3 and 41.3% of cases in the North Atlantic.

4.2 Mid-tropospheric RH (RH550)

Factors such as RH550 and SST can affect the cloud and precipitation distribution in a TC environment by triggering latent heat release in a system and hence the energy cycle of the system. Consequently, the RI of a TC is often considered to be associated with increased azimuthal and areal coverage of convection [38, 39], motivating further examination of this parameter for their role in the propagation of RI locations over NIO in the present changing climate. Over BOB, during pre-monsoon season the RH550 anomaly is mostly negative till 1998 except for 1994 (Figure 7a). During this period, the SST anomaly (Figure 6a) and RH550 show a negative relation between both of them over BOB. Similar results have been found by Liu et al. [40], and Sun and Oort [41]. The increase (decrease) in SST weakens (strengthens) the radiative cooling and relative humidity at upper atmosphere [40]. Surprisingly, no significant RI TC activities were found during these years. Moreover, in the years where remarkable RI TC activities are witnessed during the pre-monsoon season post-2004, the SST anomalies are observed to be negative whereas RH550 is found to have positive anomalies. The average RH550 during pre-monsoon is ~75% and greater than 80% during post-monsoon season over BOB.

Figure 7.

Average RH550 anomaly (%) variation during pre-monsoon (a, c) over BOB and AS and during post-monsoon seasons (b, d) over BOB and AS respectively from 1991 to 2022. Here, the anomalies are normalized by the means for the analysis period.

For the post-monsoon season, the southern and middle latitude regions of the entire BOB basin have higher RH550, i.e., 60–85%. In the years such as 1995, 1996, 1999, 2000, and 2013, where higher RI TC activities were experienced over BOB, the SST anomaly is negative, and the RH anomaly shows a rise over the basin. However, the RH550 is one of the supporting elements for the intensification of TC over BOB. Surprisingly reduced SST anomaly is found to be favorable for RI of TCs during post-monsoon season. According to Xu and Wang [35] and Park et al. [42], the maximum potential intensification rate (MIPR) of TCs is primarily associated with the regions where the SSTs are higher than 26–28°C over the Atlantic and west-north Pacific (west-north pacific) basins and similar findings can be observed for BOB basin. Furthermore, the RH550 has risen sharply over 84°E to 92°E post-2004 during pre-monsoon season, and over 84°E–100°E for post-monsoon season post-2012 for the latitudinal sections (i.e., 3–6°N, 7–10°N, 11–14°N, 15–18°N) over BOB. The detected rise in pattern over these regions was found to be supportive of the change in the RI locations over the basin.

Over AS, from Figure 7c, after 2013 a continuous rise in RH550 during pre-monsoon season could be seen over all longitude regions, though the latitude regions were different for each year. The RI TCs frequencies during 1999, 2001, 2010, 2018, and 2021 coincide with the rise in RH550 by nearly 10–15% over the ocean basin during pre-monsoon season. Analogously, for post-monsoon season, years with higher RI activities such as 2014, 2015, 2018, and 2019 are found to coincide well with the enhanced RH550 years. Until 2007, the elevated RH550 in the western longitudinal regions of the AS basin facilitated increased RI (Figure 7d). Subsequently, there is a shift towards the eastern sector of the basin as a rise in RH550 has been experienced in the eastern parts after 2010. The variations are supported by the comparative rise in SST during recent years for both seasons.

4.3 Vertical wind shear (VWS)

The apparent role of the ambient VWS in the intensification process is to encourage the growth of a storm cell by maximizing the strength of vertical motion, low-level inflow and inflow depth, helicity, convective available potential energy, and local shear in the downshear-left quadrant and near the center [43, 44, 45, 46]. The sequence of events for RI under a sheared environment as explained by Molinari and Vollaro [44] are; (i) higher vertical wind shear produced persistent downshear rainfall and subsequent downshear vortex redevelopment within the precipitation shield (ii) the tilting of this new vortex by the ambient shear created favorable conditions for a strong cell to develop downshear in the storm core and (iii) this cell dramatically enhanced the heating in the region of large kinetic energy efficiency and contributed to subsequent RI of the storm.

During pre-monsoon season, the decadal average tropospheric VWS is seen to be reducing by ~3.8 m/s over the central BOB region (Figure 8a) from the decade 1991–2000 to 2001–2010 (here onwards abbreviated as D1 and D2). The distribution of the difference is zonal, and the RI TCs during D1 is also observed to be nearly zonal (Figure 3a). Whereas, during the decade 2011–2022 (here onwards called D3), the gradient of the difference in average VWS between D2 and D3 is observed to be from tropics to central BOB region (Figure 8c). This pattern reveals decreased VWS (approximately 1.5 m/s) at lower latitudes transitioning to increased VWS (2 m/s) in the central BOB. The meridional gradient of VWS is found to complement the change in RI locations from being latitudinally aligned during D1 to longitudinal alignment post-2010 over BOB. Over AS, for pre-monsoon season the enhanced tropospheric shear distribution is observed to be NE–SW aligned (Figure 8c) during D3. This alignment coincides with the zone of high RI TCs in the current decade, as depicted in Figure 4a.

Figure 8.

Differences in average decadal tropospheric VWS (m/s) during 1991–2000 and 2001–2010 for (a) pre-monsoon and (b) post-monsoon season, and the corresponding differences for decades 2001–2010 and 2011–2022 over NIO during (c) pre-monsoon season and (d) post-monsoon seasons respectively.

During post-monsoon season, the distribution of the difference between D1 and D2 is meridional with reduced values over the northern bay of BOB to enhanced values towards the equatorial region (Figure 8b). The distribution of RI TCs location (Figure 3b) before 2005 coincides well with the observed distribution of VWS difference during D2. While experiencing the alteration in VWS pattern in D3 for the post-monsoon season, the most significant VWS gradients are noticed to be limited to the region below 15°N over the BOB with the lowest VWS (~2.5 m/s) near Odisha (20.23°N, 84.27°E) and northern Andhra Pradesh coast (Figure 8d). Surprisingly the RI TC location distribution has also shifted from a longitudinal pattern to a latitudinal one, being confined below 15°N after 2010 (Figure 3b) under the effect of changed tropospheric shear pattern.

Over AS, the change in the VWS pattern shows a reduction in values of tropospheric shear for the post-monsoon season over the latitudinal belt 13°N–20°N (Figure 8d) during D3. The region is seen to be associated with the region of higher RI TC activity locations (Figure 4b). The distribution of shear in which TC is embedded defines the track and intensity of the storm. The average tropospheric VWS over the NIO region ranges between 3 and ~20 m/s during pre-monsoon and post-monsoon seasons and 5–12 m/s over the TC formation zones. The MPIR is higher for SSTs above 28°C over the WNP than over the North Atlantic, which is shown to be related to the weaker (stronger) environmental VWS in regions with higher (lower) SSTs over the WNP than over the North Atlantic. The weaker VWS has been reported to affect the intensification process of TCs over NIO [25], hence although the annual TC frequency has reduced over BOB, the RI TCs distribution is found to be modulated by the prevailing VWS patterns.

4.4 Upper-level divergence

The divergence at higher altitudes leads to a reduction in surface pressure at the storm center. This results in an escalation of the storm’s intensity as the speed of converging surface winds rises. The converging wind accumulates enhanced heat and moisture, promoting further warming of the core of the TC. The annual trend of upper-level divergence at 200 hPa (Div200) over BOB is observed to be distributed longitudinally between 85°E and 90°E with a reduction by ~1 × 10⁻⁷ s⁻¹ per year during the pre-monsoon season and a similar reduction is also observed over AS in the latitudinal region of 10°N–18°N (Figure 9a). Figure 3a and 4a show that the distribution of RI TC locations over BOB and AS is determined by DIV200 to a great extent during the pre-monsoon season. The reduced DIV200 over both the basins help in trapping the lower-level heat flux generated within the TC core environment [47] which helps the system to strengthen the lower-level wind and momentum towards the center of the cyclone and thereby help to attain RI during its lifetime.

Figure 9.

Spatial distribution of linear trend of annual average upper-level divergence at 200 hPa over NIO region during (a) pre-monsoon season and (b) post-monsoon season.

During post-monsoon season the regions where the TCs suffered RI, the DIV200 is observed to have an increasing trend. The in-up-out circulation due to enhanced DIV200 in a convective region is associated with low to mid-level convergence and upper-level divergence [48]. The RI location gradient (Figure 3b) over BOB during recent decades is analogous to the regions where the increasing annual Div200 trends are spotted (Figure 9b). Likewise, the NE–SW aligned increasing DIV200 region from 60°E to 75°E (Figure 9b) is found to support the TCs to obtain their RI phases over the AS during post-monsoon season. Interestingly, during the post-monsoon season, the mechanism of intensification is that the increased divergence in the upper level helps in enhancing lower-level convergence by ventilating the latent heat energy from the core of the system to the upper atmosphere, and thereby ensures an undisrupted supply of water vapor, mass, and momentum to increase the lower to middle-level spin up [49] and tangential wind of the TCs to attain RI straightaway. The changing DIV200 distribution is one of the major factors helping RI to migrate towards the coastal regions of NIO. The upper-level divergence ensures the maintenance of the cumulus convection, and thus it acts as a pump for the primary ingredients [50].

5. Conclusions and discussions

The TC frequencies and distribution have clearly changed during recent warming climate scenarios. Studholme et al. [3] reported a poleward migration in latitudes of peak TC intensity at a rate of ~0.5° latitude per decade. With the poleward migration of the TCs, any change in the spatial distribution of TC genesis location or intensification would greatly affect the well-being of coastal lives. The local physics interaction with the TC distribution in the current warming climate is a major topic of discussion among researchers. Under the influence of changing climate, the tropical expansion due to Intertropical convergence zone (ITCZ) movement will be primarily driven by upper atmosphere warming whereas restrained by the surface warming patterns. Consequently, the distribution of genesis and intensification of TCs would be more controlled by the upper atmosphere variation. Similar correlations between RI locations of TCs and atmospheric patterns are observed in the present study too. Kaplan et al. [51] analyzed that VWS and upper-level divergence have the relatively highest weights in predicting the RI of TCs over the Atlantic basin. With a decrease in percentages of RI TCs over BOB and rising RI TCs over AS, presently a shift in the distribution of RI of TCs locations has become apparent. The RI location gradient during the pre-monsoon season is higher over the 85°E–90°E longitude region over BOB. Over AS, the concerned location is found to be high over the latitudinal band 10–17°N. The distribution of RI location over BOB during post-monsoon season lay parallel to the Indian coast during the decade D1 which changed to a curved pattern confined below 15°N with longitude range 75°E to 95°E during D2 and D3, thereby enhancing the vulnerability to the southern Indian coastal states, Andaman Nicobar Islands and gulf of Thailand to the effects of RI TCs. Over AS, the distribution is again observed to be latitudinal (nearly between 12°N–17°N) with increased frequencies, making the Arabian coastal states as equally vulnerable as that of the Indian coastal states. The enhanced SST is found to play a trivial role in the observed shift in the RI locations over BOB and AS during both seasons.

The average SST 27 to 29.6°C over BOB is found to be favorable for intensification of TCs. The overall rising SST over 62°E to 78°E longitudes and below 17°N latitude region during recent decades support the enhanced RI activity over AS basin, however, due to the small data size of the RI TCs the results are insignificant. Moreover, no correlation is observed between the extremely high SST anomaly during any year and the RI TC activity. During pre-monsoon season over BOB, the anomalous rise (decrease) in SST is found to be inversely related to the decrease (rise) in RH550, e.g., years such as 1995, 1998, 2000, 2008, 2010, 2016, 2019, etc. There are three possible reasons explanation behind this behavior (i) the regions with high SST experience strong subsidence, and the relative humidity at the upper level decreases as the air descends (ii) a vice-versa relation of warm SST leading to a more stable lower atmosphere leading to the inhibition of vertical motion and the lifting of moist air to middle atmosphere (iii) increased surface winds in recent climate [9] transporting the available humidity horizontally to nearby regions. The slowdown of the tropical large-scale circulation in a warming climate is linked with a weakening of convective mass flux caused by increased dry static stability and reduced meridional surface temperature gradients in the tropics [52] is another reason for the observed relation between SST and RH550. An enhanced average RH550 (up to 75%) over southern BOB between longitudes 92°E−96°E is also observed during the season. The enhanced VWS pattern over central BOB (85–90°E/10–15°N) during the pre-monsoon season of D3 by 2 m/s than D2 and largest gradient zone over BOB, host the highest number of RI TCs aligned in North-South direction during this season. Bi et al. [53] suggested that the effect of shear on the intensification process depends upon the size of the disturbance with larger TCs affected less by high VWS. The displacement of the upper-level circulation is smaller in larger TCs, indicating that larger TCs are less susceptible to VWS. The decreased DIV200 alignment again agrees well with the observed RI TC distribution over the basin in the pre-monsoon season.

During the post-monsoon season over BOB, unlike the pre-monsoon season, the observed inverse relationship between SST and RH550 is less pronounced. The average RH550 has increased (~85%) in recent years and a prominent rise is seen over southern and middle latitudes. The increasing RH550 and SST support the increasing tendency in RI TCs, despite a decrease in annual TC frequency over the basin. The increase (decrease) in RH550 during pre-monsoon (post-monsoon) over BOB and vice-versa needs further investigation which is out of scope for the present chapter. The gradient of change in decadal average VWS over BOB during post-monsoon season altered from meridional to nearly zonal from D1 to D3, consequently varying the RI TC gradient from NE-SW during early 2000 to confined below 15°N post-2005. A very clear reduction in tropospheric VWS around the coast of Odisha and West Bengal is inhibiting the intensification and landfall over the state in recent decades. High upper-level DIV200 implies large vertical motion, and more air is taken out at the top than is brought in at the bottom promoting the axisymmetric central core of the system favoring RI of the system.

Over AS, for pre-monsoon season the correlation between rise in SST and RH550 is observed for some years after 2010. However, the longitudinal regions that witnessed higher RI TC activity are mostly dominated by a decreased anomaly in RH550 with enhanced SST anomaly. The noticeable VWS gradient in 10°N–15°N facilitates intensification over the basin. The decadal average VWS over the belt is 5–10 m/s. The decreased DIV200 over the region discussed here is found to support the RI TCs. During post-monsoon season, the simultaneous increase in SST and RH550 could be seen for most of the years post-1995, and regimes of higher RI TC activity are associated with increased RH500 and SST anomalies. The rise in RH550 to 60 and 75% during pre-monsoon and post-monsoon over equatorial latitudinal zones supports higher RI activity over the eastern AS basin. The narrowing of the VWS gradient zone over the central AS during the post-monsoon season and the substantial decrease in average VWS by 2.7 m/s over 13°E−20°E favors the rapid growth of the system in the current decade D3. The orientation of low-level mean flow with respect to the vertical shear, and helicity of environmental wind has a relationship with TC intensification and expansion [54, 55, 56]. The observed linear enhancement in upper-level divergence is attributed to the observed distribution of RI TC locations which is possibly the key factor for the observed RI locations pattern in recent years. The rising upper-level divergence and reduction in VWS over the near central zone of AS also imply a potential reason behind the increase in the number of TCs over the basin during the post-monsoon period. Apart from the parameters discussed here in this chapter, the pronounced transient zonal and meridional TC migrations are also expected to occur in response to climate warming of the Hadley circulation, jet streams, El Niño–Southern Oscillation (ENSO), and Intertropical Convergence Zone, which will be covered by further extended work of this chapter.

Acknowledgments

The author acknowledges that, ‘for the purpose of open access, a Creative Commons Attribution (CC BY) license has been applied to any Author Accepted Manuscript version arising from this submission’. This research was supported by the University of Exeter’s Open Access Funds, which provided financial support for the publication fees associated with this manuscript. The author would like to thank the India meteorological department (IMD), New Delhi for the TC best track data available at, rsmcnewdelhi.imd.gov.in and ERA5 (cds.climate.copernicus.eu/) for providing the ocean and atmosphere parameters.

Conflict of interest

The authors declare no conflict of interest.