Key Messages
• Long-term observations (1951–2018) indicate a significant reduction in annual frequency of tropical cyclones
(TCs) in the North Indian Ocean (NIO) basin [−0.23 per
decade over the entire NIO; −0.26 per decade over the
Bay of Bengal]. A significant rise [+0.86 per decade] in
the frequency of post-monsoon (October–December)
season very severe cyclonic storms (VSCS) is observed in
the NIO during the past two decades (2000–2018) (high
confidence).
• Observations indicate that frequency of extremely severe
cyclonic storms (ESCS) over the Arabian Sea has
increased during the post-monsoon seasons of 1998–2018
(high confidence). There is medium confidence in
attributing this observed increase to human-induced SST
warming.
• Analyses from the observations show a decline in number
of thunderstorm days (1981–2010 relative to 1950–1980)
by 34% over the Indian region, while there is a rise in
short-span
high-intensity
rain
occurrences
(mini-cloudbursts) along the west coast of India (5 per
decade) and along the foothills of western Himalayas (1
per decade) during the 1969–2015 period (high
confidence).
• Climate model simulations project a rise in TC intensity
(medium confidence) and TC precipitation intensity
(medium-to-high confidence) in the NIO basin.
8.1 Introduction
High-impact weather phenomena associated with cyclonic
storms (synoptic-scale weather disturbances that last for a
few days), thunderstorms (occur on less than a day time
scale), and short-lived cloudbursts (a time scale of few
hours) that can produce intense rainfall amounts are generally categorized as severe or extreme weather events in the
Indian weather chronology. The extreme weather events
over the Indian region have profound socio-economic
implications (e.g., De et al. 2005). The North Indian
Ocean (NIO) rim countries (India, Bangladesh, Myanmar,
Sri Lanka, Oman; countries within the Equator region—
30° N; 50–100° E) comprising large coastal areas are
severely affected by tropical cyclones (TCs) every year (see
Singh et al. 2016; Ramsay 2017; Mohapatra et al. 2014,
2017). For example, the year 2018 witnessed four very
severe TCs over this region during the pre-monsoon
(March–May) and post-monsoon (October–December) seasons of India [Mekunu and Luban over the Arabian Sea
(AS) in May and October 2018, Titli and Gaja in October
and November 2018 over the Bay of Bengal (BOB); Source:
Annual cyclone review report, India Meteorological
Department (IMD); see also Table 8.2].
Convective storms such as thunderstorms in general are
considered as hazard to aviation, and also cause severe loss
to life, agriculture, and property. The eastern and northeastern states of India (West Bengal, Bihar, Assam, Chhattisgarh, Jharkhand, and Orissa) and adjoining regions in
Bangladesh experience violent thunderstorms known as
“Nor’wester” during the pre-monsoon season (Mukhopadhyay et al. 2005; Ghosh et al. 2008; Tyagi et al. 2012).
Rainfall occurrences of unprecedented intensity have also
been witnessed in the recent decades during the Indian
summer monsoon (ISM; June–September) season. For
example, heavy downpour that caused calamitous floods and
heavy casualties in Mumbai during July 2005 (Bohra et al.
2006), in Leh of the trans-Himalayan region during August
2010 (Thayyen et al. 2013; Rasmussen and Houze 2012), in
the northern states of Uttarakhand and Jammu and Kashmir
during June 2013 and September 2014 (Lotus 2015;
Ranalkar et al. 2016; Vellore et al. 2016, 2019; Priya et al.
2017), and in the southern state of Kerala during August
2018 (Mishra and Shah 2018) are to name a few. Various
synoptic-scale signatures have been recognized in connection with these extreme rain situations, viz localized convective instabilities, large-scale organized monsoon activity
and anomalous extratropical circulation, and their interactions with the monsoon circulation across the Himalayas
(e.g., Vellore et al. 2014, 2016; see also Krishnan et al.
2019). The aforesaid extreme storm phenomena are significant threat to lives, property, and agricultural yields and
cause huge revenue losses for the Indian subcontinent every
year. It is also noteworthy to mention that extreme rain situations over the Indian subcontinent exhibit significant
variations on a regional scale (Guhathakurta et al. 2011).
A list of extreme rain events occurred in the recent times that
resulted in calamitous flood situations over the Indian region
is documented in Table 6.4. Long-term observations show
significant rising trends in the frequency and magnitude of
extreme rain occurrences over the ISM core rain-fed regions
of central India during the later half of the twentieth century
(Goswami et al. 2006) in an unequivocally warming climate
(IPCC 2007, 2014; see also Krishnan et al. 2016; Singh et al.
2019).
Nonetheless, a clear deciphering of anthropogenic climate
change manifestations on extreme rain or storm occurrences
broadly remains elusive. Greater challenges are with detection and attribution of trends in high-impact rain events due
to representation constraints of finer scale physical processes
in the state-of-the-art climate modeling systems, as opposed
to challenges in understanding of changes in large-scale
environment in a warming climate (Mukherjee et al. 2018;
156
R. K. Vellore et al.
• Long-term observations (1951–2018) indicate a significant reduction in annual frequency of tropical cyclones
(TCs) in the North Indian Ocean (NIO) basin [−0.23 per
decade over the entire NIO; −0.26 per decade over the
Bay of Bengal]. A significant rise [+0.86 per decade] in
the frequency of post-monsoon (October–December)
season very severe cyclonic storms (VSCS) is observed in
the NIO during the past two decades (2000–2018) (high
confidence).
• Observations indicate that frequency of extremely severe
cyclonic storms (ESCS) over the Arabian Sea has
increased during the post-monsoon seasons of 1998–2018
(high confidence). There is medium confidence in
attributing this observed increase to human-induced SST
warming.
• Analyses from the observations show a decline in number
of thunderstorm days (1981–2010 relative to 1950–1980)
by 34% over the Indian region, while there is a rise in
short-span
high-intensity
rain
occurrences
(mini-cloudbursts) along the west coast of India (5 per
decade) and along the foothills of western Himalayas (1
per decade) during the 1969–2015 period (high
confidence).
• Climate model simulations project a rise in TC intensity
(medium confidence) and TC precipitation intensity
(medium-to-high confidence) in the NIO basin.
8.1 Introduction
High-impact weather phenomena associated with cyclonic
storms (synoptic-scale weather disturbances that last for a
few days), thunderstorms (occur on less than a day time
scale), and short-lived cloudbursts (a time scale of few
hours) that can produce intense rainfall amounts are generally categorized as severe or extreme weather events in the
Indian weather chronology. The extreme weather events
over the Indian region have profound socio-economic
implications (e.g., De et al. 2005). The North Indian
Ocean (NIO) rim countries (India, Bangladesh, Myanmar,
Sri Lanka, Oman; countries within the Equator region—
30° N; 50–100° E) comprising large coastal areas are
severely affected by tropical cyclones (TCs) every year (see
Singh et al. 2016; Ramsay 2017; Mohapatra et al. 2014,
2017). For example, the year 2018 witnessed four very
severe TCs over this region during the pre-monsoon
(March–May) and post-monsoon (October–December) seasons of India [Mekunu and Luban over the Arabian Sea
(AS) in May and October 2018, Titli and Gaja in October
and November 2018 over the Bay of Bengal (BOB); Source:
Annual cyclone review report, India Meteorological
Department (IMD); see also Table 8.2].
Convective storms such as thunderstorms in general are
considered as hazard to aviation, and also cause severe loss
to life, agriculture, and property. The eastern and northeastern states of India (West Bengal, Bihar, Assam, Chhattisgarh, Jharkhand, and Orissa) and adjoining regions in
Bangladesh experience violent thunderstorms known as
“Nor’wester” during the pre-monsoon season (Mukhopadhyay et al. 2005; Ghosh et al. 2008; Tyagi et al. 2012).
Rainfall occurrences of unprecedented intensity have also
been witnessed in the recent decades during the Indian
summer monsoon (ISM; June–September) season. For
example, heavy downpour that caused calamitous floods and
heavy casualties in Mumbai during July 2005 (Bohra et al.
2006), in Leh of the trans-Himalayan region during August
2010 (Thayyen et al. 2013; Rasmussen and Houze 2012), in
the northern states of Uttarakhand and Jammu and Kashmir
during June 2013 and September 2014 (Lotus 2015;
Ranalkar et al. 2016; Vellore et al. 2016, 2019; Priya et al.
2017), and in the southern state of Kerala during August
2018 (Mishra and Shah 2018) are to name a few. Various
synoptic-scale signatures have been recognized in connection with these extreme rain situations, viz localized convective instabilities, large-scale organized monsoon activity
and anomalous extratropical circulation, and their interactions with the monsoon circulation across the Himalayas
(e.g., Vellore et al. 2014, 2016; see also Krishnan et al.
2019). The aforesaid extreme storm phenomena are significant threat to lives, property, and agricultural yields and
cause huge revenue losses for the Indian subcontinent every
year. It is also noteworthy to mention that extreme rain situations over the Indian subcontinent exhibit significant
variations on a regional scale (Guhathakurta et al. 2011).
A list of extreme rain events occurred in the recent times that
resulted in calamitous flood situations over the Indian region
is documented in Table 6.4. Long-term observations show
significant rising trends in the frequency and magnitude of
extreme rain occurrences over the ISM core rain-fed regions
of central India during the later half of the twentieth century
(Goswami et al. 2006) in an unequivocally warming climate
(IPCC 2007, 2014; see also Krishnan et al. 2016; Singh et al.
2019).
Nonetheless, a clear deciphering of anthropogenic climate
change manifestations on extreme rain or storm occurrences
broadly remains elusive. Greater challenges are with detection and attribution of trends in high-impact rain events due
to representation constraints of finer scale physical processes
in the state-of-the-art climate modeling systems, as opposed
to challenges in understanding of changes in large-scale
environment in a warming climate (Mukherjee et al. 2018;
156
R. K. Vellore et al.