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Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021
https://doi.org/10.5194/nhess-21-2597-2021
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the Creative Commons Attribution 4.0 License.
Variability in lightning hazard over Indian region with respect
to El Niño–Southern Oscillation (ENSO) phases
Avaronthan Veettil Sreenath1, Sukumarapillai Abhilash1,2 , and Pattathil Vijaykumar1,2
1Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin 682016, India
2Advanced Centre for Atmospheric Radar Research (ACARR), Cochin University of Science and Technology,
Cochin 682022, India
Correspondence: Sukumarapillai Abhilash (abhimets@gmail.com)
Received: 4 September 2020 – Discussion started: 16 November 2020
Revised: 18 July 2021 – Accepted: 22 July 2021 – Published: 26 August 2021
Abstract. The El Niño–Southern Oscillation (ENSO) mod-
ulates the lightning flash density (LFD) variability over In-
dia during premonsoon, monsoon and postmonsoon seasons.
This study intends to shed light on the impact of ENSO
phases on the LFD over the Indian subcontinent using the
data obtained from Optical Transient Detector (OTD) and
Lightning Imaging Sensors (LIS) onboard the Tropical Rain-
fall Measuring Mission (TRMM) satellite. Results suggest
the LFD over northeast India (NEI) and southern peninsular
India (SPI) strengthened (weakened) during the warm (cold)
phase of ENSO in the premonsoon season. During monsoon
season, NNWI (north of northwest India) shows above (be-
low) normal LFD in the cold (warm) ENSO phase. It is strik-
ing to note that there are three hot spots of LFD over the
Indian land region which became more prominent during
the monsoon seasons of the last decade. A widespread in-
crease in LFD is observed all over India during the warm
phase of ENSO in the postmonsoon season. A robust rise in
graupel/snow concentration is found during the postmonsoon
season over SPI in the ENSO warm phase, with the lowest
fluctuations over the NEI and NNWI regions. The subtrop-
ical westerly jet stream is shifted south in association with
the warm phase, accompanied by an increase in geopotential
height (GPH) all over India for the same period. This exciting
remark may explain the indirect influences of ENSO’s warm
phase on LFD during the postmonsoon season by pushing the
mean position of the subtropical westerly towards southern
latitudes. However, the marked increase in LFD is confined
mostly over the NNWI in the cold ENSO phase.
1 Introduction
Lightning is a tremendous and inescapable atmospheric
hazard that humankind has encountered throughout history
(Cooray et al., 2007; Mills et al., 2010). The number of ca-
sualties underlines lightning hazards as a devastating phe-
nomenon, with an annual death rate of 2234 from 2001 to
2014 over India (Selvi and Rajapandian, 2016). Singh and
Singh (2015) documented the yearly number of lightning fa-
talities and lightning flashes in India from 1998 to 2005, and
they find that the fatalities increase coherently with the light-
ning flash rate. Lightning strikes over the plain terrains are
observed to be less as compared to the hilly regions. Due
to the former’s high population density, even lesser lightning
flashes take many people’s lives due to high chances of being
struck by lightning (Yadava et al., 2020).
The El Niño–Southern Oscillation (ENSO) is a naturally
occurring planetary-scale phenomenon related to the vari-
ations in sea surface temperatures over the tropical Pacific
Ocean, strongly influencing the number of flashes and aver-
age flash rate (Kumar and Kamra, 2012). It is one of the most
dynamic climatic variability modes, characterized by three
phases, namely El Niño (warm), La Niña (cold) and neutral.
The ENSO is a crucial player in the transport of heat, mois-
ture and momentum and modulates the frequency, intensity
and location of deep convection and the associated lightning
activity (Williams, 1992; Kulkarni and Siingh, 2014). Higher
lightning flash density (LFD) areas are located away from the
Equator during the warm phase and coincide with regions of
anomalous jet stream circulation enhanced by the meridional
heat transport (Chronis et al., 2008). Kandalgaonkar et al.
Published by Copernicus Publications on behalf of the European Geosciences Union.
2598 A. V. Sreenath et al.: Variability in lightning over India with ENSO phases
(2010) reported that lightning activity during the El Niño
year of 2002 increased by 18 % over the Indian land re-
gion compared to the La Niña years during 1998–2011. On a
global scale, lightning activity shows strong regional prefer-
ence during different ENSO phases.
The changes in the lower and upper air circulations asso-
ciated with different ENSO phases have been found to in-
fluence the storm frequency and intensity (Yang et al., 2002;
Hsu and Wallace, 1976), which in turn affect the lightning
activity (Goodman et al., 2000). Kent et al. (1995) observed
that ENSO could dictate the clouds’ distribution over the
tropics and subtropics. Owing to the presence of anomalous
subsidence over the western Pacific and adjacent landmass,
deep convective clouds are inhibited; hence the rainfall is
less during the warm phase (Cess et al., 2001). A south-
ward/eastward shift in the global lightning activity is visi-
ble during the warm phase, and the latitudes corresponding
to the descending limb of the Hadley circulation exhibit the
most significant contrast of LFD between the warm and cold
phase of the ENSO (Sátori et al., 2009).
Generally, lightning activity is tuned by the clouds grow-
ing deep into the atmosphere. The deep convective cores
present over India’s east coast during the premonsoon sea-
son shift to the foothills of the western Himalayas during
the monsoon (Romatschke et al., 2010). Cecil et al. (2014)
documented that India’s offshore regions and the maritime
continent are prone to deep convection. The vertical growth
of cloud systems is amplified by the intense updraught, pro-
moting ice crystals and supercooled liquid (mixed phase) in-
side the convective system. The interaction between these hy-
drometeors is mainly responsible for the electrification inside
the cloud (Takahashi et al., 1999; Williams, 2001). The atmo-
sphere’s dynamic and thermodynamic states also modulate
the lightning activity over a region (Williams, 1992; Zipser,
1994; Petersen et al., 1996; Rosenfeld, 1999). Topography
is identified as being another critical participant in develop-
ing deep convective clouds, and it impacts the distribution of
lightning activity (Kilinc and Beringer, 2007). Earlier stud-
ies have observed that elevated landmass favours the devel-
opment of deep convective clouds (Zipser et al., 2006; Houze
et al., 2007; Rasmussen and Houze Jr., 2011), thereby lead-
ing to higher LFD. In addition, aerosols are also considered
a contributor for making a decisive role in generating light-
ning flashes. Higher aerosol loading increases the available
liquid water in the mixed-phase condition, which is an es-
sential factor for cloud electrification and lightning activity
(Williams et al., 2002). Venevsky (2014) reported a signifi-
cant correlation between lightning and the concentration of
annually averaged cloud condensation nuclei over both land
and ocean.
The awareness of lightning safety among the public is rel-
atively low. The present study aims to provide vital infor-
mation to the public on the risky lightning periods over the
Indian subcontinent and how the large-scale phenomenon,
ENSO, is influencing the same periods. We are detailing
the modulation of LFD under different ENSO phases with
the help of a vertical profile of hydrometeors (graupel and
snow) inside the cloud systems and related atmospheric dy-
namics during premonsoon (March–May), monsoon (June–
September) and postmonsoon (October–December) seasons
in India. The rest of the paper is organized as follows. Sec-
tion 2 provides descriptions of the data and methodology em-
ployed in this study. Section 3 presents the results, followed
by Sect. 3.1, which depicts the composite analysis of LFD
for premonsoon, monsoon and postmonsoon seasons corre-
sponding to the three ENSO phases. The remaining subsec-
tions of Sect. 3 (3.2, 3.3 and 3.4) portray the composite anal-
ysis of anomalous LFD during the different seasons and the
significance of vertical cloud structure and associated dy-
namics in regulating their distribution. Finally, the conclu-
sions of this work are given in Sect. 4.
2 Data and methods
The Lightning Imaging Sensor (LIS) was an instrument on
board the Tropical Rainfall Measuring Mission (TRMM)
satellite launched in December 1997. This instrument senses
lightning flashes across the global tropics and subtrop-
ics (Goodman et al., 2007). The Optical Transient Detec-
tor (OTD) was the predecessor of LIS, launched with the
MicroLab-1 satellite. Combined OTD +LIS monthly aver-
aged flash density expressed as flashes per square kilome-
tre per day (km−2d−1), available from http://ghrc.nsstc.nasa.
gov/ (last access: 23 August 2021), is used in this work.
These products compute mean LFD by accumulating the to-
tal number of flashes observed and the entire observation du-
ration for each grid box (2.5◦×2.5◦) from the thousands of
individual satellite orbits. The lightning climatology derived
from OTD/LIS (Cecil et al., 2014) provides a unique obser-
vational basis for the global flash distribution in monthly time
series (Kamra and Athira, 2016), seasonal cycles (Christian
et al., 2003) or diurnal cycles (Blakeslee et al., 2014). To pro-
duce the low-resolution monthly time series (LRMTS) data,
LIS and OTD flash density and view times are smoothed pre-
cisely and are extracted for the middle day of each month
(Cecil et al., 2014). The LFD in an LRMTS has slightly
over 3 months of temporal smoothing and 7.5◦×7.5◦spatial
smoothing (Cecil et al., 2014). The data sets are described in
greater detail in a paper by Cecil et al. (2014).
The LFD data are available starting from July 1995 only.
So, the premonsoon season in our work starts in 1996
(March–May) and ends in 2013 (March–May). Due to data
unavailability, the first monsoon season includes only 3
months (July, August and September 1995). This particular
season terminates in 2013 (June, July, August and Septem-
ber). On the other hand, the postmonsoon season is pre-
pared from 1995 (October–December) to 2013 (October–
December). The LFD anomaly in this study indicates the
difference between the composite of LFD during a par-
Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021 https://doi.org/10.5194/nhess-21-2597-2021
A. V. Sreenath et al.: Variability in lightning over India with ENSO phases 2599
ticular ENSO phase in a specific season and the compos-
ite of LFD during all the three ENSO phases in that spe-
cific season. e.g., LFD anomaly during premonsoon during
La Niña =(composite of LFD during La Niña in premon-
soon) −(composite of LFD during all the three ENSO phases
in premonsoon). The anomalies of all other parameters used
in this study are calculated using the same method.
With the aid of TRMM 3A12 data, the cloud structure is
examined by evaluating the vertical profiles of hydromete-
ors (graupel and snow) and latent heat release during dif-
ferent phases of ENSO. The data set has a spatial resolu-
tion of 0.5◦×0.5◦, available from January 1998 to Decem-
ber 2013. It has 28 vertical levels, which start from 0.5 km,
and each level is separated by 0.5 km. These parameters
are averaged for the premonsoon, monsoon and postmon-
soon season from 1998 to 2013 with respect to the La Niña,
El Niño and neutral phases of ENSO over northeast India
(NEI; 85–95◦E, 20–30◦N), north of northwest India (NNWI;
25–40◦N, 65–80◦E) and southern peninsular India (SPI; 5–
15◦N, 75–80◦E) and are used in this work.
Finally, the modulation of geopotential height (GPH) at
500 hPa, wind at 200 hPa and specific humidity (SH) at
300 hPa are also examined with the ENSO phases from
July 1995 to December 2013. The above parameters are
obtained from the National Centers for Environmental Pre-
diction (NCEP) and National Center for Atmospheric Re-
search (NCAR) reanalysis data with a similar spatial and
temporal resolution to LFD. The Oceanic Niño Index (ONI)
is the standard used to identify different phases of ENSO.
The average value of ONI is determined during premonsoon,
monsoon and postmonsoon season by using Hadley Centre
Global Sea Ice and Sea Surface Temperature (HadISST) data
and is detailed in Table 1. If the ONI value is above (below)
+0.5◦C (−0.5 ◦C), it is taken as the warm (cold) phase, and
the neutral phase corresponding to the ONI index lies be-
tween −0.5 and +0.5◦C.
3 Results and discussion
3.1 Composite LFD with respect to ENSO phases
Figure 1 represents the LFD composites for premonsoon,
monsoon and postmonsoon seasons corresponding to the
three ENSO phases. Irrespective of ENSO phases, the LFD
peak is located over NEI during the premonsoon season,
while its peak shifts to the NNWI in the monsoon season.
Kamra and Athira (2016) identified a higher concentration
of LFD over northwestern and northeastern regions of India,
and it is tightly correlated with convective available potential
energy (CAPE) over those regions. They also observed that
the maxima of lightning during postmonsoon is also lying
over India’s southern and eastern regions. Ahmad and Ghosh
(2017) reported that, compared to other Indian regions, light-
ning activity is higher over the northeastern and southern
Table 1. ONI during the premonsoon, monsoon and postmonsoon
season in India from 1995 to 2013. The bold, italics and bold italic
values denote El Niño, La Niña, and neutral phases of ENSO, re-
spectively.
Year Premonsoon Monsoon Postmonsoon
1995 0.39 −0.08 −0.60
1996 −0.27 −0.10 −0.25
1997 0.51 1.81 2.41
1998 1.15 −0.57 −1.20
1999 −0.73 −0.77 −1.24
2000 −0.80 −0.39 −0.65
2001 −0.22 0.01 −0.25
2002 0.27 0.81 1.40
2003 0.10 0.15 0.46
2004 0.14 0.58 0.76
2005 0.41 0.14 −0.36
2006 −0.27 0.39 1.02
2007 −0.10 −0.40 −1.40
2008 −0.76 −0.08 −0.43
2009 −0.35 0.73 1.49
2010 0.62 −0.97 −1.52
2011 −0.61 −0.34 −0.96
2012 −0.17 0.53 0.21
2013 −0.03 0.39 0.79
parts of India during the premonsoon season. Similarly, we
have identified three hot spots of higher lightning activity
over the Indian subcontinent (Fig. 1a). They are located
in the NEI (85–95◦E, 20–30◦N), NNWI (25–40◦N, 65–
80◦E) and SPI (5–15◦N, 75–80◦E). The Himalayan orog-
raphy favours the formation of deep convective systems over
the NEI (Goswami et al., 2010) and is evidenced by the high
values of LFD over the region. Rather than the altitude, the
steep topographic gradient is responsible for producing deep
convection. Most likely, the deep convective clouds devel-
oped in the conditionally unstable atmosphere during the
premonsoon season are electrically more active (Williams
et al., 1992). Lau et al. (2008) proposed that, during the pre-
monsoon months, dust and black carbon from neighbouring
sources accumulate over the Indo–Gangetic Plain against the
foothills of the Himalayas and act as an elevated heat pump
(EHP). Accordingly, this enhanced warming of the middle
and upper troposphere contributes to the genesis of deep
clouds and higher LFD.
Compared to monsoon and postmonsoon seasons, CAPE
is higher during the premonsoon season. The seasonal aver-
age of CAPE is highest over India’s east coast, and it is near
1500 J kg−1all over southern India (Murugavel et al., 2014).
Nevertheless, large regions of India, especially the central In-
dian region, show a seasonal average of CAPE of less than
1000 J kg−1(Murugavel et al., 2014). Strikingly, the areas of
higher values of LFD (Fig. 1) during the premonsoon season
coincide with the regions of CAPE maxima reported by (Mu-
rugavel et al., 2014). Previous works ascertain that the mod-
https://doi.org/10.5194/nhess-21-2597-2021 Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021
2600 A. V. Sreenath et al.: Variability in lightning over India with ENSO phases
Figure 1. LFD composite during different ENSO phases. Coloured boxes in Fig. 1 a represents the hot spot regions of LFD (red box is NEI;
black box is NNWI; blue box is SPI).
erate updraughts limit the vertical development of convec-
tive clouds during the summer monsoon under the influence
of maritime air mass (Kumar et al., 2014; Tinmaker et al.,
2015), which leads to a decline in the cloud electrification
during the monsoon season. Among the three seasons, post-
monsoon shows a minimum of LFD over the Indian region
(Fig. 1). One possible reason for this may be the existence of
a low average value of CAPE (<500 J kg−1) over most parts
of India during this season (Murugavel et al., 2014), which is
relatively low to favour the development of deep convection
and, hence, lightning.
The relationship between LFD and graupel concentration
is examined during the three seasons by using a Pearson
correlation analysis over NEI, NNWI and SPI (Fig. 2). It
shows that the correlation between LFD and graupel concen-
tration is peaking during the postmonsoon season over NEI
(r=0.81), NNWI (r=0.64) and SPI (r=0.62). In contrast,
the correlation attained a minimum value during the mon-
soon season over these hot spot regions of LFD (NEI, where
r=0.24; NNWI, where r=0.36; SPI, where r=0.38). It is
important to note that the premonsoon season also exhibits a
Figure 2. Pearson correlation coefficient (r) between LFD and
graupel concentration over NEI, NNWI and SPI during premon-
soon, monsoon and postmonsoon seasons.
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A. V. Sreenath et al.: Variability in lightning over India with ENSO phases 2601
Figure 3. Anomaly composite of LFD during different ENSO phases with stippling indicating the statistically significant areas at a 95 %
confidence level. The box in panels (d) and (g) shows the monsoon trough and western disturbance region, respectively.
solid correlation between the LFD and graupel concentration
over NEI (r=0.64), NNWI (r=0.42) and SPI (r=0.55).
All the correlation values are strong and statistically signif-
icant at a 95 % confidence level, indicating tight linearity in
the relationship between LFD and graupel concentration over
regions of higher lightning activity over India.
3.2 Distribution of anomalous LFD during
premonsoon season with respect to ENSO phases
The LFD values are lower than normal during the premon-
soon season over NNWI when the ENSO phase is either
warm or cold (Fig. 3a,b), and it exhibits an increase in LFD
during the neutral phase (Fig. 3c). While looking into the
LFD anomaly of individual years, premonsoons of 3 years
(1997, 1998 and 2010) over NNWI have come under the
El Niño phase (Fig. 4d). The first two exhibit a decrease in
LFD, contributing to the overall reduction in the LFD over
NNWI (Fig. 3a). Out of the 4 La Niña years (1999, 2000,
2008 and 2011) of the premonsoon season, 1999, 2000 and
2011 have below-average values of LFD over the same re-
gion (Fig. 4d).
In situ airborne observations during the Cloud–Aerosol
Interaction and Precipitation Enhancement Experiment
(CAIPEEX) over various locations of India shows that con-
vective clouds during the premonsoon and monsoon period
have an ice water content of 10−4to 1 g m−3(Patade et al.,
2015). Moreover, in situ measured ice cloud properties in the
European Cloud Radiation Experiment (EUCREX) have re-
ported a similar range of ice water content inside the clouds
system (10−4to 1 g m−3; Hogan et al., 2006). From TRMM
observations and high-resolution model simulations, Abhi-
lash et al. (2008) reported ice concentrations of 10−3to
10−2g m−3for convective storms over the Indian region.
The vertical profiles for graupel and snow concentration are
shown over the hot spot domains of LFD in Figs. 5 and 6, re-
spectively, revealing a significant disparity in their seasonal
average with the observation region. These figures clearly
demonstrate that the seasonal average of graupel and snow
concentration are peaking around 6 km over NEI, NNWI and
SPI, and after that level, they show a rapid decrease with
height. Note that the seasonal average of latent heat (LH)
over these hot spot domains of LFD are peaking between a
6 and 7 km range, and this dramatically coincides with the
peaking altitude of graupel/snow concentration, mainly be-
cause of the release of energy during the phase transition of
cloud droplets to ice particles (Fig. 7). It is captivating that
https://doi.org/10.5194/nhess-21-2597-2021 Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021
2602 A. V. Sreenath et al.: Variability in lightning over India with ENSO phases
Figure 4. Anomalous LFD during the individual years with different ENSO phases. The red bar corresponds to warm phases of ENSO, the
green bar corresponds to cold phases of ENSO and the blue bar indicates the neutral phases of ENSO.
graupel/snow concentrations are prominent during all three
seasons over SPI, with a maximum value in the postmonsoon
season. In contrast to this observation, the NEI and NNWI
are showing a minimum value of graupel/snow concentra-
tions during the postmonsoon season in India.
The anomalous profile of graupel in Fig. 5a indicates that
clouds over NEI have high (low) graupel content during the
ENSO warm (cold) phase. An increase in snow content with
a peak value near 6.5 km is also observed over NEI dur-
ing premonsoon in the warm ENSO phase (Fig. 6a). The
interaction between snow and graupel and the associated
charge generation is responsible for lightning from convec-
tive clouds. Thus, the formation of the higher amount of
graupel and snow over NEI during the warm phase will re-
lease more latent heat, which is evident from Fig. 7a. Positive
(negative) anomalies of SH over NEI indicate that the con-
vective clouds formed during the ENSO warm (cold) phase
are vigorous (wimpy) and consequently responsible for the
enhanced (reduced) LFD. More importantly, graupel, snow
and latent heat profiles present below-average values in the
neutral ENSO phase, which may confirm a decrease in light-
ning events over NEI.
From Fig. 7, the anomalous latent heating exists
mostly between ±0.01. In some cases, it is extends up
to ±0.02 (K h−1); additionally, previous studies indicate that
these anomalous values are highly significant (Kumar et al.,
2014). Linked with the decrease in graupel and snow con-
tent over NNWI during the warm and cold phase of ENSO
(Figs. 5d, 6d), LH changes (Fig. 7d). The anomalous neg-
ative SH at 300 hPa manifests that the clouds are unable to
penetrate deep into the atmosphere during these two phases
over NNWI (Fig. 8a,b). As a result, LFD over NNWI in
these phases is low, especially in the cold phase. Contrarily,
higher LFD during the neutral phase points to the abundance
of graupel and snow inside the cloud system. It is noticed
that the ENSO cold phase during the premonsoon season is
favourable to LFD over central India (CI) (Fig. 3b), and the
converse is true for the ENSO warm phase (Fig. 3a).
The graupel and snow concentrations over SPI are anoma-
lously high up to 6 km during the cold phase of ENSO, and
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A. V. Sreenath et al.: Variability in lightning over India with ENSO phases 2603
Figure 5. Seasonal average (brown dotted curve) and anomaly composite of graupel concentration during different ENSO phases.
(a, d, g) Premonsoon season. (b, e, h) Monsoon season. (c, f, i) Postmonsoon season.
above that level, it decreases (Figs. 5, 6d). This particular hy-
drometeor pattern reverses during the warm phase, exhibits
below-average values beneath 6 km and rapidly increases
above that level. A similar pattern is observed in the vertical
profiles of latent heat release above 6 km. Note that the anal-
ysis presented here confirms that the warm phase of ENSO
intensifies the deep convection over SPI during the premon-
soon season and, hence, promotes LFD.
3.3 Distribution of anomalous LFD during monsoon
season with respect to ENSO phases
The LFD over the monsoon trough region of India increases
during the warm phase of ENSO (Fig. 3d), but it, remarkably,
decreases during the cold phase (Fig. 3e). Based on the 1998–
1999 El Niño event, Hamid et al. (2001) suggested that in-
tense convective storms developing over the maritime conti-
https://doi.org/10.5194/nhess-21-2597-2021 Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021
2604 A. V. Sreenath et al.: Variability in lightning over India with ENSO phases
Figure 6. Seasonal average (brown dotted curve) and anomaly composite of snow concentration during different ENSO phases. (a, d, g)
Premonsoon season. (b, e, h) Monsoon season. (c, f, i) Postmonsoon season.
nents are responsible for the increase in lightning activity de-
spite a decrease in the number of convective storms. During
the El Niño years of 1997–1998 and 2002–2003, the south-
east Asian regime exhibited an above-average value of light-
ning (Kumar and Kamra, 2012). While analysing the 300 hPa
SH variability, we noticed that the amount of SH over the
monsoon trough is higher during the warm and lower during
the cold phase (Fig. 8d,e). The NEI shows a positive anomaly
of LFD during the cold phase of ENSO. Figure 4b enforces
this result by showcasing that the majority of the years under
the cold phase (during the monsoon season) show an increase
in LFD over NEI. The vertical profile of LH shows an above-
average value during the cold phase, and this enhancement
of LH in the mid-troposphere helps to increase atmospheric
Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021 https://doi.org/10.5194/nhess-21-2597-2021
A. V. Sreenath et al.: Variability in lightning over India with ENSO phases 2605
Figure 7. Seasonal average (brown dotted curve) and anomaly composite of latent heat during different ENSO phases. (a, d, g) Premonsoon
season. (b, e, h) Monsoon season. (c, f, i) Postmonsoon season.
instability and deep convection. On the other hand, the verti-
cal distribution of hydrometers are not displaying any com-
pelling variability with ENSO phases over NEI (Figs. 5b,
6b). The observed increase (decrease) in the anomalous LFD
over NNWI during the cold (warm) period is captured well
in the vertical profiles of graupel, snow and latent heat re-
lease (Figs. 5e, 6e, 7e). There is no noticeable change in the
distribution of LFD over SPI in the three phases of ENSO
(Fig. 3d, e, f).
It is interesting to observe that, during the 12 years from
2002 to 2013, 11 years have shown above-average values of
LFD over the NEI and NNWI regions (Fig. 4b, e), registering
the intensification of deep convective cloud formation during
the recent monsoon season over respective areas. Out of the 9
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2606 A. V. Sreenath et al.: Variability in lightning over India with ENSO phases
Figure 8. Anomaly composite of specific humidity at 300 hPa during different ENSO phases with stippling to indicate statistically significant
areas at a 95 % confidence level.
years from 2005 to 2013, 8 have above-normal LFD over SPI
(Fig. 4h), thus indicating an escalation of deep convection
over SPI in that period. Specifically, the hot spots of LFD
over the Indian land region became more prominent during
the last decade’s monsoon seasons.
3.4 Distribution of anomalous LFD during
postmonsoon season with respect to ENSO phases
Western disturbances (WDs) are the vertical perturbations as-
sociated with the subtropical westerly jet stream, which is
one of the potential contributors to the rainfall over northern
India during the postmonsoon season (Dimri et al., 2016).
The jet is more intense and propagates southward during the
El Niño phase of ENSO (Schiemann et al., 2009). Our anal-
ysis shows that, at the time of the postmonsoon El Niño pe-
riod, LFD is increased throughout the country, and it is max-
imum over north–central India (Fig. 3g). In contrast, in the
cold phase, intense LFD is concentrated only over the NNWI
(Fig. 3h). Zubair and Ropelewski (2006) reported a signifi-
cant role for ENSO in controlling the postmonsoon rainfall
over SPI. The SPI shows an increase in LFD in the warm
phase of ENSO during this season due to the presence of
clouds having higher graupel and snow content over that re-
gion (Figs. 3g, 5i, 6i). The entire number of years grouped
under the warm phase of ENSO during the postmonsoon sea-
son show an increase in LFD over SPI. On the other hand,
during the cold phase, anomalous LFD displays an inconsis-
tent pattern of oscillation (Fig. 4i).
Climate variability, like ENSO, can alter the position of
jet streams and, hence, the distribution of WDs (Hunt et al.,
2018). Syed et al. (2006) identified that the intensification
of WDs during the El Niño is associated with the weak-
ening of the Siberian high. Studies signify that depressions
formed over the southern Bay of Bengal and the Arabian
Sea can also modulate the WDs’ path (Rao et al., 1969). The
500 hPa geopotential (GP) surface drops down (goes up) be-
yond 25◦N latitude and indicates the reduction in (enhance-
Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021 https://doi.org/10.5194/nhess-21-2597-2021
A. V. Sreenath et al.: Variability in lightning over India with ENSO phases 2607
Figure 9. Anomaly composite of geopotential height at 500 and 200 hPa wind during different ENSO phases. The stippling indicates statis-
tically significant areas of geopotential height at a 95 % confidence level.
ment of) convection over that region during the warm (cold)
phase of ENSO (Fig. 9a, b). Meanwhile, a higher (lower) GP
surface is visible over all of India during the warm (cold)
phase, which is an indication of an increase (decrease) in the
convective activity during the respective phases. By consid-
ering the anomalous circulation at 200 hPa level, an anoma-
lous westerly (easterly) wind is prevalent over the whole
of India during warm (cold) periods (Fig. 9). Accordingly,
upper-level wind pattern and variability in GPH together in-
dicate the southward extension of WDs during ENSO’s warm
phase. The sharp increase (decrease) in SH lies precisely over
the region of the maximum undulation of GPH over India
during the warm (cold) phase (Fig. 8g,h). This suggests that
ENSO indirectly influences the LFD over India during the
postmonsoon season by modulating the WDs’ path.
4 Conclusions
In this study, we have discussed the influence of ENSO on
LFD distribution during premonsoon, monsoon and post-
monsoon seasons over India. Regardless of ENSO phases,
the LFD is peaking at the time of the premonsoon season over
NEI and SPI. However, the NNWI exhibits a peak LFD dur-
ing the monsoon season. More importantly, the compelling
correlation values indicates the solid linear dependence of
LFD on graupel concentration over the hot spot regions of
lightning. The LFD is increased (decreased) compared to
their average values over NEI and SPI during the warm (cold)
phase of ENSO, and anomalies of the charge-generating hy-
drometeors also show a similar kind of swing during the pre-
monsoon season. An increase in graupel and snow forma-
tion above 6 km pinpoints that the warm phase of ENSO is
conducive to deep convection over SPI during the premon-
soon season. However, the neutral phase of ENSO favours
the deepening of clouds over NNWI, as evidenced by the
high values of the upper-level-specific humidity.
During monsoon season, LFD over NEI and NNWI is
higher than the average values during the La Niña periods.
The SPI is not showing a significant variation in LFD with
respect to different ENSO phases during the monsoon sea-
son. While considering the recent 12 years of this study,
irrespective of the ENSO phases, every year has displayed
above-average values of LFD over the NEI and NNWI re-
gion. Out of 9 years from 2005 to 2013, 8 displayed above-
normal LFD over SPI, which signifies the intensification of
LFD over the three hot spots during the monsoon seasons of
the last decade.
Almost all regions in India are exhibiting higher LFD dur-
ing the warm ENSO phase in the postmonsoon season. The
elevated (reduced) GPH is visible all over India during the
warm (cold) phase of ENSO, which is an indication of an
increase (decrease) in the convective activity during the re-
spective phases. Furthermore, the intensification of convec-
tion during the warm phase is advocated by a significant rise
in graupel and snow concentration over SPI. The entire years
grouped under the warm phase of ENSO during the postmon-
soon season show an increase in LFD over SPI, whereas the
years elected under the cold phase show a dispersed anoma-
lous pattern. Both the intensification and southward exten-
sion of WDs are responsible for higher LFD over India in
the warm phase, indicating an indirect interaction between
ENSO and LFD by modulating the mid-latitude westerlies.
Data availability. The LIS/OTD data and vertical profiles of
hydrometeors and latent heat are obtained from the website
https://doi.org/10.5067/LIS/LIS-OTD/DATA309 (Cecil, 2006) and
https://disc.gsfc.nasa.gov/datasets/ (last access: 23 August 2021)
(TRMM, 2011) respectively. The GPH, wind and SH data are avail-
able at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html
(last access: 23 August 2021) (National Centers for Environmen-
tal Prediction/National Weather Service/NOAA/US Department of
Commerce, 1994). The HadISST data used in this work are ac-
cessible from https://psl.noaa.gov/gcos_wgsp/ (last access: 23 Au-
gust 2021) (NOAA Physical Sciences Laboratory, 2021).
Author contributions. The paper and its methodology were concep-
tualized and developed by SA, AVS, and PV. AVS performed the
analyses, and PV curated the data. The original draft preparation
was by AVS; further reviewing and editing was by PV and SA.
AVS handled the visualization.
https://doi.org/10.5194/nhess-21-2597-2021 Nat. Hazards Earth Syst. Sci., 21, 2597–2609, 2021
2608 A. V. Sreenath et al.: Variability in lightning over India with ENSO phases
Competing interests. The authors declare that they have no conflict
of interest.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. We are grateful to NASA and the Propulsion
Systems Laboratory (PSL) for providing the LIS/ODT, TRMM and
NCEP reanalysis data products, respectively, which have been used
in this study. Support from the Department of Atmospheric Sci-
ences, Cochin University of Science and Technology, is acknowl-
edged.
Financial support. This research has been supported by the Ker-
ala State Council for Science, Technology and Environment (grant
no. KSCSTE/343/2019-FSHP-Earth).
Review statement. This paper was edited by Vassiliki Kotroni and
reviewed by two anonymous referees.
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