Indian summer Monson rainfall over a 30-year time period
(i.e., 30-year periodicity of above normal or below normal
epochs of the Indian summer monsoon rainfall). This study
showed that intense LPS (i.e., except lows) are more (less)
seen during the epochs of above (below) normal Indian
summer monsoon rainfall. A recent study by Vishnu et al.
(2018) examined the inter-decadal aspects of LPS, and they
showed that number of monsoon depressions (stronger LPS)
over BoB has out-of-phase relationship with the Pacific
Decadal Oscillation (PDO). The PDO induced warming in
the Western Equatorial Indian Ocean decreases the moisture
advection into the BoB, thereby reducing the relative
humidity and suppresses the monsoon depression activity.
This is in contrast to PDO induced cooling (i.e., over
Western Equatorial Indian Ocean) which increases the
moisture advection into the BoB, thereby enhancing the
monsoon depression activity.
LPS is generally found to be associated with heaviest rain
intensities (Sikka 2006). Despite the decreasing trend seen in
the occurrence of stronger LPS (monsoon depressions, as
noted above), the frequency of monsoon rainfall extremes
(i.e., heavy rainfall events, rainfall ! 100 mm day
−1 , and
very heavy rainfall events, rainfall ! 150 mm day
−1 ; as
defined in Goswami et al. 2006; Roxy et al. 2017; Nikumbh
et al. 2019) have increased over the central Indian landmass
since 1950. An increasing trend observed in monsoon lows
during this period (weaker LPS, see Fig. 7.1b and
Table 7.2), also implies an in-phase relationship between
lows and monsoon rainfall extremes (Ajayamohan et al.
2010). Nikumbh et al. (2019) also noticed that the monsoon
LPS, in general, (i.e., without distinguishing between monsoon lows and depressions) is conducive for increasing
occurrences of extreme events over the central part of India.
For example, the extreme rainfall events which caused
large-scale floods over central Indian landmass on 24 July
1989, 18 July 2000 and 7 August 2007, are associated with
LPS (Roxy et al. 2017).
The contrasting trends in lows and depressions imply that
the intensification from lows to depressions may be rather
constrained by certain background atmospheric or oceanic
conditions (Mandke and Bhide 2003; Rao et al. 2004; Prajeesh et al. 2013). For example, reduction in the
mid-tropospheric relative humidity over BoB is found to be
an important factor preventing the intensification of lows into
depressions, and thus the reduced frequency of monsoon
depressions (Prajeesh et al. 2013; Vishnu et al. 2016). This
was also attributed to the weakening of the low-level jet,
consistent with the weakening of summer monsoon circulation (Joseph and Simon 2005; Ramesh Kumar et al. 2009).
Though the LPSs significantly contribute to the seasonal
total rainfall, it is difficult to designate flood and drought
monsoon years in terms of LPS variability (i.e., inter-annual
variability of LPS; Sikka 2006) alone. Krishnamurthy and
Ajayamohan (2010) have shown that the LPS contribution
(to the total seasonal monsoon rainfall) remains invariant
during the periods of monsoon floods or droughts, even
though LPS frequency is slightly seen higher during flood
years. However, they have shown that the track of LPS
shows a marked difference between flood and drought years.
The LPS reach up to northwest India during the flood years,
while they are confined to central India during the drought
years.
On the large-scale modes of variability influencing LPS,
Hunt et al. (2016a) inferred a significant relationship
between El Niño–Southern Oscillation (ENSO) and LPS
activities (particularly for monsoon depressions). Their study
indicated that there are more monsoon depressions during El
Niño years (approximately 16% more) than La Niña years.
This study differs from the investigation of Krishnamurthy
and Ajayamohan (2010) which suggests that there is no
significant relationship. This may be due to the consideration
of total LPS in their study, instead of only monsoon
depressions. There are few other studies that focused on the
association of LPS activity with the Indian Ocean Dipole
(IOD). Krishnan et al. (2011) reported that positive IOD is
favorable for long-lived LPS. They found an approximate
12% increased lifetime of LPS during the positive IOD as
compared to the normal years. Hunt et al. (2016a), however,
observed that the state of IOD (i.e., positive and negative
IODs) has no significant impact on depressions. Thus far,
contrasting results from different studies imply that there is
no clear consistency to assert the association of LPS with
ENSO/IOD.
Given the prominent dependency of the Indian summer
monsoon seasonal rainfall on LPS, it is important to
understand the potential impact of climate change on LPS;
yet there are only few studies in this direction. Patwardhan
et al. (2012), with a focus on stronger LPS, showed that the
frequency (intensity) of LPS may reduce (increase) by about
9% (11%) towards the end of the twenty-first century (under
SRES-A2 scenario). They focused mainly on stronger LPS,
except lows. Although observational evidence portray significantly increasing long-term trends (Fig. 7.1b and
Table 7.2), there are no studies to diagnose the potential
future changes in monsoon lows so far. Sandeep et al. (2018)
reported that there would be about 45% reduction (significant at 5% level) in the LPS activity during the late
twenty-first century (2071–2095) following RCP8.5 scenario
(i.e., stronger warming climate scenario) from the
High-Resolution Atmospheric Model (HiRAM) simulations,
and the simulations from the fifth phase of Coupled Model
Intercomparison Project (CMIP5; Taylor et al. 2011) also
indicated weakening of LPS activity (over central India) in
the RCP8.5 simulation. They used a combined measure of
frequency, intensity, and duration of LPS to determine the
LPS activity. The HiRAM projections also showed a
148
S. Patwardhan et al.
(i.e., 30-year periodicity of above normal or below normal
epochs of the Indian summer monsoon rainfall). This study
showed that intense LPS (i.e., except lows) are more (less)
seen during the epochs of above (below) normal Indian
summer monsoon rainfall. A recent study by Vishnu et al.
(2018) examined the inter-decadal aspects of LPS, and they
showed that number of monsoon depressions (stronger LPS)
over BoB has out-of-phase relationship with the Pacific
Decadal Oscillation (PDO). The PDO induced warming in
the Western Equatorial Indian Ocean decreases the moisture
advection into the BoB, thereby reducing the relative
humidity and suppresses the monsoon depression activity.
This is in contrast to PDO induced cooling (i.e., over
Western Equatorial Indian Ocean) which increases the
moisture advection into the BoB, thereby enhancing the
monsoon depression activity.
LPS is generally found to be associated with heaviest rain
intensities (Sikka 2006). Despite the decreasing trend seen in
the occurrence of stronger LPS (monsoon depressions, as
noted above), the frequency of monsoon rainfall extremes
(i.e., heavy rainfall events, rainfall ! 100 mm day
−1 , and
very heavy rainfall events, rainfall ! 150 mm day
−1 ; as
defined in Goswami et al. 2006; Roxy et al. 2017; Nikumbh
et al. 2019) have increased over the central Indian landmass
since 1950. An increasing trend observed in monsoon lows
during this period (weaker LPS, see Fig. 7.1b and
Table 7.2), also implies an in-phase relationship between
lows and monsoon rainfall extremes (Ajayamohan et al.
2010). Nikumbh et al. (2019) also noticed that the monsoon
LPS, in general, (i.e., without distinguishing between monsoon lows and depressions) is conducive for increasing
occurrences of extreme events over the central part of India.
For example, the extreme rainfall events which caused
large-scale floods over central Indian landmass on 24 July
1989, 18 July 2000 and 7 August 2007, are associated with
LPS (Roxy et al. 2017).
The contrasting trends in lows and depressions imply that
the intensification from lows to depressions may be rather
constrained by certain background atmospheric or oceanic
conditions (Mandke and Bhide 2003; Rao et al. 2004; Prajeesh et al. 2013). For example, reduction in the
mid-tropospheric relative humidity over BoB is found to be
an important factor preventing the intensification of lows into
depressions, and thus the reduced frequency of monsoon
depressions (Prajeesh et al. 2013; Vishnu et al. 2016). This
was also attributed to the weakening of the low-level jet,
consistent with the weakening of summer monsoon circulation (Joseph and Simon 2005; Ramesh Kumar et al. 2009).
Though the LPSs significantly contribute to the seasonal
total rainfall, it is difficult to designate flood and drought
monsoon years in terms of LPS variability (i.e., inter-annual
variability of LPS; Sikka 2006) alone. Krishnamurthy and
Ajayamohan (2010) have shown that the LPS contribution
(to the total seasonal monsoon rainfall) remains invariant
during the periods of monsoon floods or droughts, even
though LPS frequency is slightly seen higher during flood
years. However, they have shown that the track of LPS
shows a marked difference between flood and drought years.
The LPS reach up to northwest India during the flood years,
while they are confined to central India during the drought
years.
On the large-scale modes of variability influencing LPS,
Hunt et al. (2016a) inferred a significant relationship
between El Niño–Southern Oscillation (ENSO) and LPS
activities (particularly for monsoon depressions). Their study
indicated that there are more monsoon depressions during El
Niño years (approximately 16% more) than La Niña years.
This study differs from the investigation of Krishnamurthy
and Ajayamohan (2010) which suggests that there is no
significant relationship. This may be due to the consideration
of total LPS in their study, instead of only monsoon
depressions. There are few other studies that focused on the
association of LPS activity with the Indian Ocean Dipole
(IOD). Krishnan et al. (2011) reported that positive IOD is
favorable for long-lived LPS. They found an approximate
12% increased lifetime of LPS during the positive IOD as
compared to the normal years. Hunt et al. (2016a), however,
observed that the state of IOD (i.e., positive and negative
IODs) has no significant impact on depressions. Thus far,
contrasting results from different studies imply that there is
no clear consistency to assert the association of LPS with
ENSO/IOD.
Given the prominent dependency of the Indian summer
monsoon seasonal rainfall on LPS, it is important to
understand the potential impact of climate change on LPS;
yet there are only few studies in this direction. Patwardhan
et al. (2012), with a focus on stronger LPS, showed that the
frequency (intensity) of LPS may reduce (increase) by about
9% (11%) towards the end of the twenty-first century (under
SRES-A2 scenario). They focused mainly on stronger LPS,
except lows. Although observational evidence portray significantly increasing long-term trends (Fig. 7.1b and
Table 7.2), there are no studies to diagnose the potential
future changes in monsoon lows so far. Sandeep et al. (2018)
reported that there would be about 45% reduction (significant at 5% level) in the LPS activity during the late
twenty-first century (2071–2095) following RCP8.5 scenario
(i.e., stronger warming climate scenario) from the
High-Resolution Atmospheric Model (HiRAM) simulations,
and the simulations from the fifth phase of Coupled Model
Intercomparison Project (CMIP5; Taylor et al. 2011) also
indicated weakening of LPS activity (over central India) in
the RCP8.5 simulation. They used a combined measure of
frequency, intensity, and duration of LPS to determine the
LPS activity. The HiRAM projections also showed a
148
S. Patwardhan et al.