important to have an estimation of likely future changes in
rainfall, particularly droughts, under warming scenario.
Additionally, quantitative information on rainfall and related
atmospheric and oceanic parameters prior to the period of
recorded meteorological data is also essential for understanding and possibly mitigating the effects of projected
climate change. Hence, a look into the palaeoclimatic
records has also been made, in the following section, for
understanding the variability of monsoon droughts in the
past.
6.2.2 Palaeoclimatic Evidences
Evidences from proxy records indicate that past monsoonal
variations were dominated by decadal- to millennial-scale
variability and long-term trends (Kelkar 2006; Sinha et al.
2018; Band et al. 2018 and references therein). Reconstruction of SW monsoon variability based on stalagmite
oxygen isotope ratios from Central India indicates a gradual
decrease in monsoon during the beginning of the
mid-Holocene from 8.5 to 7.3 ka BP (Before Present: 1950
AD), followed by a steady increase in monsoon intensity
between 6.3 and 5.6 ka BP. This overall trend of monsoon
during the mid-Holocene is punctuated by abrupt megadrought events spanning 70–100 years. During the past
1500 years, centennial-scale climate oscillations include the
Medieval Warm Period (MWP) during 900–1300 AD—with
relatively stronger monsoon, and the Little Ice Age
(LIA) during 1400–1850 AD—with the relatively weaker
monsoon (e.g. Sinha et al. 2007, 2011a, b; Goswami et al.
2015; Kathayat et al. 2017 and references therein). Severe
drought, in India, lasting decades occurred during fourteenth
and mid-fifteenth centuries in LIA. Nearly every major
famine, including the devastating Durga Devi famine during
1396–1409 AD, coincides with a period of reduced monsoon
rainfall, reconstructed from d
18 O of speleothems collected
from Central India (Sinha et al. 2007). A possible influence
of ENSO is suggested for the Indian monsoon variability
during the mid-Holocene, MWP and LIA (Mann et al. 2009;
Band et al. 2018; Tejavath et al. 2019).
Indian monsoon drought history for past 500 years and its
association with El Nino was derived by Borgaonkar et al.
(2010) from 523-year (1481–2003 AD) tree-ring chronology
from Kerala, south India (Fig. 6.4a). This chronology exhibits a significant positive relationship with the observed SW
monsoon rainfall for the instrumental period (1871–2003
AD; Fig. 6.4b, c). LIA with weaker monsoon is also evident
(Fig. 6.4a: 1600–1700 AD). Higher frequency of low tree
growth occurrences (Fig. 6.4b) was observed in years of
monsoon droughts (Fig. 6.4c), these events are associated
with El Niño since the late eighteenth century. Prior to that,
many low tree growth years were detected during known El
Niño events, probably related to deficient Indian monsoon
rainfall (Fig. 6.4a; Borgaonkar et al. 2010). It is noteworthy,
however, that the mid-eighteenth century is a time where
drought is indicated in northern Thailand (Buckley et al.
2007) and northern Vietnam (Sano et al. 2009), suggestive
of a weakened monsoon in the late eighteenth century. Most
of these periods, including those prior to the mid-eighteenth
century, have also been reported to have widespread
droughts in India (Pant et al. 1993). Aforementioned studies
thus indicate a strong influence of Indo-Pacific SSTs on past
monsoon droughts at decadal to millennial timescales.
6.3 Observed Variability of Floods
Floods, as compared to droughts, have regional characteristics and are typically confined to shorter timescales ranging
from several hours to days. Floods are classified into different types such as riverine (extreme rainfall for longer
periods), flash (heavy rainfall in cities or steep slopes), urban
(lack of drainage), coastal (storm surge) and pluvial (rainfall
over a flat surface) flooding. Regions prone to frequent
floods mainly include river basins, hilly, coastal areas and in
some instances, cities. In India, different types of floods
frequently occur primarily during the SW monsoon season,
the major rainy season. In addition, south peninsular India
experiences floods during the NE monsoon season (Dhar and
Nandargi 2000, 2003). The majority of floods in India are
closely associated with heavy rainfall events, and not all of
these heavy rain events translate into floods. Apart from the
rainfall extremes, flood occurrences are linked to other factors such as antecedent soil moisture, storm duration,
snowmelt, drainage basin conditions, urbanization, dams and
reservoirs, and also proximity to the coast (Rosenzweig et al.
2010; Mishra et al. 2012a; Sharma et al. 2018). In addition,
several other factors, such as infrastructure, siltation of rivers, deforestation, and backwater effect, can accelerate the
impacts of floods.
In India, the spatial variation of floods mostly follows the
monsoon intraseasonal oscillations. For example, Central
India experiences the majority of flood events during the
active monsoon phase, primarily due to heavy rainfall
received from monsoon disturbances (Dhar and Nandargi
2003; Kale 2003, 2012; Ranade et al. 2007; Sontakke et al.
2008). On the contrary, regions near the foothills of the
Himalayas typically experience floods during the break
monsoon condition, due to heavy rainfall associated with the
movement of monsoon trough towards the foothills, orographic uplift of moist monsoon flows and also due to
tropical and mid-latitude interactions (Dhar and Nandargi
2000; Krishnan et al. 2000, 2009; Vellore et al. 2014).
Occasionally, low pressure systems and western disturbances interact to give rise to heavy rains and floods (Sikka
6 Droughts and Floods
125
rainfall, particularly droughts, under warming scenario.
Additionally, quantitative information on rainfall and related
atmospheric and oceanic parameters prior to the period of
recorded meteorological data is also essential for understanding and possibly mitigating the effects of projected
climate change. Hence, a look into the palaeoclimatic
records has also been made, in the following section, for
understanding the variability of monsoon droughts in the
past.
6.2.2 Palaeoclimatic Evidences
Evidences from proxy records indicate that past monsoonal
variations were dominated by decadal- to millennial-scale
variability and long-term trends (Kelkar 2006; Sinha et al.
2018; Band et al. 2018 and references therein). Reconstruction of SW monsoon variability based on stalagmite
oxygen isotope ratios from Central India indicates a gradual
decrease in monsoon during the beginning of the
mid-Holocene from 8.5 to 7.3 ka BP (Before Present: 1950
AD), followed by a steady increase in monsoon intensity
between 6.3 and 5.6 ka BP. This overall trend of monsoon
during the mid-Holocene is punctuated by abrupt megadrought events spanning 70–100 years. During the past
1500 years, centennial-scale climate oscillations include the
Medieval Warm Period (MWP) during 900–1300 AD—with
relatively stronger monsoon, and the Little Ice Age
(LIA) during 1400–1850 AD—with the relatively weaker
monsoon (e.g. Sinha et al. 2007, 2011a, b; Goswami et al.
2015; Kathayat et al. 2017 and references therein). Severe
drought, in India, lasting decades occurred during fourteenth
and mid-fifteenth centuries in LIA. Nearly every major
famine, including the devastating Durga Devi famine during
1396–1409 AD, coincides with a period of reduced monsoon
rainfall, reconstructed from d
18 O of speleothems collected
from Central India (Sinha et al. 2007). A possible influence
of ENSO is suggested for the Indian monsoon variability
during the mid-Holocene, MWP and LIA (Mann et al. 2009;
Band et al. 2018; Tejavath et al. 2019).
Indian monsoon drought history for past 500 years and its
association with El Nino was derived by Borgaonkar et al.
(2010) from 523-year (1481–2003 AD) tree-ring chronology
from Kerala, south India (Fig. 6.4a). This chronology exhibits a significant positive relationship with the observed SW
monsoon rainfall for the instrumental period (1871–2003
AD; Fig. 6.4b, c). LIA with weaker monsoon is also evident
(Fig. 6.4a: 1600–1700 AD). Higher frequency of low tree
growth occurrences (Fig. 6.4b) was observed in years of
monsoon droughts (Fig. 6.4c), these events are associated
with El Niño since the late eighteenth century. Prior to that,
many low tree growth years were detected during known El
Niño events, probably related to deficient Indian monsoon
rainfall (Fig. 6.4a; Borgaonkar et al. 2010). It is noteworthy,
however, that the mid-eighteenth century is a time where
drought is indicated in northern Thailand (Buckley et al.
2007) and northern Vietnam (Sano et al. 2009), suggestive
of a weakened monsoon in the late eighteenth century. Most
of these periods, including those prior to the mid-eighteenth
century, have also been reported to have widespread
droughts in India (Pant et al. 1993). Aforementioned studies
thus indicate a strong influence of Indo-Pacific SSTs on past
monsoon droughts at decadal to millennial timescales.
6.3 Observed Variability of Floods
Floods, as compared to droughts, have regional characteristics and are typically confined to shorter timescales ranging
from several hours to days. Floods are classified into different types such as riverine (extreme rainfall for longer
periods), flash (heavy rainfall in cities or steep slopes), urban
(lack of drainage), coastal (storm surge) and pluvial (rainfall
over a flat surface) flooding. Regions prone to frequent
floods mainly include river basins, hilly, coastal areas and in
some instances, cities. In India, different types of floods
frequently occur primarily during the SW monsoon season,
the major rainy season. In addition, south peninsular India
experiences floods during the NE monsoon season (Dhar and
Nandargi 2000, 2003). The majority of floods in India are
closely associated with heavy rainfall events, and not all of
these heavy rain events translate into floods. Apart from the
rainfall extremes, flood occurrences are linked to other factors such as antecedent soil moisture, storm duration,
snowmelt, drainage basin conditions, urbanization, dams and
reservoirs, and also proximity to the coast (Rosenzweig et al.
2010; Mishra et al. 2012a; Sharma et al. 2018). In addition,
several other factors, such as infrastructure, siltation of rivers, deforestation, and backwater effect, can accelerate the
impacts of floods.
In India, the spatial variation of floods mostly follows the
monsoon intraseasonal oscillations. For example, Central
India experiences the majority of flood events during the
active monsoon phase, primarily due to heavy rainfall
received from monsoon disturbances (Dhar and Nandargi
2003; Kale 2003, 2012; Ranade et al. 2007; Sontakke et al.
2008). On the contrary, regions near the foothills of the
Himalayas typically experience floods during the break
monsoon condition, due to heavy rainfall associated with the
movement of monsoon trough towards the foothills, orographic uplift of moist monsoon flows and also due to
tropical and mid-latitude interactions (Dhar and Nandargi
2000; Krishnan et al. 2000, 2009; Vellore et al. 2014).
Occasionally, low pressure systems and western disturbances interact to give rise to heavy rains and floods (Sikka
6 Droughts and Floods
125