Key Messages
• There has been a decreasing trend in the all-India annual,
as well as summer monsoon mean rainfall during 1951–
2015, notably so over areas in the Indo-Gangetic Plains
and the Western Ghats. Increasing concentrations of
anthropogenic aerosols over the northern hemisphere
appear to have played a role in these changes (medium
confidence).
• The frequency of localized heavy rain occurrences over
India has increased during 1951–2015 (high confidence).
Urbanization and other land use, as well as aerosols,
likely contribute to these localized heavy rainfall occurrence (medium confidence).
• With continued global warming and expected reductions
of aerosol concentrations in the future, climate models
project an increase in the annual and summer monsoon
mean rainfall, as well as frequency of heavy rain occurrences over most parts of India during the twenty-first
century (medium confidence).
• The interannual variability of summer monsoon rainfall is
projected to increase through the twenty-first century
(high confidence).
3.1 Introduction
Precipitation is an important component of the global water
cycle, and the impacts of anthropogenic climate change on
precipitation have significant implications on agricultural
activities (Porter et al. 2014). Over India, the seasonal
monsoon rains during the June-September months, which
contribute to more than 75% of the annual rainfall
(Fig. 3.1a), are vital for the country’s agriculture and economy (Parthasarathy et al. 1988; Gadgil 2007). Abrupt
changes in the Indian monsoon precipitation on decadal and
centennial time-scales are evident from high-resolution climate proxy records, extending back several thousands of
years (Berkelhammer et al. 2012; Sanyal and Sinha 2010).
This chapter provides an assessment of historical changes in
precipitation, as well as projections of future changes from
climate models.
Box 3.1 Processes in the Monsoon System
The annual cycle of monsoon precipitation can be
described as a manifestation of the seasonal migration
of the Inter-Tropical Convergence Zone (ITCZ) and
the associated shift in rain band from southern to
northern Indian Ocean (Fig. 3.1a). Summer monsoon
rainfall across the Indian Subcontinent shows
substantial spatial variability with heaviest rainfall
along the Western Ghats and the Himalayan foothills
due to orographic features, and over central India due
to low-level convergence (Fig. 3.1; upper panel).
Mean Onset and Withdrawal
The onset of summer monsoon over India is characterized by the dramatic rise in mean daily rainfall over
Kerala (Ananthkrishnan and Soman 1988; Soman and
Kumar 1993). The onset of the Indian summer monsoon (ISM) is a key indicator characterizing the abrupt
transition from the dry season to the rainy season and
subsequent seasonal march (Koteswaram 1958;
Ananthakrishnan et al. 1968; Krishnamurti and
Ramanathan 1982; Ananthakrishnan and Soman 1988;
Wang et al. 2001, 2009; Pai and Rajeevan 2009). The
mean onset date of summer monsoon rainfall over
India has been around 1 June. By the first week of
July, the southwest monsoon is established over the
entire subcontinent. The southwest monsoon starts
withdrawing from the extreme northwestern portion of
India by the beginning of September.
Intra-Seasonal Oscillations (ISOs)
The strength of summer monsoon rainfall is modulated
by the intra-seasonal variability which is characterized
by the active/break spells of enhanced/decreased precipitation over India. The active/break spells over
India play an essential role in modulating mean
monsoon rainfall (e.g. Ramamurthy 1969; Sikka and
Gadgil 1980; Rodwell 1997; Webster et al. 1998;
Krishnan et al. 2000; Krishnamurthy and Shukla 2000,
2007, 2008; Annamalai and Slingo 2001; Goswami
and Ajayamohan 2001; Lawrence and Webster 2001;
De and Mukhopadhyay 2002; Goswami et al. 2003;
Waliser et al. 2003; Kripalani et al. 2004; Wang et al.
2005; Mandke et al. 2007; Goswami 2005; Waliser
2006). Active and break episodes, characteristic of
subseasonal variations of the ISM, are associated with
enhanced (decreased) rainfall over central and western
India and decreased (enhanced) rainfall over the
southeastern peninsula and eastern India (Singh et al.
1992; Krishnamurthy and Shukla 2000; Krishnan et al.
2000; Goswami et al. 2003).
ISOs are influenced by various factors such as
variations in the position and strength of the continental monsoon trough, quasi-periodic oscillations of
the monsoon, as well as synoptic systems such as lows
and depressions (Shukla 1987 Yasunari 1979; Sikka
and Gadgil 1980). These quasi-periodic fluctuations
play a major role in determining the amplitude of
seasonal mean of individual summer seasons by
modulating the strength and duration of active/break
48
A. Kulkarni et al.
• There has been a decreasing trend in the all-India annual,
as well as summer monsoon mean rainfall during 1951–
2015, notably so over areas in the Indo-Gangetic Plains
and the Western Ghats. Increasing concentrations of
anthropogenic aerosols over the northern hemisphere
appear to have played a role in these changes (medium
confidence).
• The frequency of localized heavy rain occurrences over
India has increased during 1951–2015 (high confidence).
Urbanization and other land use, as well as aerosols,
likely contribute to these localized heavy rainfall occurrence (medium confidence).
• With continued global warming and expected reductions
of aerosol concentrations in the future, climate models
project an increase in the annual and summer monsoon
mean rainfall, as well as frequency of heavy rain occurrences over most parts of India during the twenty-first
century (medium confidence).
• The interannual variability of summer monsoon rainfall is
projected to increase through the twenty-first century
(high confidence).
3.1 Introduction
Precipitation is an important component of the global water
cycle, and the impacts of anthropogenic climate change on
precipitation have significant implications on agricultural
activities (Porter et al. 2014). Over India, the seasonal
monsoon rains during the June-September months, which
contribute to more than 75% of the annual rainfall
(Fig. 3.1a), are vital for the country’s agriculture and economy (Parthasarathy et al. 1988; Gadgil 2007). Abrupt
changes in the Indian monsoon precipitation on decadal and
centennial time-scales are evident from high-resolution climate proxy records, extending back several thousands of
years (Berkelhammer et al. 2012; Sanyal and Sinha 2010).
This chapter provides an assessment of historical changes in
precipitation, as well as projections of future changes from
climate models.
Box 3.1 Processes in the Monsoon System
The annual cycle of monsoon precipitation can be
described as a manifestation of the seasonal migration
of the Inter-Tropical Convergence Zone (ITCZ) and
the associated shift in rain band from southern to
northern Indian Ocean (Fig. 3.1a). Summer monsoon
rainfall across the Indian Subcontinent shows
substantial spatial variability with heaviest rainfall
along the Western Ghats and the Himalayan foothills
due to orographic features, and over central India due
to low-level convergence (Fig. 3.1; upper panel).
Mean Onset and Withdrawal
The onset of summer monsoon over India is characterized by the dramatic rise in mean daily rainfall over
Kerala (Ananthkrishnan and Soman 1988; Soman and
Kumar 1993). The onset of the Indian summer monsoon (ISM) is a key indicator characterizing the abrupt
transition from the dry season to the rainy season and
subsequent seasonal march (Koteswaram 1958;
Ananthakrishnan et al. 1968; Krishnamurti and
Ramanathan 1982; Ananthakrishnan and Soman 1988;
Wang et al. 2001, 2009; Pai and Rajeevan 2009). The
mean onset date of summer monsoon rainfall over
India has been around 1 June. By the first week of
July, the southwest monsoon is established over the
entire subcontinent. The southwest monsoon starts
withdrawing from the extreme northwestern portion of
India by the beginning of September.
Intra-Seasonal Oscillations (ISOs)
The strength of summer monsoon rainfall is modulated
by the intra-seasonal variability which is characterized
by the active/break spells of enhanced/decreased precipitation over India. The active/break spells over
India play an essential role in modulating mean
monsoon rainfall (e.g. Ramamurthy 1969; Sikka and
Gadgil 1980; Rodwell 1997; Webster et al. 1998;
Krishnan et al. 2000; Krishnamurthy and Shukla 2000,
2007, 2008; Annamalai and Slingo 2001; Goswami
and Ajayamohan 2001; Lawrence and Webster 2001;
De and Mukhopadhyay 2002; Goswami et al. 2003;
Waliser et al. 2003; Kripalani et al. 2004; Wang et al.
2005; Mandke et al. 2007; Goswami 2005; Waliser
2006). Active and break episodes, characteristic of
subseasonal variations of the ISM, are associated with
enhanced (decreased) rainfall over central and western
India and decreased (enhanced) rainfall over the
southeastern peninsula and eastern India (Singh et al.
1992; Krishnamurthy and Shukla 2000; Krishnan et al.
2000; Goswami et al. 2003).
ISOs are influenced by various factors such as
variations in the position and strength of the continental monsoon trough, quasi-periodic oscillations of
the monsoon, as well as synoptic systems such as lows
and depressions (Shukla 1987 Yasunari 1979; Sikka
and Gadgil 1980). These quasi-periodic fluctuations
play a major role in determining the amplitude of
seasonal mean of individual summer seasons by
modulating the strength and duration of active/break
48
A. Kulkarni et al.