which is characterized by the active/break spells of enhanced
and decreased precipitation over India (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 and Waliser 2006). The relative
strength of the northward-propagating low-frequency (20–
60 days) modes has a significant decreasing trend during
1951–2010, possibly due to the weakening of large-scale
circulation in the region during the monsoon season. This
reduction is compensated by a gain in synoptic-scale (3–
9 days) variability. The decrease in low-frequency ISO
variability is associated with a significant decreasing trend in
the percentage of extreme events during the active phase of
the monsoon. However, this decrease is likely balanced by
significant increasing trends in the percentage of extreme
events in the break and transition phases. These changes are
accompanied by a weakening of low-frequency variability
(Karmakar et al. 2015, 2017). Also, while there is no change
in the distribution of the break events, the number of active
spells shows an increase of about 12% in the period
1951–2010; the increase was mainly in the short duration
(3–6 days) spells (Pai et al. 2016). A statistically significant
increase in the frequency of dry spells (27% higher during
1981–2011 relative to 1951–1980) and intensity of wet
spells and statistically significant decreases in the intensity
of dry spells have been observed in recent six decades
(Singh et al. 2014). The changes in frequency, intensity and
speed of intra-seasonal oscillations have been attributed to
Indian ocean warming (Sabeerali et al. 2015); developing
and decaying phase of ENSO (Pillai and Chowdary 2016);
increase in convective available potential energy, low-level
moisture convergence and changes in large-scale circulation
in upper atmosphere (Singh et al. 2014).
3.2.3.4 Changes in Onset Characteristics
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 ISM has been defined with various dynamic
(Koteswaram 1958; Ananthakrishnan et al. 1968; Krishnamurti and Ramanathan 1982; Wang et al. 2001; 2009; Pai
and Rajeevan 2009) and thermodynamic indices (Ananthakrishnan and Soman 1988; Fasullo and Webster 2003;
Janowiak and Xie 2003). Objective definitions of South
Asian summer monsoon onset include measures such as the
increase of rainfall above a threshold (Wang and Lin 2002),
transition in vertically integrated moisture transport (Fasullo
and Webster 2003), reversal of surface wind (Ramage 1971),
and intensification of the lower level Somali jet (Taniguchi
and Koike 2006; Wang et al. 2009). As per these different
definitions, the mean onset date of summer monsoon rainfall
over India has been stable around 1 June. In recent decades,
the monsoon onset over India is seen to be delayed to 5th
June since 1976 (Sahana et al. 2015), which can be attributed
to the net decrease in moisture supply from the Arabian Sea
in the post-1976 period. The interannual variability of the
onset date is associated with ENSO with early onsets preceded by La Nina, and late onsets preceded by El Nino (e.g.
Noska and Mishra 2016).
3.3 Projected Changes in Precipitation Over
India
Understanding the projected future changes in precipitation
has a profound importance for policy. In this report, the
assessment of rainfall changes over India is carried out based
on the multiple ensemble member simulations from CMIP5,
CORDEX-SA (COordinated Regional Downscaling
EXperiment-South Asia) and NEX-GDDP (Nasa earth
Exchange-Global Daily Downscaled Products) in which
CMIP5 is the parent GCM, CORDEX is dynamically
downscaled to 50 km  50 km grid resolution, and
NEX-GDDP is statistically downscaled to 25 km  25 km
grid resolution.
Historical and projected changes until the end of the
twenty-first century based on various simulations (28 from
CMIP5, 16 from CORDEX and 19 from NEX; see the list of
models in Tables 3.2 and 3.3) are provided in this section.
The future changes are mostly quantified as percentage
changes in the near future (2040–2069) and far future
(2070–2099) epochs. We provide our analysis for annual,
summer (JJAS) and winter (OND) seasons in all cases.
Mostly the analysis is restricted to the Indian landmass, by
masking out the seas, and regions outside the geographical
area of India. Projections are stated with respect to the
standard reference period of 1976–2005.
Mean precipitation from multi-model ensemble simulations for annual, JJAS and OND seasons is shown in
Fig. 3.6.
The change in mean precipitation over India for the
annual, summer and winter seasons is presented as
box-whiskers in Fig. 3.7. A comparison of the various
sources of climate data used in this assessment shows a
consistent enhancement in precipitation across the Indian
landmass throughout the twenty-first century. The
box-whiskers also highlight the spread among the three
suites of experiments. The variability is comparatively high
during the winter monsoon season (OND). Comparing with
the coarse resolution CMIP5 simulation, the high-resolution
CORDEX and NEX simulations show higher variability
irrespective of seasons. This increased variability in the
56
A. Kulkarni et al.
and decreased precipitation over India (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 and Waliser 2006). The relative
strength of the northward-propagating low-frequency (20–
60 days) modes has a significant decreasing trend during
1951–2010, possibly due to the weakening of large-scale
circulation in the region during the monsoon season. This
reduction is compensated by a gain in synoptic-scale (3–
9 days) variability. The decrease in low-frequency ISO
variability is associated with a significant decreasing trend in
the percentage of extreme events during the active phase of
the monsoon. However, this decrease is likely balanced by
significant increasing trends in the percentage of extreme
events in the break and transition phases. These changes are
accompanied by a weakening of low-frequency variability
(Karmakar et al. 2015, 2017). Also, while there is no change
in the distribution of the break events, the number of active
spells shows an increase of about 12% in the period
1951–2010; the increase was mainly in the short duration
(3–6 days) spells (Pai et al. 2016). A statistically significant
increase in the frequency of dry spells (27% higher during
1981–2011 relative to 1951–1980) and intensity of wet
spells and statistically significant decreases in the intensity
of dry spells have been observed in recent six decades
(Singh et al. 2014). The changes in frequency, intensity and
speed of intra-seasonal oscillations have been attributed to
Indian ocean warming (Sabeerali et al. 2015); developing
and decaying phase of ENSO (Pillai and Chowdary 2016);
increase in convective available potential energy, low-level
moisture convergence and changes in large-scale circulation
in upper atmosphere (Singh et al. 2014).
3.2.3.4 Changes in Onset Characteristics
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 ISM has been defined with various dynamic
(Koteswaram 1958; Ananthakrishnan et al. 1968; Krishnamurti and Ramanathan 1982; Wang et al. 2001; 2009; Pai
and Rajeevan 2009) and thermodynamic indices (Ananthakrishnan and Soman 1988; Fasullo and Webster 2003;
Janowiak and Xie 2003). Objective definitions of South
Asian summer monsoon onset include measures such as the
increase of rainfall above a threshold (Wang and Lin 2002),
transition in vertically integrated moisture transport (Fasullo
and Webster 2003), reversal of surface wind (Ramage 1971),
and intensification of the lower level Somali jet (Taniguchi
and Koike 2006; Wang et al. 2009). As per these different
definitions, the mean onset date of summer monsoon rainfall
over India has been stable around 1 June. In recent decades,
the monsoon onset over India is seen to be delayed to 5th
June since 1976 (Sahana et al. 2015), which can be attributed
to the net decrease in moisture supply from the Arabian Sea
in the post-1976 period. The interannual variability of the
onset date is associated with ENSO with early onsets preceded by La Nina, and late onsets preceded by El Nino (e.g.
Noska and Mishra 2016).
3.3 Projected Changes in Precipitation Over
India
Understanding the projected future changes in precipitation
has a profound importance for policy. In this report, the
assessment of rainfall changes over India is carried out based
on the multiple ensemble member simulations from CMIP5,
CORDEX-SA (COordinated Regional Downscaling
EXperiment-South Asia) and NEX-GDDP (Nasa earth
Exchange-Global Daily Downscaled Products) in which
CMIP5 is the parent GCM, CORDEX is dynamically
downscaled to 50 km  50 km grid resolution, and
NEX-GDDP is statistically downscaled to 25 km  25 km
grid resolution.
Historical and projected changes until the end of the
twenty-first century based on various simulations (28 from
CMIP5, 16 from CORDEX and 19 from NEX; see the list of
models in Tables 3.2 and 3.3) are provided in this section.
The future changes are mostly quantified as percentage
changes in the near future (2040–2069) and far future
(2070–2099) epochs. We provide our analysis for annual,
summer (JJAS) and winter (OND) seasons in all cases.
Mostly the analysis is restricted to the Indian landmass, by
masking out the seas, and regions outside the geographical
area of India. Projections are stated with respect to the
standard reference period of 1976–2005.
Mean precipitation from multi-model ensemble simulations for annual, JJAS and OND seasons is shown in
Fig. 3.6.
The change in mean precipitation over India for the
annual, summer and winter seasons is presented as
box-whiskers in Fig. 3.7. A comparison of the various
sources of climate data used in this assessment shows a
consistent enhancement in precipitation across the Indian
landmass throughout the twenty-first century. The
box-whiskers also highlight the spread among the three
suites of experiments. The variability is comparatively high
during the winter monsoon season (OND). Comparing with
the coarse resolution CMIP5 simulation, the high-resolution
CORDEX and NEX simulations show higher variability
irrespective of seasons. This increased variability in the
56
A. Kulkarni et al.