Anthropogenic aerosols modulate regional precipitation
patterns, over monsoon regions (Bollasina et al. 2011;
Krishnan et al. 2016; Undorf et al. 2018). Aerosols play an
important role in the earth-atmosphere system through their
interactions with solar radiation, clouds and the cryosphere
aerosol solar absorption over the Indian monsoon region has
a potential role in influencing the monsoon circulation and
rainfall distribution (Chap. 5 provides a summary).
Observed patterns of regional changes in precipitation are
missing from the CMIP5 (Coupled Model Intercomparison
Project 5) assessments–primarily due to the coarse resolution
of models and also due to missing local features that can be
important for such regional variability. The current generation of coupled models shows very substantial dry bias in
simulating Indian monsoon precipitation over the core
monsoon zones of central India, and the Western Ghats.
Sabin et al. (2013) used a variable resolution global atmospheric model with telescopic zooming over south Asia
(*35 km) and demonstrated that the high resolution provides particular value addition in simulating better monsoon
rainfall over the Indian region. Using the same set of model
analysis, recent changes in observed mean monsoon over
India (1951–2005) have been attributed to a combined effect
of anthropogenic aerosol, equatorial Indian Ocean warming
and land-use/land-cover change (Krishnan et al. 2016,
Fig. 3.5).
3.2.3.2 Changes in Circulation Features
The core of the Tropical Easterly Jet (TEJ) has been
shrinking over the South Asian region (Pattanaik and Satyan
2000). The strength of TEJ has been found to have been
decreasing before 2000 (Sathiyamoorthy 2005), but since
2000 has increased at the rate of 1 m/s per year (Roja Raman
et al. 2009; Venkat Ratnam et al. 2013). The weakening of
the TEJ may be attributed to increase in convection due to
the excessive warming of Indian Ocean SST (Joseph and
Sabin 2008), cooling of upper-tropospheric temperature over
the Tibetan anticyclone region, and a significant warming
over the equatorial Indian Ocean which might have resulted
in decreasing trend of the upper-tropospheric meridional
temperature gradient. These changes have caused a reduction in the strength of the easterly thermal wind at the core
region of the TEJ, after the weakening of the TEJ. Further,
the weakening of TEJ and associated decrease of easterly
shear is attributed to the reduced north-south temperature
gradient between the equator and 20
o N for the longitude belt
of 40
o E–100
o E, that is, the air temperature on the equator
side is increasing compared to the north. These variations are
particularly high above 500 hPa (Rai and Dimri 2017).
In response to the global warming, the intensity of the
summer monsoon overturning circulation (monsoon Hadley
cell) and the associated southwesterly monsoon flow
(LLJ) have significantly weakened from the 1950s (Joseph
and Simon 2005; Krishnan et al. 2013). An ultra-highresolution global general circulation model (about 20 km
resolution) also shows a stabilization (weakening) of the
summer monsoon Hadley-type circulation in response to
global warming which has resulted in a weakened large-scale
monsoon flow (Rajendran et al. 2012; Krishnan et al. 2013).
The weakening of Asian monsoon circulation (Fig. 3.5) may
be due to relatively smaller warming in Asia compared to the
surrounding regions which make the landmass a ‘heat sink’
(Zuo et al. 2012). Indeed, the tropospheric temperature over
Asia has lowered in recent decades. As a consequence, the
meridional and zonal land-sea thermal contrasts are reduced,
and the Asian summer monsoon becomes weaker.
3.2.3.3 Observed Changes in Active/Break Spells
The seasonal monsoon strength is mainly modulated by the
intra-seasonal variability of the summer monsoon rainfall
10W
30E
70E
110E
Eq
20N
40N
-3.0
-1.8
-0.6
0.6
1.8
3.0
2 m/s
Fig. 3.5 Attribution of the decline in monsoonal rainfall: The difference in mean precipitation (JJAS; mm/day) and low-level circulation at
850 hPa (m/s) between the Historical and Historical natural simulations
for the period (1951–2005) from a high-resolution simulation. Regions
with a significance level above 95% level are shown with grey dots.
Reprinted with permission from Krishnan et al. 2016
3 Precipitation Changes in India
55
patterns, over monsoon regions (Bollasina et al. 2011;
Krishnan et al. 2016; Undorf et al. 2018). Aerosols play an
important role in the earth-atmosphere system through their
interactions with solar radiation, clouds and the cryosphere
aerosol solar absorption over the Indian monsoon region has
a potential role in influencing the monsoon circulation and
rainfall distribution (Chap. 5 provides a summary).
Observed patterns of regional changes in precipitation are
missing from the CMIP5 (Coupled Model Intercomparison
Project 5) assessments–primarily due to the coarse resolution
of models and also due to missing local features that can be
important for such regional variability. The current generation of coupled models shows very substantial dry bias in
simulating Indian monsoon precipitation over the core
monsoon zones of central India, and the Western Ghats.
Sabin et al. (2013) used a variable resolution global atmospheric model with telescopic zooming over south Asia
(*35 km) and demonstrated that the high resolution provides particular value addition in simulating better monsoon
rainfall over the Indian region. Using the same set of model
analysis, recent changes in observed mean monsoon over
India (1951–2005) have been attributed to a combined effect
of anthropogenic aerosol, equatorial Indian Ocean warming
and land-use/land-cover change (Krishnan et al. 2016,
Fig. 3.5).
3.2.3.2 Changes in Circulation Features
The core of the Tropical Easterly Jet (TEJ) has been
shrinking over the South Asian region (Pattanaik and Satyan
2000). The strength of TEJ has been found to have been
decreasing before 2000 (Sathiyamoorthy 2005), but since
2000 has increased at the rate of 1 m/s per year (Roja Raman
et al. 2009; Venkat Ratnam et al. 2013). The weakening of
the TEJ may be attributed to increase in convection due to
the excessive warming of Indian Ocean SST (Joseph and
Sabin 2008), cooling of upper-tropospheric temperature over
the Tibetan anticyclone region, and a significant warming
over the equatorial Indian Ocean which might have resulted
in decreasing trend of the upper-tropospheric meridional
temperature gradient. These changes have caused a reduction in the strength of the easterly thermal wind at the core
region of the TEJ, after the weakening of the TEJ. Further,
the weakening of TEJ and associated decrease of easterly
shear is attributed to the reduced north-south temperature
gradient between the equator and 20
o N for the longitude belt
of 40
o E–100
o E, that is, the air temperature on the equator
side is increasing compared to the north. These variations are
particularly high above 500 hPa (Rai and Dimri 2017).
In response to the global warming, the intensity of the
summer monsoon overturning circulation (monsoon Hadley
cell) and the associated southwesterly monsoon flow
(LLJ) have significantly weakened from the 1950s (Joseph
and Simon 2005; Krishnan et al. 2013). An ultra-highresolution global general circulation model (about 20 km
resolution) also shows a stabilization (weakening) of the
summer monsoon Hadley-type circulation in response to
global warming which has resulted in a weakened large-scale
monsoon flow (Rajendran et al. 2012; Krishnan et al. 2013).
The weakening of Asian monsoon circulation (Fig. 3.5) may
be due to relatively smaller warming in Asia compared to the
surrounding regions which make the landmass a ‘heat sink’
(Zuo et al. 2012). Indeed, the tropospheric temperature over
Asia has lowered in recent decades. As a consequence, the
meridional and zonal land-sea thermal contrasts are reduced,
and the Asian summer monsoon becomes weaker.
3.2.3.3 Observed Changes in Active/Break Spells
The seasonal monsoon strength is mainly modulated by the
intra-seasonal variability of the summer monsoon rainfall
10W
30E
70E
110E
Eq
20N
40N
-3.0
-1.8
-0.6
0.6
1.8
3.0
2 m/s
Fig. 3.5 Attribution of the decline in monsoonal rainfall: The difference in mean precipitation (JJAS; mm/day) and low-level circulation at
850 hPa (m/s) between the Historical and Historical natural simulations
for the period (1951–2005) from a high-resolution simulation. Regions
with a significance level above 95% level are shown with grey dots.
Reprinted with permission from Krishnan et al. 2016
3 Precipitation Changes in India
55