3.2 Observed Changes in Mean Precipitation
and Circulation
3.2.1 Precipitation Records in Paleo Time
Scale—Inferences from Proxies
The paleoclimate proxy data of monsoonal record of the past
640,000 years suggest that the millennial-scale variability
arose by the solar insolation changes which are caused by
precession and obliquity (Cheng et al. 2016). In the last
11,000 years (The Holocene age), summer monsoon is
declining with variability at the multi-decadal scale to centennial scales (Chao and Chen 2001). The decline of the
summer monsoon is linked to, among other factors, the
southward migration of the ITCZ as a result of the decrease in
solar insolation (Fleitmann et al. 2007). Additionally, prolonged wet/drought periods of multi-decadal and centuryscales have occurred during the last 4000 years (Sinha et al.
2011; Prasad et al. 2014) with notable century scale long
declining trends 1550–2200 years BP (Roman Warm Period;
RWP) and 100–550 years BP (Little Ice Age; LIA) and an
increasing trend during 650–1050 year BP (Medieval warm
period; MWP) in summer monsoon (Trends marked by
arrows; Fig. 3.2). Abrupt changes in monsoon around
2800 years BP and 2350 years BP have been attributed to
solar variability (Sinha et al. 2018). Tree ring-based studies
from the Himalayan region reveal a declining trend in the
summer monsoon over the last 200 years with the possible
linkages to large-scale greenhouse warming, and anthropogenic aerosol emissions (Xu et al. 2013; Shi et al. 2017).
Speleothem (cave deposits)-based 4000-year-long monsoon
reconstructions from central, peninsular and northeast India
indicate that the monsoon has undergone multi-decadal
changes of larger magnitude in the Holocene than in the last
200 years alone (Fig. 3.2). Therefore, as per the available
paleo records, changes in the monsoon due to the regional
forcing such as anthropogenic aerosol emissions are difficult
to detect against the large multi-decadal natural variability.
3.2.2 Recent Changes in Precipitation
The mean summer monsoon rainfall (JJAS) over India from
1979 to 2005 from multiple observational datasets is shown
in Fig. 3.3. It is noted that, in general, all datasets show a
high rainfall zone over east-central India and low rainfall
zones over northwest India, northern parts of Kashmir, and
the rain shadow area of southeast India. The discrepancy in
observations occurs mainly over northern parts of India,
Himalayan region (e.g., Prakash et al. 2015) and northeast
India (Bidyabati et al. 2017).
The annual rainfall averaged over Indian landmass does
not show any trend over the period 1901–2015. However,
in the recent period 1951–2015 as well as 1986–2015 the
annual rainfall series shows decreasing trend (though not
statistically significant, or evident in all the datasets). The
summer monsoon rainfall series averaged over India landmass does not show any long-term trend on a century-scale
where it has been found that the contribution from
increasing heavy rain events has been offset by decreasing
moderate rain events (Goswami et al. 2006). However, a
downward trend of rainfall over the Indian subcontinent has
been observed in the period 1951–2004 (Kulkarni 2012).
The decreasing tendency of summer precipitation is found
Fig. 3.2 Reconstruction of the ISM of the past 4000 years from a
synthesis of records of d
18
O of speleothems from Kadapa (Andhra
Pradesh), Baratang (Andaman) and Gupteshwar (Orissa) (right y-axis)
and Dandak-Jhumar (Chhattisgarh -Meghalaya) composite (left y-axis).
Variations/trends in summer monsoon associated with major climatic
events of the past such as LIA, MWP and RWP are highlighted. Abrupt
changes in monsoonal strength (box 1 and 2), as well as major drought
periods, are also shown (Sinha 2018; marked with stars). Some gaps in
the time series are due to the poor resolution of available records for
those particular periods, thus, unaccounted
52
A. Kulkarni et al.
and Circulation
3.2.1 Precipitation Records in Paleo Time
Scale—Inferences from Proxies
The paleoclimate proxy data of monsoonal record of the past
640,000 years suggest that the millennial-scale variability
arose by the solar insolation changes which are caused by
precession and obliquity (Cheng et al. 2016). In the last
11,000 years (The Holocene age), summer monsoon is
declining with variability at the multi-decadal scale to centennial scales (Chao and Chen 2001). The decline of the
summer monsoon is linked to, among other factors, the
southward migration of the ITCZ as a result of the decrease in
solar insolation (Fleitmann et al. 2007). Additionally, prolonged wet/drought periods of multi-decadal and centuryscales have occurred during the last 4000 years (Sinha et al.
2011; Prasad et al. 2014) with notable century scale long
declining trends 1550–2200 years BP (Roman Warm Period;
RWP) and 100–550 years BP (Little Ice Age; LIA) and an
increasing trend during 650–1050 year BP (Medieval warm
period; MWP) in summer monsoon (Trends marked by
arrows; Fig. 3.2). Abrupt changes in monsoon around
2800 years BP and 2350 years BP have been attributed to
solar variability (Sinha et al. 2018). Tree ring-based studies
from the Himalayan region reveal a declining trend in the
summer monsoon over the last 200 years with the possible
linkages to large-scale greenhouse warming, and anthropogenic aerosol emissions (Xu et al. 2013; Shi et al. 2017).
Speleothem (cave deposits)-based 4000-year-long monsoon
reconstructions from central, peninsular and northeast India
indicate that the monsoon has undergone multi-decadal
changes of larger magnitude in the Holocene than in the last
200 years alone (Fig. 3.2). Therefore, as per the available
paleo records, changes in the monsoon due to the regional
forcing such as anthropogenic aerosol emissions are difficult
to detect against the large multi-decadal natural variability.
3.2.2 Recent Changes in Precipitation
The mean summer monsoon rainfall (JJAS) over India from
1979 to 2005 from multiple observational datasets is shown
in Fig. 3.3. It is noted that, in general, all datasets show a
high rainfall zone over east-central India and low rainfall
zones over northwest India, northern parts of Kashmir, and
the rain shadow area of southeast India. The discrepancy in
observations occurs mainly over northern parts of India,
Himalayan region (e.g., Prakash et al. 2015) and northeast
India (Bidyabati et al. 2017).
The annual rainfall averaged over Indian landmass does
not show any trend over the period 1901–2015. However,
in the recent period 1951–2015 as well as 1986–2015 the
annual rainfall series shows decreasing trend (though not
statistically significant, or evident in all the datasets). The
summer monsoon rainfall series averaged over India landmass does not show any long-term trend on a century-scale
where it has been found that the contribution from
increasing heavy rain events has been offset by decreasing
moderate rain events (Goswami et al. 2006). However, a
downward trend of rainfall over the Indian subcontinent has
been observed in the period 1951–2004 (Kulkarni 2012).
The decreasing tendency of summer precipitation is found
Fig. 3.2 Reconstruction of the ISM of the past 4000 years from a
synthesis of records of d
18
O of speleothems from Kadapa (Andhra
Pradesh), Baratang (Andaman) and Gupteshwar (Orissa) (right y-axis)
and Dandak-Jhumar (Chhattisgarh -Meghalaya) composite (left y-axis).
Variations/trends in summer monsoon associated with major climatic
events of the past such as LIA, MWP and RWP are highlighted. Abrupt
changes in monsoonal strength (box 1 and 2), as well as major drought
periods, are also shown (Sinha 2018; marked with stars). Some gaps in
the time series are due to the poor resolution of available records for
those particular periods, thus, unaccounted
52
A. Kulkarni et al.