Karakoram Himalayas during winter months. It is also
widely accepted that there is a significant role of HKH
and TP in maintaining the Asian summer monsoon
circulation (Nan et al. 2009; Zhou et al. 2009), i.e.,
heating of the TP in summer raises air temperatures
thereby enhancing the pressure gradient which drives
the South Asian summer monsoon, and therefore, the
HKH and TP act as a major heat sink (source) during
the winter (summer) (Yanai and Li 1994). The presence of the HKH topographical barrier restricts the
upper-level subtropical westerly winds to regions
poleward of 30° N during the boreal summer months,
thereby allowing warm and moist summer monsoon
circulation to extend northward into the Indian subcontinent. Observations and model simulations clearly
suggest that the Himalayan orography has an important role in maintaining the South Asian monsoon
circulation by insulating warm, moist air over continental India from the cold and dry extra-tropics
(Chakraborty et al. 2006; Krishnan et al. 2009;
Krishnamurti et al. 2010; Boos and Kuang 2010;
Turner and Annamalai 2012; Sabin et al. 2013).
11.2 Observed Trends in Mean Surface
Temperature and Precipitation
Significant rise in surface temperatures is noted throughout
the HKH region during the past six decades (see Kulkarni
et al. 2013; Rajbhandari et al. 2016). The warming was
reported progressively over the western and eastern Himalayan river basins, and the long-term trend of minimum
temperatures is noted slightly higher than the trend seen in
maximum temperatures (Rajbhandari et al. 2015). Using
century-long historical time series, Ren et al. (2017) showed
that there were epochs of rise and fall in temperature trend
over the HKH region, i.e., mean temperature exhibited a
moderate rising trend from 1901 to the early 1940s, while a
falling trend is seen between 1940 and 1970 followed by a
rapid warming. The spatial pattern of trends of annual mean
temperatures over the HKH region during the 1901–2014
period shows that the warming rates were more than 0.3 °C
(decade)
−1 in the TP region and about 0.2 °C (decade)
−1
over the eastern side of the HKH range (Ren et al. 2017). It
can also be noted that most of the grids consistently showed
a positive trend in the annual warming signal; however, the
warming rates are significantly different. Various studies
attributed this observed warming signal to the increase in
anthropogenic greenhouse gas concentrations (IPCC 2007,
2013; You et al. 2017).
Pepin et al. (2015) show that mountain temperatures are
increasing at a faster rate than the global average. We have
further noted that the Himalayas are also warming at a faster
rate than that of the nearby Indian land mass. Figure 11.2
shows the annual mean temperature time series averaged
over HKH and Indian land mass from 1951 to 2018, which
is indicating a warmer Himalaya comparing to the Indian
land mass. Further, the observations also reveal that the
recent warming rates are not seen to be uniform over the
HKH region where the annual average warming rates change
with altitudes which are commonly referred as the elevation
dependency of climate warming (EDW) (e.g. Liu et al. 2009;
Ren et al. 2017; Shrestha et al. 1999; Thompson et al. 2003).
Figure 11.3 shows the trend per decade for different altitude
sectors of HKH. Low-elevation sites (<500 m) show a
slower warming rate (<0.2 °C per decade) as compared to
high elevations (>2000 m) of the eastern TP where higher
warming rate (0.61 °C per decade) is seen during the past
few decades (Liu et al. 2009; Ren et al. 2017). Northern
India and the Sichuan Basin of China showed the weakest
warming trend with annual warming rates less than 0.10 °C
(decade)
−1 , as well as the Karakoram range during the
northern summer (Forsythe et al. 2017).
The spatial distribution of the trends of annual precipitation based on APHRODITE dataset during 1951–2015 is
shown in Fig. 11.4. The observed map of trend in the annual
mean precipitation depicts substantial spatial heterogeneity
over the HKH region (Fig. 11.4). While there is a subtle rise
in the precipitation trend over the Karakoram and Western
Himalayas and the eastern part of Himalayas, a declining
trend can be noticed over many areas. A wet trend is evident
over northwest China, including the TP (Ren et al. 2015;
You et al. 2015). The triangular markings in Fig. 11.4 correspond to precipitation trends per decade based on
CMA-GMLP dataset for a longer period for 1901–2013.
Both the datasets show consistency in precipitation
enhancement over WH and decline over central and north
Indian plains. The reduction in annual precipitation over
northern India is also consistent with the reported declining
trend of the Indian summer monsoon precipitation during the
post-1950 (Krishnan et al. 2013, 2016).
Recent observational studies also suggest that there is an
increasing trend in the number of wet days over the WH
during the past few decades (Klein Tank et al. 2006; Choi
et al. 2009). It has been reported that increases in wintertime
heavy precipitation over the Karakoram region of northwest
Himalayas and falling precipitation trend over Central
Himalayas (CH) are linked to an increasing trend in the
synoptic-scale activity of the western disturbances (WDs),
while the CH region experiences a falling local precipitation
trend (Cannon et al. 2015; Madhura et al. 2015; Krishnan
210
T. P. Sabin et al.
widely accepted that there is a significant role of HKH
and TP in maintaining the Asian summer monsoon
circulation (Nan et al. 2009; Zhou et al. 2009), i.e.,
heating of the TP in summer raises air temperatures
thereby enhancing the pressure gradient which drives
the South Asian summer monsoon, and therefore, the
HKH and TP act as a major heat sink (source) during
the winter (summer) (Yanai and Li 1994). The presence of the HKH topographical barrier restricts the
upper-level subtropical westerly winds to regions
poleward of 30° N during the boreal summer months,
thereby allowing warm and moist summer monsoon
circulation to extend northward into the Indian subcontinent. Observations and model simulations clearly
suggest that the Himalayan orography has an important role in maintaining the South Asian monsoon
circulation by insulating warm, moist air over continental India from the cold and dry extra-tropics
(Chakraborty et al. 2006; Krishnan et al. 2009;
Krishnamurti et al. 2010; Boos and Kuang 2010;
Turner and Annamalai 2012; Sabin et al. 2013).
11.2 Observed Trends in Mean Surface
Temperature and Precipitation
Significant rise in surface temperatures is noted throughout
the HKH region during the past six decades (see Kulkarni
et al. 2013; Rajbhandari et al. 2016). The warming was
reported progressively over the western and eastern Himalayan river basins, and the long-term trend of minimum
temperatures is noted slightly higher than the trend seen in
maximum temperatures (Rajbhandari et al. 2015). Using
century-long historical time series, Ren et al. (2017) showed
that there were epochs of rise and fall in temperature trend
over the HKH region, i.e., mean temperature exhibited a
moderate rising trend from 1901 to the early 1940s, while a
falling trend is seen between 1940 and 1970 followed by a
rapid warming. The spatial pattern of trends of annual mean
temperatures over the HKH region during the 1901–2014
period shows that the warming rates were more than 0.3 °C
(decade)
−1 in the TP region and about 0.2 °C (decade)
−1
over the eastern side of the HKH range (Ren et al. 2017). It
can also be noted that most of the grids consistently showed
a positive trend in the annual warming signal; however, the
warming rates are significantly different. Various studies
attributed this observed warming signal to the increase in
anthropogenic greenhouse gas concentrations (IPCC 2007,
2013; You et al. 2017).
Pepin et al. (2015) show that mountain temperatures are
increasing at a faster rate than the global average. We have
further noted that the Himalayas are also warming at a faster
rate than that of the nearby Indian land mass. Figure 11.2
shows the annual mean temperature time series averaged
over HKH and Indian land mass from 1951 to 2018, which
is indicating a warmer Himalaya comparing to the Indian
land mass. Further, the observations also reveal that the
recent warming rates are not seen to be uniform over the
HKH region where the annual average warming rates change
with altitudes which are commonly referred as the elevation
dependency of climate warming (EDW) (e.g. Liu et al. 2009;
Ren et al. 2017; Shrestha et al. 1999; Thompson et al. 2003).
Figure 11.3 shows the trend per decade for different altitude
sectors of HKH. Low-elevation sites (<500 m) show a
slower warming rate (<0.2 °C per decade) as compared to
high elevations (>2000 m) of the eastern TP where higher
warming rate (0.61 °C per decade) is seen during the past
few decades (Liu et al. 2009; Ren et al. 2017). Northern
India and the Sichuan Basin of China showed the weakest
warming trend with annual warming rates less than 0.10 °C
(decade)
−1 , as well as the Karakoram range during the
northern summer (Forsythe et al. 2017).
The spatial distribution of the trends of annual precipitation based on APHRODITE dataset during 1951–2015 is
shown in Fig. 11.4. The observed map of trend in the annual
mean precipitation depicts substantial spatial heterogeneity
over the HKH region (Fig. 11.4). While there is a subtle rise
in the precipitation trend over the Karakoram and Western
Himalayas and the eastern part of Himalayas, a declining
trend can be noticed over many areas. A wet trend is evident
over northwest China, including the TP (Ren et al. 2015;
You et al. 2015). The triangular markings in Fig. 11.4 correspond to precipitation trends per decade based on
CMA-GMLP dataset for a longer period for 1901–2013.
Both the datasets show consistency in precipitation
enhancement over WH and decline over central and north
Indian plains. The reduction in annual precipitation over
northern India is also consistent with the reported declining
trend of the Indian summer monsoon precipitation during the
post-1950 (Krishnan et al. 2013, 2016).
Recent observational studies also suggest that there is an
increasing trend in the number of wet days over the WH
during the past few decades (Klein Tank et al. 2006; Choi
et al. 2009). It has been reported that increases in wintertime
heavy precipitation over the Karakoram region of northwest
Himalayas and falling precipitation trend over Central
Himalayas (CH) are linked to an increasing trend in the
synoptic-scale activity of the western disturbances (WDs),
while the CH region experiences a falling local precipitation
trend (Cannon et al. 2015; Madhura et al. 2015; Krishnan
210
T. P. Sabin et al.