occurrences of WDs, suggesting that their RCM result was
sensitive to the boundary forcing from GCM. Krishnan et al.
(2019) showed a projected increase in the trend of WDs and
precipitation extremes over WH using RCP4.5 scenario (i.e.,
a warming climate scenario; see Figs. 7.4 and 11.8d for
more details) and attributed the rising trend to strong surface
warming over eastern Tibetan Plateau compared to that over
western side thus creating enhanced zonal gradient across
Tibetan Plateau. That is, the warming trend tends to alter the
background mean circulation to favor enhancements of the
WDs and the orographic precipitation over the WH (see
Fig. 11.8d for the precipitation changes). On the contrary,
Hunt et al. (2019) found a significant 10% reduction in the
annual frequency of WDs at the end of the century in future
projections (i.e., in a warming climate scenario, RCP8.5) and
attributed the changes in WD frequency to the projected
widening and weakening of the STJ. Their study also suggests that as a consequence of this falling WD activity, the
winter precipitation in north India will decrease at the end of
the century.
7.3 Knowledge Gaps
Most of the studies on climate model projections focused
mainly on stronger LPS (monsoon depressions) and not on
monsoon lows, which remains a knowledge gap, thus
requiring more studies in this direction. Further, there is no
clear understanding of the various factors causing changes
in LPS characteristics (i.e., both observed and projected
changes), thus calling for more process-oriented studies in
this direction. Secondly, the monsoon LPS associated
rainfall contribution is not well represented in climate
models, despite their prominent contribution to the monsoon seasonal rainfall. More studies to fully comprehend
the genesis mechanisms (i.e., originating mechanism of
LPS) of LPS may help in its improved representation in
climate models. Finally, it is a challenge to detect the
synoptic systems (i.e., LPS and WDs) from reanalysis and
model simulations. Particularly, detecting monsoon LPS,
having a weaker structure compared to the other tropical
storms, poses a serious problem. Further, the number of
synoptic systems detected in the reanalysis dataset, and
climate model simulations are quite sensitive to the algorithm used for its detection and tracking. The discrepancies
due to the tracking algorithm subsequently generate
uncertainty in the climate model projections of synoptic
systems. Furthermore, the model constraints to represent
the small-scale process associated with synoptic systems
(e.g. coarse resolution in CMIP5 models lead to the poor
representation of LPS) raises concerns on the reliability of
its future projections. In view of this, continued efforts are
required to find better modeling and identification strategies
for the synoptic systems.
7.4 Summary
This chapter documents a review on the current understanding of observed variability and future changes in synoptic systems that form during boreal summer and winter–
spring [i.e., monsoon low-pressure systems (LPS) and
western disturbances] seasons. Generally, majority of LPS
originates from the head BoB migrates northwestward into
the Indian subcontinent producing heavy rainfall (30–
50 cm day
−1 ), thereby significantly contributing to the total
seasonal (June–September) rainfall in India. On interannual
time scales, rainfall contribution from LPS (to the total
seasonal monsoon rainfall) remains invariant during flood or
drought years, even though the LPS frequency is slightly
higher during flood years. The frequency of LPS also shows
significant inter-decadal variations with more (less) monsoon
depressions in the epochs of above (below) normal Indian
summer monsoon rainfall. On the large-scale modes of
variability influencing LPS, there is no consistency between
the investigations to infer the associations of monsoon LPS
with ENSO or IOD. However, studies show an out-of-phase
relationship between the monsoon depressions and PDO.
There is no detectable long-term trend in the observed frequency of total LPS during both periods of our analysis
(1901–2015 and 1951–2015). The observed frequency of
monsoon depressions shows a decreasing long-term trend
(1901–2015), but the decline is more significant during the
1951–2015 period. In contrast, monsoon lows show a significant increasing trend during the 1901–2015 period. There
are only few studies that are currently available to understand the potential impact of climate change on LPS. Climate model projections generally show a weakening of LPS
activity and a poleward shift of the genesis locations of LPS
at the end of the twenty-first century. The poleward shift in
the LPS activity is expected to have profound societal
impacts, as it may possibly dry up central India as well as
increase the frequency of heavy rainfall events over northern
India.
The WDs are the synoptic weather systems that propagate
eastward from the Mediterranean region towards south Asia
7 Synoptic Scale Systems
151
sensitive to the boundary forcing from GCM. Krishnan et al.
(2019) showed a projected increase in the trend of WDs and
precipitation extremes over WH using RCP4.5 scenario (i.e.,
a warming climate scenario; see Figs. 7.4 and 11.8d for
more details) and attributed the rising trend to strong surface
warming over eastern Tibetan Plateau compared to that over
western side thus creating enhanced zonal gradient across
Tibetan Plateau. That is, the warming trend tends to alter the
background mean circulation to favor enhancements of the
WDs and the orographic precipitation over the WH (see
Fig. 11.8d for the precipitation changes). On the contrary,
Hunt et al. (2019) found a significant 10% reduction in the
annual frequency of WDs at the end of the century in future
projections (i.e., in a warming climate scenario, RCP8.5) and
attributed the changes in WD frequency to the projected
widening and weakening of the STJ. Their study also suggests that as a consequence of this falling WD activity, the
winter precipitation in north India will decrease at the end of
the century.
7.3 Knowledge Gaps
Most of the studies on climate model projections focused
mainly on stronger LPS (monsoon depressions) and not on
monsoon lows, which remains a knowledge gap, thus
requiring more studies in this direction. Further, there is no
clear understanding of the various factors causing changes
in LPS characteristics (i.e., both observed and projected
changes), thus calling for more process-oriented studies in
this direction. Secondly, the monsoon LPS associated
rainfall contribution is not well represented in climate
models, despite their prominent contribution to the monsoon seasonal rainfall. More studies to fully comprehend
the genesis mechanisms (i.e., originating mechanism of
LPS) of LPS may help in its improved representation in
climate models. Finally, it is a challenge to detect the
synoptic systems (i.e., LPS and WDs) from reanalysis and
model simulations. Particularly, detecting monsoon LPS,
having a weaker structure compared to the other tropical
storms, poses a serious problem. Further, the number of
synoptic systems detected in the reanalysis dataset, and
climate model simulations are quite sensitive to the algorithm used for its detection and tracking. The discrepancies
due to the tracking algorithm subsequently generate
uncertainty in the climate model projections of synoptic
systems. Furthermore, the model constraints to represent
the small-scale process associated with synoptic systems
(e.g. coarse resolution in CMIP5 models lead to the poor
representation of LPS) raises concerns on the reliability of
its future projections. In view of this, continued efforts are
required to find better modeling and identification strategies
for the synoptic systems.
7.4 Summary
This chapter documents a review on the current understanding of observed variability and future changes in synoptic systems that form during boreal summer and winter–
spring [i.e., monsoon low-pressure systems (LPS) and
western disturbances] seasons. Generally, majority of LPS
originates from the head BoB migrates northwestward into
the Indian subcontinent producing heavy rainfall (30–
50 cm day
−1 ), thereby significantly contributing to the total
seasonal (June–September) rainfall in India. On interannual
time scales, rainfall contribution from LPS (to the total
seasonal monsoon rainfall) remains invariant during flood or
drought years, even though the LPS frequency is slightly
higher during flood years. The frequency of LPS also shows
significant inter-decadal variations with more (less) monsoon
depressions in the epochs of above (below) normal Indian
summer monsoon rainfall. On the large-scale modes of
variability influencing LPS, there is no consistency between
the investigations to infer the associations of monsoon LPS
with ENSO or IOD. However, studies show an out-of-phase
relationship between the monsoon depressions and PDO.
There is no detectable long-term trend in the observed frequency of total LPS during both periods of our analysis
(1901–2015 and 1951–2015). The observed frequency of
monsoon depressions shows a decreasing long-term trend
(1901–2015), but the decline is more significant during the
1951–2015 period. In contrast, monsoon lows show a significant increasing trend during the 1901–2015 period. There
are only few studies that are currently available to understand the potential impact of climate change on LPS. Climate model projections generally show a weakening of LPS
activity and a poleward shift of the genesis locations of LPS
at the end of the twenty-first century. The poleward shift in
the LPS activity is expected to have profound societal
impacts, as it may possibly dry up central India as well as
increase the frequency of heavy rainfall events over northern
India.
The WDs are the synoptic weather systems that propagate
eastward from the Mediterranean region towards south Asia
7 Synoptic Scale Systems
151