Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.4.2.4.1 South and South East Asian Monsoon =====

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In AR5, South and South East Asian monsoon (SAsiaM) precipitation was projected to increase by the end of the 21st century but with a weakening of the circulation, with ''high agreement'' across the CMIP5 models (Kitoh et al. , 2013; Menon et al. , 2013; Sharmila et al. , 2015; Sooraj et al. , 2015; [[#Kitoh--2017|Kitoh, 2017]] ; Kulkarni et al. , 2020) . Since AR5, most studies have confirmed projected increases in South Asian monsoon precipitation ( ''high confidence'' ), while one high-resolution model (35 km in latitude/longitude) projects monsoon precipitation decreases during the 21st century following the RCP4.5 scenario ( [[#Krishnan--2016|Krishnan et al., 2016]] ).

Over South Asia, the moisture-bearing monsoon low-level jet is projected to shift northward in CMIP3 and CMIP5 models ( [[#Sandeep--2015|Sandeep and Ajayamohan, 2015]] ). Greater warming over the Asian land region compared to the ocean contributes to intensification of the monsoon low-level south-westerly winds and precipitation ( [[#Endo--2018|Endo et al., 2018]] ), even though the combined effect of upper and lower tropospheric warming makes the Asian monsoon circulation response rather complicated. A high resolution model projection, based on the RCP8.5 scenario, indicates that a northward shift of the low-level jet and associated weakening of the large-scale monsoon circulation can induce a large reduction in the genesis of monsoon low pressure systems by the late 21st century ( [[#Sandeep--2018|Sandeep et al., 2018]] ). Experiments with constant forcing indicate that at 1.5°C and 2°C global warming levels, mean precipitation and monsoon extremes are projected to intensify in summer over India and South Asia ( [[#Chevuturi--2018|Chevuturi et al., 2018]] ; D. [[#Lee--2018|]] [[#Lee--2018|Lee et al., 2018]] ) and that a 0.5°C difference would imply a 3% increase of precipitation ( [[#Chevuturi--2018|Chevuturi et al., 2018]] ). CMIP5 models project an increase in short intense active days and decrease in long active days, with no significant change in the number of break spells for India ( [[#Sudeepkumar--2018|Sudeepkumar et al., 2018]] ).

Future monsoon projections from CMIP6 models show an increase of SAsiaM precipitation across all the scenarios and across all the time frames (Figure 8.22) with the maximum increase at the end of the 21st century in SSP5-8.5 (Almazroui et al. , 2020c; Z. Chen et al. , 2020b; Ha et al. , 2020; Wang et al. , 2021) . Table 8.2 confirms that changes in runoff and P–E over SAsiaM region are positive and largest in the higher emissions scenarios considered, as in precipitation. On the other hand, changes in the ensemble mean for all the variables considered in the SSP1-1.9 scenario are negative for both mid and long-term periods (Table 8.2). This is also consistently reflected in the spatial map of future precipitation changes (Figure 8.15). Different near-term projections of the SAsiaM may result given the diversity in the future aerosol emissions pathways and policies for regulating air pollution ( [[#Wilcox--2020|Wilcox et al., 2020]] ). Additionally, near-term projections of SAsiaM precipitation are expected to be constrained by internal variability associated with the PDV (X. [[#Huang--2020|Huang et al., 2020]] a). CMIP6 models also indicate a lengthening of the summer monsoon over India by the end of the 21st century, at least in SSP2-4.5, with considerable inter-model spread in the projected late retreat ( [[#Ha--2020|Ha et al., 2020]] ).

In summary, consistent with AR5, there is ''high confidence'' that SAsiaM precipitation is projected to increase during the 21st century in response to continued global warming across the CMIP6 higher emissions scenarios, mostly in the mid- and long terms.

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