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

===== 8.3.2.4.2 East Asian Monsoon =====

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The AR5 reported ''low confidence'' in the observed weakening of the East Asian monsoon (EAsiaM) since the mid-20th century. Since AR5, there has been improved understanding of changes in the EAsiaM, based on paleoclimatic evidence, instrumental observations and climate modeling simulations. Rainfall reconstructions from the Loess Plateau in China indicate that the northern extent of the monsoon rain belts migrated at least 300 km to the north-west from the LGM to the mid-Holocene ( [[#Yang--2015|Yang et al., 2015]] ). Similarly, Pliocene reconstructions indicate stronger intensity of the EAsiaM with a more northward penetration of the monsoon rain belt (S. [[#Yang--2018|]] [[#Yang--2018|Yang et al., 2018]] a). EAsiaM variability has been related to Atlantic Meridional Overturning Circulation (AMOC) dynamics, especially during the last glacial period, but whether the relationship is negative or positive remains uncertain ( [[#Sun--2012|Sun et al., 2012]] ; [[#Cheung--2018|Cheung et al., 2018]] ; [[#Kang--2018|Kang et al., 2018]] ).

Long-term precipitation observations from China indicate a trend of drying in the north and wetting in the central-eastern China along the Yangtze river valley since the 1950s ( [[#Qian--2014|Qian and Zhou, 2014]] ; [[#Zhou--2017b|Zhou et al., 2017b]] ; [[#Day--2018|Day et al., 2018]] ), with a weakened EAsiaM low-level circulation that penetrates less far into northern China, increased surface pressure over north-east China and southward shift of the jet stream ( [[#Song--2014|Song et al., 2014]] ). The southward shift and enhancement of the jet stream explains the increase of rainfall especially from the Meiyu front ( [[#Day--2018|Day et al., 2018]] ) at the expense of drying over north-east China.

Anthropogenic factors such as GHGs and aerosols had an influence on the EAsiaM changes (Figure 8.11; T. Wang et al. , 2013; Song et al. , 2014; Xie et al. , 2016; [[#Chen--2017|Chen and Sun, 2017]] ; Ma et al. , 2017; L. Zhang et al. , 2017; Day et al. , 2018; Tian et al. , 2018). Increased precipitation in the southern region has been linked to increased moisture flux convergence driven by GHG forcing while changes in anthropogenic aerosols have weakened the EAsiaM and reduced precipitation in the northern regions ( [[#Tian--2018|Tian et al., 2018]] ). Aerosol-induced cooling, associated atmospheric circulation changes and sea surface temperature (SST) feedbacks weaken the EAsiaM and favour the observed dry-north and wet-south pattern of rainfall anomalies (T. Wang et al. , 2013; Song et al. , 2014; L. Zhang et al. , 2017; G. Chen et al. , 2018; X. Chen et al. , 2018; Undorf et al., 2018b).

Internal variability and volcanic eruptions also contributed to the weakened EAsiaM (Hsu et al. , 2014; [[#Qian--2014|Qian and Zhou, 2014]] ; Zhou et al. , 2017a; [[#Knutson--2018|Knutson and Zeng, 2018]] ). Since the late 1970s, the EAsiaM weakening has been also linked to SST changes in the Pacific Ocean with warm conditions in the central-eastern tropical part and cold ones in the north, similar to a positive phase of the Pacific Decadal Variability (PDV; Section AVI.2.6; Z. Li et al. , 2016b; Zhou et al. , 2017a ). In the late 1990s the transition from a positive to a negative PDV has been associated with the recent recovery observed in the EAsiaM strength ( [[#Zhou--2017a|Zhou et al., 2017a]] ). Atlantic Multi-decadal Variability (AMV) also has an influence on the EAsiaM via the global teleconnection pattern propagating from the North Atlantic through the westerly jet ( [[#Zuo--2013|Zuo et al., 2013]] ; [[#Wu--2016a|Wu et al., 2016a]] , b). This North Atlantic influence has contributed to the increase of precipitation over the Huaihe-Huanghe valley since the late 1990s (Y. [[#Li--2017|]] [[#Li--2017|]] [[#Li--2017|]] [[#Li--2017|Li et al., 2017]] ). When PDV and AMV are in opposite phase, the former has a larger influence in driving the southern flooding and northern drought pattern over the region (Q. [[#Yang--2017|]] [[#Yang--2017|Yang et al., 2017]] ).

In summary, there is strong evidence of a stronger EAsiaM and northward migration of the rainbelt during warmer climates based on paleoclimate reconstructions. There is ''high confidence'' that anthropogenic forcing has been influencing historical EAsiaM changes with drying in the north and wetting in the south observed since the 1950s, but there is ''low confidence'' in the magnitude of the anthropogenic influence. The transition towards a positive PDV phase has been one of the main drivers of the EAsiaM weakening since the 1970s ( ''high co'' ''nfidence'' ).

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