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

===== 8.3.2.4.6 Australian and Maritime Continent Monsoon =====

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Since AR5, several studies have examined observed variability and changes in the Australian and Maritime Continent monsoon (AusMCM) using paleoclimate records, instrumental observations and modeling studies ( [[#Denniston--2016|Denniston et al., 2016]] ; [[#Zhang--2016|Zhang and Moise, 2016]] ). Paleoclimate reconstructions and modelling indicate that the Indo–Australian monsoon may vary in or out of phase with the EAsiaM, depending on whether there is a meridional displacement or expansion of the tropical rainfall belt ( [[#Ayliffe--2013|Ayliffe et al., 2013]] ; [[#Denniston--2016|Denniston et al., 2016]] ). For instance, mid-Holocene simulations suggest that the AusMCM weakens and contracts due to a decreased net energy input and a weaker dynamic component ( [[#D’Agostino--2020b|D’Agostino et al., 2020b]] ).

Rainfall increases have been observed over northern Australia since the 1950s, with most of the increases occurring in the north-west ( [[#Dey--2019a|Dey et al., 2019a]] , b; [[#Dai--2021|Dai, 2021]] ) and decreases observed in the north-east (J. [[#Li--2012|]] [[#Li--2012|Li et al., 2012]] ) since the 1970s. There is also a trend towards more intense convective rainfall from thunderstorms over northern Australia ( [[#Dowdy--2020|Dowdy, 2020]] ). There is no consensus on the cause of the observed Australian monsoon rainfall trends, with some studies suggesting changes are due to altered circulation driving increased moisture transport or increased frequency of the wettest synoptic regimes ( [[#Catto--2012|Catto et al., 2012]] ; [[#Clark--2018|Clark et al., 2018]] ). Other studies find that model simulations that include anthopogenic aerosols ( [[#Rotstayn--2012|Rotstayn et al., 2012]] ; [[#Dey--2019a|Dey et al., 2019a]] ) are better able to capture observed Australian monsoon rainfall trends than simulations with natural or GHG forcing only ( [[#Knutson--2018|Knutson and Zeng, 2018]] ).

The Maritime Continent (MC) experiences the influence of both the Asian and the Australian monsoons, with rainfall peaking during boreal winter/austral summer ( [[#Robertson--2011|Robertson et al., 2011]] ). Reductions in land rainfall and marine cloudiness over the MC and weakening of surface moisture flux convergence have been observed in the period 1950 – 1999 (Tokinaga et al., 2012; [[#Yoden--2017|Yoden et al., 2017]] ). These trends are indicative of a slowdown of the Walker Circulation, with positive sea level pressure trends over the MC and negative trends over the central equatorial Pacific (Tokinaga et al., 2012). More recently (1981 – 2014), a trend of increasing annual rainfall over large areas of the MC has been identified (Hassim and Timbal, 2019). Given the large variability in MC rainfall on interannual time scales, the choice of time period may influence the calculated rainfall trend (Hassim and Timbal, 2019).

During 1951 – 2007 daily rainfall extremes did not increase over the MC, in contrast to the rest of South East Asia ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] ; [[#Villafuerte--2015|Villafuerte and Matsumoto, 2015]] ). Rainfall extremes in Indonesia increased in austral summer, as evidenced from station weather observations for the period 1983 – 2012 (Supari et al., 2018).

In summary, notable rainfall increases have been observed in parts of northern Australia since the 1970s, although there is ''low confidence'' in the human contribution to these changes. Rainfall changes have been observed over the MC region but there is ''low confidence'' in the identification of trends because of large variability at interannual time scales.

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