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

===== 8.4.2.4.6 Australian and Maritime Continent Monsoon =====

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The AR5 concluded that projected changes in Australian and Maritime Continent monsoon (AusMCM) rainfall and seasonality are uncertain in the CMIP5 models, with some projecting increases and others projecting decreases for the range of emissions scenarios. Models that perform better at simulating present day regional climate project little change or an increase in Australian monsoon rainfall ( [[#Jourdain--2013|Jourdain et al., 2013]] ; CSIRO and BoM, 2015; [[#Brown--2016b|Brown et al., 2016b]] ). CMIP6 models project increased AusMCM precipitation in the 21st century but with a more robust signal in SSP2-4.5 and SSP5-8.5 rather than in lower emissions scenarios (Figure 8.22). A reduced range of CMIP6 rainfall projections but continued disagreement on the sign of change is reported over Australia ( [[#Narsey--2020|Narsey et al., 2020]] ).

The northern and eastern parts of the Maritime Continent have projected increases in rainfall in CMIP5 models ( [[#Siew--2014|Siew et al., 2014]] ), while there are projected decreases over Java, Sulawesi and southern parts of Borneo and Sumatra. Rainfall changes are correlated with the extent of warming in the western tropical Pacific in CMIP5 models ( [[#Brown--2016b|Brown et al., 2016b]] ) but inter-model differences are also related to modelled large-scale zonal mean precipitation response in both CMIP5 and CMIP6 model ensembles ( [[#Narsey--2020|Narsey et al., 2020]] ). Decomposition of projected rainfall changes indicates that the largest source of model uncertainty is associated with shifts in the spatial pattern of convection ( [[#Chadwick--2013|Chadwick et al., 2013]] ; [[#Brown--2016b|Brown et al., 2016b]] ). Uncertainties in capturing the spatial and temporal features of the Maritime Continent monsoon depend also on the horizontal resolution of coupled climate models (e.g., [[#Jourdain--2013|Jourdain et al., 2013]] ).

The role of anthropogenic aerosol forcing in future projections of the Australian monsoon has been investigated for CMIP5 models ( [[#Dey--2019a|Dey et al., 2019a]] ); decreases in anthropogenic aerosol concentrations over the 21st century are expected to produce relatively greater warming in the NH than SH, favouring a northward shift of the tropical rain belt (e.g., [[#Rotstayn--2015|Rotstayn et al., 2015]] ).

There are some clear projected changes in the rainfall variability and extremes of the Australian monsoon. Rainfall variability in the Australian monsoon domain increases on time scales from daily to decadal in CMIP5 models ( [[#Brown--2017|Brown et al., 2017]] ), indicating either more intense wet days or more dry days or both. There is also a projected increase in the intensity of extreme rainfall but a reduction in the frequency of heavy rainfall days for the Australian monsoon (Dey et al. , 2019a) . This is consistent with [[#Moise--2020|Moise et al. (2020)]] , who found an increase in Australian monsoon active phase or ‘burst’ rainfall intensity but a reduction in the number of burst days and events.

H. [[#Zhang--2013|]] [[#Zhang--2013|Zhang et al. (2013)]] examined changes in Australian monsoon onset and duration in CMIP3 models and found model agreement on a delay in onset and shortened duration to the north of Australia, but less agreement over the interior of the continent. An updated study of CMIP5 models found similar mean changes with delayed onset and shortened duration, but substantial model disagreement (H. [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|Zhang et al., 2016]] ).

In summary, CMIP6 projections show an increase of AusMCM precipitation across all emissions scenarios. There is strong model agreement on an increase in monsoon precipitation over the Maritime Continent while there is ''low agreement'' on the direction of change over northern Australia. There is a projected increase in rainfall variability over northern Australia, with increased intensity of rainfall during the active or ‘burst’ phase ( ''medium con'' ''fidence'' ).

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