The projected future of the South American monsoon
Authors: Grimm Alice M , Padoan Dayane 
Published: February 2023

&lt;p&gt;The South American monsoon (SAM) is the main source of precipitation over most of the continent, including subtropical regions, affecting the most populated areas and those with largest contribution to agricultural production and hydroelectric power generation. Precipitation extremes with strongest social and economic impacts occur during the monsoon season. Therefore, there is great interest for reliable projections of its future behavior and for a theoretical framework able to explain it. Over the tropical region, observed trends in precipitation are not consistent and vary according to the period of analysis, while in the subtropical southeast South America (SESA) a positive trend has been observed. Most recent projections from CMIP6 multimodel ensembles show very weak signal of SAM precipitation change. The IPCC AR6 findings indicate that there are no projected precipitation changes, but that the onset of monsoon will be delayed into the 21st century and extreme events will be more frequent.&lt;/p&gt;
&lt;p&gt;The fact that the models do not show a significant future change in the total precipitation accumulated in the entire monsoon season does not mean that there are no changes of great interest in different phases of this season. Detecting them requires analysis of greater temporal resolution and an additional focus: El Ni&amp;#241;o (EN). There are two aspects that prompted the approach used in the present study: i) model projections of future SST indicate a warming pattern in the central-east equatorial Pacific leading to a more El Ni&amp;#241;o-like future climate; ii) the El Ni&amp;#241;o effects over South America (SA) display a tendency to spring-summer reversal of precipitation anomalies in central-east SA (CESA), which results in little or no change in the total monsoon precipitation.&lt;/p&gt;
&lt;p&gt;A set of twelve CMIP6 selected models was evaluated not only for their simulation of South American climatology, but also for their simulation of ENSO and its impacts on SA. Several of them did not produce a satisfactory ENSO. The changes projected by the ensemble of seven models that best reproduced ENSO and also the climatology of SA indicate a more EN-like future climate. Consistently, the main changes projected for the SAM resemble the observed EN impacts, remarkably including the tendency to spring-summer reversal of precipitation anomalies in CESA, from dryer spring to wetter summer. While the total monsoon precipitation shows little or no change in this region, there is reduction of early monsoon rainfall and increase of the peak season rainfall, which results in a delay and shortening of the monsoon season. The dynamical effect of the EN-like SST changes shapes the spring response and thermodynamical processes trigger the changes from spring to summer. In central-northern-eastern Amazon dry conditions prevail throughout the season, while in SESA, precipitation increases.&lt;/p&gt;
&lt;p&gt;The results of the all-model ensemble, although consistent with the seven-model ensemble, are weaker and less satisfactory. The most intense changes are projected by the model that best simulates ENSO and its impacts on AS.&lt;/p&gt;