Towards robust and actionable information on monsoon climate change in South America

Although the South American monsoon (SAM) is the main source of precipitation over most of the continent, the effects of anthropogenic climate change on it remain unclear. Most recent projections from CMIP6 multimodel ensembles show very weak signal of total SAM precipitation change, and sometimes climate change information from different sources seems confusing and contradictory for the public and decision makers. As SAM affects the most populated areas and those with largest contribution to agricultural production and hydroelectric power generation, its future behavior should be clearly detailed and supported by a dynamical framework able to explain it, so as to better serve decision-makers in planning actions to respond to climate change and adopt effective policies for climate adaptation.

The existence of a dynamic framework that explains the major climate changes projected by the best-performing models gives coherence to the different monthly changes throughout the monsoon season, which otherwise seem incomprehensible and can lead to discrepant interpretations if not understood within a correct dynamic context. The lack of significant future change in total monsoon precipitation does not mean that there are no changes of great interest in different phases of the monsoon season.

There are two aspects that prompted the approach of the present study: i) model projections of future SST indicate an El Niño-like warming pattern in the central-east equatorial Pacific; ii) the impacts of the present climate El Niño events on 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 in this region.

Twelve CMIP6 selected models were 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 satisfactory ENSO. The changes projected by the ensemble of seven models that best reproduced ENSO and the climatology of SA indicate a more EN-like future climate. Consistently, the main climate 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 via teleconnection, and thermodynamical processes trigger the changes from spring to summer in CESA, which is part of the core monsoon region. Also coherently with EN impacts, drier conditions prevail in central-northern-eastern Amazon throughout the monsoon season thanks to changes in the Walker circulation, while in southeast SA, precipitation increases due to tropics-extratropics teleconnection.

The changes projected by the all-model ensemble are much weaker and confusing. This clear description of climate change throughout the monsoon season and its connection with intensified EN effects is easy to understand and use, as these effects are reasonably known.