Assessment of anthropogenic and natural influences on the observed precipitation trends in Southern South America

LEANDRO B. DÍAZ; CAROLINA VERA

Kigali

Conferencia; 2nd Open Science Conference of the World Climate Research Programme (WCRP).; 2023

World Climate Research Programme (WCRP)

Southern South America (SSA) is one of the regions of the world with the highest precipitation trends over the last 120 years. Southeastern South America (SESA), encompassing Uruguay, southern Brazil and the eastern portions of Argentina and Paraguay, has been affected by a large positive trend in austral summer precipitation. On the other hand, a significant negative trend in rainfall has been observed in the Southern Andes (SAn), encompassing the extratropical mountain region of Argentina and Chile. Despite the relevance of these trends for the region due to their impacts on regional socioeconomic activities, there are still open questions related to the attribution of them. For instance, IPCC AR6 WGI Ch.10 states that there is high confidence that SESA summer precipitation has increased and that anthropogenic forcing has contributed to the increase since 1950, but there is very low confidence on the relative contribution of each driver to the precipitation increase. The objective of this work is to assess the attribution to anthropogenic and natural forcings of the observed trends in austral summer precipitation in SSA using the Detection and Attribution Model Intercomparison Project (DAMIP) simulations from sixth phase of the Coupled Model Intercomparison Project (CMIP6).From the set of CMIP6 simulations, we used the historical simulations considering all observed external forcings and the following DAMIP experiments considering only one forcing: historical simulations with only natural forcing (hist-nat), historical simulations with only solar forcing (hist-sol), historical simulations with only volcanic forcing (hist-volc), historical simulations with only aerosol forcing (hist-aer), historical simulations with only greenhouse gas forcing (hist-GHG) and historical simulations with only stratospheric ozone forcing (hist-stratO3). Positive (negative) rainfall austral summer trends in SESA (SAn) are identified in most historical simulations for the period 1901-2014. For both regions, hist-GHG simulations show trends consistent with historical simulations, while hist-nat simulations exhibit negligible values. SESA (SAn) shows negative (negligible) trends associated with hist-aer simulations and high uncertainty negative trends for hist-stratO3 simulations. It is also found a negative high (positive low) correlation between simulated rainfall trends in SAn (SESA) and simulated trends of the Southern Annular Mode (SAM) among the different simulations. Thus, SAn negative rainfall trends seem to be connected to the positive trend of the SAM. Overall, our results provide evidence for anthropogenic influences on SSA rainfall trends. Attribution studies in the region will benefit from having a larger number of models (and more members) with single-forcing simulations.