Gross discrepancies between observed and simulated secular precipitation trends over the 20th-21st centuries in Southeastern South America

Abstract:

Southeastern South America (SESA; encompassing Paraguay, Southern Brazil, Uruguay, and northern Argentina) has experienced a 28% increase in austral summer precipitation from 1901-2018, one of the largest observed trends in precipitation globally. Previous research has identified Atlantic Multidecadal Variability and anthropogenic forcing from stratospheric ozone depletion as playing important roles in forcing precipitation trends in SESA. Radiative forcing from anthropogenic emissions of greenhouse gases has also been identified as a contributing factor. We analyze multi-model ensemble simulations from Phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) and find that not only do many Earth System models simulate much weaker positive precipitation trends over SESA during the 20^th century, some models persistently simulate negative trends over the region. Similarly, ensemble simulations from two versions of the National Center for Atmospheric Research atmospheric models forced with observed global ocean sea surface temperatures simulate much weaker trends that are all outside the range of uncertainty in the observational trends. Pre-industrial control runs from CMIP6 indicate that some models do occasionally simulate centennial-scale trends in SESA that fall within the range of uncertainty of the observed trend. Moreover, while 21^st-century projections from CMIP6 yield positive ensemble mean precipitation trends over SESA that grow with increasing greenhouse-gas emissions, forcing from the highest emission scenario is required in order to produce any model simulations of future trends as steep as those observed over the past century. Results point to significant uncertainties in the attribution of anthropogenically forced influences on the observed precipitation trend over SESA and suggest internal variability may have a large contribution in generating secular trends over the region.