Influence of Anthropogenically-Forced Global Warming and Natural Climate Variability in the Rainfall Changes Observed Over the South American Altiplano

SAURRAL, RAMIRO I.; DÍAZ, LEANDRO B.; VERA, CAROLINA S.

Frontiers in Environmental Science

Frontiers

Lugar: Laussane; Año: 2019 vol. 7

Changes in the summer rainfall and 200-hPa zonal winds (U200) in the South AmericanAltiplano are studied from 1902 to 2018 using three different reanalysis datasetsand simulations from 14 climate models of the fifth phase of the Coupled ModelIntercomparison Project (CMIP5). No significant trend in rainfall was identified fromGPCCreanalysis data over that period. On the other hand, regional U200 trends estimated from20C and ERA20C reanalyses and from CMIP5 Historical simulations are significant andpositive over the 1902?2005 period. However, the trends seem to be dependent on thereanalysis dataset and period considered. While no significant U200 trend is detectedin simulations forced only by external natural sources, the mean trend is positive andsignificant in simulations forced only by the increment of anthropogenic greenhousegas emissions. Therefore, a signal associated with the anthropogenic forcing of climatechange has been detected in U200 trends in the Altiplano, but it is weak as comparedwith the internal climate variability. Singular value decomposition analyses based onboth reanalyzed and simulated data were performed to describe the co-variabilitybetween rainfall in the Altiplano, regional U200 and global sea surface temperature(SST). The analysis confirms that negative rainfall anomalies in the Altiplano, associatedwith positive U200 anomalies, are related with positive SST anomalies mainly in thetropical Pacific-Indian Oceans. Simulations can reproduce observed relationships andconfirm that natural variability explains the observed year-to-year variability. Simulationsalso confirm that anthropogenic forcing is a necessary condition to explain the positivetrends detected in the co-variability between tropical SST and regional U200 anomalies.However, the large influence exerted by the South American Monsoon over the regioncan also affect sign and magnitude of the changes in the Altiplano. No significantrelationship was found from CMIP5 simulations between poleward displacements ofthe global Hadley cell and regional U200 changes. Instead, South American Hadley cell displacements are significantly correlated with regional U200 changes. The latter mightbe an additional evidence of the combined influence of both tropical surface oceanand South America Monsoon on the circulation changes in the Altiplano in the globalwarming context.