Influence of anthropogenically-forced global warming and natural climate variability in the rainfall changes observed over the South American Altiplano

VERA, C.; DÍAZ, L.; SAURRAL, R.

Frontiers in Environmental Science

Frontiers Media

Año: 2019

Changes in the summer rainfall and 200-hPa zonal winds (U200) in the South American Altiplano are studied from 1902 to 2018using three different reanalysis datasets and simulations from 14 climate models of the fifth phase of the Coupled ModelIntercomparison Project (CMIP5). No significant trend in rainfall was identified from GPCC reanalysis data over that period. On theother hand, regional U200 trends estimated from 20C and ERA20C reanalyses and from CMIP5 Historical simulations are significantand positive over the 1902-2005 period. However, the trends seem to be dependent on the reanalysis dataset and periodconsidered. While no significant U200 trend is detected in simulations forced only by external natural sources, the mean trend ispositive and significant in simulations forced only by the increment of anthropogenic greenhouse gas emissions. Therefore, a signalassociated with the anthropogenic forcing of climate change has been detected in U200 trends in the Altiplano, but it is weak ascompared with the internal climate variability.Singular value decomposition analyses based on both 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 negativerainfall anomalies in the Altiplano, associated with positive U200 anomalies, are related with positive SST anomalies mainly in thetropical Pacific-Indian Oceans. Simulations can reproduce observed relationships and confirm that natural variability explains theobserved year-to-year variability. Simulations also confirm that anthropogenic forcing is a necessary condition to explain thepositive trends detected in the co-variability between tropical SST and regional U200 anomalies. However, the large influenceexerted by the South American Monsoon over the region can also affect sign and magnitude of the changes in the Altiplano.No significant relationship was found from CMIP5 simulations between poleward displacements of the global Hadley cell and regionalU200 changes. Instead, South American Hadley cell displacements are significantly correlated with regional U200 changes. The lattermight be an additional evidence of the combined influence of both tropical surface ocean and South America Monsoon on thecirculation changes in the Altiplano in the global warming context.