Future Projections of Summer precipitation-driving Mechanisms over the South American Altiplano

The South American Altiplano is a high-altitude plateau (3800 m MSL) located in the central Andes between 15ºS and 22ºS. Bounded to the west by the coastal desert of Peru-Chile and to the east by the hyper-humid lowlands of Peru-Bolivia, the Altiplano exhibits a semi-arid climate, following a pronounced annual cycle, with over 70% of rainfall occurring during the austral summer (December-January-February). Associated with factors such as convective activity occurring west of the Amazon Basin, the generation of convective clouds over the central Andes occurs when eastward winds encounter the orographic barrier on the eastern slope of the Andes. This process represents the primary mechanism governing precipitation variability in the Altiplano. Previous studies analyzing future projections anticipate that the central Andes will become warmer during the 21st century, impacting the population, ecosystems, and glaciers of the South American Altiplano. This is particularly relevant since agriculture is the main economic activity in this region and depends directly on precipitation.

Summer precipitation over the Altiplano has shown a strong dependence on the magnitude of zonal flow in the free troposphere (200 - 300hPa). Nevertheless, General Circulation Models (GCMs) suggest a continuous increase in westerly flow over the central Andes, hindering moisture transport from the interior of the continent. Minvielle and Garreaud (2011) suggest a significant reduction (10%-30%) in Altiplano precipitation by the end of this century under moderate to strong greenhouse gas emission scenarios. More recently, Segura et al., (2020) found that precipitation variability in the Altiplano is also associated with upward motion over the western Amazon (WA). Thus, DJF precipitation over the Altiplano seems to respond directly and primarily to the upward motion over the WA, since the early 21st century.

Using a set of 13 GCMs, this study aims to explore possible future projections in precipitation processes under the SSP3-7.0 scenario. This study focuses on the evolution of two previously established mechanisms driving austral summer precipitation over the Altiplano: 1) easterly winds at upper levels over the central Andes, and 2) upward motion over the WA. As preliminary conclusions of this work, models indicate that both mechanisms appear to weaken for the future period analyzed (2050-2084), suggesting a reduction in summertime precipitation by the mid-21st century. Additionally, models project a more stable atmosphere over the central Andes for the future period, also indicating a reduction in precipitation in the region, reinforcing the initial conclusion.