Exploring the causes of multicentury pluvials in the Altiplano with a climate modelling experiment

Abstract

Abstract. Proxy records have long documented the existence of multicentury hydroclimate anomalies in the Altiplano of South America. However, the causes and mechanisms of these events are still largely unknown. Here we present the results of an innovative climate modelling experiment that explores the oceanic drivers and atmospheric mechanisms conducive to long-term precipitation variability in the southern part of the Altiplano (18–25° S). For doing so, we performed 100-yr-long hydroclimate simulations using a regional climate model forced by climatological conditions that resulted in pluvial December-January-February (DJF) seasons during historical times. Our modelling simulations produce long-term negative DJF precipitation trends for the southern Altiplano, suggesting that the mechanisms leading to historical wet summers are unable to sustain century-scale pluvials such as the ones documented in paleoclimate records. Our simulations show that permanent La Niña conditions, as well as SSTs anomalies in the southern tropical Atlantic, progressively reinforce upper and lower-troposphere features that inhibit moisture transport towards the Altiplano. We further observed increases in March-April-May precipitation, suggesting the emergence of a long-term seasonal shift. Our simulations reproduce a sustained northward migration of the Atlantic trade winds, resulting in contrasting hydroclimate responses between the Altiplano and the tropical Andes. The atmospheric processes associated with these differences provide a useful analogue for explaining divergences in proxy records. Our study shows how regional climate modelling can be used to test paleoclimate hypothesis, emphasizing the necessity of combining proxy and modelling data to improve our understanding of long-term hydroclimate change.