On the importance of moisture conveyor belts from the tropical eastern Pacific for wetter conditions in the Atacama Desert during the mid-Pliocene

Geomorphic and sedimentologic data indicate that the climate of today's hyper-arid Atacama Desert (northern Chile) was more humid during the mid-Pliocene to Late Pliocene. The processes, however, leading to increased rainfall in this period are largely unknown. To uncover these processes we use both global and regional kilometre-scale model experiments for the mid-Pliocene (3.2 Ma). We found that the PMIP4–CMIP6 (Paleoclimate Modelling Intercomparison Project–Coupled Model Intercomparison Project) model CESM2 (Community Earth System Model 2) and the regional model WRF (Weather Research and Forecasting) used in our study simulate more rainfall in the Atacama Desert for the mid-Pliocene in accordance with proxy data, mainly due to stronger extreme rainfall events in winter. Case studies reveal that these extreme winter rainfall events during the mid-Pliocene are associated with strong moisture conveyor belts (MCBs) originating in the tropical eastern Pacific. For present-day conditions, in contrast, our simulations suggest that the moisture fluxes rather arise from the subtropical Pacific region and are much weaker. A clustering approach reveals systematic differences between the moisture fluxes in the present-day and mid-Pliocene climates, both in strength and origins. The two mid-Pliocene clusters representing tropical MCBs and occurring less than 1 d annually on average produce more rainfall in the hyper-arid core of the Atacama Desert south of 20∘ S than what is simulated for the entire present-day period. We thus conclude that MCBs are mainly responsible for enhanced rainfall during the mid-Pliocene. There is also a strong sea-surface temperature (SST) increase in the tropical eastern Pacific and along the Atacama coast for the mid-Pliocene. It suggests that a warmer ocean in combination with stronger mid-tropospheric troughs is beneficial for the development of MCBs leading to more extreme rainfall in a +3 ∘C warmer world like in the mid-Pliocene.