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

Abstract. Geomorphic and sedimentologic data indicate that the climate of today’s hyper-arid Atacama Desert (Northern Chile) was more humid during the Mid-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 BP). We found that the PMIP4-CMIP6 model (CESM2) and the regional model (WRF) used in our study simulate more rainfall in the Atacama Desert for the mid-Pliocene in accordance to 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 conveyer belts (MCBs) originating in the tropical East 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 one day per year 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 SST increase in the tropical East 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 K warmer world like in the mid-Pliocene.