The Overlooked Role of Moist Northerlies as a Source of Summer Rainfall in the Hyperarid Atacama Desert

Abstract

In the Atacama Desert, one of the driest places on Earth, the persistent absence of water preserves the record of environmental change, making it an invaluable proxy for studying the evolution of life on Earth. Due to the scarcity of in situ measurements and difficulties in satellite remote sensing, information on precipitation characteristics is limited even for the present climate. Guided by a case study of extreme precipitation in late January 2019, we derive a conceptual framework to explain how moisture transport combined with the diurnal circulation produces rainfall. We found a synoptic-scale weather pattern that we named “moist northerlies” (MNs) based on surface observations, reanalysis, and high-resolution simulation. During an MN event, moisture transport from the tropical Pacific is observed in the lower free-troposphere in the forefront of an 850 hPa low-pressure offshore Atacama. The diurnal circulation along the western Andean slope transports the moist free tropospheric air above the marine boundary layer inland, triggering clouds and storms. A trough over the southeast Pacific and a southward displaced Bolivian High seem to drive the MNs dynamically. Long-term observations (1960–2020) show that most of the rainy days in the hyperarid core (75%) are triggered by MNs, occurring more frequently during neutral/La Niña conditions and phases 7-8-1 of the Madden-Julian oscillation (MJO). A trend analysis (1991–2020) reveals that summer water vapor along the west coast of South America has increased rapidly due to the MNs, enhancing summer rainfall in Atacama. The implications of climate change and other climate variability modes are discussed.