The overlooked role of westerly moisture 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 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 (Rutllant Cell) transports the moist free tropospheric air
inland above the coastal marine boundary layer, triggering clouds and
storms. Long-term observations (1960–2020) show that most of the rainy
days in the hyperarid core (75%) are triggered by MNs. A trough over
the southeast Pacific and a southward displaced Bolivian High
dynamically drives them, occurring more frequently during the
neutral-cold phase of El Niño-Southern Oscillation (ENSO) and phases
7-8-1 of the Madden-Julian Oscillation (MJO). A trend analysis
(1991–2020) reveals that summer water vapor along the subtropical west
coast of South America has increased rapidly due to the MNs, enhancing
summer rainfall in the Atacama. The implications of climate change and
other variability modes are discussed.