“[…] the Atacama Desert, a place I know so well because it was my home for a few years and a frequent destination in Chile.”
José Vicencio Veloso’s publication “The Overlooked Role of Moist Northerlies as a Source of Summer Rainfall in the Hyperarid Atacama Desert” published in the Journal of Geophysical Research: Atmospheres, on behalf of American Geophysical Union, has ranked within the top 10% of most-viewed papers published by the journal in 2024.
For CESOC member José and his co-authors Christoph Böhm, Jan Schween, Ulrich Löhnert, and Susanne Crewell (University of Cologne), the most striking finding was realizing that the Atacama’s summer storms are largely fed by moisture coming from the Pacific, not from across the Andes as long assumed. They found that a synoptic pattern we call “moist northerlies” triggers about 75% of summer rainy days in the hyperarid core in the last 40 years: a low-pressure system offshore drags tropical Pacific moisture southward, and the daytime heating of the Andean slope lifts it inland, where it forms the storms that pop up in the middle of the driest desert on Earth. The conventional Andes-crossing explanation never quite fit what we were seeing, and finally having a coherent picture was very satisfying.
Within CESOC, this work sits right at the intersection of both Earth System Observations and Computational Analysis. On the observation side, the team combined a growing network of stations installed by the Atacama Project (CRC1211) in recent years with conventional weather stations, daily radiosondes launched along the Chilean coast by the national weather service, and satellite retrievals. On the computational side, our researchers used ERA5 reanalysis to identify and classify 96 summer rainfall events over 1961–2020, and high-resolution WRF simulations to resolve the local circulation around a strong rainfall case (January 2019). Bringing observations, reanalysis, and simulations together was essential, and none of these alone would have revealed the moist-northerly mechanism.
The trend analysis over 1991–2020 shows that summer water vapor along the subtropical west coast of South America has been increasing rapidly, which means moist northerlies have more fuel available and rainfall events could become more frequent. “We suspect this is connected to the poleward expansion of the Hadley cell, but the long-term dynamics still need to be worked out – both the mechanism itself and its downstream effects on rainfall patterns along the western Andes,” says José. The signs of change are already visible on the ground: greening of the pre-Andean cordillera, plant blooms, expanding agricultural frontiers, and even consequences for the astronomical observatories that rely on Atacama’s exceptional dryness. Quantifying how robust these trends are, and how they project under future climate scenarios, is the natural next step.
Read this publication here.
