Impacts of Atmospheric Rivers on Precipitation in Southern South America

This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from 6-hourly CFSR reanalysis data over a 16-yr period (2001-16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27 degrees to 55 degrees S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38 degrees and 50 degrees S, averaging 35-40 days yr(-1). This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of similar to 43 degrees S. ARs contribute 45%-60% of the annual precipitation in subtropical Chile (37 degrees-32 degrees S) and 40%-55% along the midlatitude west coast (37 degrees-47 degrees S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2-3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.