Extreme rainfall characteristics in central-southern Chile and its relationship with Atmospheric Rivers

Abstract

Central-southern Chile is the narrow strip of land along the west coast of South America, bounded to the east by the Andes cordillera. Precipitation is largely produced by mid-latitude storms, ranging from ~100 mm/year around 30°S to more than 2000 mm/year around 41°S. Extreme events have a disproportionally large impact in this region given its complex topography. In this study, we employ 67 years of daily rainfall observations (1950-2016) between 30°S and 41°S to examine the temporal and spatial characteristics of extreme rainfall (using 90th or larger percentiles) in this region. Climate Forecast System Reanalysis (CFSR) products between 1979-2011 allow us to assess the relationship between extreme daily rainfall events, integrated water vapor transport (IVT), and atmospheric rivers (ARs) making landfall along the Chilean coast. Monthly average number of extreme rainy days (RD) increase significantly southward but the extreme rain intensity (RI) has a peak around 36°S. There is a seasonal cycle in RD with a peak in austral wintertime, whereas RI shows no seasonal cycle. Extreme rainfall at selected reference stations (SRS) is relatively homogenous within ~100-250 km of radius. The joint distribution of CFSR-derived zonal- and meridional-IVT and daily rainfall indicates that no straightforward relationship between extreme rainfall and IVT can be established. The mean spatial structure of IVT for all extreme rainfall days at SRS shows that AR-like structures are present during these events. However, mean AR structures from northward locations are composed by extreme days with both AR and NO-AR structures, while southward locations include almost exclusively AR structures. Since ARs are coupled with cold-frontal dynamics, these results suggest that extreme rainfall in northward locations is likely associated with IVT from AR and non-AR sources (e.g. cut off lows), while southern locations are more frequently associated with ARs from landfalling cold fronts.