Extreme daily rainfall in central-southern Chile and its relationship with low-level horizontal water vapor fluxes

Abstract

In this study we explore extreme daily rainfall along central-southern Chile (27º-45ºS) and their relationship with horizontal water vapor fluxes, synoptic-scale patterns and hydrological impacts. Daily rainfall accumulation is retrieved from over 300 rain gauges covering a period between 20–50 years. In addition, the integrated water vapor column (IWV) and transport (IVT) derived from Climate Forecast System Reanalysis products are employed to unravel the dependency between extreme rainfall and horizontal water vapor fluxes. Our results indicate that extreme rainfall concentrates in very specific areas where terrain has higher altitude, especially along the Andes foothills. The majority of these events concentrates in austral winter, last a single day and encompass a north-south band of about 200 km in length . Composited synoptic analyses identified extreme rainfall cases dominated by northwesterly (NW) and westerly (W) moisture fluxes. Some features of the NW group include a 300-hPa trough projecting from extratropic to subtropics, a surface–level depression, and cyclonic winds at 850-hPa along the coast associated with IWV > 30 mm. Conversely, features in the W group include both a very weak 300-hPa trough and surface depression, as well as coastal westerly winds associated with IWV > 30 mm. About half of extreme daily rainfall is associated with an atmospheric river. Extreme rainfall observed in W (NW) cases have a strong orographic (synoptic) forcing. In addition, W cases are, in average, warmer than NW cases, leading to an amplified hydrological response.