Modeling an Extreme Precipitation Event over the Atacama Desert: The Impact of Warmer SST Anomalies in the Eastern Tropical Pacific

Abstract:

Northern Chile (NC), located along the west coast of South America between 28^°S-20^°S, hosts the driest place on Earth, Quillagua in the Atacama Desert (annual mean precipitation of 0.05 mm for 1969-2000 period). Nonetheless, an extreme precipitation event affected the region on 24-26 March 2015 with 1-day accumulated precipitation exceeding 40 mm in several locations and 4 mm in Quillagua, producing floods and resulting in casualties and significant damage. Two favorable conditions existed during the event: (i) a specific synoptic condition dominated by a cut-off low (COL) pressure system off the coasts of NC and (ii) positive sea surface temperature (SST) anomalies over eastern tropical Pacific favored by the onset of El Niño pattern. The main issue of the present study is to disentangle the relative role of SST anomaly and the COL in explaining this extreme precipitation event. The event is investigated using a regional climate model and its sensitivity to SST anomalies in the eastern tropical Pacific is tested. The ICTP-RegCM4 model is applied in a nested configuration with two domains at 60- and 10-km resolutions using different convective precipitation schemes. Boundary conditions are provided by ERA-Interim and NCEP/NCAR reanalysis data and different temporal SST input fields (weekly and daily) are used. Results indicate that simulations driven by ERA-Interim reanalysis and daily SST data realistically represent the precipitation distribution and associated convective systems over NC under the mixed convective scheme. The precipitation maxima appeared to be in the coastal areas and propagate inland, following the location of the COL center that allowed northerly moisture transport towards the Atacama Desert. In order to test sensitivity of precipitation to SST, the warm anomaly of SST in the eastern tropical Pacific is removed, which leads to a cooler SST forcing in the sensitivity experiment. The cooler SST simulation reproduced a very similar COL dynamics, however, suppressed the precipitation by 60-80% over NC and 100% in the hyper-arid Atacama Desert due to the decreased availability of precipitable water. The results indicate that the warm coastal SST anomaly favored by the onset of El Niño was instrumental to the extreme precipitation event by providing an anomalous source of water vapor.