Summer precipitation events over the western slopes of the subtropical Andes

VIALE, M.; GARREAUD, R. D.

Santiago

Congreso; III Congreso de Oceanografia Fisica, Meteorologica y Clima del Pacifico Sur Orienta; 2013

Summertime (DJF) precipitation over the western slopes of the subtropical Andes (32°-36°S) accounts for less than 10% of the annual accumulation but it mostly occurs as rain and may triggers landslides leading to serious damages. Based on 13-year of reanalysis, in-situ observations, and satellite imagery, a synoptic climatology and physical diagnosis reveal two main weather types lead to distinct precipitation systems. The most frequent type (~80% of the cases) occurs when a short-wave mid-level trough with weak winds and thermally-driven mountain winds favor the development of convective precipitation during the daytime. The trough progresses north-west of a long-lasting warm ridge, which produces low-level easterly airflow that enhances its buoyancy as moves over the arid land of western Argentina toward the Andes. The weak winds aloft facilitate the penetration of the moist easterly flow into the Andes. Mid-level flow coming from the west side of the Andes is decoupled from the low-level maritime air by a temperature inversion, and thus provides little moisture to support precipitation. The less frequent type (~20% of the cases) occurs when a deep mid-level trough and strong westerly flow produces stratiform precipitation. This type has a baroclinic nature akin of winter storms, except that they are rare in summer and there is no evidence of the frontal passage at low levels. The lifting and cooling ahead of the trough erode the typical temperature inversion over the Pacific coast, and thus allows upslope transport of low-level marine air by the strong westerlies forming a precipitating cloud cap on the western slope of the Andes.