CONICET | Buscador de Institutos y Recursos Humanos

The winter orographic precipitation over the Andes between 30ºS and 37ºS is examined using precipitation gauges in the mountains and adjacent lowlands. Due to the few precipitation gauges available, large-scale variation in cross-barrier precipitation is focuses on this paper, not taking account the fine ridge-valley scale. The maximum amount of precipitation was observed on upslope of the mountain range before the crest, and was about twice larger than that observed on windward lowlands between 33º-34ºS. To immediately leeward of the crest, the precipitation amount sharply drops generating a strong gradient transversal to the topography, with differences of 100 mm or more in less than 25 km. The rain shadow effect is greater in the north (32º-34.5ºS) than in the south (35º-36.5ºS) of the lee side, probably due to more baroclinic activity to southernmost latitudes and a southward decrease in the height of the Andes enabling more spillover precipitation. The blocking effect of the Andes on winter precipitation is so marked, that it modifies the precipitation regimes in the low adjacent areas at windward and lee side north of 35ºS. The heavy orographic precipitation events are strongly related to strong water vapor transport from Pacific Ocean in the pre-cold-frontal environment of extratropical cyclones, which would have the form of atmospheric rivers as depicted in the reanalysis and rawinsonde data. The composite fields revealed marked difference between two groups of heavy precipitation events. The extreme events are associated with deeper cyclones than that for intense events. This leads to much stronger plumes of water vapor content and cross-barrier moisture flux against the high Andes that result in heavier orographic precipitation for extreme episodes. In addition, local air flow characteristics suggested that the low-level flow is typically blocked and diverted poleward in the form of an along-barrier jet. In the lee side, the downward flow dominate during heavy events producing a prominent rain shadow effects, as denoted the domain of downslope winds extending to low-laying areas.

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