“Strong Influence of the Subtropical Central Andes on the wintertime precipitation

MAXIMILIANO VIALE; FEDERICO AUGUSTO NORTE

Santiago de Chile, Chile.

Conferencia; 4th Alexander von Humboldt International Conference “The Andes: Challenge for Geosciences”.; 2008

European Geophysical Union

This study documents the strong influence of the Andes on winter precipitation in the Subtropical Central region (32º-35ºS) using daily precipitation gauge observations for 1970-1976. During the winter, the approaching storms from Southern South Pacific Ocean are profoundly modified and blocked when it impinge on the high barrier of the Andes in this region. The typical orographic precipitation pattern with enhanced and reduced precipitation on windward (Chilean) and leeward (Argentinean) slopes was explored by the cross-mountain section of winter mean precipitation around 33ªS and 35ªS. On the northernmost sector, the precipitation rate is enhanced on upslope by upwind effect in a factor of 2,5 in relation to lowlands of Chile, and abruptly reduced by downwind effect in a factor of 3, immediately lee of the highest crests, and up to 13, on lee-side lowlands of northern Mendoza province in Argentina. On the southernmost sector, the precipitation decrement in lee side is lower due to the lower mean height of the mountains, and thus lower blocking effect. In order to examine the precipitation decrement in lee-side lowlands of northern sector according to the precipitation intensity in the high mountains, the areal daily mean precipitation in the high mountains was classified in weak and moderate, intense, and extreme. It was defined as “weak and moderate” those days with precipitation lower than 14 mm, “Intense” when the amount was between 14mm and 45 mm, and “extreme” if precipitation was greater than 45 mm. These thresholds represent the 80%, 15%, and 5% of the daily precipitation distribution; and the 25%, 40% and 35% of total winter precipitation, respectively. It was found that the precipitation intensity (frequency) decrement in lee side increase (decrease) when increase the precipitation in high mountain from weak and moderate to extreme cases. The rain shadow in lee-side lowlands only represent 1% of total precipitation accumulated in high mountains for extreme cases.