Episodes of Strong Flow down the Western Slope of the Subtropical Andes

Abstract

Nocturnal flows down the narrow Andean valleys within the western slope of the subtropical Andes (central Chile) are episodically enhanced by easterly downslope winds that flow into the Santiago basin over the radiatively cooled air above the surface. Local, regional, and large-scale data have been used here to characterize the mean features of these episodes. About 80% of easterly downslope flow episodes in austral winter are forced by a reversal in the sea level pressure gradient along the coast of south-central Chile, when a midlatitude cold high migrates from southern Chile eastward across the Andes under midtroposphere SW winds associated with a warm ridge aloft. Under these circumstances low-level, easterly (offshore) flow sets in, producing a compensating downslope flow that subsides over central Chile. The remaining cases are associated with prefrontal conditions under a midlatitude trough with NW winds aloft. Since in most of these cases the easterly low-level flow occurs beneath westerly flow higher above, these episodes classify as strong windward downslope flows. Within the Andean valleys and canyons, the near-surface air experiences a sensible warming and drying at night and early morning during these episodes, as the strong downvalley winds tend to destroy the surfacebased radiative inversion and mix down warmer air from aloft. At the exit region of these valleys into the central basin, these downslope flows in austral winter are not able to flush the cold air pool there. Hence, dawn surface temperatures over the basin tend to be lower than average as clear skies and dry subsiding air aloft favor surface radiative cooling. The resulting enhancement of the near-surface static stability hampers the subsequent development of the mixed layer, leading to severe air pollution events in Santiago and other cities in central Chile. A comparative discussion on governing mechanisms with respect to apparently similar phenomena, as gap flow and shallow foehn, is included.