Climatological Characterization of Puelche Winds down the Western Slope of the Extratropical Andes Mountains Using the NCEP Climate Forecast System Reanalysis

The existence of strong easterly winds down the western slope of the south-central Andes in Chile, locally known as Puelche winds, has been known by the meteorological community since at least the mid-twentieth century. However, this is the first time that a climatological characterization of them is presented. The analysis is based on 36 yr of daily CFSR-NCEP reanalyzed data, validated by surface weather observations. Puelche winds are present all year round. The main synoptic-scale forcing of Puelche winds in south-central Chile is the passage of cold anticyclonic systems across the Andes Mountains. As these systems progress into the South American continent, a zonal surface circulation crossing from Argentina (upslope) to Chile (down-slope) develops. Unlike terral and raco, other foehnlike winds at subtropical latitudes in Chile, the Puelche winds are forced by both meridional and zonal pressure gradients. Presumably, the smaller altitude of the Andes Mountains south of 35(circle)S allows the air crossing from east to west in response to the presence of the migratory high pressure system over Argentina. As in other places where foehnlike winds develop, the warming extends far from places where the Puelche is actually observed, that is, to the west of the Andes into the surface at the coastal and the central depression areas. This "foehn clearance'' is the result of cloudless sky and drier atmosphere that would allow an increase in the solar radiation reaching the surface and a subsequent warming of the near-surface air. The foehn clearance also drives an enhanced nighttime cooling, especially on the days after the onset of the Puelche event.