The time-mean regional flow near the western side of the subtropical Andes (27°-37°S) features a marked diurnal cycle with westerly (valley-to-mountain) flow during daytime and weaker easterly (mountain-to-valley) flow during nighttime, resulting from the superposition of the large-scale westerlies (altered by the topographic blocking) and thermally-driven circulations at lower levels. This typical diurnal cycle is occasionally disrupted by episodes of forced downslope flow in which the low-level easterlies become very intense and persist beyond the nocturnal period.

These episodes exhibit föhn wind characteristics (that is, low-level adiabatic warming and drying), tend to last for 2-4 days and occur a few times per month. The warm downslope flow produces a very stable condition over the plains downstream of the mountains, hindering the dispersion of air pollutants during wintertime and increasing the risk of forest fires during summertime. Within the Andean valleys, the strength of the downslope wind can reach 20-30 ms-1, producing hazardous conditions, leading to local warming as large as 10°C/h, and likely accelerating snowmelting at low elevations.

The typical regional features and attending large-scale circulation associated with forced downslope flow are presented in this work. To this effect, 50 episodes over 3 years were selected from 15' near surface wind measurements on the entrance of an Andean valley in central Chile, complemented with observations from a network of automatic weather stations, wind-profiler data, on-demand research pilot balloons and NCEP/NCAR reanalysis data. Our key findings include (a) the low-level jet structure of the flow downstream of the Andes that tends to surmount the pools of cold air over the nearby valleys; (b) the non-linear superposition of the forced flow with the termally-driven circulations; (c) the close relationship between the wind and temperature fields; and (d) the identification of three synoptic-scale patterns than can force the downslope flows (coastal lows, incoming synoptic-low, easterly flow atop of the Andes).