The Andes cordillera, running continuously from the north of the equator down to the southern tip of South America, represents a formidable obstacle to the tropospheric circulation and acts as a climatic wall between the sharply contrasting conditions over the continental lowlands to the east and the Pacific rim to the west. Along its central portion (15°S-22°S), the widening of the Andes produces distinctive meteorological conditions, that we refer to as the climate of the Altiplano, described in this work using local observations, gridded atmospheric reanalysis, and satellite-based cold cloud measurements (a rainfall proxy in the tropics).

At a local scale, deep moist convection (and hence, convective rainfall) over the Altiplano is largely controlled by the moisture content within the ABL, with an activation threshold on the order of 5 g/Kg. This high moisture value (with respect to the free tropospheric air at the same level) indicates moisture transport from the very humid continental lowland to the east of the central Andes, which is accomplished by regional-scale circulation that develops in responce to the daytime heating of the sloping terrain. The intensity and duration of the easterly upslope flow (and its counterpart over the dry, western slope) is however, modulated by the sign and intensity of the mid- and upper-level, large-scale zonal flow over the central Andes. Easterly (westerly) flow aloft favors the easterly (westerly) upslope flow, increasing the transport of moist (dry) air into the Altiplano.

The large-scale zonal flow is in turn produced by the superposition of transient (synoptic or intraseasonal) disturbances and the seasonal mean flow. Transient disturbances are associated with Rossby wave propagating from the SH midlatitudes and amplifying over subtropical South America, resulting in episodic fluctuations of the zonal flow with a period on the order of 1-2 weeks. These fluctuations lead to actual easterly winds over the central Andes only under favorable background flow: easterly or weak westerly. This later condition is meet in a fairly narrow window of time, December to March, during which the rainy season develops with its typical episodic evolution. The mean easterly flow aloft during summer is associated with large-scale circulation changes ultimately driven by the solar cycle. On the interannual timescales, the large-scale zonal flow at tropical-subtropical latitudes further weaken during La Niña years, because of the cooling of the tropical troposphere, fostering a extended or more active rainy season over the Altiplano. The opposite effect tends to occur during El Niño years. This mechanism explains the well-documented tendency to deficit (excess) of summer rainfall during El Niño (La Niña) years.