Intraseasonal Variability of Moisture and Rainfall over the South American Altiplano

Abstract

Precipitation over the South American Altiplano (about 4000 m above sea level) is mostly concentrated during the austral summer (December?January?February) when mean easterly flow in the middle and upper troposphere favors the moisture transport from the interior of the continent toward the central Andes. Within the wet season, rainy days tend to cluster in rainy episodes of about a week long, interrupted by somewhat longer dry periods. Based on one-site, research-quality observations over the western Altiplano, it has been suggested that occurrence of deep, moist convection is largely controlled by the availability of water vapor in the local boundary layer. In this work the author evaluates the representativeness of the observations in the western Altiplano in a regional context and investigates if the hypotheses derived from these data are generally applicable to the rest of the plateau. The study is based on surface synoptic data and atmospheric reanalysis. The relationship between moisture fluctuations on the Altiplano and the lowlands to the east of the central Andes is also addressed. It is found that intraseasonal moisture fluctuations tend to be coherent on the Altiplano and closely related to basinwide episodes of active/suppressed moist convection. On the other hand, near-surface moisture variability over the lowlands to the east of the central Andes is too small and noisy to explain the persistent, large-amplitude fluctuations in the Altiplano. Thus, moisture and rainfall variability over the Altiplano is strongly dependent on the intensity of the moisture transport over the eastern slope of the Andes rather than the precise low-level conditions on the central part of the continent.