Interannual rainfall variability over the South American Altiplano

Several issues have motivated a growing interest in the rainfall variability over the South American Altiplano, the high-level plateau entrenched in the central Andes between 15°-20°S. First, convective rainfall during the austral summer (December-January-February, DJF) is the primary source of water for this semiarid region. On the other side, intense or persistent summertime storms can trigger flash floods over the Altiplano and the western slope of the Andes. It is also recognized that convection over the central Andes plays a key role in the evolution of the wet season of tropical and subtropical South America, throughout the intense diabatic heating of the tropical troposphere. Finally, paleorecords from dry lakes and low-latitude glaciers in the Altiplano have revealed important clues about past environmental conditions and climate variability in the tropics.

Summertime (DJF) rainfall in the Altiplano exhibits a pronounced year-to-year variability (seasonal amounts ranging from 100 to 600 mm), broadly distributed between 2 and 5 years and with no apparent long-term tendency. For most cases, the interannual variability is coherent among different stations in the Altiplano. A few studies have also indicated a tendency to dry conditions during the warm phase of the El Niño-Southern Oscillation (ENSO) phenomenon, and a somewhat weaker association between cold ENSO episodes and above than normal summertime precipitation.

In this work, satellite measurements of outgoing longwave radiation (OLR) is used as an estimate of the convective rainfall over the Altiplano, complementing Lake Titicaca level increments and a few rainfall records. A reasonable correlation was found among these different rainfall estimates. To describe the large-scale circulation anomalies during wet and dry conditions over the Altiplano we have used the NCEP-NCAR reanalyzes for the period 1958-1999.

There is an inverse correlation between anomalies of convection over the Altiplano and the zonal wind in the middle and upper troposphere over the central Andes, best defined in January and February. This association is consistent with previous findings and the notion that moisture that precipitates over the Altiplano originates over the tropical lowlands to the east of the central Andes. For instance, transport of moist air toward the highlands is enhanced during summers (or months) with mean easterly flow aloft, increasing the rainfall over the Altiplano. Interannual fluctuations of the zonal flow over the central Andes seems largely produced by changes in the meridional pressure gradient between the tropical and the subtropical atmosphere off the west coast of South America. Regression analysis shows this anomaly encompasses the whole globe, resembling a classical feature the ENSO phenomenon. It is hypothesized that a fraction of the interannual rainfall variability over the Altiplano is explained by the ENSO cycle throughout this mechanism.