Interannual Rainfall Variability over the South American Altiplano

Abstract

Summertime (December–February) precipitation is virtually the only water resource over the South American Altiplano, a semiarid, high-level plateau entrenched in the central Andes. On the interannual timescale, Altiplano rainfall exhibits pronounced fluctuations between drought and very wet conditions, with subsequent impacts on agriculture and hydrology. In this work, the large-scale patterns of convective cloudiness and circulation associated with interannual variability of the summer rainfall over this region are investigated using a regression analysis between relevant atmospheric fields (NCEP–NCAR reanalysis, outgoing longwave radiation) and an index of convection over the Altiplano. It is found that the seasonal-mean, large-scale zonal flow over the central Andes is directly related with the number of days with easterly flow within the season, that, in turn, favor the occurrence of summertime deep convection on the Altiplano by transporting moist air from the interior of the continent. Consequently, interannual variability of the seasonal-mean zonal wind explains nearly half of the variance of summertime convection over the Altiplano through an easterly/wet–westerly/dry pattern. The circulation anomalies are in geostrophic balance with changes in the meridional baroclinicity at the southern border of the tropical belt. Thus, a previously documented relationship between El Niño–Southern Oscillation (ENSO) phenomenon and interannual rainfall variability over the Altiplano is explained by the generalized warming (cooling) of the tropical troposphere during the negative (positive) phase of ENSO and the associated strengthening (weakening) of the westerlies over the central Andes.