8 - Amazonia Rainfall and Its Variability

Summary

ABSTRACT

The hydrological processes in Amazonia vary widely from year to year due to natural changes in the atmospheric conditions. This paper reviews the dynamic mechanisms that produce rainfall in the region, its characteristics and fluctuations due to inter-annual variability of the large-scale atmospheric circulations associated with the El Niño-Southern Oscillation phenomenon (ENSO) and blocking patterns of the atmospheric flow. It is hypothesized that the observed trends in precipitation, as well as runoff, reported by Rocha et al., (1989), may be related to a higher frequency of positive phases of ENSO and/or the presence of volcanic aerosols in the stratosphere over Amazonia. Such aerosols could influence the heating of the Andean Altiplano and, in turn, the seasonal development of the upper tropospheric anticyclone over tropical South America (Bolivian High). The effects of large-scale deforestation on local hydrology as well as the possible impacts on global climate are also discussed.

INTRODUCTION

The hydrologic cycle is an integrated product of the climate and of the biogeophysical attributes of the surface. On the other hand, it exerts an influence on climate which goes beyond the interaction between the atmospheric moisture, rainfall and runoff. It is the major single heat source for the atmosphere, in the form of latent heat which is released, mainly in the tropics, through the condensation of atmospheric moisture into clouds and rainfall.

Attempts to identify patterns in climatic fluctuations are mostly inconclusive because the climate, hence the hydrologic cycle, presents an intrinsic variability, both in space and time, which is not adequately known.