RAINFALL CLIMATE VARIABILITY DINAMICS OVER SOUTH AMERICA

ABSTRACT
Intraseasonal time-scales of climate variability over Brazil, and interannual and larger time-scales climate variability over South America were studied using daily (1979-1990) and monthly (1951-1990) mean rainfall data and several statistical methods. The results show that the seasonal and interannual rainfall variability over South America have intrinsic regional features with prominent and persistent annual cycle. The semi-annual cycle is predominant over northern Amazon (high persistence) and Southern Brazil (low persistence). The seasonal mean rainfall distribution is strongly related to the statistics of probabilities in all seasons. With respect to intraseasonal variability over Brazil, it was found that there are two dominant oscillations with periods of 20 and 40 days with distinct characteristics. The 20 day oscillation shows greater dependence on baroclinic waves, while the 40 day seems to be more related to the global scale oscillations and apparently interacts with greater intensity with the tropical atmosphere. Regarding the interannual variability, it is observed that precipitation over South America is modulated by El Niño/South Oscillation (ENSO). However, this modulation is present in the distinct periods, 3.7 and 5 years. The 5 years oscillation is predominant over equatorial regions, while the 3.7 years oscillation is predominant over Southern Brazil. Both are modulated by Tropical Atlantic, but with different intensities. The linear trends and climate change over South America show coherent space-temporal distributions. Negative trends of the annual precipitation over the maximum rainfall area, related to the climatological region of the South Atlantic Convergence Zone (SACZ), and positive trends over Southern Brazil were observed. These patterns are results of the intense negative tendency occurred in summer over the maximum rainfall area and during spring season over Southern Brazil. The trends seem to be associated with the observed climate change in the mid 70's, which is related to the sea surface temperature change, mainly in the Pacific Ocean. This abrupt climate change affected almost the whole South America, except Northeast Brazil, where precipitation shows high interannual variability and apparent quasi-decadal variability.