Features of the rainy season in Amazonia are examined within the context of the circulation and convection anomalies, the impacts of tropical Pacific and Atlantic sea surface temperatures (SST), and the interannual variability associated to extremes of the Southern Oscillation (SO). The focus is on the onset of the rainy season, and the antecedent and subsequent conditions over six different regions inside the Brazilian Amazonia, chosen based on the different rain producing mechanisms affecting them. Additionally, the demise and the intensity and length of the rainy season are also assessed. The onset is determined using an objective rainfall criteria, based on 5-days averaged precipitation for the period 1979-96. Composites of outgoing longwave radiation (OLR), SST from tropical Pacific and Atlantic, and circulation from the NCEP reanalysis for the period 1979-96 provide a comprehensive picture on the evolution of the rainy season in Amazonia.
In general, the onset of the rainy season is shown to occur first in southwest Amazonia, with onset dates occurring progressively later toward the northwest. The demise of the rainy season occurs first in southeast and progressively later to the northwest. The variability of the characteristics of the rainy season is larger in northern Amazonia, as compared to southern part of the basin. Associations with the extremes of the SO index are found better in central Amazonia, and the mouth of the Amazon, where years with the low/high SO phase are consistent with anomalously dry/wet rainy seasons on those two regions, due mainly to a late/early onset of the rainy season. Indices of SST for the Pacific (NINO 3), and the Atlantic (SST dipole) confirm these findings. Moisture input from the North Atlantic feeds convection and rain on these regions, and the variability of these circulation features affects the onset and intensity of the rainy season. Southeast Amazonia rainfall seems independent from the extremes of the SO, and the moisture from the Amazon basin feeds convection and rainfall on this region, which is also an integral components of the South American monsoon. In general, there is a good correlation between the intensity of the rainy season and the length of it.