While numerous detailed studies have been conducted of the annual cycle of convection over other regions (e.g., the Asian summer monsoon and the West African summer monsoon regions), the annual cycle and its modulation in the tropical South American region has received attention only relatively recently. Most of the annual total rainfall observed over tropical South America occurs during the austral summer and autumn months. The large-scale meteorological systems that modulate rainfall during these periods are linked to the strength and movement of large-scale climatological features -- in particular, the Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone (SACZ). It is well known that the anomalous patterns related to the El Niño/Southern Oscillation (ENSO) influence the ITCZ and SACZ patterns, with strong interannual and seasonal variations over tropical and subtropical South America.
The goal of this chapter is to analyze the influence of ENSO events on the regional Hadley and Walker cells and their respective impacts on South American seasonal rainfall. As is well documented, ENSO events influence regional precipitation patterns over South America, with the strongest influences in the Amazon/Northeast Brazil and southern South America.
Basically, two separate responses can be composited for each phase of the ENSO cycle. El Niño (La Niña) Composite 1 is the canonical ENSO warm (cold) event with well-known impacts on large-scale atmospheric circulation and regional precipitation patterns over South America, indicating that the central-eastern Pacific sea surface temperature anomaly (SSTa) is the dominating feature in this case. On the other hand, the El Niño and La Niña Composite 2 analyses characterize the influence of the intertropical Atlantic SST gradients as being significant in modulating the influence of ENSO by intensifying the SACZ and ITCZ in some cases. For these latter composites, evidence of a completely reversed atmospheric circulation and regional precipitation patterns is found during the summer and autumn seasons. The analysis demonstrates that interaction of ENSO events with the South American monsoon produces changes in the time and space evolution of convection and circulation over northern South America, which can also be reinforced by the Atlantic. Thus, depending on conditions in the Atlantic, the South American rainy season may be strongly affected. These results suggest that some care always must be taken in producing precipitation (and impacts) forecasts based on ENSO indices and composites alone.