The Hadley and Walker Regional Circulations and Associated ENSO Impacts on South American Seasonal Rainfall

Abstract

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 aus-tral 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 Nino/Southern Oscillation (ENSO) influence the ITCZ and SACZ patterns, with strong interan-nual 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 AmazonINortheast Brazil and southern South America. Basically, two separate responses can be composited for each phase of the ENSO cycle. El Nino (La Nina) Composite 1 is the canonical ENSO warm (cold) event with well-known impacts on large-scale atmospheric circulation and regional precipitation patterns over 203 H.F. Diaz and R.S. Bradley (eds.). The Hadley Circulation: Present, Past and Future, 203-235.