11.6.1 Key Processes
Over much of Central and South America, changes in the intensity and location of tropical convection are the fundamental concern, but extratropical disturbances also play a role in Mexico’s winter climate and throughout the year in southern South America. A continental barrier over Central America and along the Pacific coast in South America and the world’s largest rainforest are unique geographical features that shape the climate in the area.
Climate over most of Mexico and Central America is characterised by a relatively dry winter and a well-defined rainy season from May through October (Magaña et al., 1999). The seasonal evolution of the rainy season is largely the result of air-sea interactions over the Americas’ warm pools and the effects of topography over a dominant easterly flow, as well as the temporal evolution of the ITCZ. During the boreal winter, the atmospheric circulation over the Gulf of Mexico and the Caribbean Sea is dominated by the seasonal fluctuation of the Subtropical North Atlantic Anticyclone, with invasions of extratropical systems that affect mainly Mexico and the western portion of the Great Antilles.
A warm season precipitation maximum, associated with the South American Monsoon System (Vera et al., 2006), dominates the mean seasonal cycle of precipitation in tropical and subtropical latitudes over South America. Amazonia has had increasing rainfall over the last 40 years, despite deforestation, due to global-scale water vapour convergence (Chen et al., 2001; see also Section 3.3). The future of the rainforest is not only of vital ecological importance, but also central to the future evolution of the global carbon cycle, and as a driver of regional climate change. The monsoon system is strongly influenced by ENSO (e.g., Lau and Zhou, 2003), and thus future changes in ENSO will induce complementary changes in the region. Displacements of the South Atlantic Convergence Zone have important regional impacts such as the large positive precipitation trend over the recent decades centred over southern Brazil (Liebmann et al., 2004). There are well-defined teleconnection patterns (the Pacific-South American modes, Mo and Nogués-Paegle, 2001) whose preferential excitation could help shape regional changes. The Mediterranean climate of much of Chile makes it sensitive to drying as a consequence of poleward expansion of the South Pacific subtropical high, in close analogy to other regions downstream of oceanic subtropical highs in the Southern Hemisphere (SH). South-eastern South America would experience an increase in precipitation from the same poleward storm track displacement.