DevelopmentsinEarthSurfaceProcesses ...

Climate and Geomorphologic-related Disasters in Latin America 15
These results are broadly consistent with the current view of the region’s
climate. Over tropical South America during El Niño events, the rising motion
tends to be weaker than average and rainfall over the eastern Amazon and northeast
Brazil tends to be below average, with broadly the reverse situation during La
Niña events (Marengo and Hastenrath 1993). One would therefore expect a
tendency of stronger rainfall and higher stream flows during cold events and the
opposite during warm events. In general, however, ENSO only explains a small
fraction of the interannual variance of rainfall in the Amazon (Marengo et al. 1993,
2001; Rao et al. 1996; Dettinger et al. 2000). Therefore, the consequences for
stream flows have small statistical significance. The Tocantins and San Francisco
rivers are in the northeastern part of South America, where the regional climate is
also influenced by sea-surface temperature (SST) and intertropical convergence
zone (ITCZ) anomalies in the Atlantic. The La Plata Basin’s climate, however, is
significantly affected by ENSO through atmospheric teleconnections (Cazes et al.,
2003).
At the same time, as discussed in this book by Islas and Schnack (Chapter 16),
the South American coast is particularly exposed to ENSO-triggered effects. The
effects are different in different regions. In Colombia, ENSO produces seasonal
increases in mean sea level, altering the dynamics of the barrier islands. In Ecuador,
coastal and island populations were badly affected during the 1997–1998 El Niño
season by wave action and increasing sea levels, due to Kelvin waves from storms in
the northern Pacific. They produced coastal erosion along the shoreline and
destroyed structures along the beaches. In Peru, ENSOs trigger strong inputs of
sediment to the coast, while similar processes impact the estuarine complexes of
Lagoa dos Patos (southern Brazil) and cause Paraná River/Rio de la Plata floods
(Argentina–Uruguay). In northern Brazil, on the other hand, dry conditions during
El Niño favor the migration of coastal dunes.
3.2. Mountain Areas and Intense Tropical Rainfall
High and intense rates of tropical rain falling on mountain landscapes with steep
slopes are a deadly combination, generating disasters. As analyzed in the chapters
devoted to Colombia (Hermelin and Hoyos, Chapter 7), Venezuela (Bezada,
Chapter 6), and Brazil (Coelho Netto and Avelar, Chapter 12 and Stevaux et al.,
Chapter 13), landslides and flash floods full of detritus flowing along mountain river
valleys can attain catastrophic dimensions during periods of intense rainfall, especially
for densely populated regions. Extreme landslide events produce excessive
sediment yields, which converge into the channel network that drains the watersheds,
causing immediate mass sedimentation of drainage systems in the river
valleys, the piedmont plains, or coastal flatlands.
In Brazil, the majority of these phenomena occur in the steep mountainous
lands of the serras and coastal ranges along the Brazilian Atlantic region (São Paulo,
Rio de Janeiro, Minas Gerais, Espirito Santo, Rio Grande do Sul, Santa Catarina,
and Paraná states). The relief is higher than 1200 m and close to the coast, but inland
the mountainous escarpments can reach altitudes of up to 2000 m and relief
amplitudes of more than 700 m. The combination of the steep relief, plus the