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Figure 9.6. Time series of normalized departures of rainfall in northern Amazonia (NAR index) and southern Amazonia (SAR index). The domains of the regions appear in the map inside each panel. Source: Marengo (2004a).

suggest the important role played by warming of the tropical central and eastern Pacific on decreasing rainfall in northern Amazonia, due to more frequent/intense strong El Nino events during the relatively dry period 1975-98. This decadal-scale variability has also been detected in other regions in South America, and it seems that other regions—such as the La Plata Basin—also experience these shifts. They have been linked to phases of the Pacific Decadal Oscillation (PDO) (Zhang et al., 1997). The positive PDO phase started in the mid-1970s and apparently ended in the early 2000s. A negative PDO phase started in the mid-1940s and extended until 1975-76, a period with more frequent and intense El Nino events.

Callede et al. (2004) suggested a reduction in the interannual variability of mean discharge during 1927-70 that is consistent with the positive phase of the PDO, and positive/negative rainfall departures in northern/southern Amazonia as shown by Marengo (2004a). Their analysis of mean annual discharge and average annual rainfall during 1945-98 demonstrates an increase in flow in relation to rainfall, which could be, according to the authors, the consequence of Amazonian deforestation.

Amazonia Rainfall
Southern Amazonia Rainfall Index (SAR)

Figure 9.6. Time series of normalized departures of rainfall in northern Amazonia (NAR index) and southern Amazonia (SAR index). The domains of the regions appear in the map inside each panel. Source: Marengo (2004a).

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