Under the present climate the Amazon Basin behaves as a moisture sink (P > E), and therefore the basin receives moisture from sources such as the tropical rainforest by means of intense recycling and by transport from the tropical Atlantic by near-surface easterly flows or trade winds. The former has generated plenty of concern due to the possible impact of deforestation on the hydrological cycle of the basin (see Section 9.2.5). In this context, water that evaporates from the land surface is lost to the system if it is advected out of the prescribed region by atmospheric motion, but recycled in the system if it falls again as precipitation (Brubaker et al., 1993). Studies on recycling of water in the hydrological cycle of the Amazon Basin have been performed since the mid-1970s by Molion (1975), Lettau et al. (1979), Salati et al. (1979), Salati and Voce (1984), Salati (1987), and Eltahir and Bras (1993) among others. All indicate the active role of evapotranspiration from the tropical forest in the regional hydrological cycle. During highly active precipitation episodes, moisture convergence can account for 70 to 80% of precipitation. However, on the monthly or longer term, mean evapotranspiration is responsible for approximately 50% of the precipitation, especially in the southeastern parts of the basin.
Bosilovich et al. (2002) adapted the passive tracer methodology developed by Koster et al. (1986) to the NASA GEOS climate model in order to identify the sources of precipitation in various continental-scale basins, among them the Amazon Basin. They found that the largest contribution to rainfall in the Amazon Basin comes from the South American continent (45.5%) and the tropical Atlantic contributes 37%. The continental sources for Amazon precipitation are large throughout the year, but the oceanic sources vary with the seasonal change of the easterly flow over the tropical Atlantic. In the Amazon, significant amounts of water are not transported from very long distances, contrasting with the Mackenzie River in Canada that receives a significant contribution from Asian sources, or the North American sources for rainfall in the Baltic Sea region in Europe.
In the context of regional circulation in South America, the role of moisture transport from the tropical North Atlantic to the Amazon region has been documented in previous studies (see reviews in Hastenrath, 2001), and the interannual variability of rainfall anomalies in the region has been linked to variability in moisture transport and the intensity of the trade winds in the tropical Atlantic sector. Furthermore, the Amazon Basin is a region that provides moisture to regions in subtropical South America—such as southern Brazil and the La Plata River Basin—as has been shown in various studies.
A relevant feature of South American low-level circulation during the wet warm season is a poleward warm and moist air stream immediately to the east of the Andes often referred to as a low-level jet, because of its resemblance to the U.S. Great Plains Low-Level Jet east of the Rocky Mountains. This moist air current is referred as the "South American Low-Level Jet east of the Andes'' or SALLJ—a component of the seasonal low-level circulation in the region that is detected all year long but mostly during the warm season (Berbery and Barros, 2002; Marengo et al., 2004a). Figure 9.4 shows a conceptual model of the SALLJ. It illustrates moisture transport reaching the Amazon Basin by means of tropical North Atlantic easterly trade winds. The moisture transport typical of austral summer time is enriched by evapotranspiration from the Amazon Basin. Once the trade winds reach the Andes they are deflected by the mountains, changing the near-surface flow from northeast to southeast. During winter, subtropical Atlantic highs move towards southern Brazil and northern Argentina, and the winds from the northwest—at the western flank of this anticyclone—seem to replace the northwestern flow from Amazonia typical of summer. This winter flow carries less moisture than the summer flow even though it can sometimes be stronger. In both seasons, this northwest stream at the exit region of the jet converges with air masses from the south, which can favor the development of convective activity and rain at the exit region of the jet in southeastern South America over the La Plata River Basin. The conceptual model also shows the effects of topography in the SALLJ through dry and moist processes, the impact of the energy balance terms (sensible and latent heat) released from the Bolivian Plateau, while the near-surface heat low is important in terms of the impacts of transients (cold fronts, cyclogenesis, etc.) on the SALLJ (see reviews in Nogues-Paegle et al., 2002; Marengo et al., 2004a).
SALLJ variability in time and space is relatively poorly understood because of the limited upper-air observational network in South America east of the Andes, which
seems to be unsuitable to capture the occurrence of the low-level jet, its horizontal extension and intensity, or temporal variability. Regarding time variability, SALLJ events seems to occur all year long, being more intense in terms of wind speed and moisture transport during the austral summer. As shown in Figure 9.4, more frequent wintertime SALLJs are related to the intensity and position of the subtropical South Atlantic anticyclone, and the source of moisture is the tropical-subtropical South Atlantic. During the summertime, the most important source of moisture is the tropical Atlantic-Amazon Basin system, when northeast winds coming from the tropical North Atlantic are deflected to the southeast by the Andes and in doing so get enriched by moisture from the Amazon Basin. This SALLJ, which brings tropical moisture from the Amazon to southern Brazil and northern Argentina, is more frequent in the warm season.
From the moisture budget calculations of Saulo et al. (2000), using regional models, a net convergence of moisture flux is found over an area that includes the La Plata Basin, with a maximum southward flux through the northern boundary at low levels that represents the moisture coming from Amazonia via the SALLJ. While there is evidence to suggest that this model provides a realistic description of the local circulation, it is emphasized that observational data are needed to gain further understanding of the behavior of the South American Low-Level Jet and its role in the regional climate.
Another feature of the low-level circulation in South America is the semipermanent South Atlantic Convergence Zone (SACZ). The SACZ is influenced by SST anomalies over the southwestern tropical Atlantic, has a strong impact on the rainfall regime over southern northeast Brazil, southeast and southern Brazil, and contributes to modulate underlying SSTs over the southwest tropical Atlantic (Chaves and Nobre, 2004). There is evidence that the phases and location of the SACZ respond to Rossby wave activity (Liebmann et al., 1999) and to Madden Julian Oscillation (MJO) (Carvalho et al., 2004). On the other hand, its intensity depends on moisture coming from the Amazon region during summertime. Analyses performed by Nogues-Paegle (2002), Herdies et al. (2002), and Marengo et al. (2004a) suggest that there is an out-of-phase relationship between the SALLJ and SACZ. Moisture transport and possibly rainfall downstream of the jet or in the SACZ show a contrasting pattern, with enhanced convection and rainfall due to enhanced SALLJ consistent with periods of weak SACZ and vice versa. The SALLJ and SACZ are components of the South American Monsoon System (SAMS).
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