Interannual variability El Nino and tropical Atlantic impacts

Rainfall variability in various timescales in Amazonia has been the subject of several studies regarding physical causes that could include remote and local forcings. At seasonal and interannual timescales, remotely forced seasonal variations are usually linked to SST anomalies in the tropical Pacific and Atlantic Oceans. The Southern Oscillation (SO) and its extremes—linked to anomalies in the tropical Pacific (El Nino or La Nina at interannual scales), and to sea surface temperature (SST) anomalies and meridional contrasts in the tropical Atlantic—have been associated with rainfall anomalies in the Amazon Basin. Various papers have been devoted to studies on the impact of regional and global SST anomalies in the tropical Pacific and Atlantic Oceans on rainfall anomalies in the region (Ropelewski and Halpert, 1987, 1989; Aceituno, 1988; Richey et al., 1989; Rao and Hada, 1990; Marengo, 1992, 2004a; Meggers, 1994; Nobre and Shukla, 1996; Rao etal., 1996;Guyot etal., 1997; Marengo et al., 1998a, b; Uvo et al., 1998; Fu et al., 1999, 2001; Botta et al., 2002; Foley et al., 2002; Ronchail et al., 2002).

The low SO phase, which is associated with the El Nino phenomenon, is related to negative rainfall anomalies in northern and central Amazonia and anomalously low river levels in the Amazon River, while the high SO phase (related to the La Nina phenomenon) features anomalously wet seasons in northern and central Amazonia. Below-average southern summer rainfall throughout the Amazon Basin during seasons with weaker northeast trades, due to reduced moisture flux from Amazonia, has been identified during extreme El Nino years. In fact, a tendency towards drier rainy seasons and lower Rio Negro levels was detected during El Nino events in 192526,1982-83, and more recently during 1997-98, while wetter conditions were observed during La Nina years in 1988-89 and 1995-96 (Figure 9.5).

The drought of 1998 in north and central Amazonia is generally considered as the most intense of the last 118 years. Kirchoff and Escada (1998) described the "wildfire of the century''' in 1998 as one of the most tragic that have ever occurred in Brazil.

Figure 9.5. Time series of annual maximum {top) and annual minimum (bottom) river stage (in cm) of the Rio Negro at Manaus, Amazonas. Source: Williams et al. (2005). Arrows indicate occurrence of the El Nino phenomenon. The 1903-2004 long-term mean stage is 23.22 cm.


Figure 9.5. Time series of annual maximum {top) and annual minimum (bottom) river stage (in cm) of the Rio Negro at Manaus, Amazonas. Source: Williams et al. (2005). Arrows indicate occurrence of the El Nino phenomenon. The 1903-2004 long-term mean stage is 23.22 cm.

However, Williams et al. (2005) have suggested that the most severe drought in tropical South America during the 20th century occurred in 1926 during the El Nino of 1925-26. They established that dryness in the northern portion of the Rio Negro basin in 1925 also contributed to the major drought in 1926, through both depletion of soil moisture and possibly a negative feedback on rainfall from the abundant smoke aerosol (see below for elaboration). Annual rainfall deficits are broadly consistent with the reduction in annual discharge for 1926—estimated as 30-40%. The reduction in peak discharge during 1926 is closer to 50%. Sternberg (1987) describes an unparalleled drop in the high-water levels of the Rio Negro at Manaus during the El Nino event in 1926, during a severe dry season in which a great fire blazed for over a month, scorching the vegetation along the main channel. The drought also affected the Orinoco Basin with widespread and drought-related fires in the savannas. Evidence for a dry year in 1912 is also apparent in the Amazon discharge record in Figure 9.5, as is the minimum discharge prior to 1926, 1983, and 1998.

Regional forcing is associated with the systematic buildup of planetary boundary layer moisture and can affect the onset of the rainy season (Fu et al., 1999; Marengo et al., 2001). The atmosphere in southern Amazonia is quite stable; therefore, a great amount of surface heating is required to force the transition between dry and wet seasons, with soil moisture seeming to play a role in the predictability of the rainy season in southern Amazonia (Koster et al., 2000; Goddard et al., 2003; Marengo et al., 2003). Near the equator, stability during the dry season is weaker and the transition from the dry to the wet season is more dependent on adjacent SST anomalies. Recent studies have demonstrated that the transition between the dry and wet seasons in southern Amazonia is also dependent on the presence of biogenic aerosol and aerosol produced by biomass burning in the region, which play a direct role in surface and tropospheric energy budget due to their capacity to scatter and absorb solar radiation (Artaxo et al., 1990). This aerosol can also influence atmospheric thermodynamic stability in as much as it tends to cool the surface (by scattering radiation that would otherwise be absorbed at the surface) and by warming atmospheric layers above by absorption. Recent modeling and observational results have indicated that the aerosol plume produced by biomass burning at the end of the dry season is transported to the south and may interact with frontal systems, thus indicating a possible feedback to the precipitation regime by affecting the physics of rainfall formation (Freitas et al., 2004) through the radiative forcing of cloud micro-physical processes (Silva Dias et al., 2002).

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