Rainfall distribution and seasonal rainfall and river seasonal variability

The Amazon River drains an area of 6.2 x 106 km2 and discharges an average of 6,300 km3 of water to the Atlantic Ocean annually. The annual cycle of rainfall in the region has been extensively described in Rao and Hada (1990), Marengo (1992), Figueroa and Nobre (1999), Marengo and Nobre (2001), Liebmann and Marengo (2001), Marengo (2004b). The spatial distribution of rainfall shows three centers of abundant precipitation in the Amazon Basin. One is located in northwest Amazonia, with more than 3,600 mm per year. Another region with abundant rainfall is the central part of Amazonia around 5°S with 2,400 mm per year. A third center is found close to the mouth of the Amazon River near Belem, with more than 2,800 mm per year. In the Rio Negro basin area, in northwestern Amazonia, the rainfall is abundant throughout the year reaching its maximum in April-June, while southern Amazonia's rain peaks earlier (January-March). The extreme high and localized values of precipitation in narrow strips along the eastern side of the Andean slopes are thought to be due to upglide condensation and a rain shadow effect on the lee side, so the localized maximum is due to the easterly winds being lifted when they flow over the Andes. The coastal maximum is caused by nocturnal convergence between the trade winds and the land breeze. In the central-north and south-southeast sections, rainfall is lower—in the region of 1,500 mm. The peak of the rainy season occurs earlier (December-February) in southern Amazonia, while northern and central Amazonia experience maximum rain in March-May (Figure 9.1, see color section).

The seasonal variability in rainfall in Figure 9.1 can be better understood by viewing it with Figure 9.2, which shows the mean seasonal distribution of rainfall. The austral summer—that is, December to February—season is characterized by the peak of the rainy season in southern Amazonia and the dry season in the Amazon region north of the equator, with less than 360 mm in the entire season. March-May represents the peak of the rainy season in central Amazonia, all the way from western Amazonia to the mouth of the Amazon River, and June-August represents the dry season over most of the region, with less than 180 mm in the entire season in southern and eastern Amazonia and less than 360 mm in the entire season in central Amazonia, while the extreme north of Amazonia experiences its wet season.

Following the annual cycle of rainfall, river discharge peaks first in southern and eastern Amazonia (January-March)—as in the Tocantins-Araguaia and Madeira Rivers in Figure 9.3—while the Negro and Amazon River peak in March-May. Measurements taken at Obidos integrate the contributions of the Solimees River (southern and western Amazonia) and the Rio Negro (northern Amazonia). Chu (1982) shows that almost 70% of the contribution to Obidos measurements comes from the waters of the Solimees River. The records of the Amazon, Negro, Xingu and Tocantins Rivers (Marengo et al., 1998b) are displayed for two El Nino years (198283, 1986-87) and La Nina years (1975-76, 1988-89). Extreme years were chosen since they may better show the associations between El Nino and rain in their basins. The

Figure 9.2. Seasonal cycle of rainfall in various stations across the Amazon Basin. Location of stations is indicated by a small square, and units are in mm month-1. Scale is shown at the lower left side of the panel. Modified from Marengo (1992).

year before an El Nino peak, Amazon River discharges at Obidos are anomalously high, while in actual El Nino years the discharges are lower than average. During La Nina years, the discharges at Obidos are more than 7,000 m3 s-1 above the normal.

River discharge peaks earlier in southern and eastern Amazonia (Tocantins and Madeira Rivers) than in northern Amazonia (Rio Negro) (Figure 9.2). Discharges of the Amazon River measured at Obidos (200 km inland from the mouth of the Amazon) do not represent the true amount of water that reaches the mouth of the Amazon, since they do not include the waters of the Xingu and Tocantins Rivers (Marengo et al., 1994). Mean discharge at the Obidos gauging station is 175,000 m3 s-1 (or 2.5 mm day-1), while the correct value at the mouth of the Amazon (Roads et al., 2002; Marengo, 2004b) is 210,000 m3 s-1 (or 2.9 mm day-1).

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