Precipitation

The Amazon River discharges roughly 20% of Earth's river water that reaches the ocean, even though its basin occupies just 2% of continental land area. The spectacular discharge rate is due to the deep convection that forms over the Amazon Basin, the moisture brought in from the Atlantic by easterly winds, and the huge amounts of orographic rainfall generated as water vapor from the Atlantic is forced upwards by the Andean massif. While the mean annual rainfall is ^2,400 mm yr-1, precipitation is not uniformly distributed over the Amazon Basin, either in space or in time (Sombroek, 2001; Malhi and Wright, 2004; Marengo, 2004). The number of dry months (months averaging <100-mm precipitation) at Amazonian sites ranges from 0 to 8, and annual precipitation over closed-canopy humid forest and rainforest varies from 1,200 mm yr-1 in the southeastern Amazon Basin to >8,000 mm yr-1 in the Andean foothills.

Total rainfall in the Amazon Basin is high in a large region of upper Amazonia in northeastern Peru, eastern Ecuador, eastern Colombia, and western Amazonas state in Brazil, as well as near the mouth of the Amazon in eastern Para and Amapa states. High-rainfall areas are also found along the base of the Andes to from to ~16°S. Rainfall amounts drop rapidly south of to 8° in the western and central Amazon. A region of lower rainfall in central Para state, the Roraima territory of Brazil, and adjacent Guayana and Surinam is known as the transverse dry belt (Nimer, 1977; Pires and Prance, 1977, Davis et al., 1997), though total annual rainfall in this area is as high as much of the upper-Amazonian forests in southeast Peru and Acre, Brazil.

Precipitation varies temporally in the Amazon Basin on all timescales, even though the effects of super-annual variation on diversity have been little studied. Variability ranges from daily cycles and anomalies in rainfall rate due to local climatic processes, to intra-annual variation in rainfall, to among-year variation in rainfall driven by changes in sea surface temperature—such as ENSO and the North Atlantic Oscillation (Marengo et al., 2001; Pezzi and Cavalcanti, 2001; Marengo, 2004). On even longer timescales, precipitation changes are driven by the orbital parameters of Earth and their interaction with proximate climate drivers—such as the South American Low-Level Jet and the South American Convergence Zone (Baker et al., 2001; Cruz et al., 2005). Far from being static, detailed paleoclimatological reconstructions show that the main atmospheric and oceanic features influencing climate in the Amazon Basin show large variations through time. Indeed, modern studies correlating these drivers with historical weather patterns show that temporal variation also translates into differential spatial effects across the Amazon Basin (Giannini et al., 2001; Cruz et al., 2005; Vuille and Werner, 2005).

Previous empirical hypotheses of tree diversity suggested that tree diversity increased with precipitation amount and decreased with the number of months having <100-mm precipitation, a figure where evapotranspiration exceeds precipitation in a typical lowland ecosystem (Gentry, 1988; Clinebell et al., 1995; ter Steege et al., 2003). Annual precipitation amount and dry-season length are also inversely correlated with each other (Figure 10.2a).

Alpha Diversity Tropical Rainforest

Figure 10.2. (a) Local (alpha) diversity derived from 423 1-ha Amazonian tree plots. Isoclines drawn from universal kriging fit for 1° grid-cells. Figure adapted from Amazon Tree Diversity Network. Shaded regions show areas with no inventory plots. (b) Local diversity versus latitude. (c) Local diversity versus longitude.

Figure 10.2. (a) Local (alpha) diversity derived from 423 1-ha Amazonian tree plots. Isoclines drawn from universal kriging fit for 1° grid-cells. Figure adapted from Amazon Tree Diversity Network. Shaded regions show areas with no inventory plots. (b) Local diversity versus latitude. (c) Local diversity versus longitude.

Precipitation regimes have traditionally been characterized in terms of monthly and yearly averages interpolated from existing gauges, with varying degrees of quality and duration (e.g., Sombroek, 2001). Much of the Amazon Basin remains wilderness, and large areas are without rain gauges, particularly in upper Amazonia. In the present analysis we use satellite measurements of total monthly precipitation on a 0.25° x 0.25° grid collected over 7 years by the Tropical Rainfall Monitoring Mission (Chiu et al., 2006). These data give complete temporal and spatial coverage for precipitation across the tropics in grid-cells just less than 30 km per side, or for every ^900 km2, and allow examination of precipitation trends in the Amazon with unprecedented clarity.

From these data we derive four measures of precipitation amount and variability: average annual precipitation; average dry-season length, taken as the average number of months with precipitation below 100 mm (Clinebell et al., 1995; Sombroek, 2001; Ter Steege et al., 2003); inter-annual variability, taken as the coefficient of variation of rainfall among years; and total variability, taken as the coefficient of variation among months over the 7-year period.

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Responses

  • samppa
    What is the total number of precipitation in the amazon rain forest yearly?
    8 years ago

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