Amazonia

Amazonia lies athwart the equator (Figure 11.46) and contains some 30 per cent of the total global biomass. The continuously high temperatures (24 to 28°C) combine with the high transpiration to cause the region to behave at times as if it were a source of maritime equatorial air.

Important influences over the climate of Amazonia are the North and South Atlantic subtropical high-pressure cells. From these, stable easterly mT air invades Amazonia in a shallow (1000 to 2000 m), relatively cool and humid layer, overlain by warmer and drier air from which it is separated by a strong temperature inversion and humidity discontinuity. This shallow airflow gives some precipitation in coastal locations but produces drier conditions inland unless it is subjected to strong convection when a heat low is established over the continental interior. At such times, the inversion rises to 3000 to 4000 m and may break

Figure 11.46 Mean annual precipitation (mm) over the Amazon basin, together with mean monthly precipitation amounts for eight stations.

Source: From Ratisbona (1976), with kind permission from Elsevier Science NL, The Netherlands.

down altogether associated with heavy precipitation, particularly in late afternoon or evening. The South Atlantic subtropical high-pressure cell expands westward over Amazonia in July, producing drier conditions as shown by the rainfall at inland stations such as Manaus (see Figure 11.48), but in September it begins to contract and the buildup of the continental heat low ushers in the October to April rainy season in central and southern Amazonia. The North Atlantic subtropical high-pressure cell is less mobile than its southern counterpart but varies in a more complex manner, having maximum westward extensions in July and February and minima in November and April. In northern Amazonia, the rainy season is May to September. Rainfall over the region as a whole is due mainly to a low-level convergence associated with convective activity, a poorly defined equatorial trough, instability lines, occasional incursions of cold fronts from the southern hemisphere, and relief effects.

Strong thermal convection over Amazonia can commonly produce more than 40 mm/day of rainfall over a period of a week and much higher average intensities over shorter periods. When it is recognized that 40 mm of rainfall in one day releases sufficient latent heat to warm the troposphere by 10°C, it is clear that sustained convection at this intensity is capable of fuelling the Walker circulation (see Figure 11.50). During high phases of ENSO, air rises over Amazonia, whereas during low phases the drought over northeast Brazil is intensified. In addition, convective air moving poleward may strengthen the Hadley circulation. This air tends to accelerate due to the conservation of angular momentum, and to strengthen the westerly jet streams such that correlations have been found between Amazonian convective activity and North American jet stream intensity and location.

The intertropical convergence zone (ITCZ) does not exist in its characteristic form over the interior of South America, and its passage affects rainfall only near the east coast. The intensity of this zone varies, being least when both the North and South Atlantic subtropical high-pressure cells are strongest (i.e. in July), giving a pressure increase that causes the equatorial trough to fill. The ITCZ swings to its most northerly position during July to October, when invasions of more stable South Atlantic air are associated with drier conditions over central Amazonia, and to its most southerly in March to April (Figure 11.47). At Manaus, surface

Figure 11.47 The synoptic elements of Brazil. The seasonal positions of the coastal intertropical convergence zone; the maximum northerly extension of cool southerly mP airmasses; and the positions of a typical frontal system during six successive days in November as the centre of the low pressure moves southeastward into the South Atlantic.

Source: From Ratisbona (1976), with kind permission from Elsevier Science NL, The Netherlands.

Figure 11.47 The synoptic elements of Brazil. The seasonal positions of the coastal intertropical convergence zone; the maximum northerly extension of cool southerly mP airmasses; and the positions of a typical frontal system during six successive days in November as the centre of the low pressure moves southeastward into the South Atlantic.

Source: From Ratisbona (1976), with kind permission from Elsevier Science NL, The Netherlands.

winds are predominantly southeasterly from May to August and northeasterly from September to April, whereas the upper tropospheric winds are northwesterly or westerly from May to September and southerly or southeasterly from December to April. This reflects the development in the austral summer of an upper tropospheric anticyclone that is located over the Peru-Bolivia Altiplano. This upper high is a result of sensible heating of the elevated plateau and the release of latent heat in frequent thunderstorms over the Altiplano, analogous to the situation over Tibet. Outflow from this high subsides in a broad area extending from eastern Brazil to West Africa. The drought-prone region of eastern Brazil is particularly moisture-deficient during periods when the ITCZ remains in a northerly position and relatively stable mT air from a cool South Atlantic surface is dominant (see Chapter 9B). Dry conditions may occur between January and May during strong ENSO events (see p. 306), when the descending branch of the Walker circulation covers most of Amazonia.

Significant Amazonian rainfall, particularly in the east, originates along mesoscale lines of instability, which form near the coast due to converging trade winds and afternoon sea breezes, or to the interaction of nocturnal land breezes with onshore trade winds. These lines of instability move westward in the general airflow at speeds of about 50 km hr-1, moving faster in January than in July and exhibiting a complex process of convective cell growth, decay, migration and regeneration. Many of these instability lines reach only 100 km or so inland, decaying after sunset (Figure 11.48). However, the more persistent instabilities may produce a rainfall maximum about 500 km inland, and some remain active for up to forty-eight hours such that their precipitation effects reach as far west as the Andes. Other meso- to synoptic-scale disturbances form within Amazonia, especially between April and September.

Precipitation also occurs with the penetration of cool mP airmasses from the south, especially between September and November, which are heated from below and become unstable (see Figure 11.47).

Surges of cold polar air (friagens) during the winter months can cause freezing temperatures in southern Brazil, with cooling to 11°C even in Amazonia. In June to July 1994, such events caused devastation to Brazil's coffee production. Typically, an upper-level trough crosses the Andes of central Chile from the eastern South Pacific and an associated southerly airflow transports cold air northeastward over southern Brazil. Concurrently, a surface high-pressure cell may move northward from Argentina, with the associated clear skies producing additional radiative cooling.

Figure 11.48 Hourly rainfall fractions for Belem, Brazil, for January and July. The rain mostly results from convective cloud clusters developing offshore and moving inland, more rapidly in January.

Source: After Kousky (1980).

The tropical easterlies over the northern and eastern margins of Amazonia are susceptible to the formation of easterly waves and closed vortices, which move westward generating rain bands. Relief effects are naturally most noteworthy as airflow approaches the eastern slopes of the Andes, where large-scale orographic convergence in a region of significant evapotranspiration contributes to the high precipitation all through the year.

G EL NINO-SOUTHERN OSCILLATION (ENSO) EVENTS

1 The Pacific Ocean

The Southern Oscillation is an irregular variation, see-saw or standing wave in atmospheric mass and pressure involving exchanges of air between the subtropical high-pressure cell over the eastern South Pacific and a low-pressure region centred on the western Pacific and Indonesia (Figure 11.49). It has an irregular period of between two and ten years. Its mechanism is held by some experts to centre on the control over the strength of the Pacific trade winds exercised by the activity of the subtropical high-pressure cells, particularly the one over the South Pacific. Others, recognizing the ocean as an enormous heat energy source, believe that near-surface temperature variations in the tropical Pacific may act somewhat similar to a flywheel to drive the whole ENSO system (see Box 11.1). It is important to note that a deep (i.e. 100 m+) pool of the world's warmest surface water builds up in the western equatorial Pacific between the surface and the therm ocline. This is set up by the intense insolation, low heat loss from evaporation in this region of light winds, and the piling up of surface water driven westward by the easterly trade winds. The warm pool is dissipated periodically during El Niño by the changing ocean currents and by release into the atmosphere - directly and through evaporation.

The Southern Oscillation is associated with the phases of the Walker circulation that have already been introduced in Chapter 7C.1. The high phases of the Walker circulation (usually associated with non-ENSO or La Niña events), which occur on average three years out of four, alternate with low phases (i.e. ENSO or El Niño events). Sometimes, however, the Southern Oscillation is not in evidence and neither phase is

Figure 11.49 The correlation of mean annual sea-level pressures with that at Darwin, Australia, illustrating the two major cells of the Southern Oscillation.

Source: Rasmusson (1985). Copyright © American Scientist (1985).

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