Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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Figure 3.1 Depiction of topography over the Congo Basin and surrounding regions. Solid line approximates the boundary of the Congo River drainage basin. Dashed line approximates the extent of tropical rainforest.

3.2. METEOROLOGICAL BACKGROUND

      The Congo Basin, i.e., the drainage basin of the Congo River, lies primarily in the equatorial latitudes between roughly 10°N and 14°S, bounded in the north and south by highlands over the Central African Republic and Zambia, respectively (Figure 3.1). Its eastern boundary is the Rift Valley highlands, at roughly 34°E, while highlands over Cameroon and Gabon lie along its western edge around 14°E. The rainforest itself extends throughout most of the basin, generally where elevation is below about 500 meters.

      Traditionally, the rainfall regime of the Congo Basin is assumed to be associated with the twice‐annual equatorial transit of the Intertropical Convergence Zone or ITCZ, producing an annual cycle with peaks in the two transition seasons. Recent work has shown that there is little evidence of such a discrete zone of low‐level convergence over equatorial Africa during the rainy season (Nicholson, 2018; Yang et al., 2015). Moreover, low‐level divergence prevails over much of the region during those seasons.

      Clearly the factors creating the meteorological regime are much more complex. Essentially, there is a broad zone of rainfall, termed the tropical rain belt or equatorial rain belt by some authors, that moves latitudinally with the seasons. Several factors make this region conducive to the production of rainfall. Longandjo (2018), for example, shows that the rainbelt over the Congo Basin is associated with a maximum in moist static energy (MSE, a measure of bulk atmospheric energy content) with wind shear in the lower‐ and mid‐troposphere playing a role.

      The results of research on eastern equatorial Africa are also relevant here. Liebmann et al. (2017) suggested control via upper‐level winds, low‐level specific humidity, and convective available potential energy (CAPE). Their findings are consistent with those of Yang et al. (2015), who demonstrate the importance of MSE, saturation moisture static energy, and vertically integrated moisture in the lower‐ to mid‐troposphere in the development of both rainy seasons over East Africa. Yang et al. further conclude that the annual cycle of MSE is prescribed by a combination of monsoon winds and tropical Indian Ocean SSTs (sea‐surface temperatures). The monsoon winds do not extend beyond the highlands, so that their full explanation of the seasonal cycle is not applicable to the central equatorial Africa (i.e., the Congo Basin).

      Equatorial Africa extends between the Atlantic and Indian Oceans, suggesting that both oceans would influence the climatology of the Congo Basin, particularly its moisture supply. However, there is some disagreement about the region’s moisture sources. Using water tracers in an Earth system model, Dyer et al. (2017) found that the Indian Ocean and local evaporation were the dominant moisture sources in the Congo Basin. The recycling ratio was found to be 25%. The Atlantic source was comparatively small, as moisture transported from the Atlantic into the basin is recirculated back to the Atlantic. The Indian Ocean source was found to become particularly important in wet years. Sori et al. (2017), using the Lagrangian FLEXPART model, estimated a recycling ratio of 50% and found that it increases/decreases in wet/dry years. They also found that the eastern equatorial Atlantic and land areas east of the Congo Basin are important sources of moisture for the basin. The sources appear to vary seasonally (Balagizi et al., 2018), with the equatorial Atlantic serving as a source in summer (Neupane, 2016).

The region is strongly influenced by the Walker circulation, a pattern of east–west oriented vertical circulation cells, with a primary cell in the Pacific and secondary cells primarily over the Atlantic and Indian Oceans. However, there is considerable disagreement about the seasonal development of the component cells, especially over equatorial Africa. Hastenrath (2007) claims that the cell over the Indian Ocean and eastern Africa exists only during the boreal autumn. Cook and Vizy (2016) conclude that a Walker‐type cell exists over central equatorial Africa only during the boreal summer, while Neupane (2016) and Washington et al. (2013) provide evidence of Walker‐type overturning during the boreal spring and autumn as well. Dezfuli et al. (2015) showed a Walker‐type overturning in the boreal winter, but in the south of the Congo Basin (0° to 10°S).