Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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).

Figure 3.2 Schematic of the Congo Basin cell, the “pseudo” Central Africa cell, and the Walker cells over the Atlantic and Indian Oceans

      (adapted from Longandjo & Rouault, 2020; Nicholson et al. 2018a. ©American Meteorological Society. Used with permission).

Longandjo and Rouault (2020) pointed out that the Congo Basin lies in between the Atlantic and Indian Ocean Walker cells, the resultant circulation being what they termed the “pseudo” Central Africa cell (see also Pokam et al., 2014). They further suggested that, in addition to the main Walker cells, a low‐level Walker‐type cell exists over equatorial Africa. They term this cell, which is capped at roughly 750 hPa, the Congo Basin cell. Figure 3.2 is a schematic illustrating these cells.

The Congo Basin cell is also seen in the mean vertical motion for October–November (ON) and for March‐to‐May (MAM) (Figure 3.3). The Congo Basin cell is seen as a peak in rising motion from the surface to 750 hPa at ~35° to 40°E and low‐level subsidence over the continent just to the west. Overriding this cell is a strong area of ascent, with a maximum around 300 hPa. To the east lies the descending branch of the Indian Ocean cell, which extends to eastern equatorial Africa in ON but appears to be limited to the western Indian Ocean in MAM.

Two other features appear to play a role in determining the characteristics of the rainfall regime over the Congo Basin. These include a mid‐level jet stream and topography. The African Easterly Jet‐South (AEJ‐S) is the Southern Hemisphere counterpart of the better‐known AEJ over West Africa (Nicholson & Grist, 2003). This jet (Figure 3.4), with a core around 600 hPa, is a response to the temperature gradient between the tropical rainforest and the woodlands to the south. Consequently, the jet is seasonal, being well developed at the end of the dry season in the woodlands, i.e., during September to November. Its strength is maintained by an anticyclonic circulation associated with the mid‐level high pressure cell over southern Africa (Kuete et al., 2020). The AEJ‐S is characterized by a jet streak circulation (e.g., Uccellini & Johnson, 1979), such that there is convergence in the right entrance quadrant of the jet promotes ascent and convection (Jackson et al., 2009).

The impact of topography is illustrated by Figure 3.5, which depicts the mean vertical motion field during September‐to‐November (SON) at 850 hPa and 1800 UTC (roughly 2000 local time). The pattern shows two concentric rings with rising motion over the surrounding highlands and subsidence further towards the center of the basin plus rising motion over the center of the basin (Jackson et al., 2009). The large‐scale winds tend to blow toward the highlands from the east, west, and north, creating rising motion over the highlands. These interact with the more local upslope afternoon flow (Tripoli & Cotton, 1989a,b), the result being intense convection over the terrain or in the lee but compensatory subsidence further over the plain. The low‐level divergence produced by upslope winds around the basin enhances the subsidence. By early evening, downslope winds commence and converge into the center of the basin, producing the core of rising motion over the center of the basin. Note that this pattern is consistent with the regions of rising and sinking motion in Figure 3.3 during both MAM and ON and is probably the origin of the Congo Basin Walker cell described by Longandjo and Rouault (2020).

3.3. DATA

      The data utilized in this study include two satellite estimates of precipitation and two gauge‐data sets. This array of indicators is considered for several reasons. For one, the satellite record includes only a relatively recent period while the gauge data is very sparse in recent years. Also, the various satellite products differ with respect to temporal and spatial resolution and are hence applicable to different analyses. Finally, because of uncertainties in all of the data products, the most realistic characterization of the Congo hydrologic regime can be gleaned by the combination.

Figure 3.3 Vertical profiles of omega (hPa/s × 10^–2, negative values = ascent) during MAM (bottom) and ON (top) averaged between 10°N and 10°S (from Nicholson, 2017, based on NCEP Version 1). Similar patterns are evident in MERRA 2 and ERA5.

      Source: Jackson et al., 2009. ©American Meteorological Society. Used with permission.

Figure 3.4 The African Easterly Jet‐South: mean wind (m/s) at 600 hPa during October (from Jackson et al., 2009).

      Source: Nicholson et al., 2019. © American Meteorological Society. Used with permission.

The lack of gauge availability in recent years is a serious problem. Besides the lack of raw rainfall records, this impacts most satellite estimates because they are merged with or adjusted by gauge data (Nicholson et al., 2019). Data were plentiful throughout equatorial Africa during the period 1947–1972 but gauge networks in the region have steadily declined since that time (Camberlin et al., 2019; Malhi et al., 2013; Nicholson et al., 2018a; Washington et al., 2013). Figure 3.6 illustrates the steady decline in gauge availability. It shows the stations available