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Figure 3.20 AMJ rainfall trends 1985 to 2012 for the individual regions in and around the Congo Basin (from Nicholson et al., 2018a; © American Meteorological Society. Used with permission). An asterisk indicates regions with trends significant at or above the 10% level. Units are standard departures per year averaged over the analysis period.

Hua et al. (2016) presented evidence of a longer term trend towards drier conditions. They specifically evaluated AMJ rainfall over the period 1950 to 2014, using GPCC data. A downward trend was evident throughout the Congo Basin. This trend was even stronger in eastern equatorial Africa. The NIC131‐gridded data set suggests an even longer term trend and a very sharp discontinuity in the mid‐1960s. The trend was evaluated for a somewhat longer season (March to June) over the period 1921 to 2014, the period of record for the NIC131‐gridded data set (Figure 3.21). A decline is seen in all but 3 of the 35 2.5‐degree grid boxes covering the Congo Basin. It significant at the 10% level in half of those regions.

A complete explanation for the drier conditions is beyond the scope of this chapter. Detailed analyses can be found in Camberlin et al. (2001), Todd and Washington (2004), Balaset al. (2007), Dezfuli and Nicholson (2013), Nicholson and Dezfuli (2013), and Hua et al. (2016, 2018). Overall, the causes are complex, with changes in SSTs and large‐scale circulation patterns being implicated. However, the links vary greatly by region and season (Balas et al., 2007). The factor that encompasses the region as a whole appears to be the Walker Circulation, which is strongly impacted by SSTs and in particular by the SST contrast between the various ocean basins (e.g., Hua et al., 2016, 2018; Williams & Funk, 2011).

Figure 3.21 Spatial patterns of linear trend per decade (1985 to 2012) and regional mean anomalies in rainfall during March through June, as evidenced in the NIC131‐gridded data set (from Liming Zhou, personal communication). The regional time series represents the area enclosed in bold in the figure on the left.

Figure 3.22 Five‐year averages of (left) relative number of MCSs per year and (right) total volumetric rainfall (km² mm/h × 10⁴) from MCSs (from Jackson et al., 2009, based on TRMM data; © American Meteorological Society. Used with permission). All data are averaged for a 1° × 1 °grid box.

3.6. CONVECTIVE ACTIVITY

      The Congo Basin is considered to be the site of the world‘s most intense thunderstorms. The thunderstorms are associated with MCSs. The precise definition of an MCS varies among authors, but a common one, from Nesbitt and Zipser (2003) is a precipitation feature with “at least 2000 km² of contiguous area with a 85‐GHz polarization corrected temperature of ≤250 K and 185 km² ≤ 225 K.” These systems are characterized by ice crystals and an anvil cloud in the upper levels. They develop in the late afternoon, producing primarily convective rainfall. At night, as the anvil spreads, a large proportion of the rainfall is stratiform.

Figure 3.22 shows the average number of MCSs per year over equatorial Africa. The number is termed “relative” because of the low sampling frequency of the TRMM satellite and the narrowness of the swath of its radar instrument. Over the central areas of the basin there is a distinct maximum in the number of MCSs per year and in the amount of rainfall from MCSs. Within that area are two local maxima at roughly 20°E and 28°E. Other areas with a distinct maximum include the mountainous region of Cameroon just north of the equator at roughly 8°E, over the Ethiopian highlands, and over Lake Victoria near the equator at 32°E. All MCS maxima correspond closely to the annual rainfall maxima described in Section 4.1 and evident in Figure 3.8a. Maxima in lightning frequency (Figure 3.23) also coincide with the maxima in MCS occurrence.

Figure 3.23 Five‐year averages of relative number of lightning flashes per year (from Jackson et al. 2009, based on TRMM data © American Meteorological Society. Used with permission). All data are averaged for a 1° × 1 °grid box, with a three‐point smoothing applied.

Figure 3.24 shows the diurnal cycle of MCS activity and associated rainfall. These are averages in two latitudinal spans across Africa: 0° to 10°N and 10°S to 0°. There is a minimum in MCS activity in the morning hours, then an increase to a maximum in late afternoon and early evening. The MCS count remains high through the night. The MCS count is notably lower in the southern latitude span than in the northern. The mean volumetric rainfall (total of stratiform plus convective rainfall) per MCS has a minimum in the afternoon, during the hours when the percent convective rainfall is highest. It is highest between midnight and 9 a.m., when convective rainfall is augmented by stratiform rainfall. The stratiform rainfall reaches 40 to 45% during this time period. The volume of rainfall per MCS during the nighttime is lower for the more northern areas.

Figure 3.24 Diurnal cycle of convection: annual average of the relative number of MCSs, the mean volumetric rainfall per MCS, and the percentage of convective rainfall in three‐hour intervals (from Jackson et al., 2009; © American Meteorological Society. Used with permission). Solid line represents averages across Africa in the latitudes 0° to 10°N. Dashed line represents averages for 10°S to 0°. Time indicated is local time. Volumetric rainfall is km² mm/h × 10⁴.

Hartman

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