Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

2020) studied the interannual variability of MCS characteristics over the Congo Basin during the period 1984 to 2015. Some of the variables considered were total number, mean duration, minimum cloud‐top temperature (CTT, a measure of MCS intensity), and area. Hartman’s calculations for the latitudes of 0° to 5°N (region 1) and 0° to 5°S (region 2) are shown in Figure 3.25. For both regions there is a slight tendency for the number of MCSs to increase over time. The increase is largest in MAM in region 2, the area with strongest MCS activity. In region 1 there is a tendency for increasing/decreasing duration and area in MAM/SON. Minimum CTT decreases over time in both seasons, suggesting more intense MCSs. In region 2, the duration generally increases and the area generally decreases over time in both seasons. As in region 1, CCT decreases over time in both seasons. In both regions the decrease is greater in MAM than in SON.

The noted increase in MCS intensity over the Congo is consistent with the findings of Raghavendra et al. (2018), who found an increase in the intensity of thunderstorms over the Congo Basin during JJAS over the period 1982 to 2016. Taylor et al. (2018) also found an increase in MCS intensity over the Congo Basin during February. Hartman’s results for MAM, in particular the increased number and intensity of MCSs, appear to be at odds with the decrease in MAM rainfall. A paper by Hamada et al. (2015) might provide an explanation for this apparent paradox. They found that throughout the tropics the heaviest rain events are mostly associated with less intense convection. They also found contrasts in environmental conditions between the most extreme rainfall events and the most intense convection. The extreme rain events have lower moist static energy and temperatures (i.e., are less convectively unstable), higher relative humidity, and higher moisture flux convergence than the extreme convective events. Hamada et al. (2015) concluded that the contribution of warm‐rain processes might help to explain the weak relationship between extreme rainfall and extreme convection.

Figure 3.25 Interannual variability of select characteristics of MCSs over the Congo Basin during the period 1984 to 2015 (from Hartman, 2016). These include number, duration, minimum cloud‐top temperature, and area. Trend lines based on least squares regression are indicated. MAM and SON are shown separately. Region 1 extends from 0° to 5°N and region 2 extends from 0° to 5°S.

3.7. COMPARISON WITH THE AMAZON

      As mentioned earlier, one of the open questions concerning the Congo Basin is why rainfall is relatively low compared to other equatorial areas, such as the Amazon. Here a brief comparison is made between the Congo Basin and the Amazon, in order to examine some aspects of that question.

Figure 3.26 presents mean annual rainfall over the Amazon Basin and mean rainfall for four months. TRMM 3B43 V7 is used here instead of CHIRPS2 because it has been more extensively validated over the Amazon (e.g., Zulkafli et al., 2014). Note that the means over the Congo Basin from TRMM are nearly identical to those from CHIRPS, so that the comparison with rainfall over the Congo Basin in Figure 3.8 is valid. In the Amazon, annual rainfall exceeds 1750 mm nearly everywhere, and over most of the region it exceeds 2000 mm. Rainfall is on the order of 2500 mm to over 3000 mm in vast regions in the northwest. By comparison, over the Congo Basin mean annual rainfall is as low as 1250 mm along its periphery, and few areas receive over 1750 mm. Monthly rainfall is also generally higher over the Amazon, with large areas receiving 300 to 400 mm on average in some months. Over the Congo few areas receive more than 150 to 200 mm per month except during October. Then monthly rainfall is typically 150 to 300 mm per year, with only a very small area receiving more than 300 mm.

Figure 3.26 Mean rainfall (mm/mo)over the Amazon based on TRMM 3B43 V7 for the period 1998 to 2014.

Figure 3.27 shows the diurnal cycle of rainfall over the Amazon for two of the rainiest months, March and November. Over most of the region the maximum falls within the afternoon hours (18 to 21 UTC). The portion with a rainfall maximum at night (dark blue) is considerably smaller than the area of with a nocturnal maximum over the Congo Basin. As MCS activity tends to peak in the night or early morning hours (Nesbitt & Zipser, 2003), this suggests a lower contribution of MCS activity over the Amazon. This is confirmed in the analysis of Zipser et al. (2006), showing a much lower storm intensity over the Amazon than over the Congo. The storm intensity maximum over South America is in a drier region much further south.

McCollum et al. (2000) pointed out the contrast in atmospheric humidity between the Congo Basin and the Amazon. As an example, total column water vapor averages 40–50 kg/m² over equatorial South America but only 30–40 kg/m² over equatorial Africa. They pointed out the much more arid conditions east of the highlands and suggested that this feature and the restriction of moisture transport from the Indian Ocean by the highlands are major factors in the drier atmosphere and relatively low amounts of rainfall over equatorial Africa. Jackson et al. (2009) suggested additional reasons. One is the advection of relatively dry air from North Africa into the northern rim of the Congo Basin. The other is the mesoscale situation described by Tripoli and Cotton (1989a, b) to explain convection in the lee of high terrain. In areas of subsidence in the lee (i.e., within the Congo Basin), dry air off the highlands is mixed with the surface air and adiabatic heating further reduces the relative humidity.

Figure 3.27 The diurnal cycle of rainfall over the Amazon, as in Figure 3.14.

      The drier conditions over the Congo Basin might provide some explanation for the more intense convective activity compared to the Amazon. Hamada et al. (2015) underscored the contrast between factors enhancing convection (i.e., MCSs) and those enhancing rainfall. Lower relative humidity and lower moisture flux convergence favor the convection. Notably, the atmospheric moisture content over the Congo Basin is similar to that in the area of most extreme MCSs over South America.

3.8. SUMMARY AND CONCLUSIONS

      3.8.1. Controls on the Rainfall Regime

      The rainfall regime over the Congo Basin is shaped by a number of factors on both global and local scales. Over‐riding factors are orographic effects, which appear to produce a local and shallow Walker‐type circulation over the Congo Basin, the Walker cells over the Indian and Atlantic Oceans, tropical sea‐surface temperatures, and a mid‐level easterly jet stream that is present only during the SON rainy season.

      Most of the rainfall, as much as 60–70%, is associated with intense MCSs. These systems include a layer of ice and a large anvil cloud, and produce both stratiform and convective rainfall. The stratiform component is strongest at night. The systems over the Congo appear to be the strongest in the world and are associated with the world’s highest frequency of lightning.

      3.8.2. Mean Rainfall and the Seasonal Cycle

      The