Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов
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Название: Congo Basin Hydrology, Climate, and Biogeochemistry
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119656999
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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 paucity of rain gauges in the Congo Basin in recent years has hindered climatological analysis. For this reason, this study considered both gauge data and satellite estimates from the CHIRPS2. The pattern of mean annual rainfall is extremely similar for both, despite the means for CHIRPS2 being based on the period 1981 to 2019 and those from gauges based on the period 1945 to 1984, when a dense gauge network was available. This suggests a rainfall regime that is relatively stable on a multi‐decadal time scale.
Throughout the Congo Basin mean annual rainfall exceeds 1250 mm, but it exceeds 1500 mm over most of the region. Within this central region are three rainfall maxima, within which mean annual rainfall exceeds 2000 mm in at least some areas. The rainfall maxima coincide with maxima in MCS activity and lightning frequency.
The seasonal cycle of rainfall in the region is traditionally assumed to be bimodal, with peak rainfall being associated with the twice‐annual equatorial transit of the ITCZ. Detailed analyses show this scenario to be inadequate. For one, there is no discrete low‐level convergence zone in the region. On the contrary, divergence and subsidence prevail at low levels over much of the region. Moreover, the pattern of seasonality is complex and varies significantly over relatively short distances.
While there is no distinct ITCZ, there is a well‐defined rainbelt and it does move seasonally, reaching its northernmost position in the boreal summer and its southernmost position in the boreal winter. The factors producing this broad region of rainfall are not yet well understood, but they likely include the atmospheric energy content (e.g., moist static energy) and total atmospheric moisture, in conjunction with low‐level circulation.
Over most of the region the seasonal cycle is weak and only 30–40% of the rainfall is concentrated in the wettest portion of the year. The two rainy seasons are generally considered to be MAM and SON, with the latter being the more important season, and the main dry season being JJA. This generalization holds mainly in the latitudes 0° to 5°S, but the second peak tends to fall in ON. The relative importance of the two seasons changes cross the east–west extent of the basin, with MAM becoming the dominant season further east. To the north, the dry season is DJF and the bimodality is weak, with only a slight reduction evident in the boreal summer. In the northernmost region of the basin, in the latitude span of 5° to 10°N, the pattern becomes unimodal with a peak in the boreal summer, generally in August. In the southernmost region, in the latitude span of 5° to 10°S, the seasonality tends to be bimodal but with only one pronounced maximum.
3.8.3. Spatial and Temporal Variability
One of the very unusual characteristics of the rainfall regime over the Congo is the extremely low spatial variability. Elsewhere in Africa rainfall variability on interannual time scales is coherent over large areas. Over the Congo, variability is localized and the correlation between individual stations is exceedingly low in both rainy seasons.
Because of the paucity of gauge data in the Congo Basin, it is difficult to provide a detailed and reliable picture of the interannual variability of rainfall. Six sectors were examined, three of which fall largely within the Congo Basin. In most of the regions a shift to drier conditions occurred around 1970. It was apparent in MAM in all areas but the central Congo Basin, in annual rainfall in the northern and southern СКАЧАТЬ