Читать книгу Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов - Страница 47

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.