Hydrodynamics of regional and seasonal variations in Congo Basin precipitation

Abstract

The processes that determine the seasonality of precipitation in the Congo Basin are examined using the atmospheric column moisture budget. Studying the fundamental determinants of Congo Basin precipitation seasonality supports process-based studies of variations on all time scales, including those associated with greenhouse gas-induced global warming. Precipitation distributions produced by the ERA5 reanalysis provide sufficient accuracy for this analysis, which requires a consistent dataset to relate the atmospheric dynamics and moisture distribution to the precipitation field. The Northern and Southern Hemisphere regions of the Congo Basin are examined separately to avoid the misconception that Congo Basin rainfall is primarily bimodal. While evapotranspiration is indispensable for providing moisture to the atmospheric column to support precipitation in the Congo Basin, its seasonal variations are small and it does not drive precipitation seasonality. During the equinoctial seasons, precipitation is primarily supported by meridional wind convergence in the moist environment in the 800-500 hPa layer where moist air flows into the equatorial trough. Boreal fall rains are stronger than boreal spring rains in both hemispheres because low-level moisture divergence develops in boreal spring in association with the developing Saharan thermal low. The moisture convergence term also dominates the moisture budget during the summer season in both hemispheres, with meridional convergence in the 850-500 hPa layer as cross-equatorial flow interacts with the cyclonic flow about the Angola and Sahara thermal lows. Winter precipitation is low because of dry air advection from the winter hemisphere subtropical highs over the continent.

Divergent Representation of Precipitation Recycling in the Amazon and the Congo in CMIP6 Models

“…The correspondence between E estimates from catchment‐balance, reanalyzes and CMIP6 models shown in Figure 3e suggests that the simulated seasonal cycle in Congo E is relatively robust. E, which might be expected to positively correlate with ρ , broadly follows the same seasonal cycle as precipitation in the Congo (Figure S4b in Supporting Information S1 ), providing essential atmospheric moisture without being the primary driver of precipitation seasonality (Cook & Vizy, 2021 ). An imbalance in E between the two rainfall peaks has been linked to a difference in solar radiation (Burnett et al., 2020 ; Crowhurst et al., 2021 ; Crowhurst et al., 2020 ).…”

Processes behind the decrease in Congo Basin precipitation during the rainy seasons inferred from ERA‐5 reanalysis

“…Notably, the highest rainfall values occur in November, which differs from the pattern observed in the northern part, where they peak in October. Simultaneously, when examining the seasonality of precipitation across the range of 5 S to 5 N (Figure 1c), there is an apparent, though erroneous (Cook & Vizy, 2021), suggestion of a bimodal distribution. A spatial analysis (Figure 2) reveals maximum rainfall in both the eastern and western regions of the Congo Basin during MAM and SON, with ERA5 reanalysis values (Figure 2a,b) showing a slight increase compared to CHIRPS (Figure 2c,d).…”

“…This comparison focuses on the primary rainy seasons of March-April-May (MAM) and September-October-November (SON). The decision to divide the Congo Basin into various latitudinal levels is informed by prior research (Cook & Vizy, 2021), which demonstrated that this region does not exhibit a clear bimodal rainfall distribution. Specifically, in the Northern Hemisphere latitudinal band of 0 -5 N (Figure 1a), two distinct rainfall maxima are observed in April and October.…”

On the Mechanisms that Control the Rainy Season Transition Period in the Southern Congo Basin

“…Finally, we calculate the moisture convergence term (-𝑞 × 𝛻. 𝑉) and moisture transport vectors (𝑞𝑉) at four pressure levels to represent near surface, lower-troposphere, mid-troposphere, and upper troposphere, respectively: 925 hPa, 850 hPa, 700 hPa, and 650 hPa, similar to that in Cook and Vizy (2022) and Nicholson (2022). This is because moisture transport varies substantially between near surface atmosphere, lower troposphere, and middle troposphere, including its source regions.…”

“…Previous studies point to atmospheric hydrodynamics as key controlling factors in determining seasonal rainfall (e.g., Cook and Vizy, 2022). Low-level divergence within the equatorial region during the rainy seasons shows that the Intertropical Convergence Zone does not play a strong role in determining the seasonal migration of rainfall (Nicholson, 2018, Yang et al, 2015.…”