| 27 Nov 2025
On the Mechanisms that Control the Rainy Season Transition Periods in the Equatorial Congo Basin
Abstract. The Congo Basin equatorial region (2° S–2° N, within the watershed), experiences semi-annual rainy seasons (RSs) in boreal spring and fall. Previously, the mechanisms driving the transitions to these RSs have not been investigated systematically. We show that both RS transitions begin with increases in the low-level atmospheric moisture transport from the Atlantic Ocean into the region, ~1.5 months prior to the spring RS and ~2 months prior to the fall RS. Evapotranspiration contributes the most to atmospheric moisture but does not change significantly throughout both transition periods. Sharp precipitation jumps 10 days before the start of the spring and fall RSs result from boundary layer moisture increases orographically uplifted to the lower troposphere by the East African Rift and Congo Basin Cell ascending branch. This destabilizes the lower free troposphere, lowering the level of free convection and decreasing convective inhibitive energy. Meanwhile, the African Easterly Jet-North and westerly return flow of the Congo Basin Cell induce vertical shear for the spring and fall RSs, respectively. Mid-tropospheric convergence strengthens due to return flow at ~700 hPa from shallow meridional cells that direct low-level moisture towards the West African Heat Low prior to the spring RS, and to the Congo Air Boundary prior to the fall RS. Therefore, the RS onsets over the equatorial region are driven by seasonal changes in large-scale atmospheric circulation, contrasting with the pivotal role of increasing evapotranspiration in driving the transition to the RS onset over the southern Congo Basin.
This manuscript investigates the mechanisms controlling the transition into the equatorial Congo Basin’s biannual rainy seasons. It leverages satellite observations, water vapor isotopic data, and reanalysis to diagnose how moisture transport and atmospheric instability evolve in the weeks leading up to rainy season onsets. The topic is scientifically important and timely, as the processes initiating rainy seasons in this region have been poorly understood. The analysis is comprehensive and yields interesting results, notably that increased low-level Atlantic inflow and mid-tropospheric moistening appear to trigger the onset of deep convection in both spring and fall. Overall, this study provides important analyses, but several issues should be addressed to strengthen the manuscript.
Specific comments:
1. While the study is thorough, some findings (e.g., the importance of Atlantic moisture influx and mid-level moisture convergence for convective onset) are not entirely surprising in light of prior studies. For example, it was already recognized that near-surface divergence and mid-tropospheric moisture convergence play a role in equatorial rainfall (Nicholson 2018; Pokam et al., 2014). The manuscript would benefit from a discussion to demonstrate the unique contribution.
Nicholson, S. E. (2018). The ITCZ and the seasonal cycle over equatorial Africa. Bulletin of the American Meteorological Society, 99(2), 337-348.
Pokam, W. M., Bain, C. L., Chadwick, R. S., Graham, R., Sonwa, D. J., & Kamga, F. M. (2014). Identification of processes driving low-level westerlies in west equatorial Africa. Journal of Climate, 27(11), 4245-4262.
2. The study’s reliance on reanalysis and satellite datasets may raise concerns about data uncertainty. The equatorial Congo is data-sparse, and different reanalyses can diverge in their depiction of rainfall and moisture transport. The authors may acknowledge the uncertainties inherent in their data and methods, such as how sensitive are the moisture convergence patterns to the choice of reanalysis or to errors in satellite precipitation?
3. The manuscript examines many interacting processes, such as Atlantic inflow, the Congo Basin Cell, shifting heat lows, the Congo Air Boundary, AEJ-N/S jets, orographic uplift, etc. Is it possible to clearly distinguish the primary drivers of the rainy season onset from secondary or contextual factors? For instance, if low-level westerly inflow and subsequent mid-level moistening are the essential triggers, those should be highlighted as the key results.
4. A few terms and conceptual elements would benefit from clearer definition, such as the shallow meridional overturning cell. Providing these definitions up front would make the physical interpretation clearer and help readers follow the proposed mechanism.