Abstract
The West African Monsoon (WAM) is a complex system depending on global climate influences and multiple regional environmental factors. Central and Southern African biomass-burning (SABB) aerosols have been shown to perturb WAM during episodic northward inter-hemispheric transport events, but a possible dynamical connection between the core of the SABB aerosol outflow and the WAM system remains unexplored. Through regional climate modeling experiments, we show that SABB aerosols can indeed impact WAM dynamics via two competitive regional scale and inter-hemispheric dynamical feedbacks originating from (i) enhanced diabatic heating occurring in the Southeastern Atlantic low-cloud deck region, and (ii) aerosol and cloud-induced sea surface temperature cooling. These mechanisms, related to aerosol direct, semi-direct, and indirect effects, are shown to have different seasonal timings, resulting in a reduction of June to September WAM precipitation, while possibly enhancing late-season rainfall in WAM coastal areas.
Highlights
The understanding of climate variability in West Africa is of prime importance for regional socioeconomic sustainability, especially in the context of climate change and population growth
The conclusions of this experiment are that i) the southern Africa biomass-burning (SABB) aerosol diabatic warming and the Aerosol Diabatic Heating Feedback (ADHF) triggered in the South East Atlantic (SEA) outflow, which depend on a lowlevel cloud deck presence and feedback, are of prime importance for the impact on West African Monsoon (WAM), and ii) that the continental SABB aerosol radiative forcing, while likely inducing the dry anomaly over southern Africa through increased stability[7], might only be a secondary contributor to the drying obtained over the WAM domain
We describe mechanisms by which SABB aerosols are likely to modulate the WAM dynamics and precipitation over southern West Africa
Summary
The understanding of climate variability in West Africa is of prime importance for regional socioeconomic sustainability, especially in the context of climate change and population growth. Recent studies have pointed out that a fraction of SABB particles, once northwardly advected over West Africa, could contribute to impacting the local climate via cloud interactions[8] or modification of local atmospheric stability[9,10], Here, we show that SABB can have an important effect on WAM through a regional inter-hemispheric dynamical feedback originating from the highly concentrated SABB aerosol outflow located over the South Eastern Atlantic (SEA) ocean (Fig. 1, region A). The low-level cloud feedback mostly results from the diabatic heating produced by shortwave absorption in the SABB aerosol layer, materialized by extinction contours in Fig. 1b, in relationship with a positive DE above the bright low-level cloud deck
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