The Radiative Effects and Climatic Feedbacks of Anthropogenic Aerosols in South Africa

Abstract:

Using multi-year runs of coupled regional climate-chemistry model, the present study investigates the radiative effects and climatic feedbacks of Anthropogenic Aerosols (AAs) [i.e., sulphate, Black Carbon (BC), Organic Carbon (OC) and all together (SBO)] in South Africa. The results show that the eastern regions of South Africa experience higher loads of AAs and their Surface Radiative Forcing (SRF): up to 12.6 mg/m² [-16.8 W/m²] for sulphate and 16.9 mg/m² [-19.6 W/m²] for SBO, during austral summer, as well as, up to 1.2 mg/m² [-2.8 W/m²] for BC and 3.1 mg/m² [-1 W/m²] for OC during austral winter. Contrary to sulphate, both BC and OC aerosols reduce incoming solar radiation reaching the ground via enhancing shortwave radiative heating in the atmosphere (up to: BC ~ +3.7 W/m², OC ~ +0.46 W/m² and SBO ~ +4.5 W/m²). This is attributed to short-wave absorption primarily by BC and slightly by OC aerosols. These radiative perturbations of AAs instigate alterations on thermal and dynamical fields of the atmosphere. This resulted changes in background aerosol concentrations, hence promoted climate signals of AAs in areas far away from their main loading zones. The overall feedback of the climate system to the radiative effects of AAs resulted both positive and negative changes on Net Atmospheric radiative Heating Rate (NAHR: from -0.6 to +0.42 K/day). Generally, areas that experience a reduction in NAHR exhibited an increase in column integrated Cloud Cover (CC: up to ~ +8.4%). However, during the NAHR enhancement, CC over arid areas decreased and over the wet regions increased. The AAs induced statistically significant changes on surface temperature (from -0.5 to +0.55 K) and surface sensible heat flux (from -6.3 to +7 W/m²), are more closely correlated with AAs’ induced CC alterations than their direct radiative forcing. Moreover, the AAs radiative feedbacks induced convection process and thermodynamic adjustments also consequence statistically significant changes on boundary layer height (from -42 to +50 m), surface pressure (from -0.07 to +0.045 hPa) and surface wind fields. Overall, the present contribution underscores the importance of AAs radiative feedbacks on thermodynamic structure and cloud fields – which both play a far-reaching role in moderating other climatic anomalies.