Assessments for the Effect of Mineral Dust on the Spring Heat Waves in the Sahel

Abstract

The physical mechanisms associated with heat waves (HWs) are well known in the midlatitudes but still under-documented in the Sahel. Specifically, the role of anthropogenic and natural changes in tropospheric aerosols regarding HWs remains an issue to address. Our study focuses on the characterisation of the dusty HWs in the Sahel, which generally occur from March to June. The goal is to reinforce or invalidate the assumption proposed in previous studies recently carried out in southern Europe and according to which mineral dust may locally change irradiance at the surface, thus atmospheric temperatures at 2 m, intensifying the HW. The work is carried out in three steps: (i) detect and describe the HW over the 2003–2014 period based on maximum daily 2-m temperatures (T (Formula presented.)) from ERA-Interim reanalyses; (ii) characterise the dust optical properties during the HW using the Deep Blue aerosols products from MODIS (Moderate Resolution Imaging Spectroradiometre): the Aerosol Optical Depth at 550 nm (AOD (Formula presented.)), the Angstrom Exponent (AE (Formula presented.)) and the Single Scattering Albedo at 412 nm (SSA (Formula presented.)) as a proxy of quantity over atmospheric column, size and absorption of aerosols, respectively; (iii) relate HW intensity to the aerosol conditions during the HW. Over the 12-year study period, 14 HWs are detected when T (Formula presented.) exceeds the 90th percentile (P90). The HWs are dusty with AOD (Formula presented.) ranging between 0.46 and 1.17 and all the dust types are absorbent with a SSA (Formula presented.) value of 0.93 (round to hundredths). The HW classification according to aerosol conditions gave three HWs: Type 1 corresponds to Pure Dust Situation (PDS with AE (Formula presented.) = 0.1), Type 2 and Type 3 are associated with Mixed Situation (MS) with dominance of Coarse Particles (CP with AE (Formula presented.) = 0.35) and Fine Particles (FP with AE (Formula presented.) = 0.65), respectively. The main result obtained is that the intensity of the dusty HW, computed as the difference between daily T (Formula presented.) and its P90 (T (Formula presented.) −P90)), is higher for Type 1 HW (+1.1 °C) in the case of the most absorbent aerosol situation (SSA (Formula presented.) = 0.931). A non-significant difference between Type 2 and Type 3 especially for temperature (+0.5 °C and +0.4 °C, respectively) and SSA (0.938 and 0.935, respectively) is observed and, during these mixing situations, the HWs are less intense than those during the PDS. Finally, the analysis of two huge Type 1 HWs in 2007 and 2010 shows that dust mass concentrations at the surface were particularly high, up to 214 μg/m3 on average. These findings enable us to assess that highly absorbent and concentrated pure dust situations observed in spring in the Sahel may have a potential warming effect at the surface.