Sahel's Rainfall Variability: Aerosol Emissions

The anthropogenic and volcanic aerosols emissions also play significant roles in Sahel's rainfall variability. 

As mentioned in the overview, the Intertropical Convergence Zone (ITCZ) location greatly controls Sahel's seasonal rainfall. Hwang et al. (2013) found that sulphate aerosol-induced cooling in the Northern Hemisphere during the late 20th century shifted the tropical rain belt southward and contributed to the profound 1970s-1980s Sahel drought. As shown in Figure 1, results from different models all show that there was drying on the northern side of the tropical rain belt and wetting on the southern side in the late 20th century. 

Figure 1. Time series of zonal mean precipitation anomaly relative to the 20th-century mean based on a) the Global Historical Climatology Network (GHCN) gridded products, b) the 20th-century reanalysis project (20CR), and c) the ensemble mean of the 20th-century climate simulation from 14 GCMs in CMIP3 and 12 GCMs in CMIP5 (Hwang et al., 2013). 

Sulphate aerosols can induce a cooling of the Earth's surface directly and indirectly. The direct radiative effect is through scattering incoming solar radiation back to space. The sulphate aerosol particles can also act as could condensation nuclei, creating more but smaller could droplets, which increase the cloud surface area and subsequently the cloud albedo. An increase in albedo means more reflection of incoming solar radiation and cooling of the Earth's surface (Myhre, Samset & Storelvmo, 2013). 

In the second half of the 20th century, especially during the 1970s-1990s, massive sulphate aerosols were emitted in North America and Western Europe due to rapid industrial developments. This led to a cooling of the Northern Hemisphere relative to the Southern Hemisphere because aerosol particles have a very short lifetime of 1-2 weeks, the cooling effect is concentrated locally. Baroclinic eddies spread this cooling throughout the Northern Hemisphere tropics. To keep the tropospheric temperature gradients within the tropic relatively flat, an abnormal Hadley circulation is produced to transport energy from the Southern Hemisphere to the Northern Hemisphere. And because the lower troposphere contains most of the water vapour, this abnormal Hadley circulation also conveys an anomalous southerly moisture flow causing the tropical rain belt to migrate southward (Figure 2; Hwang et al., 2013).

Figure 2. Mechanism of the southward shift of tropical rain belt due to sulphate aerosols induced cooling of the Northern Hemisphere (Hwang et al., 2013) 

Since the 1990s, Europe and North America have begun to regulate aerosol emissions to control their air quality due to the adverse health impacts of aerosols. The implementation of the Clean Air Act has reduced Northern Hemisphere aerosol emissions significantly. Several studies show that this reduction has subsequently shifted the ITCZ northward and is related to the partial recovery of Sahel's rainfall since the early 1990s (such as Allen, 2015 and Giannini & Kaplan, 2018). 

There are also other studies such as Haywood et al., 2013 and Iles & Hegerl, 2014 examining the effect of volcanic aerosols on the position of ITCZ and the Sahel's rainfall variability, and they all suggest that the volcanic eruptions in the Northern Hemisphere lead to the southward migration of ITCZ. Haywood et al. (2013) also related three of the four driest summers in the Sahel with volcanic eruptions in the Northern Hemisphere.