Fig. 5 Relationships between wind erosion and mean annual maximum wind speed in southern Africa from 1991 to 2015. (a) Temporal changes in mean annual maximum wind speed and (b) spatial patterns of the partial correlations between wind erosion and mean annual maximum wind speed.

4. Discussion

4.1 Climate dynamics in southern Africa

Since the 1870s, the increase in atmospheric water demand and changes to atmospheric circulation patterns have led to more frequent droughts in Africa (Dai, 2011). In particular, since 1980, the area suffering from severe drought has continued to increase (Rouault & Richard, 2005), and the irrational utilization of limited natural resources and population growth will further aggravate the risk of drought in the future (Ahmadalipour, Moradkhani, Castelletti, & Magliocca, 2019). Southern Africa was subjected to severe droughts in 2003 and 2007 (Rouault & Richard, 2005; Winkler, Gessner, & Hochschild, 2017); and serious drought events have occurred in local areas (Masih, Maskey, Mussá, & Trambauer, 2014), such as Namibia in 1995, South Africa in 1995 and 2003, and Lesotho and Eswatini in 2007. These events may have led to greater soil erosion across southern Africa in these years. Southern Africa has a noticeable rising temperature trend with the minimum temperature rising higher than the maximum temperature, especially in Namibia (Collins, 2011; Hulme et al., 2001). There are great uncertainties surrounding the multi-year rainfall intensity in southern Africa, but the average precipitation in the region has significantly decreased, the rainy season has shortened, and the rainfall intensity has weakened (Kusangaya et al., 2014; Nicholson, 2001a, 2001b). Over the last 30 years, the near-surface wind speed in southern Africa has shown a downward trend (Horton, Skinner, Singh, & Diffenbaugh, 2014; Torralba, Doblas-Reyes, & Gonzalez-Reviriego, 2017; Wu, Zha, Zhao, & Yang, 2018), and aerosol emissions have decreased the near-surface wind speed in sub-Saharan Africa by 0.05 m/s (Bichet, Wild, Folini, & Schär, 2012). During 1993–2010, the data derived from meteorological stations in the South Africa showed an average decrease in near-surface wind speed of 0.21 m/s/decade (Kruger, Goliger, Retief, & Sekele, 2010), which may be one of the reasons for the decrease in annual average soil erosion in southern Africa. In addition, vegetation productivity and desertification in sub-Saharan Africa may be influenced by global climate change, which is partly caused by the North Atlantic Oscillation and the El Niño Southern Oscillation (ENSO) (Oba, Post, & Stenseth, 2001). For example, in 2002, 2003, and 2015, the ENSO led to drought in southern Africa (Gizaw & Gan, 2017; Winkler et al., 2017), which was probably further linked to soil wind erosion.

4.2 Relationship between vegetation and wind erosion

Temperature and precipitation can have a direct impact on soil wind erosion. However, vegetation growth, which is dominated by the temporal and spatial water and heat patterns, can also indirectly affect soil wind erosion (Miao, Yang, Chen, & Gao, 2012). A correlation analysis between the average annual fractional vegetation cover (FVC), annual temperature, and annual precipitation showed that in southern Africa, temperature and FVC were negatively correlated across 91.8% of the total area (Fig. 6a). However, in most areas (approximately 88.3% of the total area), FVC had a positive correlation with precipitation (Fig. 6b). The correlations between different FVC values, and temperature and precipitation were also different (Figs. 6c, 6d). When the FVC is below 0.4: the negative correlation between FVC and temperature increases. In contrast, when FVC is greater than 0.4: the influence of temperature on FVC decreases as FVC increases. For example, the Kalahari Basin, which is in the middle of the study area and sparsely covered by shrubs and herbs, has a vegetation coverage of between 0.2 and 0.4 (Fig. 7a). The negative correlation between FVC and temperature and the positive correlation with precipitation were clearly more robust than in other regions (Figs. 6a, 6b). The high temperatures and reduced rainfall make this area hotter and drier, which reduces vegetation growth. In contrast, an increase in precipitation promotes the growth of vegetation. However, in the southern coastal area with a marine climate, the rise in temperature under warm and humid climate conditions increases FVC, but the increase in precipitation actually inhibits vegetation growth.