Abstract
Abstract The eastward shift of semi-arid climate across South Africa is studied using satellite assimilated cloud cover, vegetation temperature and potential evaporation 1981–2019, and 21st century coupled model projections. Semi-arid thresholds over the plateau have shifted hundreds of kilometers eastward in the Vaal River catchment for potential evaporation, cloud fraction, and vegetation temperature. Coastal cloudiness has also changed due to sea breezes modified by shelf zone sea temperatures. Processes underlying the spread of semi-arid conditions across South Africa are quantified. Desiccation is related to greater westerly airflow, as the atmospheric boundary layer over the Kalahari preferentially links with the upper-level circulation. Warm dry spells and climate change enhance the meridional temperature gradient and accelerate the sub-tropical jet at both short- and long timescales. According to observations and reanalysis, dry westerlies prevail during the afternoon and induce +0.2 °C/year trends in vegetation temperature over the Highveld during the study period. Coupled model projections show that semi-arid conditions expand eastward from Bloemhof (25.5°E) by 50,000 km2, altering future adaptation strategies.
Highlights
The semi-arid climate of South Africa can be traced to high elevation, a sub-tropical latitude, and the inflow of dry air from the South Atlantic (Vigaud et al 2009) which is prominent during winter and in the Pacific El Niño phase
The results progress from climatology and trend maps to shifts in semi-arid thresholds across South Africa, to the underlying processes involving diurnal advection and observed trends, and to hydrology projections and inter-comparisons
Mayaud et al (2017) found that changes in vegetation cover in 21st century projections depend on land use
Summary
The semi-arid climate of South Africa can be traced to high elevation, a sub-tropical latitude, and the inflow of dry air from the South Atlantic (Vigaud et al 2009) which is prominent during winter and in the Pacific El Niño phase. Moisture is locally dissipated via evaporative fluxes compounded by anthropogenic stress (Chikoore & Jury 2010). Understanding shifts in climate can forewarn of socio-economic consequences that could be mitigated by adaptive resource management (Tyson & Crimp 1998; Scholes et al 2002; Ringrose et al 2003). Past research has found trends of ∼0.02 °C/ year in air temperatures (Jury 2018) consistent with global averages, but regional studies on evaporative stress under climate change are limited. It is conjectured that the Kalahari Desert is spreading eastward into agriculturally productive lands, threatening livelihoods and resources in southern Africa (Tyson & Crimp 1998), presenting a motivation for this research. Coupled model intercomparison project (CMIP) resources (Taylor et al 2011) enable long-range projections of climatic shifts as a basis for informed decisions on mitigating actions
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