Topographic modulation of drought propagation establishes low-elevation hotspots and mid-elevation climatic refugia in Southern Africa

Abstract

Drought dynamics across Southern Africa remain poorly understood, particularly regarding the topographic mechanisms that influence their spatial development and ecological consequences. Yet, the mechanisms by which local topography modulates drought propagation across elevation gradients, a key control on regional vulnerability, remain uncertain. We precisely quantify this critical mechanism. This study integrates high-resolution climate data (1950–2022) with future Standardized Precipitation Evapotranspiration Index (SPEI) projections to quantify three key aspects: 1) multi-year drought regimes, 2) topographic controls on drought propagation mechanisms, and 3) identification of hotspots of hydroclimatic vulnerability across various topographical complexities. We found the most severe long-term aridification occurs at higher elevations (2000–2500 m), where SPEI24 trends are strongest ( r = 0.6). In contrast, mid-elevation zones (1000–1500 m) displayed a persistent hydroclimatic buffer, exhibiting the slowest drought propagation rates and a decadal-scale shift toward shorter drought timescales, a pattern projected to intensify, with low-elevation zones accelerating toward more persistent drought conditions. This buffering effect, driven by orographic precipitation and reduced evaporative demand, creates distinct climatic refugia. Regionally, we identified a significant post-2000 expansion in the dominance of short-term droughts (SPEI01, SPEI03), with the fastest drought propagation in low-elevation and regional hotspots (e.g., Angola and Namibia). Our results demonstrated that drought development is fundamentally controlled by the interaction of timescale and topography. We, therefore, conclude that effective early warning systems must be spatially and temporally tailored by prioritizing rapid-onset agricultural alerts for lowlands while monitoring long-term hydrological deficits in high-elevation areas to mitigate ecological and societal consequences across Southern Africa.