Spatio-temporal patterns of rainfall trends and the 2015-2017 drought over the winter rainfall region of South Africa

We analyse long-term (1900-2017) rainfall data in the southern part of the winter rainfall region of southern Africa to understand the spatial patterns of recent and long-term trends and contextualize the 2015-2017 rainfall anomalies which led to the so-called "Day Zero" drought in Cape Town. Our analyses reveal cohesive spatial patterns and seasonal differences in rainfall trends across a range of timescales. These suggest that rainfall is subject to regional driving mechanisms, predominantly manifested at the 20-50 year timescale, but the influence of these mechanisms is modified by subregional and seasonally specific processes, frequently resulting in trends of different magnitudes and even sign. Trend patterns are consistent with multidecadal-scale quasi-periodicity, with only the most recent phase (post-1981 drying) corresponding to the expected regional response to hemispheric processes linked to anthropogenic climate change. The spatial and seasonal patterns of drying observed since 1981 alone do not explain the pattern of 2015-2017 drought anomalies, although they share a strong autumn and weak mid-winter signal. These results have implications to the interpretation of drought in the context of observed rainfall trends. Furthermore, we identify directions for improvement of the conceptual understanding of drivers of rainfall variability and the role of anthropogenic climate change in the winter rainfall region of South Africa.