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 quasiperiodicity, 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 midwinter 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.
Abstract. A renewed focus on southern Africa's winter rainfall zone (WRZ) following the Day Zero drought and water crisis has not shed much light on the spatial patterns of its rainfall variability and climatological seasonality. However, such understanding remains essential in studying past and potential future climate changes. Using a dense station network covering the region encompassing the WRZ, we study spatial heterogeneity in rainfall seasonality and temporal variability. These spatial patterns are compared to those of rainfall occurring under each ERA5 synoptic-scale wind direction sector. A well-defined “true” WRZ is identified with strong spatial coherence between temporal variability and seasonality not previously reported. The true WRZ is composed of a core and periphery beyond which lies a transition zone to the surrounding year-round rainfall zone (YRZ) and late summer rainfall zone. In places, this transition is highly complex, including where the YRZ extends much further westward along the southern mountains than has previously been reported. The core receives around 80 % of its rainfall with westerly or north-westerly flow compared to only 30 % in the south-western YRZ incursion, where below-average rainfall occurs on days with (usually pre-frontal) north-westerly winds. This spatial pattern corresponds closely to those of rainfall seasonality and temporal variability. Rainfall time series of the core and surroundings are very weakly correlated (R2<0.1), also in the winter half-year, implying that the YRZ is not simply the superposition of summer and winter rainfall zones. In addition to rain-bearing winds, latitude and annual rain day climatology appear to influence the spatial structure of rainfall variability but have little effect on seasonality. Mean annual rainfall in the true WRZ exhibits little association with the identified patterns of seasonality and rainfall variability despite the driest core WRZ stations being an order of magnitude drier than the wettest stations. This is consistent with the general pattern of near homogeneity within the true WRZ, in contrast to steep and complex spatial change outside it.
Abstract. The 2014–2018 drought over South Africa's winter rainfall zone (WRZ) created a critical water crisis which highlighted the region's drought and climate change vulnerability. Consequently, it is imperative to better understand the climatic characteristics of the drought in order to inform regional adaptation to projected climate change. In this paper we investigate the spatio-temporal patterns of drought intensity and the recent rainfall trends, focusing on assessing the consistency of the prevailing conceptual model of drought drivers with observed patterns. For this we use the new spatial subdivision for the region encompassing the WRZ introduced in our companion paper (Conradie et al., 2022). Compared to previous droughts since 1979, the 2014–2018 drought in the WRZ core was characterised by a markedly lower frequency of very wet days (exceeding the climatological 99.5th percentile daily rainfall – including dry days) and of wet months (SPI1>0.5), a sub-seasonal attribute not previously reported. There was considerable variability in the spatial footprint of the drought. Short-term drought began in the south-western core WRZ in spring 2014. The peak intensity gradually spread north-eastward, although a spatially near-uniform peak is seen during mid-2017. The overall drought intensity for the 2015–2017 period transitions radially from most severe in the WRZ core to least severe in the surroundings. During 2014 and 2015, the drought was most severe at those stations receiving the largest proportion of their rainfall from westerly and north-westerly winds; by 2018, those stations receiving the most rain from the south and south-east were most severely impacted. This indicates an evolving set of dynamic drivers associated with distinct rain-bearing synoptic flows. No evidence is found to support the suggestion that the drought was more severe in the mountain catchments of Cape Town's major supply reservoirs than elsewhere in the core nor that rain day frequency trends since 1979 are more negative in this subdomain. Rainfall and rain day trend rates also exhibit some connections to the spatial seasonality structure of the WRZ, although this is weaker than for drought intensity. Caution should be applied in assessing South African rain day trends given their high sensitivity to observed data shortcomings. Our findings suggest an important role for zonally asymmetric dynamics in the region's drought evolution. This analysis demonstrates the utility of the spatial subdivisions proposed in the companion paper by highlighting spatial structure in drought intensity evolution linked to rainfall dynamics.
Phillip Mukwenha for assistance with software, hardware and data Chris Jack & Pierre-Louis Kloppers for data formatting & cleaning Birgit Erni for assistance with gap-filling techniques South African Weather Service (SAWS) for providing long-term station records for many locations; City of Cape Town & SA Dept. of Water and Sanitation (DWS), for additional station data Financial assistance towards the PhD research project from the SA National Research Foundation (NRF) and CSAG CSAG and UCT PGFO for sponsoring attendance at this conference Conclusion: Circulation Contribution to Drought Most rainfall in extended winter associated strong troughs in mid-tropospheric westerlies Circulation type frequency variation explains 2−36% of rainfall variability across WRZ Explanatory power greater in recent years (50%) ≈ 55% of Day Zero Drought shortfall explained by trough/ridge variability & trend. Unprecedented autumn (AM) frequency of most intense ridge node during severe drought year 2017, consistent with increasing trend (p < 0.01) over 39 yrs
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