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

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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.
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RESEARCH ARTICLE
Spatio-temporal patterns of rainfall trends and the
20152017 drought over the winter rainfall region of
South Africa
Piotr Wolski | Stefaan Conradie | Christopher Jack | Mark Tadross
Climate System Analysis Group,
University of Cape Town, Cape Town,
South Africa
Correspondence
Piotr Wolski, Climate System Analysis
Group, University of Cape Town, P. Bag
X3, 7701 Rondebosch, Cape Town, South
Africa.
Email: wolski@csag.uct.ac.za
Funding information
RSA National Research Foundation
Abstract
We analyse long-term (19002017) 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 20152017 rainfall anoma-
lies which led to the so-called Day Zerodrought 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 2050 year timescale,
but the influence of these mechanisms is modified by subregional and season-
ally 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 anthropo-
genic climate change. The spatial and seasonal patterns of drying observed
since 1981 alone do not explain the pattern of 20152017 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.
KEYWORDS
drought, Mediterranean climate, rainfall trend
1|INTRODUCTION
In the years of 20152017, the southwestern region of
southern Africa experienced a rainfall deficit, the severity
of which was estimated to be a 1 in 100 to 1 in 300 year
event (Otto et al., 2018; Wolski, 2018). That anomaly trig-
gered a water shortage in Cape Town water supply reser-
voirs, and a water crisis that culminated with the
imminent threat of a shut-down of supply to the roughly
4 million inhabitants of that city. That event gained
worldwide attention from the media and has come to be
known as the Day Zerodrought. The crisis highlighted
the vulnerability of Cape Town (and also likely many cities
in the world) that result from the interplay of four factors:
rapid urban population growth, economic policy, infra-
structural and water resource development and constraints,
and anthropogenic climate change (Scott et al., 2019).
Climatic factors underlying the 20152017 drought
have been the subject of several studies. Botai et
al. (2017) mapped the rainfall anomalies in the Western
Cape region of South Africa (where Cape Town is
located) using the standardized precipitation index. Sousa
Received: 20 November 2019 Revised: 16 July 2020 Accepted: 21 July 2020 Published on: 19 August 2020
DOI: 10.1002/joc.6768
Int J Climatol. 2021;41 (Suppl. 1):E1303E1319. wileyonlinelibrary.com/journal/joc © 2020 Royal Meteorological Society E1303
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