South African winter rainfall zone shifts: A comparison of seasonality metrics for Cape Town from 1841–1899 and 1933–2020

Abstract

Mounting evidence across South Africa's southwestern winter rainfall zone (WRZ) re ects consistent drying since ~1980 and projected trends suggest this will continue. However, limited evidence exists for the region's rainfall seasonality changes. To improve our understanding of these WRZ drying trends, especially within the context of Cape Town's 2015-2017 "Day Zero" drought, it is necessary to explore long-term rainfall seasonality trends. Thus, we use the longest WRZ meteorological record from the South African Astronomical Observatory (SAAO) in Cape Town to investigate rainfall seasonality shifts during 1841-2020. Consistent with recorded poleward migrations of the subtropical high-pressure belt and mid-latitude westerlies, known drivers behind the drought and drying trends, calculated trends demonstrate strengthening of WRZ conditions, primarily from a later start-date trend leading to a shorter wet-season. Long-term drying trends are quanti ed for the wet-and dry-seasons, however, analysis of trend evolution reveals much variability, re ecting that drying has only persisted since ~1892. Comparative analyses of the rst and last 59 years of 1841-2020 reveals a rainfall decline of ~10% across both seasons -highlighting that the extreme "Day Zero" drought was not only driven by wet-season rainfall declines. Results demonstrate that these drying trends were consistently driven by a long-term decline in rain day counts and a more recent decline in average rainfall per rain day. Correspondence between our results and projected rainfall seasonality trends suggests the trends we quanti ed will likely continue, thus improvements and continuation of existing water conservation and management strategies are imperative for Cape Town. IntroductionThe southwestern region of South Africa's winter rainfall zone (WRZ; Fig. 1) experienced below-average rainfall during the 2015-2017 April-September winter wet-seasons, which resulted in the worst drought and water shortages across the region since 1904 (Botai et al. 2017;Wolski 2018). These de cits, which were most pronounced for the autumn (March-May) and spring transition (September-November) seasons, led to supply dam water levels dropping to ~ 20% capacity during May 2018 (Burls et al. 2019;Pascale et al. 2020). For the ~ 4 million inhabitants of the City of Cape Town, this culminated in an imminent threat that dam levels would fall below ~ 10%, marking the level at which the city's municipal water supply would have been disconnected, before the 2018 winter wet-season (Sousa et al. 2018). The fear surrounding this gathered much attention, with media and municipal authorities terming this event "Day Zero", and forced the city to enforce strict water restrictions, from pre-drought levels of ~ 200L per person per day to 50L per person per day (Muller 2018; Wolski 2018). Fortunately, "Day Zero" was averted as good early winter rains and near-average rainfall for the 2018 winter season led to dam levels rising to ~ 70% by October 2018 (Sousa et al. 2018;Burls et al. 2019). This c...