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) reflects 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 quantified for the wet- and dry-seasons, however, analysis of trend evolution reveals much variability, reflecting that drying has only persisted since ~1892. Comparative analyses of the first 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 quantified will likely continue, thus improvements and continuation of existing water conservation and management strategies are imperative for Cape Town.