The Cape Town “Day Zero” drought and Hadley cell expansion

Abstract

In early 2018, Cape Town (population ~3.7 million) was at risk of being one of the first major metropolitan areas in the world to run out of water. This was due to a severe multi-year drought that led to the levels of supply dams falling to an unprecedented low. Here we analyze rainfall data from the city catchment areas, including rare centennial records from the surrounding region, to assess the severity of the 2015–2017 drought. We find that there has been a long-term decline in the number of winter rainfall days, but this trend has been generally masked by fluctuations in rainfall intensity. The recent drought is unprecedented in the centennial record and represents a combination of the long-term decline in rainfall days and a more recent decline in rainfall intensity. Cold fronts during the winter months are responsible for most of the rainfall reaching Cape Town and our analysis shows no robust regional trend in the number of fronts over the last 40 years. Rather, the observed multidecadal decline in rainfall days, which threatens to increase the occurrence of severe drought, appears to be linked to a decrease in the duration of rainfall events associated with cold fronts. This change in rainfall characteristics associated with fronts appears to be linked to Hadley Cell expansion seen across the Southern Hemisphere and an increasing trend in post-frontal high-pressure conditions that suppress orographically enhanced rainfall.

How will southern hemisphere subtropical anticyclones respond to global warming? Mechanisms and seasonality in CMIP5 and CMIP6 model projections

“…Doyle and Barros 2002 ; Reboita et al 2010 ), Southern Africa (e.g. Burls et al 2019 ). Understanding how SAs will respond under global warming condition is a crucial element of future climate projections for these regions.…”

“…Recent studies, using observed and reanalysis datasets, show that there is a significant SLP trend in recent decades in the SH subtropical regions (Gillett et al 2003 ; Gillett and Stott 2009 ; Grise et al 2018 ; Staten et al 2018 ; Vizy et al 2018 ; Burls et al 2019 ). The area and intensity of the SH SAs have increased in strength and extended towards the pole during both summer and winter seasons.…”

Drought in the Eastern Cape region of South Africa and trends in rainfall characteristics

“…5 of Blamey et al 2018a ). In February 2018, the Western Cape Province was declared a disaster area after a severe drought which occurred between 2015 and 2018 (Pienaar and Boonzaaier 2018 ; Sousa et al 2018 ; Mahlalela et al 2019 ; Burls et al 2019 ). In October 2019, the Eastern Cape Province (see Fig.…”

A SOM‐based analysis of the drivers of the 2015–2017 Western Cape drought in South Africa

“…Also, Burls et al . (2019) linked the drought to a long‐term decrease in rainfall events during cold fronts over the Western Cape region. They associated these changes in rainfall characteristics to Hadley Cell expansion over the Southern Hemisphere and to the upward trend in high‐pressure conditions in days following the passage of a frontal system.…”