Cape storm: A dynamical study of a cut-off low and its impact on South Africa

Abstract

An intense surface low pressure system and associated cut-off low (COL) that affected the south-western Cape of South Africa on 6–7 June 2017 are analysed. The storm, locally named Cape Storm, was associated with heavy rains, strong winds, runaway fires, storm surge and extremely large waves. These extreme meteorological conditions resulted in a loss of life and damage to infrastructure in various forms around the province. The societal impacts that occurred have been collected and summarized in this study. Cape Storm was tracked back to its initial development as a COL extension to a surface low pressure system to the south-west of South America. ERA-Interim reanalysis is analysed from a potential vorticity (PV) perspective to assess the factors that influenced the development of this impactful storm. The analysis found that the system was associated with a Rossby wave breaking (RWB) event and an intrusion of high-potential vorticity (PV) from the stratosphere, assisting the development of the surface low pressure system. The COL migrated across the South American continent and made its way across the South Atlantic Ocean. As it migrated towards South Africa, two more COLs associated with RWB events and stratospheric intrusions maintained and deepened the surface low pressure system, increasing its eventual impact on South Africa. The surface cyclone deepening events were both associated with high-PV in both the upper to mid-levels and lower levels of the troposphere. One of these events was associated with a deep, tightly wrapped, cyclonic intrusion of high-PV air into the troposphere and higher PV values in the lower tropospheric, resulting in rapid and intense surface cyclogenesis.