A severe weather system accompanied by a stratospheric intrusion during unusual warm winter in 2015 over the South Africa: An initial synoptic analysis

Abstract

This study aims to highlight the abnormally warm winter experienced in South Africa during 2015. This work also investigates the synoptic and dynamic aspects of the specific severe weather system (late July 2015) which affected the western, southern, and eastern provinces including some parts in the Northern Cape, Western Cape, Lesotho, KwaZulu-Natal, and Eastern Cape. This is followed by an investigation of the stratospheric-tropospheric transport of the high-level ozone associated with a deep cut-off low weather system over the southern and central regions of South Africa which occurred on 24–25 July 2015. The case characteristics of the dynamical analysis of the weather system associated with a stratospheric-tropospheric intrusion was also determined. This research illustrated that this system was accompanied by deep upper and mid-tropospheric troughs which developed over the eastern part of the country. This resulted in tropopause folding controlled by the upper meandered tropospheric jet stream axis in the east of the upper and mid-tropospheric troughs. This caused the strong downward injection of stratospheric ozone-rich air with a maximum ozone value of 320 ppb at a level of 300 hPa observed using MERRA reanalysis data in the north of the North Cape province together with very dry higher tropospheric air at 300 hPa. In the lower troposphere, ozone levels as high as 61 ppb at 850 hPa were found within these intrusions in the northern-central region during the active cut-off low weather system. Using the Aura-Microwave Limb Sounder (MLS) to determine the vertical ozone profile at different pressure levels over South Africa indicated that a maximum value of approximately 270 ppb ozone was recorded on 24 July 2015. The observational measurements from this study show good agreement with the findings from the reanalysis models together with the results taken from the MERRA database for the specific day of peak ozone activity.