Preprint Article Version 1 This version is not peer-reviewed

Stratosphere–Troposphere Exchange and O3 Decline in the Lower Stratosphere over Irene SHADOZ Site, South Africa

Version 1 : Received: 17 February 2020 / Approved: 18 February 2020 / Online: 18 February 2020 (11:25:41 CET)

How to cite: Mkololo, T.; Mbatha, N.; Venkataraman, S.; Begue, N.; Coetzee, G.; Labuschagne, C. Stratosphere–Troposphere Exchange and O3 Decline in the Lower Stratosphere over Irene SHADOZ Site, South Africa. Preprints 2020, 2020020268 (doi: 10.20944/preprints202002.0268.v1). Mkololo, T.; Mbatha, N.; Venkataraman, S.; Begue, N.; Coetzee, G.; Labuschagne, C. Stratosphere–Troposphere Exchange and O3 Decline in the Lower Stratosphere over Irene SHADOZ Site, South Africa. Preprints 2020, 2020020268 (doi: 10.20944/preprints202002.0268.v1).

Abstract

This study aims to investigate the Stratosphere-Troposphere Exchange (STE) events and ozone trends over Irene (25.5°S, 28.1°E). Twelve years of ozonesondes data (2000–2007, 2012–2015) from Irene station operating in the framework of the Southern Hemisphere Additional Ozonesodes (SHADOZ) was used to study the troposphere (0–16 km) and stratosphere (17– 28 km) ozone (O3) vertical profiles. Ozone profiles were grouped into three categories (2000–2003, 2004–2007 and 2012–2015) and average composites were calculated for each category. Fifteen O3 enhancement events were identified over the study period. These events were observed in all seasons (one event in summer, four events in autumn, five events in winter and five events in spring), however, they predominantly occur in winter and spring. The STE events presented here are observed to be influenced by the Southern Hemisphere polar vortex. During the STE events, the advected potential vorticity maps assimilated using Modélisation Isentrope du transport Méso–échelle de l’Ozone Stratosphérique par Advection (MIMOSA) model for the 350 K (~12–13 km) isentropic level indicated a transport of high latitude air masses which seems to be responsible for the reduction of the O3 mole fractions at the lower stratosphere over Irene which takes place at the same time with the enhancement of ozone in the upper troposphere. In general, the stratosphere is dominated by higher Modern Retrospective Analysis for Research Application (MERRA-2) potential vorticity (PV) values compared to the troposphere. However, during the STE events, higher PV values from the stratosphere were observed to intrude the troposphere. Ozone decline was observed from 12 km to 24 km with highest decline occurring from 14 km to 18 km. An average decrease of 6.0 and 9.1% was calculated from 12 to 24 km in 2004–2007 and 2012–2015 respectively. The observed decline occurred in the upper troposphere and lower stratosphere with winter and spring showing more decline compared with summer and autumn.

Subject Areas

ozone enhancement; Irene; ozone decline; potential vorticity; ozonesondes

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