Stratospheric ozone intrusion events and their impacts on tropospheric ozone in the Southern Hemisphere

Jesse W Greenslade,Robyn Schofield,Andrew K Klekociuk,Simon P Alexander,Jenny A Fisher

doi:10.5194/acp-17-10269-2017

Abstract

Abstract. Stratosphere-to-troposphere transport (STT) provides an important natural source of ozone to the upper troposphere, but the characteristics of STT events in the Southern Hemisphere extratropics and their contribution to the regional tropospheric ozone budget remain poorly constrained. Here, we develop a quantitative method to identify STT events from ozonesonde profiles. Using this method we estimate the seasonality of STT events and quantify the ozone transported across the tropopause over Davis (69° S, 2006–2013), Macquarie Island (54° S, 2004–2013), and Melbourne (38° S, 2004–2013). STT seasonality is determined by two distinct methods: a Fourier bandpass filter of the vertical ozone profile and an analysis of the Brunt–Väisälä frequency. Using a bandpass filter on 7–9 years of ozone profiles from each site provides clear detection of STT events, with maximum occurrences during summer and minimum during winter for all three sites. The majority of tropospheric ozone enhancements owing to STT events occur within 2.5 and 3 km of the tropopause at Davis and Macquarie Island respectively. Events are more spread out at Melbourne, occurring frequently up to 6 km from the tropopause. The mean fraction of total tropospheric ozone attributed to STT during STT events is ∼ 1. 0–3. 5 % at each site; however, during individual events, over 10 % of tropospheric ozone may be directly transported from the stratosphere. The cause of STTs is determined to be largely due to synoptic low-pressure frontal systems, determined using coincident ERA-Interim reanalysis meteorological data. Ozone enhancements can also be caused by biomass burning plumes transported from Africa and South America, which are apparent during austral winter and spring and are determined using satellite measurements of CO. To provide regional context for the ozonesonde observations, we use the GEOS-Chem chemical transport model, which is too coarsely resolved to distinguish STT events but is able to accurately simulate the seasonal cycle of tropospheric ozone columns over the three southern hemispheric sites. Combining the ozonesonde-derived STT event characteristics with the simulated tropospheric ozone columns from GEOS-Chem, we estimate STT ozone flux near the three sites and see austral summer dominated yearly amounts of between 5. 7 and 8. 7 × 1017 molecules cm−2 a−1.

Highlights

Tropospheric ozone constitutes only 10 % of the total ozone column but is an important oxidant and greenhouse gas which is toxic to life, harming natural ecosystems and reducing agricultural productivity
We find that the mean ozone flux associated with Stratosphere-to-troposphere transport (STT) events is ∼ 0.5−2.0×1016 molecules cm−2
Stratosphere-to-troposphere transport can be a major source of ozone to the remote free troposphere, but the occurrence and influence of STT events remains poorly quantified in the southern extratropics

Summary

Introduction

Tropospheric ozone constitutes only 10 % of the total ozone column but is an important oxidant and greenhouse gas which is toxic to life, harming natural ecosystems and reducing agricultural productivity. Greenslade et al.: Southern Hemisphere stratospheric ozone intrusions important in the upper troposphere (Jacobson and Hansson, 2000, and references therein). The contribution of this source to overall tropospheric ozone budgets remains uncertain (Škerlak et al, 2014), especially in the Southern Hemisphere (SH). Models show that stratospheric ozone depletion has propagated to the upper troposphere (Stevenson et al, 2013). Work based on the Southern Hemisphere Additional OZonesonde (SHADOZ) network suggests stratospheric mixing may be increasing upper-tropospheric ozone near southern Africa (Liu et al, 2015; Thompson et al, 2014). We use a multi-year record of ozonesonde observations from sites in the SH extratropics, combined with a global model, to better characterise the impact of stratospheric ozone on the tropospheric ozone budget in the SH

Methods

Results

Conclusion