Hemispheric differences in ozone across the stratosphere–troposphere exchange region

Abstract

Ozone changes in the upper troposphere–lower stratosphere (UTLS) resulting from dynamical and chemical processes strongly affect the atmosphere's radiative forcing. This study analyzed intra- and inter-hemispheric ozone differences in the UTLS within the 45–60° latitude band, distinguishing between years disrupted by sudden stratospheric warming (SSW) events from 2002 to 2022. We utilized measurements from IAGOS commercial aircraft (45–60° N), long-term ozonesonde records (45–60° S) and unique in situ measurements from the high altitude and long range (HALO) research aircraft deployed over the southernmost region of South America (45–60° S) in September–November 2019 as a part of the Southern Hemisphere Transport, Dynamics, and Chemistry (SouthTRAC) research campaign. The mission period enabled us to examine the impact of two Southern Hemisphere (SH) SSW events that developed during 2002 and 2019. To enhance the spatial coverage, we incorporated Copernicus Atmosphere Monitoring Service reanalysis data. Stratospheric air origin was assigned using relative humidity (<20 %) and carbon monoxide (<50 nmol mol−1) thresholds. Our results show that air masses of stratospheric origin had higher ozone abundances in the Northern Hemisphere (NH) UTLS than in the SH (between 300–200 hPa and 45–60° latitude): in high ozone depletion years in the stratospheric vortex, the SH ozone median (184 nmol mol−1) was only 54 % of that in the NH (341 nmol mol−1), while in low depletion years, SH ozone median (214 nmol mol−1) reached 57 % of the NH values (371 nmol mol−1). Notably, the SSW events (2002 and 2019) increased SH UTLS ozone by 24 % (43 nmol mol−1) compared to high depletion years, while in the NH, the increase was 9 % (31 nmol mol−1).