A document by Daniel J Zawada. Click on the document to view its contents.

Following the Hunga Tonga–Hunga Ha’apai (HTHH) eruption in January 2022, stratospheric ozone depletion was observed in the Southern Hemisphere mid-latitudes and Antarctica during the 2022 austral wintertime and springtime. This eruption injected sulfur dioxide and unprecedented amounts of water vapor into the stratosphere. This work examines and quantifies the chemistry contribution of the volcanic materials to the ozone depletion using chemistry-climate model simulations with nudged meteorology. Simulated 2022 ozone and nitrogen oxides (NOx) anomalies show a good agreement with satellite observations. We find that chemistry only contributes up to 6% and 20% ozone destruction at mid-latitudes wintertime and Antarctic springtime respectively. The majority of the ozone depletion is attributed to the internal variability and dynamical changes forced by the eruption. Both the simulation and observations show a significant NOx reduction associated with the HTHH aerosol plume, indicating the enhanced dinitrogen pentoxide hydrolysis on sulfate aerosol.

The phase and amplitude relationships between dynamical quantities and ozone within the Quasi-biennial Oscillation (QBO) are explored. An initial assessment of this is done by applying a Principal Oscillation Pattern analysis to observations (SWOOSH for Ozone) and reanalysis data (ERA5). This analysis highlights features of the ozone and temperature variability including two peaks in amplitude in the QBO region as well as more subtle phase differences that cannot be explained by a simple QBO theory. We also quantify the sizes of the ozone and temperature advection terms and show that the contribution of background upwelling on variations in ozone gradient is not negligible (~25%). A radiative-convective equilibrium and photochemical equilibrium model, with the imposed ERA5 QBO variation in upwelling and OSIRIS NOx variations, is used to further understand ozone and temperature changes. The results show that photochemistry and transport are important at all levels and it is misleading to divide the QBO into separate regimes. Prominent aspects of the variability can be reproduced if ERA5 upwelling is reduced by ~60% between 15 and 50 hPa where ERA5 is likely over-predicting the strength of the secondary meridional circulation. Finally, we demonstrate that non-locality in the vertical plays a major role in QBO dynamics. This arises from ozone transport, the dependence on column ozone of photochemical production and radiative transfer between layers.

We use trace gas profiles from Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS) satellite measurements and the TOMCAT three-dimensional chemical transport model to diagnose stratospheric trends in O3, HCl and N2O. We find that the 2004-2021 ACE-FTS trends exhibit a clear lower stratosphere (LS) interhemispheric asymmetry with positive (negative) O3 and N2O (HCl) trends in the Southern Hemisphere (SH), and trends of opposite sign in the Northern Hemisphere (NH). The trends are larger for the shorter time period of 2004-2018. TOMCAT qualitatively agrees with the ACE-FTS LS N2O and HCl trends, confirming that transport variability drives such patterns, despite some discrepancies for O3. An additional model simulation is used to quantify the sensitivity of O3 to long-term changes in chlorine and bromine and thus determine the chemical contribution of the spatially varying halogen trends to both observed and modelled O3 trends. Overall, the recent dynamically induced variation in mid-latitude LS halogen abundance has, through chemical feedback, accentuated the O3 recovery signal in the SH and delayed it in the NH, reflecting the enhanced dynamical variability of the NH. These results further indicate the complexities that exist in the search for the signal of ozone recovery in the mid-latitude lower stratosphere.