Perturbations in stratospheric aerosol evolution due to the water-rich plume of the 2022 Hunga-Tonga eruption

The Hunga Tonga-Hunga Ha'apai (HTHH) volcanic eruption in January 2022 was a significant event, injecting substantial amounts of volcanic material into the stratosphere. Unlike many eruptions, HTHH's impact was not primarily due to sulfur dioxide (SO2) emissions, but rather to an unprecedented amount of water vapor. Previous research has established the importance of both stratospheric water vapor and volcanic aerosols in influencing atmospheric chemistry and radiative balance. Changes in water vapor can affect the rate of chemical reactions, including the oxidation of SO2 to sulfate aerosols, a crucial process impacting climate. The sheer scale of water injection from the HTHH eruption presents a unique opportunity to study the impact of increased stratospheric water on aerosol formation and evolution. This study investigates the extent to which the anomalous water injection altered the typical stratospheric aerosol lifecycle and subsequently influenced the climate system, utilizing state-of-the-art climate models and satellite observations to achieve this.

Prior research highlights the significant role of stratospheric water vapor and volcanic aerosols in both atmospheric chemistry and radiative forcing. Studies have demonstrated the influence of stratospheric water vapor on temperature profiles and ozone concentrations. The impact of volcanic eruptions on stratospheric aerosol loading and subsequent radiative effects has been extensively investigated, with models predicting significant alterations to global climate following major eruptions. However, the relatively unique composition of the HTHH plume, with its exceptionally high water content, necessitates a deeper understanding of the processes at play, going beyond previously established paradigms. This study builds upon the existing literature by focusing on the interaction between the unusually high water vapor and the resulting aerosol dynamics.

The study employed the Whole Atmosphere Community Climate Model (WACCM) within the Community Earth System Model version 2 (CESM2) framework. WACCM's high vertical resolution (1-1.5 km in the stratosphere) is critical for capturing the detailed processes involved in the water vapor and aerosol interactions. The model was initialized with data from a standalone ocean model and land model, utilizing reanalysis data for forcing. For the period of January 3 to March 30, 2022, atmospheric data were nudged to match GEOS5 meteorological analysis data. After April 1, the model ran with fully interactive atmosphere and ocean components, generating three ensemble members to account for uncertainty in the nudging period. Three distinct simulations were performed: a control simulation without any volcanic injection, a simulation with SO2 injection only, and a simulation with both SO2 and water injection. The amount of SO2 injected was adjusted based on TROPOMI and OMPS observations, while the water vapor injection amount was tuned to reproduce the observed water vapor enhancements from MLS data. The model's ice cloud formation parameterization was modified to incorporate both homogeneous and heterogeneous nucleation. Satellite data from instruments like MLS, OMPS (LP and NM), and CALIOP were used for model validation and comparison, providing crucial observational constraints.

The model simulation successfully reproduced the observed persistent water vapor enhancements at pressure levels around 30 hPa for three months after the eruption. The inclusion of water vapor in the simulation led to a significantly shorter SO2 lifetime (12 days compared to 28 days in the SO2-only simulation) due to the enhanced hydroxide (OH) concentrations. This faster SO2 conversion resulted in a substantially higher concentration of sulfate aerosol particles, which in turn underwent faster coagulation, leading to larger particle sizes. Consequently, the stratospheric aerosol optical depth (sAOD) was approximately doubled in the simulation with both SO2 and water injection compared to the SO2-only case. Analysis of the radiative effects showed a negative forcing of about -1 to -2 W m⁻² in the impacted areas during January and February, consistent with a moderate-sized volcanic eruption. The enhanced water vapor, however, slightly offset the increased negative radiative forcing from the doubled sAOD. Forecasts showed that the volcanic water vapor and sulfate would persist near the Antarctic polar vortex until at least October 2022. This prolonged residence time will likely impact ozone chemistry due to increased surface area available for heterogeneous reactions on the aerosol particles and water's influence on polar stratospheric cloud formation.

The findings demonstrate the pivotal role of the unusually large water vapor injection from the HTHH eruption in shaping the evolution of the stratospheric aerosol cloud. The significantly shorter SO2 lifetime and the doubling of sAOD highlights the non-linear interactions between water vapor and aerosol formation and growth. The impact of this eruption extends beyond the immediate aftermath, with the model projecting a considerable influence on the Antarctic polar vortex and ozone chemistry well into the latter half of 2022. This demonstrates that even eruptions with relatively modest SO2 emissions can have significant climate effects if accompanied by large-scale water injection into the stratosphere. The results underscore the need for refined models that accurately capture the complex interactions between water vapor and volcanic aerosol evolution to better predict the climatic and atmospheric consequences of future volcanic events.

This study provides compelling evidence for the profound influence of water vapor on stratospheric aerosol evolution following the 2022 Hunga Tonga eruption. The significantly altered aerosol properties, driven by the interaction between water vapor and SO2, resulted in considerably amplified radiative effects and ozone chemistry modifications. The results highlight the need for enhanced modeling capabilities to capture the intricate dynamics involved in these complex processes. Further investigations should focus on the long-term implications of this unique eruption on the climate system and the Antarctic ozone hole. Detailed analysis of the temporal evolution of ozone changes, combined with more sophisticated modeling of aerosol microphysics and heterogeneous chemistry, would considerably enhance our understanding of such events.

The study relies on a global climate model with inherent limitations in resolving small-scale processes such as the initial plume spreading dynamics immediately after the eruption. While the model captures large-scale patterns well, fine-scale features might be less accurately represented. The uncertainty in the exact amount and distribution of injected water vapor also introduces some uncertainty in the results. Future work could incorporate higher-resolution models and more comprehensive satellite data to further refine the model simulations and reduce uncertainty.