Plain Language Summary
The 15 January 2022 eruption of the Hunga Tonga-Hunga Ha’apai underwater volcano injected a record amount of water directly into the stratosphere. This study attempts to quantify this impact on the temperature, as well as the subsequent changes to the stratospheric circulation, during the months following the eruption. The extreme nature of the stratospheric temperature, wind, and circulation changes are tracked through comparisons of the first six months of 2022 with the past 42 years. Details of the stratospheric perturbations in latitude and pressure are presented for June 2022, where anomalously low temperatures are found at near 20 km altitude from 60oS to 30oS. In response to this cooling the atmosphere adjusts by creating strong westerly winds above the temperature anomaly and large changes to the downward and poleward mean meridional circulation.
1 Introduction
The 15 January 2022 eruption of the Hunga Tonga-Hunga Ha’apai underwater volcano injected an unprecedented amount of water directly into the stratosphere (Millán et al., 2022; Xu et al., 2022; Carr et al., 2022). While the initial injection plume at 20oS reached to the upper stratosphere (Carr et al., 2022), Millán et al. (2022), showed that after three months this excess water vapor settled near 20 hPa altitude in a latitude band from 30oS to 5oN. With no major thermodynamic or photochemical sinks, this excess moisture is expected to remain in the stratosphere for two to three years. Water vapor is radiatively active in the infrared, contributing to the total radiative cooling in the stratosphere, which is dominated by the effects of carbon dioxide and ozone (e.g., Gille and Lyjak, 1986). These large perturbations in water vapor are expected to increase the amount of radiation lost to space, locally cooling the stratosphere. This study attempts to quantify this impact on the temperature, as well as the subsequent changes to the stratospheric circulation, during the first six months after the eruption.
The MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2) reanalysis (Gelaro et al., 2017) provides the circulation fields (temperatures and winds) for this study. While MERRA-2 assimilates a number of in-situ and space-based observations that constrain tropospheric moisture, stratospheric water vapor observations are not assimilated. Stratospheric moisture is closely constrained to monthly climatologies imposed by a relaxation constraint in the GEOS model, so that the stratospheric water vapor in MERRA-2 does not respond to the volcanic perturbation. This precludes the use of MERRA-2 to directly infer the thermal impacts of the water vapor increase on the circulation. However, numerous nadir- and limb-sounding microwave observations are assimilated in the stratosphere (McCarty et al., 2016) and these data will constrain the analyzed temperatures, which can be used to indirectly infer the thermal and dynamical response of the stratosphere in MERRA-2. Additionally, a more recent data assimilation system, M2-SCREAM (described below), does contain additional water vapor and is used to compare with the MERRA-2 analysis and radiative temperature tendencies.
2 Assimilation Products
The 43-year (1980-2022) climate record from MERRA-2 is used to assess the anomalies of 2022. Two sets of monthly averaged files are used: The assimilation files for winds and temperatures (GMAO, 2015a), and the temperature tendency files to obtain the analysis temperature tendencies (GMAO, 2015b). The residual zonal mean circulations for each month are calculated from the monthly averaged assimilation files as, in addition to winds and temperatures, these files also contain the heat and momentum fluxes needed for evaluation of the residual circulation (see Andrews et al., 1987, page 128). For this study we examine the stream function of the residual circulation as well as the residual mean meridional and vertical winds.
The MERRA-2 Stratospheric Composition Reanalysis of Aura Microwave Limb Sounder (M2-SCREAM: Wargan et al., 2022) is a new stratosphere-focused reanalysis product developed at NASA’s Global Modeling and Assimilation Office. M2-SCREAM assimilates stratospheric profiles of ozone, water vapor, hydrogen chloride, nitric acid, and nitrous oxide from version 4.2 retrievals of the Microwave Limb Sounder (MLS: Waters et al., 2006; Livesey et al., 2020) observations, the same as those used in Millán et al. (2022), alongside total ozone observations from the Ozone Monitoring Instrument (Levelt et al., 2006, 2018). Temperature, winds, surface pressure, and tropospheric water vapor in M2-SCREAM are constrained by the MERRA-2 assimilated fields. M2-SCREAM covers the MLS period from September 2004 to May 2022. Details of the M2-SCREAM system are described in Wargan et al., (2020; 2022). Because this reanalysis assimilates MLS data, unlike MERRA-2, it contains a representation of the water vapor enhancement from the Hunga Tonga eruption in agreement with MLS. The total increase of stratospheric water vapor mass calculated from M2-SCREAM is 10% as in Millán et al. (2022). As shown below, the radiative transfer model in M2-SCREAM responds to the moisture enhancement by producing long wave cooling in better agreement with the observations.
3 Results