The eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km, but also an unprecedented and much larger amount of water vapor into the stratosphere. Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground-based Water Vapor Millimeter Wave (WVMS) instruments began to measure record-high amounts of water vapor in the mesosphere over a wide range of latitudes. While there are indications that some of this mesospheric increase in water vapor was probably caused by the Hunga Tonga eruption, the dynamical situation in 2022 also played an important part in establishing the unusually large water vapor mixing ratios, both in the upper and lower mesosphere.

The eruption of Hunga Tonga in January 2022 injected a large amount of water into the stratosphere. Satellite measurements from Aura Microwave Limb Sounder (MLS) show that this water vapor (H2O) has now spread throughout the stratosphere and into the lower mesosphere, resulting in an increase of >1 ppmv throughout most of this region. Measurements from three ground-based Water Vapor Millimeter Wave Spectrometer (WVMS) instruments and MLS are in good agreement, and show that in 2023 there was more H2O in the lower mesosphere than at any time since the WVMS measurements began in the 1990’s. At Table Mountain, California all WVMS H2O measurements at 54 km since June 2023, and all of the measurements from Mauna Loa, Hawaii, since the resumption of measurements in September 2023, show larger mixing ratios than any previous measurements. At 70 km several recent ~1 week WVMS retrievals in the last few months show the largest anomalies ever measured. The MLS measurements show that maximum H2O anomalies have occurred throughout almost all of the stratosphere and lower mesosphere since the eruption. As of November 2023, almost all of the ~140 Tg of water originally injected into the stratosphere by the Hunga Tonga eruption remains in the middle atmosphere at pressures below 83 hPa (altitudes above ~17 km). The eruption occurred during a period when stratospheric H2O was already slightly elevated above the 2004-2021 MLS average, and the November 2023 anomaly of ~160 Tg represents ~15% of the total mass of H2O in this region.

We present a reanalysis of the ground-based microwave measurements of upper stratospheric ClO from Mauna Kea from 1992-2023. In order to reduce instrumental baseline artifacts from the Chlorine Oxide Experiment (ChlOE) measurements, the retrieval makes use of the difference of daytime and nighttime spectra to produce a day minus night ClO mixing ratio. The trend in the upper stratospheric ChlOE measurements from 1997-2023 was found to be -0.4+/-0.3%/yr (2σ). Comparisons of ChlOE measurements with a combined coincident UARS and Aura Microwave Limb Sounder (MLS) dataset shows a relative trend of +0.3%+/-0.3%/yr (2σ) from 1992-2023. Restricting the comparisons to ChlOE and Aura MLS from 2004-2023 results in relative trends that varied by pressure level (3.2hPa, 4.2 hPa, and 5.6 hPa), from +0.15%/yr to +0.42%/yr, all agreeing to within the 2σ uncertainty. From 2004-2021 the average ChlOE trend for the three levels was -0.3+/-0.4%/yr, but, with the addition of the 2022 and 2023 measurements the trend from 2004-2023 becomes 0.0+/-0.4%/yr. The increase in ClO during 2022 is, at least in part, caused by dynamical variations that affect CH4 mixing ratios, which in turn affect the ClO chemistry. The higher ClO in 2023 is closely correlated with the presence of increased H2O from the Hunga eruption which also affects the ClO chemistry.

The eruption of Hunga Tonga in January 2022 injected an amount of water vapor into the stratosphere that is unprecedented in the satellite era. In the ensuing months Aura Microwave Limb Sounder (MLS) measurements showed that this plume of water vapor spread from its original injection site at 20.5⁰S to Mauna Loa, Hawaii at 19.5⁰N, where an increase was observed in April by the ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments. Interannual variations in water vapor occur over Mauna Loa due to both dynamical variations in the tropical stratosphere and variations in the amount of water vapor crossing the tropical tropopause, and we place the observed stratospheric water vapor increase from Hunga Tonga into context of these other variations that have been observed since 2013.