We use Aura Microwave Limb Sounder (MLS) trace gas measurements to investigate whether water vapor (H2O) injected into the stratosphere by the Hunga Tonga-Hunga Ha’apai (HTHH) eruption affected the 2022 Antarctic stratospheric vortex. Other MLS-measured long-lived species are used to distinguish high HTHH H2O from that descending in the vortex from the upper-stratospheric H2O peak. HTHH H2O reached high southern latitudes in June–July but was effectively excluded from the vortex by the strong transport barrier at its edge. MLS H2O, nitric acid, chlorine species, and ozone within the 2022 Antarctic polar vortex were near average; the vortex was large, strong, and long-lived, but not exceptionally so. There is thus no clear evidence of HTHH influence on the 2022 Antarctic vortex or its composition. Substantial impacts on the stratospheric polar vortices are expected in succeeding years since the H2O injected by HTHH has spread globally.

The January 2022 eruption of Hunga Tonga-Hunga Ha’apai (HT-HH) caused the largest enhancement in stratospheric aerosol loading in decades and produced an unprecedented enhancement in stratospheric water vapor, which led to strong stratospheric cooling that in turn induced changes in the large-scale circulation. Here we use satellite measurements of gas-phase constituents together with aerosol extinction to investigate the extent to which the thick aerosol, excess moisture, and strong cooling enabled heterogeneous chemical processing. In the southern tropics, unambiguous signatures of substantial chlorine and nitrogen repartitioning appear over a broad vertical domain almost immediately after the eruption, with depletion of N2O5, NOx, and HCl accompanied by enhancement of HNO3, ClO, and ClONO2. After initially rising steeply, HNO3 and ClO plateau, maintaining fairly constant abundances for several months. These patterns are consistent with the saturation of N2O5 hydrolysis, suggesting that this reaction is the primary mechanism for the observed composition changes. The southern midlatitudes and subtropics show similar but weaker enhancements in ClO and ClONO2. In those regions, however, effects of anomalous transport dominate the evolution of HNO3 and HCl, obscuring the signs of heterogeneous processing. Perturbations in chlorine species are considerably weaker than those measured in the southern midlatitude stratosphere following the Australian New Year’s fires in 2020. The moderate HT-HH-induced enhancements in reactive chlorine seen throughout the southern middle and low-latitude stratosphere, far smaller than those in typical winter polar vortices, do not lead to appreciable chemical ozone loss; rather, extrapolar lower-stratospheric ozone remains primarily controlled by dynamical processes.

The 2022 Hunga eruption led to extraordinary water vapor enhancement throughout the stratospheric vortex at the beginning of the 2023 Antarctic winter. Although the dynamical characteristics of the vortex itself were generally unexceptional, the excess moisture initially raised the threshold temperatures for the formation of polar stratospheric clouds (PSCs) above typical values over a broad vertical domain. Low temperatures, especially during an early-July cold spell, prompted ice PSC formation and unusually severe irreversible dehydration at higher levels (500–700 K), while atypical rehydration occurred at lower levels (380–460 K). Heterogeneous chemical processing was more extensive, both vertically (up to 750–800 K) and temporally (earlier in the season), than in prior Antarctic winters. The resultant HCl depletion and ClO enhancement both redefined their previously observed ranges at and above 600 K. Albeit unmatched in the satellite record, the early-winter upper-level chlorine activation was insufficient to induce substantial ozone loss. Chlorine activation, denitrification, and dehydration processes saturated in midwinter, with trace gas evolution essentially following the climatological mean thereafter. Chlorine deactivation started slightly later than in most years. While cumulative ozone losses at 410–550 K were relatively large, probably because of the delayed chlorine deactivation, they were not unprecedented. Thus, ozone depletion was unremarkable throughout the lower stratosphere. Although Hunga hastened the onset of and increased the vertical extent of PSC formation and chlorine activation in early winter, saturation of lower stratospheric chemical processing (as is typical in the Antarctic) prevented an exceptionally severe ozone hole in 2023.

Aura Microwave Limb Sounder (MLS) measurements show that chemical processing was critical to the observed record-low Arctic stratospheric ozone in spring 2020. The 16-year MLS record indicates more denitrification and dehydration in 2019/2020 than in any Arctic winter except 2015/2016. Chlorine activation and ozone depletion began earlier than in any previously observed winter, with evidence of chemical ozone loss starting in November. Active chlorine then persisted as late into spring as it did in 2011. Empirical estimates suggest maximum chemical ozone losses near 2.8 ppmv by late March in both 2011 and 2020. However, peak chlorine activation, and thus peak ozone loss, occurred at lower altitudes in 2020 than in 2011, leading to the lowest Arctic ozone values ever observed at potential temperature levels from ~400–480 K, with similar ozone values to those in 2011 at higher levels.

Following the 15 January 2022 Hunga Tonga-Hunga Ha’apai eruption, several trace gases measured by the Aura Microwave Limb Sounder displayed anomalous stratospheric values. Trajectories and radiance simulations confirm that the H2O, SO2, and HCl enhancements were injected by the eruption. In comparison with those from previous eruptions, the SO2 and HCl injections were unexceptional, although they reached higher altitudes. In contrast, the H2O injection was unprecedented in both magnitude (far exceeding any previous values in the 17-year MLS record) and altitude (penetrating into the mesosphere). We estimate the mass of H2O injected into the stratosphere to be 146+-5 Tg - ~10% of the stratospheric burden. It may take several years for the H2O plume to dissipate. This eruption could impact climate not through surface cooling due to sulfate aerosols, but rather through surface warming due to the radiative forcing from the excess stratospheric H2O.

The exceptionally strong and long-lived Arctic stratospheric polar vortex in 2019/2020 resulted in large transport anomalies throughout the fall-winter-spring period from vortex development to breakup. These anomalies are studied using Aura MLS long-lived trace gas data for N2O, H2O,and CO, ACE-FTS CH4 , and meteorological and trace gas fields from reanalyses. Strongest anomalies are seen throughout the winter in the lower through middle stratosphere (from about 500K through 700K), with record low (high) departures from climatology in N2O and CH4 (H2O). CO also shows extreme high anomalies in midwinter through spring down to about 550K. Examination of descent rates, vortex confinement, and trace gas distributions in the preceding months indicates that the early-winter anomalies in N2O and H2O arose primarily from entrainment of air with already-anomalous values (which likely resulted from transport linked to an early January sudden stratospheric warming the previous winter during a favorable quasi-biennial oscillation phase) into the vortex as it developed in fall 2019 followed by descent of those anomalies to lower levels within the vortex. Trace gas anomalies in midwinter through the late vortex breakup in spring 2020 arose primarily from inhibition of mixing between vortex and extravortex air because of the exceptionally strong and persistent vortex. Persistent strong N2O and H2O gradients across the vortex edge demonstrate that air within the vortex and its remnants remained very strongly confined through late April (mid-May) in the middle (lower) stratosphere.