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.

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.

The mainstream media and popular science platforms are rife with misunderstandings about what a “polar vortex” is. The term most aptly describes the stratospheric polar vortex, a single feature dominating the cool-season circulation at ∼15–50 km altitude. Regional upper tropospheric jet stream variations dominate the tropospheric circulation, which is not well-described by the idea of a polar vortex; indeed, there is no single consistent definition of a tropospheric polar vortex in the literature. Stratospheric polar vortex disturbances profoundly influence extreme weather events such as cold air outbreaks (CAO). How the stratospheric polar vortex affects the tropospheric jets, local excursions of which drive CAOs, is not yet fully understood. The most public-facing parts of publications describing research on this topic are sometimes unclear about how the “polar vortex” is defined; greater clarity could help improve communications both within the community and with non-specialist audiences.

The winter of 2019-20 was dominated by an extremely strong stratospheric polar vortex and positive tropospheric Arctic Oscillation (AO). Here, we analyze forecasts from 6 different prediction systems contributing to the C3S seasonal forecast database. Most performed very strongly, with consistently high skill for January-March 2020 from forecasts launched through October–December 2019. Although the magnitude of the anomalies was underestimated, the performance of most prediction systems was extremely high for a positive AO winter relative to the common hindcast climate. Ensemble members which better predicted the extremely strong stratospheric vortex better predicted the extreme tropospheric state. We find a significant relationship between forecasts of the anomalous mid-latitude tropospheric wave pattern in early winter, which destructively interfered with the climatological stationary waves, and the strength of the stratospheric vortex later in the winter. Our results demonstrate a strong interdependence between the accuracy of stratospheric vortex and AO forecasts.

The Northern Hemisphere (NH) polar winter stratosphere of 2019/2020 featured an exceptionally strong and cold stratospheric polar vortex. Wave activity from the troposphere during December-February was unusually low, which allowed the polar vortex to remain relatively undisturbed. Several transient wave pulses nonetheless served to help create a reflective configuration of the stratospheric circulation by disturbing the vortex in the upper stratosphere. Subsequently, multiple downward wave coupling events took place, which aided in dynamically cooling and strengthening the polar vortex. The persistent strength of the stratospheric polar vortex was accompanied by an unprecedentedly positive phase of the Arctic Oscillation in the troposphere during January-March, which was consistent with large portions of observed surface temperature and precipitation anomalies during the season. Similarly, conditions within the strong polar vortex were ripe for allowing substantial ozone loss: The undisturbed vortex was a strong transport barrier, and temperatures were low enough to form polar stratospheric clouds for over four months into late March. Total column ozone amounts in the NH polar cap decreased, and were the lowest ever observed in the February-April period. The unique confluence of conditions and multiple broken records makes the 2019/2020 winter and early spring a particularly extreme example of two-way coupling between the troposphere and stratosphere.

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.

Regionally and seasonally resolved relationships of upper tropospheric jet variability to El Niño / Southern Oscillation (ENSO) in multiple reanalyses are presented, with subtropical and polar jets analyzed separately. Previously reported results confirmed herein include strengthening of tropical jets associated with monsoons and Walker circulation during La Niña and a statistically significant subtropical jet latitude decrease (increase) during El Niño (La Niña) in the zonal mean view in both hemispheres. However, subtropical jet latitudes increase significantly during El Niño over the NH eastern Pacific in DJF, and in different limited SH regions in MAM and SON. Subtropical jet altitudes increase significantly during El Niño in the zonal mean in all seasons (DJF / MAM) in the NH (SH). Subtropical jet windspeed correlations with ENSO vary, showing increasing windspeed during El Niño in both hemispheres in DJF and MAM. Polar jet correlations with ENSO are typically not significant in the zonal mean, but there are a few regions/seasons with significant correlations with ENSO, particularly in the SH, where polar jet latitudes decrease over Asia and the western Pacific in DJF, and increase over the eastern Pacific in JJA and SON, during El Niño. Typically, significantly weaker (stronger) polar jet windspeeds are associated with El Niño (La Niña) in the western than in the eastern hemisphere in both NH and SH. All reanalyses analyzed agree well. This work highlights the importance of regional and seasonal variations in the upper tropospheric jet response to ENSO and provides new information for model evaluation.

A moments/area study of meteorological reanalyses (focusing on MERRA-2, ERA-Interim, and JRA-55) allows a novel investigation of the climatology of and interannual variability and trends in the Asian summer monsoon anticyclone (ASMA). The climatological ASMA is nearly elliptical, with its major axis aligned along its centroid latitude and an aspect ratio of ∼5–8. The ASMA centroid shifts northward with height, northward and westward during development, and in the opposite direction as it weakens. ASMA position and seasonal evolution generally agree among the reanalyses, except that MERRA-2 shows over 40% larger area at 350 K. No evidence of climatological bimodality is seen in the ASMA, consistent with previous studies using modern reanalyses. ASMA moments trends are mostly neither statistically significant nor consistent among reanalyses, but area and duration increase significantly over 1979–2018, and over 1958–2018 in JRA-55; JRA-55 trends are largest for 1979–2018, suggesting that reanalysis trends may have accelerated in recent decades. ASMA centroid latitude is significantly negatively (positively) correlated with subtropical jet core latitude (altitude), and significantly negatively correlated with concurrent ENSO. Other ASMA moments and area are not strongly correlated with concurrent ENSO, but ASMA area is significantly positively correlated with ENSO two months previously. Significant (negative) correlations of ASMA area with QBO are seen only during June at 370, 390, and 410 K. These results provide a unique and comprehensive view of the structure and evolution of the ASMA and introduce new tools that can be used to further explore ASMA characteristics and impacts.