Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (~0–50km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are fundamentally caused by the breaking of planetary-scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid warming of the polar air column. This rapid warming and descent of the polar air column affects tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode (NAM), including increased frequency of cold air outbreaks over North America and Eurasia. SSWs affect the whole atmosphere above the stratosphere producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (non-ionized) particle and electron densities, and electric fields. These effects span the surface to the thermosphere and across both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal predictions. This work reviews the current knowledge on the most important aspects related to SSWs from the historical background to involved dynamical processes, modelling, chemistry and impact on other atmospheric layers.

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.

The Quasi-Biennial Oscillation (QBO) is a major mode of climate variability with periodically descending westerly and easterly winds in the tropical stratosphere, modulating transport and distributions of key greenhouse gases such as water vapor and ozone. In 2016 and 2020, anomalous QBO easterlies disrupted the QBO’s 28–month period previously observed. Here, we quantify the impact of these QBO disruptions on lower stratospheric circulation, and water vapour and ozone using reanalyses and satellite observations, respectively. Both constituents decrease globally from early spring to late autumn during 2016, while they only weakly decrease during 2020. These dissimilarities result from differences in upwelling and cold-point tropopause temperatures caused by anomalous planetary and gravity wave forcing. Our results highlight the need for a better understanding of the causes of QBO disruptions, their interplay with other modes of climate variability, and their impacts on water vapor and ozone in the face of a changing climate.