Using potential vorticity to define Northern Hemisphere (NH) stratospheric polar vortex strength and position, the influence of the QBO on the polar vortex in winter is analyzed. The results show that the weakened/enhanced NH polar vortex in the lower stratosphere during easterly/westerly QBO (E/WQBO) phases is more noticeable in January and February than in November and December. Furthermore, the NH polar vortex shows a shift toward the Eurasian continent and away from North America in winter during EQBO phases compared with that during WQBO phases, with a greater shift in January and February than in November and December. The weaker QBO impact on the zonal mean zonal wind and temperature in February found in previous studies may be related to the cancelling effects of opposite‐signed anomalies over Eurasia and North America induced by the polar vortex shift associated with the QBO. During EQBO phases the upper stratospheric easterly anomalies in early winter increase the frequency of negative refractive index in the middle and high latitudes in late winter. Less planetary wave 1 propagates upward into the upper stratosphere and more wave 1 accumulates in the lower stratosphere, leading to the weakening and shift of the polar vortex in late winter. In addition, the poleward displacement of the subtropical jet during EQBO phases causes more poleward propagation of synoptic‐scale waves in the lower stratosphere, and thus more Rossby wave breaking events over Eurasia in late winter than during WQBO phases, further weakening and shifting the polar vortex in EQBO phases.

Using ozone observations, reanalysis data, and climate model simulations, this study investigates in detail the independent and joint influences of the eastern Pacific (EP) El Niño-Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO) on stratospheric ozone in the Northern Hemisphere (NH) during winter. Statistically, the stratospheric ozone in the NH in winter increases during El Niño events but decreases during La Niña events. Stratospheric ozone increases in the east wind phases of the QBO (EQBO) and decreases in the west wind phases of the QBO (WQBO). The stratospheric ozone anomalies in the middle and high latitudes caused by ENSO activities are clearly larger than those related to the QBO phase. Since the phases of wave-1 and wave-2 planetary waves anomalies related to ENSO activities are broadly similar to those of QBO phases, the joint effect of ENSO and QBO on stratospheric ozone is approximately equal to the linear superposition of their independent impacts. This means that during EQBO phases, the stratospheric ozone anomalies are increased during El Niño events but reduced during La Niña events, and vice versa during WQBO phases. Numerical sensitivity experiments are performed to further investigate the independent and joint influences of EP ENSO and QBO on stratospheric ozone in the NH during winter. The results from simulations agree well with the results from statistical analysis. K E Y W O R D S eastern Pacific El Niño-southern oscillation, independent and joint influences, quasi-biennial oscillation, stratospheric ozone 1 | INTRODUCTION Ozone protects life on Earth by absorbing ultraviolet radiation (e.g., Lubin and Jensen, 1995; Chipperfield et al., 2015; Zhang et al., 2018). It has recently been found to significantly influence tropospheric climate change. For instance, many studies have pointed out that Antarctic ozone depletion has substantial influence on Southern Hemispheric climate change (

Using the NCEP–NCAR reanalysis dataset, this study classifies stratospheric northern annular mode (NAM) anomalies during the negative or positive phase into two categories—anomalies extending into the troposphere [trop event (TE); referred to as negative or positive TEs] and those not extending into the troposphere [nontrop event (NTE); referred to as negative or positive NTEs], and the corresponding tropospheric environments during the TEs and NTEs are identified. Compared with that for the negative NTEs, the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs. Furthermore, the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation, which is manifested by fewer occurrences of negative refractive index squared in the mid- to high-latitude troposphere and stronger wave intensity in the mid- to high-latitude troposphere. However, the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave propagation in modulating the upward wave fluxes into the stratosphere. Stronger and more persistent upward wave fluxes in the negative TEs, particularly wave-1 fluxes, are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors. These negative (positive) geopotential height anomalies over the North Pacific (Euro-Atlantic) are related to the positive (negative) diabatic heating anomalies and the decreased (increased) blocking activities in the mid- to high latitudes. The subtropical diabatic heating could also impact the strength of the mid- to high-latitude geopotential height anomalies through modulating horizontal wave fluxes. For positive NAM events, the results are roughly similar to those for negative NAM events, but with opposite signal.

Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) dataset and the Specified Chemistry Whole Atmosphere Community Climate Model (WACCM-SC), the impacts of sea ice reduction in the Barents-Kara Seas (BKS) on the East Asian trough (EAT) in late winter are investigated. Results from both reanalysis data and simulations show that the BKS sea ice reduction leads to a deepened EAT in late winter, especially in February, while the EAT axis tilt is not sensitive to the BKS sea ice reduction. Further analysis shows that the BKS sea ice reduction influences the EAT through the tropospheric and stratospheric pathways. For the tropospheric pathway, the results from a linearized barotropic model and Rossby wave ray tracing model reveal that long Rossby wave trains stimulated by the BKS sea ice loss propagate downstream to the North Pacific, strengthening the EAT. For the stratospheric pathway, the upward planetary waves enhanced by the BKS sea ice reduction shift the subpolar westerlies near the tropopause southward. With the critical lines displaced equatorward, the poleward transient eddies break at lower latitudes, shifting the eddy momentum deposit throughout the troposphere equatorward. Tropospheric westerlies maintained by eddy momentum deposit are also shifted southward, inducing the cyclonic anomalies over the North Pacific and deepening the EAT in late winter. Nudging experiments show that the tropospheric pathway only contributes to around 29.7% of the deepening of the EAT in February induced by the BKS sea ice loss, while the remaining 70.3% is caused by stratosphere-troposphere coupling.

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