The influence of El Niño–Southern Oscillation (ENSO) on northern midlatitude ozone during the period January–March (JFM) is investigated using various observations and a chemistry–climate model. The analysis reveals that, during El Niño events, there are noticeable anomalously high total ozone column (TOC) values over the North Pacific, the southern United States, northeastern Africa, and East Asia but anomalously low values in central Europe and over the North Atlantic. La Niña events have almost the opposite effects on TOC anomalies. The longitudinal dependence of midlatitude ozone anomalies associated with ENSO events during the period JFM is found to be related to planetary waves. Planetary waves excited by tropical convection propagate into the middle latitudes and give rise to longwave trains (Pacific–North American pattern) and shortwave trains along the North African–Asian jet. These wave trains affect ozone in the upper troposphere and lower stratosphere (UTLS) by modulating the midlatitude tropopause height and cause TOC anomalies by changing the vertical distributions of ozone. In addition, synoptic-scale Rossby wave breaking increases on the poleward flanks of the enhanced westerly jet during El Niño events, leading to a stronger eddy-driven meridional circulation in the UTLS and hence causing TOC increases over the North Pacific, the southern United States, northeastern Africa, and East Asia and vice versa for La Niña events. It is also found that the contribution of changes in Brewer–Dobson circulation due to anomalous planetary wave dissipation in the stratosphere during ENSO events to TOC changes in the middle latitudes for the period JFM is small, not more than 1 Dobson unit (DU) per month.

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.

The Montreal Protocol has succeeded in limiting major ozone-depleting substance emissions, and consequently stratospheric ozone concentrations are expected to recover this century. However, there is a large uncertainty in the rate of regional ozone recovery in the Northern Hemisphere. Here we identify a Eurasia-North America dipole mode in the total column ozone over the Northern Hemisphere, showing negative and positive total column ozone anomaly centres over Eurasia and North America, respectively. The positive trend of this mode explains an enhanced total column ozone decline over the Eurasian continent in the past three decades, which is closely related to the polar vortex shift towards Eurasia. Multiple chemistry-climate-model simulations indicate that the positive Eurasia-North America dipole trend in late winter is likely to continue in the near future. Our findings suggest that the anticipated ozone recovery in late winter will be sensitive not only to the ozone-depleting substance decline but also to the polar vortex changes, and could be substantially delayed in some regions of the Northern Hemisphere extratropics.

Recently, observations and simulations have shown that Arctic stratospheric ozone (ASO) variations affect the middle-high latitude tropospheric climate in the Northern Hemisphere. In particular, a connection from the ASO to El Niño-Southern Oscillation (ENSO) has been reported. However, no detailed study has been made of a key process in the connection, the influence of ASO on the North Pacific sea surface temperature (SST) and its underlying mechanism. Using observations, reanalysis and simulations, it is found that the ASO changes in March have the strongest connection with North Pacific SST variations in April. This implies a leading effect of ASO on North Pacific SST. The stratospheric circulation anomalies caused by March ASO changes can rapidly extend to the lower troposphere in the region 60 •-90 • N, 180 •-120 • W. Nevertheless, a theoretical analysis indicates that circulation anomalies from the region 60 •-90 • N, 180 •-120 • W in the lower troposphere would take about a month to propagate horizontally to the North Pacific middle latitudes (30 •-60 • N, 180 •-120 • W).