The Southern Hemisphere (SH) surface westerlies fundamentally control regional patterns of air temperature, storm tracks, and precipitation while also regulating ocean circulation, heat transport and carbon uptake. Wind‐forced ocean perturbation experiments commonly apply idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude and wind intensification factors of between 10% and 300%. In addition, changes in winds are often prescribed ad hoc as a zonally uniform anomaly that neglects important regional and seasonal differences. Here we quantify historical and projected SH westerly wind changes based on examination of CMIP5, CMIP6, and reanalysis data. We find a significant reduction in the location bias of the CMIP6 ensemble and an associated reduction in the projected poleward shift compared to CMIP5. Under a high emission scenario, we find a projected end of 21st Century ensemble mean wind increase of ∼10% and a poleward shift of ∼0.8° latitude, although there are important seasonal and regional variations.

The benefits of the 1987 Montreal Protocol in reducing chlorofluorocarbon emissions, repairing the stratospheric ozone hole, shielding incoming UV radiation, reducing the incidence of skin cancer and mitigating negative ecosystem effects are all well documented. Projected future climate impacts have also been described, mainly focused on a reduced impact of the mid-latitude jet as the ozone hole gradually repairs. However, there is little appreciation of the surface warming that has been avoided as a result of the Montreal Protocol, despite CFCs being potent greenhouse gases. Instead, the issue of ozone depletion and climate change are often thought of as two distinct problems, even though both ozone and CFCs impact Earth's radiation budget. Here we show that a substantial amount of warming has been avoided because of the Montreal Protocol, even after factoring in the surface cooling associated with stratospheric ozone depletion. As of today, as much as 1.1°C warming has been avoided over parts of the Arctic. Future climate benefits are even stronger, with 3°C-4°C Arctic warming and ∼1°C global average warming avoided by 2050; corresponding to a ∼25% mitigation of global warming. The Montreal Protocol has thus not only been a major success in repairing the stratospheric ozone hole, it has also achieved substantial mitigation of anthropogenic climate change both today and into the future.

The Southern Hemisphere (SH) surface westerlies are the strongest time averaged surface winds affecting the open ocean (Russell et al., 2006). The surface westerlies affect the distribution of clouds, precipitation, and the position and intensity of storm tracks in the Southern Hemisphere high latitudes (e.g.,

The spring 2019 stratospheric warming event (SWE) in the Southern Hemisphere (SH) was accompanied by a vertical dipole in polar cap (60-90°S) geopotential height throughout September and early October, with positive anomalies in the stratosphere and negative anomalies in the troposphere (Figure 1a). As the event evolved in time, the stratospheric positive anomalies started to descend, and by late October the tropospheric anomalies switched signs to become positive as well (Figure 1a). Positive polar cap geopotential height anomalies correspond to the negative phase of the Northern and Southern Annular Modes (NAM/SAM; Gerber et al., 2010) and have long been associated with SWEs (Baldwin & Dunkerton, 2001). SWEs have attracted interest for their potential to improve seasonal forecasting in both hemispheres (Domeisen et al., 2020;Sigmond et al., 2013). Specifically, the negative phase of the SAM is associated with warmer and drier than usual conditions over much of Australia and South Africa, and the inverse for southern South America and New Zealand (Gillett et al., 2006).However, the observation of a prominent and persistent positive phase of the tropospheric SAM in the spring of 2019 was surprising (Figure 1), and in particular its persistence (∼5 weeks) made it extraordinary. Based on daily reanalysis data (ERA5, see Methods), the SAM was above 0.5 for periods longer than three weeks only six times in September-November (SON) since 1979, with one other occurrence during a year of an SWE (1988), and the last time more than 25 years ago in 1995. In contrast, most forecasting systems predicted a neutral SAM with a much faster transition to the negative phase during 2019 spring (Figure 1c; Rao et al., 2020). Previous studies describe the specific conditions of 2019, including the evolution of the stratosphere (