A large polar stratospheric warming is under way in the Southern Hemisphere. It is expected to be on a par with the last large event, in 2002, according to the Australia Bureau of Meteorology, the US National Oceanic and Atmospheric Administration and NASA, the UK Met Office and the European Centre for Medium-Range Weather Forecasts, and the Japan Meteorological Agency. The ability to forecast these rare occurrences has improved hugely in the past decade owing to dramatic advances in the fusion of weather and climate prediction.

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Sudden stratospheric warmings can affect surface weather for weeks to months. Typically occurring once every two years or so in the Northern Hemisphere, the 2002 event is the only comparable one to be observed in the Southern Hemisphere. It was associated with hot and dry extremes across Australia, as well as rises in Antarctic ozone concentrations (R. S. Stolarski et al. J. Atmos. Sci. 62, 716–720; 2005). The 2019 event is predicted to be no less remarkable (see go.nature.com/2kjhupn).

A weakening of the vortex in the upper stratosphere in late August has already resulted in record-high polar stratosphere temperatures for early September. This weakened vortex is forecast to move downwards towards the surface over the following months. It is expected to reduce the severity of this year’s Antarctic ozone hole, as well as affect Antarctic sea ice and weather across the Southern Hemisphere over this year’s austral spring.