Arctic polar vortex strengthens, El Niño

The Northern Hemisphere's stratospheric polar vortex is currently stronger than the long-term average. According to NASA's GEOS Forward Processing data, zonal-mean winds at a key diagnostic point reached 10.40 meters per second on September 15, 2026, exceeding the climatological mean of 8.10 m/s for that date.

This strengthening follows the vortex's return to westerly circulation over the Arctic in late August, a normal part of the seasonal cycle as the polar stratosphere cools. NASA describes the vortex as the cold air poleward of the strong stratospheric jet stream.

Current vortex development

Analysis from Severe Weather Europe shows a closed low-pressure system established over the polar region at an altitude of approximately 30 kilometers. Their extended forecast guidance suggests the vortex will deepen and expand through late September, developing a more organized ring of winds. The circulation is described as slightly stronger than normal for this time of year, with models indicating a return closer to average conditions during October.

Data from NASA Ozone Watch tracks the vortex's transition from weak easterly to westerly winds in late August, followed by steady strengthening through early September.

Seasonal signal for mid-winter

The longer-range seasonal outlook presents a different picture. Output from the European Centre for Medium-Range Weather Forecasts (ECMWF), analyzed by Severe Weather Europe, shows a stronger vortex early in the winter followed by a marked reduction in stratospheric winds beginning around January 2027. The ensemble mean falls below the long-term reference during January and February.

A corresponding shift from negative to positive geopotential-height anomalies over the polar stratosphere around January is interpreted as consistent with a weaker or more disturbed vortex. UK Met Office seasonal output presented in the same analysis also shows weaker westerly circulation in the middle and latter part of winter. This January-February signal is reported as Severe Weather Europe's analysis of Copernicus Climate Change Service seasonal guidance, which incorporates both ECMWF and Met Office systems.

Copernicus cautions that such forecasts describe broad probabilities and circulation tendencies, not day-to-day weather months in advance. Uncertainty increases with lead time, and the output can change with each monthly update.

Understanding a weakening signal

A reduction in the seasonal ensemble mean for stratospheric winds is not a forecast for a specific, disruptive event. "The current output indicates a possible weakening regime, not a forecast of a specific major sudden stratospheric warming," notes the Severe Weather Europe analysis. A major sudden stratospheric warming is officially identified when winds reverse from westerly to easterly, accompanied by rapid warming and major disruption of the polar circulation.

The previous winter demonstrated how such events unfold. A series of warmings in February and March 2026 ultimately led to a wind reversal and a split of the vortex into two lobes by early March. Major vortex disruptions can alter surface weather patterns by increasing the probability of cold-air intrusions into mid-latitude regions, though the regional response is never guaranteed.

The role of El Niño

The developing very strong El Niño is a key background factor for the upcoming winter. On September 10, NOAA stated the event had a greater than 90% chance of becoming very strong during the Northern Hemisphere fall and winter. Research published in the Journal of Geophysical Research: Atmospheres in 2024 found that strong El Niño events can produce larger average stratospheric responses than weaker events, partly through changes in planetary-wave activity.

The study's authors, led by E. Manzini, noted substantial differences among climate models. This means El Niño can support a tendency for a weaker vortex but cannot determine whether or when a major sudden stratospheric warming will occur. The next stages of the vortex will be tracked through polar-stratospheric temperatures, geopotential height, planetary-wave activity, and zonal winds. As of September 18, the vortex continues to develop normally and remains somewhat stronger than average.