A strong polar vortex event is likely to come

The stratospheric polar vortex could soon reach or even exceed the thresholds for a so called “strong polar vortex event” (criterion: the zonal mean zonal wind at 10hPa/60 N exceeds the 80th percentile NDJFM climatology (41,2 m/s) for at least 2 days), which corresponds to our expectations and, to some extent, explains a higher probability of the development of a more pronounced positive NAO/AO weather pattern at the end of November and especially through early December.

 

The first graphic shows ECMWF’s forecast of the zonal mean zonal winds in the middle stratosphere.

 

 

What does it mean for the prevailing circulation patterns in the North Atlantic/European sector?

The following graphic shows the GFS forecast of the Northern Annular Mode (NAM) Index, which is similar to the AO index.

 

 

During late November and early December, an active North Atlantic pattern could continue, bringing wetter, windier weather to the northern half of Europe.

This graphic shows what a strong stratospheric polar vortex event could mean. However, it is not an exact physically plausible explanation, i.e. there is no downward propagation of a positive NAM/AO signal, since it is wave activity flux and propagation that are behind these processes. The graph indicates a higher probability of a positive AO index within the troposphere, especially in early December.

 

Given this trend, milder, wetter and windier conditions with an active North Atlantic pattern could set in or continue at the end of November and through early December.

 

Nonetheless, a stronger positive AO index could indicate that the low pressure or upper air trough center is shifting toward far northwest Europe/near Greenland, causing a partial northward shift of the westerly jet in the eastern North Atlantic sector. The latter conditions carry the risk of a stronger and possibly longer-lasting high-pressure ridge downstream (e.g. a European or Scandinavian high-pressure regime could occur at times).

 

Also refer below to the latest DTN temperature forecast chart for December.

 

About the author

Dr. Jens Bonewitz

Dr. Jens Bonewitz is an experienced long-range energy meteorologist at DTN. He consults with utility and renewable energy clients to help plan and prepare for the impact of weather. He combines research and its practical applications to produce meaningful weather forecasts within the S2S range.