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Sudden Stratospheric Warmings and cold-air outbreaks

Sudden Stratospheric Warmings and cold-air outbreaks

The SSWs are confirmed to be early precursors of cold weather. However the researchers also found that the most severe cold air outbreaks in Eurasia tend to occur not after - but before SSWs.

In a recent study the researchers from Finnish Meteorological Institute analysed the link between surface air temperatures and SSWs in observations covering 35 Sudden Stratospheric Warmings (SSW) events. They confirmed the utility of SSW as early precursors of cold weather; however they also found that the most severe cold air outbreaks in Eurasia tend to occur not after but before SSWs. The researchers suggested that these cold air outbreaks are associated with significant disruptions of the atmospheric circulation in the lower atmosphere, called blockings, which themselves can trigger SSWs.

"The scientific and public interest to SSWs is driven in particular by the fact that they are often followed by cold air outbreaks near the surface which can last for several weeks and bring cold weather to Northern Europe and eastern coast of North America", says researcher Alexey Karpetchko from Finnish Meteorological Institute. For this reason SSWs are considered as early precursors of cold weather events in these regions and so the information about stratospheric conditions can be used to improve climate forecasts up to 60 days ahead.

Sudden Stratospheric Warmings (SSW) occur every second winter

During winter in the stratosphere at the altitudes 20 - 40 km above the surface, the polar night conditions lead to a significant cooling of the air and formation of a cold air mass roughly centred at the pole. This air mass, called polar vortex, is surrounded by strong westerly winds (i.e. winds blowing from west to east) which have typical speed of 20-50 m/s. Occasionally in the middle of winter the temperatures in the polar vortex rise by several tens of degrees within just a few days and the winds change their direction from westerlies to easterlies. Such periods are called Sudden Stratospheric Warmings (SSW) and they occur in the Arctic stratosphere on average every second winter.

Forecast models usually have difficulties to predict strength and duration of blocking events because of the complexity of involved dynamical processes. Despite the progress achieved in long-range climate predictability by the use of stratospheric information, some of most severe cold air outbreaks in Eurasia will likely remain to be more difficult to predict beyond the typical weather predictability limit of approximately 10 day.

The study is published in the Journal of Geophysical Research - Atmospheres.

More information:

Dr. Alexey Karpechko, tel. +358-503613901

Researcher Ilari Lehtonen, tel. +358 50380 2870

Lehtonen I. and A.Yu. Karpechko (2016), Observed and modeled tropospheric cold anomalies associated with sudden stratospheric warmings, J. Geophys. Res. Atmos., 121, 1591–1610, doi:10.1002/2015JD023860.


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