Warm here, cold there, a large atmospheric wave event

On returning from Los Angeles to Zürich on January 2nd, I was surprised to find that it was quite warm. It got up to 13°C yesterday (Kloten Airport, weatherspark.com), that's quite warm for Switzerland, 11°C above average. Given the current hype about a 'polar vortex' over North America, I wanted to see a global map of temperature anomaly, so here it is. Cold over much of North America, warm over Europe, Cold again over Central Asia. What this is showing is how much warmer or colder the average daily temperature for each day is than the normal for the first week of January*. It is calculated from NCEP reanalysis, which uses a model to interpret available data in a physically valid way (data available here). Fahrenheit users beware, a difference in temperature of 10°C is 18°F.

As you click through the days here, you can see that all the patterns move around. You can also see (most easily on January 5) that the Northern hemisphere goes through hot cold hot cold about 4 times around the globe. This means that the jet stream has a large wave in it, with a wavelength of about a quarter of the way around the Earth. The cold spots in this wave are where cold air from near the pole are brought down to lower latitudes by equatorward wind. This happens all the time, but now this wave is particularly strong. There is some debate about whether Arctic warming is leading to changes in the jet stream and changes in the frequency and extent of these cold anomalies. Recent work by Elizabeth Barnes suggests that there is not a clear trend and that many factors need to be considered to understand what is happening to extreme weather at these latitudes and why.

And as to the polar vortex name, in this case, 'polar vortex' refers to a cold meander of the jet stream that tends to stay in place because of its large size. This long term weather patterns stays over Northeast Canada and Northeast Siberia, not over the North Pole. I'm not sure the origin of this name. Most in the climate community would say the polar vortex is a stratospheric vortex over the poles that is strongest in winter. This stratospheric vortex probably had a minor role, if any, in the cold weather over the US. Name confusion aside, a large meander in the jet stream is bringing cold air into the US that would normally be brought to Northeastern Canada instead. So those of you in the Eastern US, you are enjoying a taste of the Northeastern Canadian winter. Stay warm!

If you want more on this, I would recommend Cliff Mass' Weather Blog.

* Normal for the first week of January means January 1-7 averaged over 1958 - 2011