There have been a number of news articles that claim that a global average surface temperature trend (i.e. “global warming) explains the extreme cold weather and snow that has occurred recently; e.g. see
In this post I want to illustrate why it is the location of the westerlies that determine areas that have extreme cold weather and snowstorms.
The first image below presents the heights of the 500mb pressure surface and the temperatures at 850mb from the ECMWF analysis for January 28 2011 at noon GMT.
The 500mb level is used as it is about halfway through the depth of the atmosphere. The distances between the lines of equal height are proportional to the speed of the winds at that level. Since, in the Northern Hemisphere, winds blow counterclockwise around regions of lower heights, the wind field (not shown) is predominately westerly. This is why the middle and higher latitudes are often referred to as the “westerlies”. Winds at this spatial scale blow almost parallel to lines of constant height. When the height contours are close together, we refer to the higher winds that result as the “polar jet stream”.
Clearly evident in the example below is the progressively cooler 850mb temperatures and lower 500mb heights as one progresses to higher latitudes. Also, clearly seen are the regions of colder air (and corresponding lower heights) that extend towards lower latitudes. When these large equatorward excursions of the westerlies occur, extreme cold weather often happens. On the east side of these cold pockets, where there is a strong contrast with warmer air to the east, winter storms occur. If the temperatures are cold enough, precipitation can fall as heavy snow. These large excursions of the westerlies explains why there have been several extreme snowstorms in the eastern USA and western Europe in recent months.
To illustrate the dynamic character of the westerlies, I have presented below the ECMWF 500mb height and 850mb temperature forecast for next Friday [February 7 2011]. Compare the above figure with the one below. Note, for example, the large excursion of cold air and, therefore, westerlies southward to over the central USA. If this forecast verifies, it will be an extreme cold outbreak with considerable snow (and ice storms) on the southeast flank of this cold region.
It is not scientifically accurate to attribute “global warming” of a few tenths of a degree to explain these extreme weather events.
Moreover, in the latest measurements, the lower tropospheric temperatures are actually cooler than the long-term average! [e.g. see
For other excellent discussions of the recent extreme winter weather, see the posts by Joe Daleo; e.g.