Recent weather news in the United States and North America has been dominated by one major theme: COLD temperatures. At the end of December 2013 and the beginning of January 2014, much of the United States and Canada were subjected to low temperature extremes that have not been experienced for decades.
The two days before Christmas and Christmas Day 2013 in the United States brought cold weather misery, with low temperatures compounded by icy precipitation. This outbreak of cold air combined with rain, snow, and ice was an unwelcomed holiday visitor, as the associated power outages left hundreds of thousands of homes without power and heat. Hardest hit was the state of Michigan, states in the northeastern U.S., and southern Ontario and Quebec.
Power outages darken Christmas for thousands (USA Today)
Ice storm means dark Christmas for thousands of Canadians (CBC News)
The Atmospheric Infrared Sounder (AIRS) on NASA’s Aqua satellite provides daily measurements of many different atmospheric variables related to weather. One of these variables is the Surface Air Temperature (SurfAirTemp) which is the atmospheric temperature at the surface of the Earth. A map of surface air temperature for December 23-25 over North America (Fig. 1) shows the characteristic pattern of cold temperatures that interacted with atmospheric moisture to produce the ice storm. The thick blue line on the map shows the 270 K isotherm, the line where the temperatures averaged 270 K (-3 °C or 26 °F). Most locations north of this line experienced ice and snow when the cold Arctic air interacted with warmer moist air flowing northward from the south. Note that the cold air penetrated as far south as northern New Mexico, where temperatures were actually lower than those in the Aleutian Islands.
Figure 1. Image of AIRS surface temperature data (SurfAirTemp), averaged over the period December 23-25, 2013. The blue boundary line is the 270 K (-3 °C or 26 °F) isotherm. Click the image to view it full-size.
With the advent of remote sensing observations from space, the cause of these cold air outbreaks has been traced to the Polar Vortex, although the link is yet to be fully understood. The Polar Vortex is a cyclonic feature in the atmosphere above both poles, and has been shown to especially influence the Northern Polar Jet Stream. A slowing Arctic Polar Vortex may impact the northern jet stream by forcing it to “buckle,” or meander, which is a manifestation of growing Rossby waves. Some of these meanders can bring bitterly cold polar air deep into the south. Contrary to the climate inferences these cold air outbreaks may generate, their increasing frequency is a sign of decreasing north-south atmospheric temperature gradients, under which the Polar Vortex inevitably slows down.
Figure 2. Image of the upper atmospheric circulation over the Northern Hemisphere, showing the northern jet stream (red shades indicate highest wind speeds). Rossby waves create peaks and troughs in the polar atmospheric circulation, directly affecting the position of the jet stream. The underlying image, created by the NASA Goddard Scientific Visualization Studio, shows the presence of more peaks and troughs than were present in December 2013 and early January 2014. Click the image to view it full-size.
In December 2013 and January 2014, the Rossby waves were unusually long and strong, resulting in only two wave troughs in the entire Northern Hemisphere (normally there are more than two) where the Arctic air penetrated southward. One trough descended over central North America, illustrated by the 270 K line on the map of AIRS surface temperature (Fig. 1). On the other side of the Northern Hemisphere, a similarly-shaped trough dipped south over Siberia (Fig. 3 and Fig. 4). This trough wasn’t very newsworthy in Siberia and the Tibetan Plateau, where cold winter temperatures are the typical state of the weather.
The AIRS instrument provides many more data variables than surface air temperature and provides these data from the Earth’s surface to the top of the stratosphere. These atmospheric data profiles allow meteorologists and climate scientists to examine the multitude of factors that influence Earth’s variable weather and changing climate. One outcome of this research could be an assessment of whether North Americans should expect more (or fewer) events like the Christmas ice storm of 2013 and the cold snap of early January 2014.
Figure 3. Global image of AIRS surface temperature data (SurfAirTemp), averaged over the period December 23-25, 2013. The two Rossby wave troughs are indicated by the colder temperatures (blue) over North America and Siberia. Between the troughs, warm surface air temperatures were present over most of Europe and the U.K and extended to the lower Baltic Sea. The color scale used to depict the temperatures is the same as that shown in Figure 1. Click on the image to view it full-size.
Figure 4. North polar projection of AIRS surface temperature data (SurfAirTemp), averaged over the period December 23-25, 2013. The unusual Rossby wave pattern, characterized by only two cold troughs in the Northern Hemisphere over North America and Siberia, is clearly shown in this image. The color scale used to depict the temperatures is the same as that shown in Figure 1. Click on the image to view it full-size.