A recent weather pattern brought frigid temperatures and life-threatening wind chills to parts of the Midwest, and freezing temperatures as far south as Florida and the Gulf of Mexico. Many sources, including The Weather Channel, said this was the coldest arctic air to reach the contiguous United States since the 1990’s.
The question is: what caused this bitterly cold air to advance past the Canadian border? Many online and print news sources have been blaming the polar vortex for the outbreak. It is true that an irregularity in the polar vortex aided in the weather pattern, and that it is very unusual for the vortex to drift as far south as it did. But, it did not simply “cause” the cold weather.
The Polar Vortex is a planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere. The westerly airflow is largely a manifestation of the thermal wind above the polar frontal zone of middle and subpolar latitudes. The vortex is strongest in winter when the pole-to-equator temperature gradient is strongest. In the Northern Hemisphere, the vortex has two centers in the mean, one near Baffin Island and the other over northeast Siberia. The Polar Vortex in the Southern Hemisphere is more uniform, strong and tends to last longer than in the Northern Hemisphere. The polar vortex begins to form during the last Fall, lasts throughout the Winter months, and then completely dies out in the Spring, disappearing only to reform again in the late Fall.
The high winds or the stratospheric jet associated with the polar vortex is called the polar night jet, which flow around the polar regions. A polar vortex can have waves, splits, and breaks that cause its formation to become irregular, rather than perfectly circular and symmetrical. This same phenomenon occurs in the polar jet stream (referred to as “polar jet”), which is found in the troposphere generally around 30,000 feet mean sea level (MSL). This polar jet is the main driver of our weather systems in the middle latitudes. The changes in shapes are often referred to as ridges and troughs.
It is very rare for the polar vortex and its associated polar night jet to extend as far south as it did into the United States in early January of 2014.
The GFS (Global Forecast System) model wind and temperature data were analyzed to diagnose the actual cause of the prolonged arctic blast. Vertical cross sections along the 93.27W longitude transect between the North Pole and the Equator were produced using the 1800 UTC January 5, 2014 GFS (1300 EST January 5, 2013) analysis data, and the GFS forecast 96-hour (4 days) from that same time. This slice passes near various locations in North America before reaching near the Galapagos Islands at the Equator. These locations are labeled on the map and associated cross-sections below.
The wind cross-section shows the polar night jet and polar vortex near 45°N latitude. The polar jet is shown lower in the atmosphere, and between 30°N and 45°N. Both are labeled on the diagram.
The actual cause of the cold outbreak was dominant high-pressure sinking southward and transporting arctic air from south-central Canada into the Midwest. Ultimately, the polar vortex exacerbated and greatly extended the region and the timeframe that this cold outbreak affected. In this type of event, it is able to do this because it thermally isolates the lower latitudes and regions surrounding the outside of the vortex area. This is because the polar night jet (stratosphere) and the polar jet (troposphere) form an allied front preventing any frontal systems from entering the region and interfering with the very deep (and cold) high. That is how temperatures were bitterly cold, often in record-breaking fashion, for such a prolonged period. Temperatures near the surface can be seen below -10°F farther south than Minneapolis, Minnesota, and that does not factor in the wind chill.
The GFS 1800 UTC January 9, 2014 (1300 EST January 9, 2014) 96-hour forecast temperature cross-section shows the frigid air retreating northward back into Canada and the far northern latitudes. The wind cross-section shows a slightly weaker polar vortex and polar jet, which by then had enabled the breakdown of the allied front, caused the high to be pushed out, and allowed a less frigid pattern to enter the region.
Click on any of the thumbnails below to view enlarged image.