This past December was the deadliest for tornadoes in the U.S. in over 60 years.
The United States experiences ten times more tornadoes every year than any other country, however December is usually one of the quietest months — averaging 24, compared to 276 once May hits. However, with over 20 deaths this past month alone, it has been the second deadliest December on record.
The tornado that made its way through Garland and Rowlett on December 26 was rated an EF4 — the strongest December tornado the local National Weather Service office has ever recorded.
So what is different this year?
Temperatures this past December were unseasonably warm, resembling temperatures in the Spring: warm, muggy air flowing north from the Gulf of Mexico, colder air flowing south from Canada, and a low pressure system moving in from the west. These three ingredients often lead to the development of thunderstorms and tornadoes.
El Niño, the cyclic warming of the equatorial Pacific Ocean, is responsible for the some of the spring-like temperatures occurring in most of the southern and eastern U.S. this fall and winter, however it is not the only large-scale weather phenomenon responsible.
El Niño (ENSO)
During El Niño, the subtropical jet stream, which is located across the far southern portion of the US, tends to strengthen, leading to an increase in moist, unstable air and deep “troughs” (regions of low atmospheric pressure). But, the warming half of the cycle, what we are experiencing now, is often associated with a drop in the number of twisters over some parts of the U.S. and an increase in tornadoes in states closer to the Gulf coast, such as Texas. Research is currently being conducted to determine whether there is a relationship between tornado frequency and El Niño and La Niña conditions.
Pacific Decadal Oscillation (PDO)
The PDO is a climate variability very similar to ENSO, but varies in phase for 20 to 30 years, compared to ENSO’s 6 to 18 months. PDO consists of a warm and cool phase which can alter upper level atmospheric winds, which can have a significant impact on global climate. Many experts believe the PDO can diminish or enhance the impacts of ENSO depending on its phase. If both PDO and ENSO are out of phase, each may offset the effects of the other, however PDO is currently experience a warm-phase, as is El Niño, which could explain why we are having such a mild winter in southeastern North America.
Arctic Oscillation (AO)
The AO represents the state of the atmospheric circulation over the Arctic, consisting of a positive and negative phase. In the positive phase, the polar low pressure system (also known as the “polar vortex”) is stronger, forcing cold air south, however in the negative phase, this vortex is weaker and cannot push cold Arctic air into the U.S. The AO was experiencing a positive phase for much of the fall and winter, which may explain the large warming and melting experienced last week in the North Pole and storms remaining further south in North America.
North American Oscillation (NAO)
The NAO is closely related to AO and consists of two pressure centers in the North Atlantic: a low pressure area typically located near Iceland and a high pressure area typically over the Azores. The strength of these two pressure systems significantly alters the jet stream, especially over the eastern U.S., impacting the temperature and amount of precipitation. The NAO also has a positive and negative phase. In the positive phase, the low over Iceland and the high over the Azores both strengthen, resulting in a strong pressure gradient that causes the westerlies to increase. This results in cold air remaining further north and above average temperatures over much of North America. The opposite is true for the negative phase, which allows cold air to build up over Canada and south into the U.S. As you can guess, the NOA is currently experiencing a positive phase.
So there you have it, there is more to this strange sequence of weather than just El Niño. NASA posted this video showing the growth of the storms that occurred over the U.S. between December 26th and 28th, 2015.