There are two signals that are currently suggesting cold weather for the eastern US during the month of February and they include the Madden-Julian Oscillation (MJO) index and the recent major Sudden Stratospheric Warming (SSW) event.This blog will focus on the SSW event.
There has been a major SSW event in the northern and middle latitudes during the past several weeks and it has set off a chain of events in the atmosphere that has lead to a much colder temperature pattern in the central and eastern US.A review of a couple of highly analogous past SSW events suggest that the change to much colder weather in the eastern US may be quite sustained lasting well into the month of February and that the impact of this latest major SSW event may not have even reached its peak yet.
The phenomenon of stratospheric warming was first discovered in 1952 and there have been about 30 events registered since then or about one every two winters. Research suggests that the SSW events are a consequence of the interaction between the North Atlantic, the troposphere and the stratosphere, and there tends to be an increased number with a warm North Atlantic.During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius.The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex.The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes.This dominant structure is sometimes disrupted in some winters or even reversed.Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50 degrees in just a few days.This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. In this recent SSW event, the polar vortex split into two and that opened the floodgates for Arctic air to move southward.In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes.The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take several weeks to unfold.
Two relatively recent major SSW events that had lots of similarities with the current situation provide us with some clues as to how long an impact this latest event may have on the temperature pattern in the central and eastern US.The first recent warming event took place during December 1984 and it impacted temperatures greatly in the central and eastern US during January 1985. The second significant upper atmospheric warming event occurred in December 1998, and it helped to flip the temperature pattern in the central and eastern US during January 1999. Both of these wintertime SSW events resulted in dramatic turnarounds in the overall temperature pattern from “well above” to "well below” normal several weeks after the initial stratospheric warming, and in both cases, the temperature pattern change was rather sustained lasting for several weeks. This doesn’t mean that each and every day will be below normal as that will not be the case. However, it does suggest that, based on historical similarities, we could be looking at an overall below-normal temperature pattern in the central and eastern US continuing well into the month of February. Indeed, the very latest NCEP Couple Forecast System (CFS) v2 temperature anomaly forecast (upper, left in the figure below) for the month of February is colder-than-normal for the eastern US.A video discussion at thesiweather.com breaks down each of these recent highly analogous SSW events and the clues that they may hold for the current setup.