The “stratospheric warming” phenomenon
One of the driving factors for a change back to colder-than-normal in much of the US is an ongoing stratospheric warming event that featured one “burst” which lasted from late November into mid-December and then a second one that took place through the early and middle parts of January. There is usually a lag time of several weeks for a stratospheric warming event to have an impact on temperature patterns across the US. In this winter’s on-going event, the first “burst” likely contributed to a very cold middle part of January for much of the nation and the second wave should contribute to a colder-than-normal pattern again beginning in early-to-mid February and perhaps taking us right into March. Numerous teleconnection indices support the idea of a change back to colder-than-normal for the central and eastern US as we progress through the month of February.
One way to monitor the potential for wintertime Arctic air outbreaks into the central and eastern U.S. is to track the temperature pattern in the stratosphere over the polar region of the Northern Hemisphere. The stratosphere is the second major layer of the atmosphere just above the troposphere and below the mesosphere. It occupies the region of atmosphere from about 12 to 50 km, although its lower boundary tends to be higher nearer the equator and lower nearer the poles. The stratosphere defines a layer in which temperatures rise with increasing altitude. At the top of the stratosphere, the thin air may attain temperatures close to 0°C. This rise in temperature is caused by the absorption of ultraviolet (UV) radiation from the sun by the ozone layer. Such a temperature profile creates very stable atmospheric conditions and the stratosphere lacks the air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds or other forms of weather.
Sudden stratospheric warmings (SSWs) are among the most impressive dynamical events in the physical climate system. During the winter months in the polar stratosphere, temperatures are typically at or below minus 70°C. The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex which 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 by being displaced, split apart or even reversed. Under these circumstances, the winds can decrease or change directions and the temperatures in the lower stratosphere can rise by more than 50°C in just a few days.
In response to the stratospheric warming (and associated layer expansion) at the high latitudes, the troposphere cools down dramatically underneath (with layer contraction) at the high latitudes and pressures rise. This tropospheric cold air can then be more easily transported from the high latitudes to the middle latitudes given the “high-latitude blocking” that often sets up over places like Greenland and northeastern Canada. 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. [For more information on the stratospheric warming phenomenon visit our “Meteorology 101” page for an extended video discussion on the topic].
2023-2024 stratospheric warming event
During the latter part of November, the polar vortex normally positioned near or over the North Pole began to get “stretched out” and a warming area aloft pushed to near the North Pole. This initial stratospheric warming event lasted well into December and then there was a second “burst” of warming that took place in early and middle January. Typically, the impact on US temperature patterns by a stratospheric warming event can take on the order of 4-6 weeks. Sure enough, a widespread cold wave hit much of the US in middle January which took place several weeks after the late November to mid-December event. The recent “burst” of warming in early-to-mid January is likely going to show up in the US with colder-than-normal temperatures by the early-to-middle of February and that pattern change can last right into March.
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