A new precursor for North American cold spells

Strong stratospheric waves are associated with sub-seasonal fluctuations between warm and cold spells over North America, according to an analysis of observations and climate models.
A new precursor for North American cold spells
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Despite the prevailing expectation that global warming would lead to a reduction in the frequency of cold spells, recent extreme cold events such as the 2021 and 2014 North American cold waves, and the 2016 East Asian and  2017 European cold waves, serve as vivid reminders that we cannot overlook cold extremes yet. The exceptional North American cold wave in February 2021 in particular, brought about record-breaking or nearly record-low temperatures not seen in decades or even a century in the U.S. Southern Plains. The severity of this event was further compounded by the collapse of the Texas energy infrastructure, leaving millions of people without power and causing unprecedented damage. Hence, the causes of these winter “beasts” have garnered much public attention and scientific debate.

Fig 1 Surface air temperature on Feb 15, 2021 (image from https://earth.nullschool.net)

Meet the precursors

The drivers for wintertime North American cold spells are multifarious, including mid-latitude dynamics, ENSO, and Arctic sea ice. One well-known precursor you may have heard from media coverage is the “polar vortex”. Although the polar vortex is actually a common and normal feature in the atmosphere, the highly weakened and disrupted state of the polar vortex in the stratosphere has been shown to be associated with surface cold-air outbreaks. The exceptional 2021 North American cold wave was suggested to be related to such a weakened polar vortex, also known as sudden stratospheric warming (SSW). However, numerical weather forecasts suggest the SSW exerted a very limited influence on this cold wave. The ongoing scientific debate shows that it is not well understood how robustly and on which timescales the stratosphere contributes to the surface conditions.

A new precursor

Recent studies have shed new light on the role of strong stratospheric waves in shaping surface weather conditions. While traditionally associated with the occurrence of SSWs, these waves have now been recognized for their direct influence on surface air temperature. Building upon this understanding, our research provides compelling evidence for the connection between strong stratospheric waves and sub-seasonal fluctuations between warm and cold spells over North America in observations and climate models. Particularly, when strong stratospheric wave events occur, there is an increased risk of cold extremes over North America 5–25 days later. Compared to all the winter days, strong stratospheric waves enhance the risk by about 30% across much of Canada and the northeastern U.S. (Fig. 2a). Furthermore, the distribution of area-averaged temperature anomalies (red boxes in Fig. 2a) clearly shifts towards colder temperatures during days 5–25 (blue) compared with normal winter days (black) (Fig. 2b). These findings are very different from the surface signatures one typically expects from SSWs, which exhibit more persistent cold spells that are weaker in magnitude over North America, pointing to a distinct physical mechanism.

Fig. 2 Risk ratio of extreme cold days and probability density function (PDF) of NA SAT anomalies following strong stratospheric wave events. a-b, The spatial pattern of the risk ratio of extreme cold days (a) and the PDF of NA SAT anomalies (b) in ERA5 during days 5–25 after the onset of strong stratospheric wave events, compared with the statistics of all winters. The risk ratio in (a) is defined as the probability of cold days (i.e., SAT is at least 1.5 SD below its climatology) in days 5–25 divided by the probability of cold days in any random 21-day period in winter. Stippling indicates where the risk ratio is significant at the 95% confidence interval based on a Student’s t-test. c-d, As in (a-b), but for CMIP6 models. The red boxes in (a) and (c) indicate the region where the NA SAT anomalies are calculated. The vertical gray lines in (b) and (d) denote -1, -1.5, and -2 SD of NA SAT anomalies in all the winter days, and the values in red depict the risk ratios of the exceedance frequency due to strong wave events.

Mounting evidence

The good agreement between observations and climate models gives us confidence in the robustness of the connection between stratospheric waves and surface temperature. This encouraging agreement also motivates us to delve deeper into the underlying mechanism driving this linkage. It is crucial to rule out the possibility that the occurrence of strong stratospheric waves is merely coincidental with surface fluctuations between warm and cold spells without any causal contribution. To address this, we analyzed idealized simulations using an atmospheric model that accurately captures the dynamics of the stratosphere, revealing that strong stratospheric waves indeed play a significant role in driving North American cold spells through vertical wave coupling.

Concluding remarks

Our research establishes a solid relationship in observations and climate models and offers new insights into the role of stratospheric waves in North American cold spells. However, it is important to exercise caution when interpreting individual cases, as strong tropospheric variability can lead to varied surface patterns. Furthermore, the application of our results to real-time weather forecasts requires future work. 

These findings ultimately have the potential to improve the predictability of severe winter cold events in the U.S. and Canada. Such improved predictability can have far-reaching benefits, particularly in sectors such as agriculture, transportation, energy planning and utilization, and human health, which are highly susceptible and sensitive to cold temperature extremes during the winter season. By gaining a deeper understanding of the underlying mechanisms driving these cold events, we can better prepare and mitigate their impacts, ultimately fostering resilience and well-being in affected regions.

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Earth and Environmental Sciences
Physical Sciences > Earth and Environmental Sciences

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