Snow falls in Texas, February 15, 2021. (Alexander Hatley/Flickr)
Temperatures across the United States this winter have been a roller coaster.
For example, average December temperatures in Minnesota were 14 degrees Fahrenheit warmer than normal, but the following month, they dropped below the 30-year average before swinging back to break their all-time high in January 31, reaching a balmy 55 degrees. – Nine degrees higher than the previous daily record. Parts of Montana experienced temperature swings of 90 to 100 degrees in a single month.
It wasn't just the northern states that saw the yo-yo temperature. Freezing Arctic air has swept across much of the United States. At least 55 deaths were attributed to January's cold and winter storms. The Houston, Texas airport reached an all-time low of 19 degrees Fahrenheit on January 16. As in Minnesota, temperatures in Houston quickly rebounded to warmer than average.
“Flash weather” events like this can be costly, even deadly. Although there is no precise definition yet, weather injury generally refers to a sudden shift from one set of persistent weather conditions to an entirely different set. For example, long-term drought followed by heavy rains can damage crops. Orchards can be devastated by the arrival of a severe cold snap after a sustained warm spell in early spring. Long summer heatwaves and drought have also ignited wildfires across Canada and Mediterranean Europe in recent years, and if these conditions suddenly turn into a stormy pattern with heavy rain, the barren burn scars will absorb little moisture, increasing the risk of flooding. Surprise. All of these types of neck injuries have occurred several times in recent years, and some research suggests they may occur more often in the future.
Researchers have used different metrics to identify weather injury events. Some studies focus on jumps in temperature while others target changes in extreme precipitation rates. Some metrics measure infection in one location while others target an area. No matter how they are measured, sudden weather events are often highly destructive to agriculture, ecosystems, infrastructure, and human activities.
My research team took a different tack, focusing on abrupt shifts in continental-scale patterns in the jet stream — the powerful river of westerly to easterly winds that surrounds the northern and southern hemispheres at altitudes where jets fly. We used an artificial intelligence tool to identify typical jet stream patterns consisting of north-to-south fluctuations or waves that can form in different locations across North America or Eurasia. These waves are important because they create and direct the weather systems we experience at the surface — those areas of high and low pressure that appear on TV weather maps. For example, when a northward bulge in the jet stream (called a ridge) is located over the western states, generally dry and warm conditions prevail in the region as storms are directed northward into western Canada. In contrast, a large southward dip in the jet stream (trough) allows unusually cold Arctic air to penetrate farther south in the winter months, bringing debilitating cold snaps in southern states ill-equipped to handle them.
The “Texas Freeze” of February 2021, which killed 246 people and cost about $200 billion, is an excellent example of a highly disruptive climate event. In early February (Figure 1), temperatures were well above normal across much of the United States west of the Mississippi River, while temperatures in southeastern states were much colder than normal. Just a few days later (Figure 2), a major pattern shift sent frigid Arctic air reaching the U.S.-Mexico border, wreaking havoc on power utilities that couldn't keep up with heating demand, especially in Texas.
The size and location of jet stream waves determine the weather in the midlatitudes (the region between the polar regions and the tropics), so identifying and tracking their patterns tells us a lot about weather systems and their changes over time. We have identified a set of twelve typical jet stream patterns using daily maps of the upper atmosphere since the late 1940s (specifically, upper-level pressure anomalies). The next step was to look for cases in which the weather became “stuck” in one pattern for four or more days. When these long-duration events occur, surface weather conditions tend to be constant, either hot, dry, cold or wet depending on the location of the jet stream's ridges and troughs. We then determined what pattern the atmosphere turned into after the jet stream separated, and measured how different the new pattern was from the previous stable pattern. If it was different enough (statistically speaking), we considered the shift to be a sudden weather event. We counted events during each season and year, then tracked changes in frequency based on the pattern in which they started.
Interestingly, we found that when the initial persistent pattern of an injury event was characterized by anomalously warm temperatures in the Pacific and Atlantic sectors of the Arctic Ocean (note that the Arctic is warming three to four times faster than the Earth as a whole!) the events occurred More often in recent years. When the Arctic is unusually cold (which happens less and less), fewer injury incidents are produced. We found that injury events have become more frequent in recent years when the Pacific and Atlantic sectors of the Arctic are abnormally warm, and less frequent when it is colder than normal.
One advantage of this method is that we can analyze past conditions as well as future conditions based on simulations performed by climate models. Our research suggests that if we continue to release greenhouse gases into the atmosphere and cut down forests as we do, future changes in the frequency of sudden weather events will show similar but much stronger trends, especially in the summer months. We know that rapid warming in the Arctic will worsen in the future as human activities continue to increase the amount of greenhouse gases in the atmosphere. So, according to our findings, we should also expect sudden weather events to occur more frequently, along with the devastating effects they often cause.
