A commonly accepted explanation for the unusually cold weather over the United States in the early 2021 – including the unprecedented and brutal cold snap that gripped Texas in February – posits a “sudden stratospheric warming” as the main cause behind these weather events.
However, a new study led by the National Center for Atmospheric Research (NCAR) has challenged this assumption, arguing that the tropospheric circulation and bidirectional coupling between the troposphere and stratosphere were the dominant contributors to the unusual weather patterns during that period.
Sudden stratospheric warmings are breakdowns of the winter stratospheric polar vortex. During the Northern Hemisphere winters, when the North Pole remains shrouded in darkness, a frigid mass of cold air forms in the stratosphere above the pole. This cold air is locked into place by a jet stream known as a stratospheric polar vortex.
About once every two years, the polar vortex is disrupted by planetary-scale waves that propagate upward from the troposphere (the lowest layer of the atmosphere) and warm the vortex, weakening it or causing it to be displaced or split in two – a phenomenon known as a sudden stratospheric warming. In the month after such events occur, it is usually warmer than normal in Canada, Alaska, and the Middle East, and colder than normal in Siberia, Europe, and the United States.
However, scientists from NCAR argue that this phenomenon has not been the main cause behind the early 2021 cold waves over the United States. “I think everyone imagined that a pinball is shot up from the troposphere, hits the polar vortex, and breaks it apart,” said study lead author Nicholas Davis, an expert in climate dynamics at NCAR.
“And then another pinball shoots back down and changes the weather. But this study shows that it’s not so simple. I think it possible that the events in the troposphere and the stratosphere are feeding back on one another and reinforcing what’s happening.”
According to Dr. Davis and his colleagues, the initial state of the troposphere was the primary driver of the US cold snap, rather than the onset of the sudden stratospheric warming. However, a disturbed polar vortex following stratospheric warmings can reflect planetary waves back down into the troposphere, where they can intensify weather systems and create or exacerbate cold air outbreaks.
Thus, the complex interactions between the troposphere and the stratosphere are most likely behind the unusual weather patterns from early 2021, rather than the simpler form of causality connecting stratospheric warmings with the onset of the cold wave.
“There is overall insufficient dynamical evidence to conclude that the sudden stratospheric warming in particular, or the stratosphere in general, had a substantial impact on the surface circulation or surface temperatures in the month after the event. The tropospheric circulation, surface forcings, and their bidirectional coupling with the stratosphere played an equal if not more important role,” concluded the authors.
The study is published in the journal Nature Communications.