When the atmosphere gives rise to surprisingly similar weather situations

When the atmosphere gives rise to surprisingly similar weather situations


Satellite views, the meteorological situation of last October 11 in North America was surprisingly similar to that of November 1, 1991. If today such similarities are a mere anecdote, they once were able to question the potentially periodic nature of atmospheric flow.

Although the atmosphere is organized on a large scale, it remains an extraordinarily turbulent fluid. In fact, the panel of possible combinations regarding the placement of anticyclones and depressions presents an unspeakable wealth.

Nevertheless, from time to time it happens that situations are repeated with a disconcerting similarity. A little as if the atmosphere was lacking inspiration and served us a simple copy and paste. A recent example based on satellite images is presented below.

A striking similarity
On the first vignette, the situation of October 11, 2019 between the center of the United States and the West Atlantic. There is an early blizzard in the west. The latter brought historic snow totals to North Dakota. In addition, a subtropical depression sits in the east. She was named Melissa by the National Hurricane Center (NHC).

On the second vignette, the situation of November 1, 1991. The similarity is striking. Indeed, there is an early blizzard in the west and a subtropical depression in the east. However, at the time it was not officially named. Note the similar positioning of cloud structures – taking into account that the angle of view is not quite the same.

Does extratropical atmospheric flow have a periodic behavior?
Nowadays, this kind of similarity is a simple anecdote. However, during the twentieth century, the hypothesis that the atmospheric flow of mid-latitudes could be periodic has been studied with interest (Laval, 2013). More precisely, one wondered if the film of the situations met was repeated after a certain time interval. It goes without saying that such behavior would have greatly facilitated the forecast. In particular, at a time when powerful calculators were not yet available.

In the end, it turned out that this was probably not the case and that configurations close to each other never really changed in the same way. For example, the two snapshots presented above may be very similar, but the weather scenario that follows will be significantly different. Thus, any operational application based on this principle was doomed to failure.

In summary, the turbulent nature of the air does not allow an approximately similar situation to lead to an approximately similar future evolution. Said in a more poetic way, it’s the famous butterfly effect. An emblem of chaos theory. Meteorologists have therefore had to develop schemes to circumvent as much as possible this difficulty. Research on predictability issues is still active today.