The ring of winds that circles the Arctic through the depth of the stratosphere is forecast to break apart in the first days of November, and if it does, it will be the earliest event of its kind ever recorded.
Forecasts published this week by Severe Weather Europe show stratospheric winds over the pole slowing sharply and collapsing towards a full reversal, splitting the polar vortex into two separate cores. The analysis was written by the site's meteorologist Andrej Flis.
The polar vortex is a broad winter circulation over the polar regions, rising from the surface into the stratosphere more than 50km (31 miles) up and trapping the coldest air inside. A strong, stable vortex acts as a barrier, holding Arctic air over the pole and leaving the United States and Europe milder. When it is disrupted, that barrier breaks and cold air spills south into the mid-latitudes.
Such disruptions usually begin with a rise in stratospheric pressure and temperature known as a Sudden Stratospheric Warming. Most of them happen in January and February, when the vortex is fully established.
Forecasters track the vortex by measuring winds around the polar circle. Strong westerly winds indicate a stable, strong vortex; weak or easterly winds indicate a reversal and a breakdown. On the latest runs, mid-stratospheric winds at the 10mb level, about 30km (19 miles) up, fall below the historical minimum for the time of year.
The two main models, the ECMWF and the GFS, show a reversal or a near reversal, and the extended-range GEFS also approaches one before recovering rapidly. At the same level, the stratospheric temperature wave is forecast to push temperatures almost 22C (40F) above normal.
The ensemble forecasts, which run the models many times from slightly different starting conditions, do not entirely agree. The ECMWF ensemble puts the chance of a wind reversal at 96%. The GEFS puts it at 71%. The AIFS ensemble puts it at 37%.
The timing is what makes the event unusual. Flis compared the first Sudden Stratospheric Warming of every season on record and found that a reversal in the current event, centred on about 4 November, would beat the earliest previously recorded by at least three weeks. The earliest to date is 28 November.
The consequences are harder to call. “We've never seen anything like this so early in the season,” Flis wrote, noting that a mid-autumn atmosphere behaves differently from a mid-winter one. A Super El Niño is also shaping the pattern, with peak anomalies in the eastern Pacific exceeding 8C (14.4F) above normal. The forecasts show early, indirect signs of cooler trends over parts of Europe and the eastern United States in the coming weeks.
It is worth being precise about what currently holds the record. The earliest Sudden Stratospheric Warming ever recorded has a date, a margin and three published probabilities. It has not happened.
This newspaper regards that as an advance. Meteorology has reached the point where a record can be entered in the book before the atmosphere has been consulted, which removes the single greatest source of delay in record-keeping.
There remains the question of custody. The record is 96% true according to the ECMWF ensemble, 71% true according to the GEFS and 37% true according to the AIFS. Our analysts take the mean, which is 68%, and conclude that the earliest stratospheric warming on record is at present slightly more than two-thirds of a record, the remaining third being held by nobody.
The incumbent is also entitled to consideration. The 28 November event actually occurred. It was observed, measured and written down in full, and it is now being displaced by a wind reversal that three computers cannot agree on. The Department of Unnecessary Research has written to ask whether an event that happened may appeal against an event that has not.
The date is 4 November. The stratosphere has until then to confirm it.