There is no doubt that a significant widespread pattern change is now underway across the entire northern hemisphere. The culprit behind this significant synoptic-scale pattern change which is impacting North America, Europe, and Asia has to do with the dramatic reversal of the Arctic Oscillation (AO) during the past few days. We begin our discussion with a look at the recent trend with the Arctic Oscillation..
This image shows the trend with Arctic Oscillation since 11/1/20. The graph clearly shows that the AO has been moderate to strongly Positive since the Summer and the entire Autumn in the northern hemisphere. However just as the winter began around 12/2/20, the AO shifted to neutral and then dropped into negative territory during the first week of December. Indeed, for the past 60 days, the Arctic Oscillation has been consistently negative except for a brief interval of the NEUTRAL phase at the beginning of the New Year. The combination of the moderate to strong negative AO, along with the weakening of the moderate La Nina, and the NAO / North American Oscillation which has also been consistently been in the negative since 12/1/20, have all combined to produce the recent, widespread, active cold pattern in North America, Europe and in much of Asia. The chart on the right however shows the recent trend since the middle of February where the AO has shifted to Neutral for the past 7 days — FEB 15–22. But most of the data is clearly moving the AO into a sustained positive phase as we close out the month of February into the first week of March 2021.
This dramatic and significant shift in the current phase and intensity of the Arctic oscillation has major implications around the world.
This image shows the 500mb pattern in Western Russia and all of Europe on February 11t and the corresponding temperature anomalies as of February 11. Notice the extent of extreme temperature anomalies of -6 to -12 CELSIUS extend from central Russia across Poland, and into France. Note that the Arctic air mass in this outbreak did not extend into Spain, Italy, or Southwest Russia.
Here are some MIN temperatures reported in Europe on February 13 and 14. It is safe to say –with sub-zero or readings and temperatures in the single digits in much of central and eastern Europe as well as the Baltic the Ukraine and Belarus -that Europe was at this time, consumed by a significant Arctic outbreak.
These maps show MIN temperature readings on the morning of February 15 and February 18 in all of Ukraine and most of western and central Russia. Widespread Sub-Zero readings were common in the coldest air east of the Ural Mountains in central Russia. Not record-shattering cold but clearly, a widespread significant Arctic outbreak enhance by the strongly negative AO.
However, with the breakdown of the sustained strongly negative Arctic Oscillation, the pattern in Europe and western Russia is undergoing a dramatic shift. This image shows the anticipated 500mb map from the GFS model valid for 2/24/21.
As you can clearly see, a massive powerful Ridge has developed across all of Europe as well as into the Baltic and most of the Ukraine. The closed 500 LOW in the central North African coast establishes this strong Ridge in Europe as a REX BLOCK — which strongly implies that it is likely to last in this position for several days. The image on the right shows the temperature anomalies anticipated today 2/22/21 in Europe. Notice the widespread + 9 to + 15 degrees Celsius temperature anomalies; Not actual surface temperatures but readings above normal by +9 to +15 degrees C.
A truly stunning pattern reversal flip from last week.
In China , the warming pattern has been underway for the better part of the last 10 days. This 500mb map shows a surprisingly mild pattern for most of eastern Asia including a good portion of China and far eastern Siberia with temperatures well above normal. Note the large expensive Ridge from Kazakhstan into central and northeast China and then into eastern Siberia. Notice that temperature anomalies are simply off the charts here In Mongolia and Northern China ranging from + 5 to + 12 degrees C
The extremely mild pattern continued and if anything intensified over the past several days so that on 2/20/21 northern China set a record for the warmest February temperature ever reported in Beijing — at 78 degrees F . Note the model data that si indicative of widespread temperature anomalies between + 10 + + 16 degrees C above-normal and more + 10 degrees above the previous record
And finally here in the CONUS, we can see the dramatic change in the temperatures when we compare them to actual readings from 2/15//21 — when the AO was still negative , vs the forecast temperatures for the afternoon of February 26th
I am not saying that the winter is coming to an end because certainly, that is not true.
But what I am saying is that the risk of seeing widespread significant cold air outbreaks which will last for an extended interval is going to be dramatically restricted because of this change in both the Arctic Oscillation and the North American Oscillation.
There is still a reasonable possibility that either one of these teleconnections or both may flip back into a negative phase by the middle of March. That would set the stage for one last winter hurrah in the northern hemisphere