Leave it to me to write about this mild winter on the coldest day of the season. There is our mean warming climate involved, but I would be hesitant to assign all the anomalous warmth this winter to global climate warming. I’m not trying to play word games, as the mean warming is certainly there in the background. What admittedly has me puzzled is how the upper air pattern came to being so different from most winters over the last 10 years.
I’ve written extensively on arctic latitude warming episodically weakening the polar jet stream because of the reduced temperature contrast between the high latitudes and the mid-latitudes. Here is one such article from Dec. 23.
What’s got me puzzled is why this winter features a greatly strengthened jet stream. Last week, the jet stream was helped out from an incredibly strong North Atlantic storm, and brought along a British Airways jet to a ground speed of more than 800 mph, setting a new transatlantic flight time record of 4 hours and 56 minutes from JFK to Heathrow. What’s made this mild winter different from most other mild winters in this last decade is the renewed strength of the polar vortex. When the vortex is strong, it tends to center itself closer to the north pole. In doing so, it bottles up most of the true polar air in the far north latitudes. Michael Ventrice of IBM’s The Weather Company just posted this forecast polar vortex position and amplitude for next Thursday evening.
If you look at those contour lines, it should be evident the flow around the arctic is more west to east rather than traversing high amplitude ridges and troughs as it has in most of our recent winters. Pacific air has been flooding the lower 48, and much of Europe and even Asia have been spared any lasting arctic blasts.
With this very strong polar vortex has come the lack of blocking patterns in the northern hemisphere, which ordinarily result in the periodic delivery of arctic blasts. Ideally, for North America, a blocking ridge of high pressure near Greenland and another over the western part of the continent tends to force the polar jet to buckle southward. In this NOAA diagram, the basic circulation of a strong polar vortex is on the left, and a weakened polar vortex is on the right.
Before mean global warming had accelerated, the strong polar vortex was often dominant for large portions of the winter, with occasional weakening resulting in polar surges southward. In recent years arctic warming has reached unprecedented levels, making a weak polar vortex more common. This winter the polar vortex took up a position with unexpected (early on) persistence. Again, from IBM’s The Weather Company here is the February upper air pattern in the European ensemble.
There are some subtle changes which show up, in which the polar vortex starts to split and buckle. However, the limited blocking which may result from time to time during this month is in all the wrong places for persistent cold for the United States. Instead of a blocking high over Greenland, there is low pressure. The western ridge is out over the Pacific instead of western North America.
One byproduct of the persistent strong polar vortex is sea ice at polar latitudes is the most widespread in 11 years, after near record low levels in September. Some of the new ice will be thin because starting out from near record low ice mass doesn’t leave as much time for refreezing in what had been warmer waters.
Snowfall amounts are coming up short at many locations. Buffalo, at 56.2” is not that far off the average mark of 70” for Feb. 13th. But a few other locations in the east have amazing snow deficits. Philadephia: 0.3” for the season, NYC at Central Park: 4.8”, and National Airport in D.C. has only 0.6” so far.
Should this persistently strong polar vortex finally break down in April, that could spell problems for growers. From the US Geological Survey, here is an animated map of premature leaf growth which has already occurred over much of the south, where it has been warmer than compared to their normal winters.
This means should some April frosts occur, growers could take a hit, as our region did in April 2012, following a record warm March.
What I’ve told you here are some of the “whats” of the mild winter pattern. As for the “whys” of regained dominance of a strong polar vortex, the answers are not yet in. Most of you have heard of El Nino, which we are not currently experiencing. There are quite a few other oscillating air/ocean patterns which have played a role in the global circulation this winter. The phases of these oscillations (the Arctic Oscillation, the North Atlantic Oscillation, and the Madden-Julian Oscillation, to name a few) have appeared to work in concert for favorable conditions for the lack of blocking and the strong polar vortex.
In the meantime, local skiers still have pretty decent conditions of their own for skiing this weekend.