Tuesday, September 30, 2014

Weak Stratospheric Warmings Suggest Turbulent Winter Ahead

The first substantial stratospheric warming event appears to be underway in the Northern Hemisphere, and could be a signal for what's ahead this winter.

The animation above shows temperature anomalies at the 10 millibar level of the stratosphere across the world over the past month. The Southern Hemisphere has been on the receiving end of more than one powerful sudden stratospheric warming (SSW) event, but we'll focus our attention on the Northern Hemisphere for now.

Take a look at what's been happening over Canada and into Greenland in recent days. We've been seeing warmer than normal temperature anomalies push northward from Canada into Greenland, and now across the Arctic Circle. In the winter, when these events happen, outbreaks of cold air may be expected at lower latitudes approximately 2-4 weeks after the event. This warming is pretty minor, so I wouldn't expect any significant cold to come from it, but there's more to this whole process.

If we look at the same animation, now at the 50 millibar level of the stratosphere (generally considered the middle of the stratosphere), we still see that swath of warmth pushing north from Canada into Greenland, and actually extending westward into the Pacific as the warmth is shoved into the Arctic. The presence of this warmth across the upper and middle stratosphere means that this isn't just a fluke warming, but in terms of an early-season stratospheric warming event, this one's pretty substantial.

All of this may be hinting at what kind of winter we'll see this season. If the stratosphere is already rather vulnerable to warm air invasions, it could mean that the polar vortex, which spans the troposphere and stratosphere, may be weakened significantly this winter, possibly leading to more frequent wintry outbreaks. The chance of this polar vortex weakening is raised by the expectation for stratospheric winds to be unfavorable for the formation of a strong polar vortex.

More on this will be analyzed in the near future.