Monday, July 29, 2013

2013-2014 Winter Update: Polar Vortex Likely to be Anomalously Weak This Winter

The polar vortex, a tight low pressure system in the Arctic Circle whose strength determines how much cold air North America may receive in the winter, appears to be headed for a weaker than normal stature this winter.

Two factors are contributing to this new development: the QBO and the stratosphere.

The QBO, also known as the Quasi-Biennial Oscillation, is an atmospheric oscillation that has a positive and negative phase. In the positive phase, a certain wind pattern in the stratosphere enhances the polar vortex, which in turn restricts cold air that may move to North America. On the other hand, the negative QBO permits a weaker polar vortex, resulting in more opportunities for cold weather in the winter.

This image may look a little intimidating, but bear with me, I'll explain it. This chart, developed by Mike Ventrice, shows eight different phases the QBO can take. As is shown on the left and right sides of this diagram, Phases 7, 8, 1 and 2 tend to be associated with the Easterly (Negative) QBO, while Phases 3, 4, 5 and 6 correspond with the Westerly (Positive) QBO. As the chart tells, we are in the mature stage of this positive QBO, something winter weather lovers don't want to see when winter comes around. I posted a few days ago about how the positive QBO is surviving in a very hostile environment, and this should allow the mature +QBO to stay close to the circle in the middle of the chart (this circle indicates the QBO is too weak to be in a certain phase) as it progresses into Phases 6, 7 and 8 by late fall. When November rolls around, I expect the QBO to be transitioning to a negative phase, which will give the green light for a weaker polar vortex.

While the QBO is in kind of a wait-and-see mode for when/if it will move to its Easterly (negative) phase, the current lower stratosphere temperatures are already hinting at a weaker than normal polar vortex this winter.

This image shows past observed temperatures at the 70 millibar level in the stratosphere; considering the stratosphere runs from the 100mb level to the 1mb level, these temperatures are considered to be in the lower stratosphere. Looking at the most recent plot of the red line (observed temperatures), we see it to be above the green dashed line, which defines the average temperature for that time of year. This means the stratosphere is running warmer than normal this summer. A warmer than normal stratosphere in the winter means the polar vortex is favored to be weaker than normal- in the same sense, below normal stratospheric temperatures would favor a stronger than normal polar vortex, and thus fewer opportunities for cold weather in lower latitudes of the northern hemisphere.

When looking at past years with stratospheric temperatures comparable to what we have seen this past spring and into summer, the following years came up:


So, I took a look at the winters of 2003-2004 and 1997-1998 in the stratosphere. Both winters had rather significant stratospheric warming events in the early winter, with warmings continuing into the later winter. This bodes well for the idea of a weaker than normal polar vortex this winter.

A new item I found was how the winter stratosphere (December-January-February) tends to reflect stratospheric height anomalies in the summer months (June-July-August).

Analysis of the geopotential height anomalies for the stratosphere from June 1 to July 26 shows a very strong tendency for positive height anomalies over the Northern Hemisphere. The strongest positive anomalies are observed over northeast Asia into Alaska and the Bering Sea. Lower height anomalies are displayed over Greenland and into eastern North America. While this summer to winter stratosphere correlation does not necessarily keep location in mind, the general gist you can get out of this image is that we are likely going to see positive height anomalies (high pressure) battling the polar vortex during the upcoming winter, aiding in the bid to weaken the vortex. If we see these positive height anomalies continue through August, I will be more comfortable with this idea of a weaker polar vortex for the upcoming winter.

The QBO moving from positive to negative (if not neutral-negative), along with an already-favorable stratosphere, tells me the chances of an anomalously weak polar vortex are on the 'likely' side.