Tuesday, December 22, 2015

Upper Stratospheric Polar Vortex Warming in Short Term

(Note: This post discusses the stratospheric polar vortex in the short term (out to Day 10). The other stratospheric post discusses expectations next month.)

The upper stratospheric polar vortex is forecasted to experience some intensive warming, in the atmosphere's attempt to get this winter started.

The image above shows a graph of the forecasted Wave-1 temperature outlook from the ECMWF model, 10 days from today. While daunting at first glance, it can be explained.
The different levels of the atmosphere are listed on the left-hand legend. We can see the graph covers the atmosphere from the 1000-millibar level up to the 1-millibar level. Note the warmest colors centered right around the 3-millibar line. The bottom legend shows lines of longitude.

Let's depart this forecast for a moment and discuss different modes of stratospheric vortex disruption. There are two primary modes of disruption: Wave-1 and Wave-2. In a Wave-1 stratospheric polar vortex disruption event, a single ridge / body of warm temperatures forms aloft and singlehandedly tries to displace the polar vortex. This singular body of warmth / ridging is why it is called a Wave-1 event- Wave-1 events are also the strongest type of disruption event. Similarly, Wave-2 events involve two bodies of ridging / warmth trying to squeeze into the Arctic Circle, usually to split the polar vortex into two vortexes. This is weaker than a Wave-1 event, but still wields significant power in the atmosphere.

This temperature chart above shows a temperature spike at the 3-millibar level. Remembering this chart specifically identifies Wave-1 events, we can then deduce that the ECMWF model is expecting a Wave-1 disruption attempt at the 3-millibar level of the atmosphere. We can look at this on a 3-millibar forecast map below, valid for the same timeframe as the graph above:

Note how we see a single body of warm temperatures on the right-hand side of the hemisphere (if you look closely, the warmth is centered over Eurasia), visually showing that Wave-1 pattern I discussed earlier.

Why is this important? It means that the atmosphere is applying pressure to the stratosphere to try and make the pattern more conducive for wintry weather here in the troposphere, something that's been lacking this December. Now, vortex disruption at the 3-millibar level won't do much down here at the surface, but if it can expand to lower levels of the stratosphere (ideally 30, 50, and/or 100-millibar levels), the influence becomes greater on us here at the surface.

To summarize:

- The upper portion of the stratospheric polar vortex looks to experience warming in the next 10 days and beyond.
- While insignificant at that height of the stratosphere (3-millibar level), it could indicate a pattern more conducive for wintry weather may set up down the road (i.e. into next month).


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