Stratospheric Warming Events May Be Suppressed This Winter

Latest data suggests the probability for stratospheric warming events is relatively low for much of the winter. While they are not completely ruled out of possibility, the likelihood of them happening is reduced.

One culprit of this suppression of sudden stratospheric warming events (SSW events) is the below-normal state of the 100 millibar poleward eddy heat flux. To summarize, the poleward eddy heat flux, when above normal, helps initiate SSW events and helps to weaken the polar vortex by enhancing these attempts to push warm air into the stratosphere to kick off those SSWs. Below normal heat flux values suggest a stronger polar vortex, as the willingness of the stratosphere to kickstart SSW events is lower than normal. We have seen a rise in the lower stratosphere eddy heat flux values, but I doubt we will be seeing significant warming events in the stratosphere until this factor, among others, complies.

Keeping with the observed conditions in the stratosphere, temperature data across the entirety of the stratosphere indicates temperatures are well below normal. While this chart for 50 millibars (commonly identified as the middle of the stratosphere) tells the story of a colder than normal stratosphere, you could look at any level of the stratosphere from the 1 millibar level (the top of the stratosphere) to the 70 millibar level (one of the lower regions of the stratosphere) and find the same result, which says the stratosphere is colder than normal. This really isn't good news for those wanting some stratospheric activity. It's one thing for one region of the stratosphere to be colder than normal, but it's a whole different matter when the entire stratosphere is colder than normal. If a specific region is colder than normal, it isn't a gigantic block to SSW chances. But with the entirety of the stratosphere colder than normal, the chances of warming events are significantly diminished. Add to that the below-normal eddy heat flux values and the final factor we will discuss below, and the stratosphere really isn't looking so hot, both literally and figuratively.

To add to all the troubles the stratosphere is having, the Quasi-Biennial Oscillation (QBO), a wind-related oscillation entrenched in the stratosphere, is doing nothing to help matters. The QBO switches between positive/westerly and negative/easterly phases, where the positive QBO strengthens the polar vortex and the negative QBO weakens it. The Quasi-Biennial Oscillation had previously been in a deep negative trend, as shown by the solid negative phases and significantly weakened positive phases over the past few years. Now, the new positive QBO is breaking through that barrier and is at levels not seen for several years. To summarize, this positive QBO is bad for stratospheric warming prospects, especially now that the +QBO continues to strengthen.

Michael Ventrice has been able to divide the QBO into eight phases, not unlike the Madden-Julian Oscillation. Each phase has its own mid-level (500 millibar) height anomaly distinction, as Mr. Ventrice's composite chart for December-January-February shows below.

As you can see, Phase 6 of the QBO (which we are currently in) promotes significantly below-normal height anomalies across the Arctic Circle, leading to ridging over the Southeast and East Coast, in addition to ridging over the Gulf of Alaska. This has been displayed well in the recent weather pattern, and medium-range forecasts show no end to the Phase 6 QBO. We are all looking forward to when (or if) the QBO enters Phase 7, which is projected to happen later on this winter. When the QBO shifts to phase 7, the composite chart tells us that high pressure will dominate the upper latitudes, leading to a favorable winter pattern over the Central and Eastern US. Now, this does not mean that the weather pattern will always be high pressure over the Arctic and cold in the Lower 48. But it does mean that those conditions become more possible/probable with the emergence of that Phase 7 QBO. However, until that Phase 7 evolves, we're really out of luck in the stratosphere, as the unfavorable QBO, anomalously weak eddy flux values and colder than normal stratosphere temperatures combine to create a trifecta of unfavorable SSW conditions.

Andrew

ALSO: ICYMI:
-December 2-4 Potential Blizzard
-December 5-7 Potential Winter Storm
-December 8-9 Potential Southeast Snowstorm/Ice Storm

Data in this post was contributed by Eric Webb.