When Does A Displaced Polar Vortex Become A Split Vortex?

By WUWT Regular “Just The Facts”

As discussed last week, the Northern Stratospheric Polar Vortex appears to have been displaced in January and now it appears to be splitting into two discrete lobes, i.e. see the image above with two areas of blue / cold air descending within the funnels/lobes of the Polar Vortex at 10 hPa/mb – 31 km – 102K feet. What follows is succinct summary of Polar Vorticity, followed by various current observations. If you aren’t familiar with Stratospheric Polar Vortexes, you can get acquainted here, here and here.

Planetary Vorticity is “generated by the rotating earth”, it “is zero at equator”, is at it’s “maximum at pole (one revolution per day)” and “is always positive (cyclonic [counter-clockwise in the northern hemisphere])”, Lyndon State College Atmospheric Sciences i.e.:

Polar Vortices are “caused when an area of low pressure sits at the rotation pole of a planet. This causes air to spiral down from higher in the atmosphere, like water going down a drain.” Universe Today

“The polar vortex extends from the tropopause (the dividing line between the stratosphere and troposphere) through the stratosphere and into the mesosphere (above 50 km). Low values of ozone and cold temperatures are associated with the air inside the vortex.” NASA

Below is Northern Hemisphere Area Where Temperature is Below 195K or -78C, and it shows very cold air within the Polar Vortex descended from 10 hPa/mb –  31 km – 102K feet down to 250 hPa/mb – 10 km – 34K feet, twice during January, 2014.

“During extreme variability of the Arctic polar vortex termed a “weak vortex event,” anomalies can descend from the upper stratosphere to the surface on time scales of weeks. Subsequently the outbreak of cold-air events have been noted in high northern latitudes, as well as a quadrupole pattern in surface temperature over the Atlantic and western European sectors, but it is currently not understood why certain events descend to the surface while others do not.” “The subdivision of such events into vortex displacements and vortex splits has important implications for tropospheric weather patterns on weekly to monthly time scales.” “Using reanalysis data it is found that vortex splitting events are correlated with surface weather and lead to positive temperature anomalies over eastern North America of more than 1.5 K, and negative anomalies over Eurasia of up to −3 K. The corresponding signals are weaker during displacement events, although ultimately they are shown to be related to cold-air outbreaks over North America.”  Mitchell et al. 2012 – Paywalled

Onto the observations. If you look at the following 4 National Weather Service – Climate Prediction Center Northern Hemisphere Temperature Analyses at 10 hPa/mb –  31 km – 102K feet showing the cold Northern Stratospheric Polar Vortex on January 7th and 11th, and February 7th and 8th, 2014 you can see the progression as the Polar Vortex was first displaced/squeezed and now spliting into two lobes:

Above you can also see an area of high pressure and warm air building between the lobes of the vortex. Polar Wind at 10 hPa/mb –  31 km – 102K feet clearly shows the two lobes of the Northern Stratospheric Polar Vortex spinning counter-clockwise (Click the Pic to Animate):

Also interesting is that Ozone Mixing Ratios at 10 hPa/mb –  31 km – 102K feet show the “Ozone Hole” within the Polar Vortex splitting:

The two lobes of the Polar Vortex aren’t just visible at 10 hPa/mb –  31 km – 102K feet, the funnels of the vortex also extend both up and down, as you can see in the following National Weather Service – Climate Prediction Center Height Analysis, which starts at 1 hPa/mb – 50 km – 164K feet and extends down to 100 hPa/mb – 15 km – 49K feet. The Vortex appears to split into two lobes/funnels at about 5 hPa – 35 km – 115K feet:

For those unfamiliar with the variation of pressure with height, this graphic may prove helpful:

So what is the result of this Polar Vortex behavior? “Large regions in northern Asia, Europe and North America have been found to cool during the mature and late stages of weak vortex events in the stratosphere. A substantial part of the temperature changes are associated with changes in the Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure patterns in the troposphere.” Kolstad et al. 2010

Here is Northern Hemisphere – Vertical Cross Section of Geopotential Height Anomalies and the Northern Annular Mode (NAM) or Arctic Oscillation (AO) Index, which shows large positive Height Anomalies and the AO swinging negative in January and early February:

And here is North Atlantic Oscillation (NAO) Index for the prior 4 Months, showing a positive swing in mid-January and remaining there until present:

Lastly, the causes of this year’s weak vortex events were discussed in depth last week on this thread, however two key drivers of recent Polar Vortex behavior appear to be Eddy Heat and Planetary Waves. In terms of Eddy Heat, i.e. “strong negative fluxes indicate poleward flux of heat via eddies. Multiple strong poleward episodes will result in a smaller polar vortex, Sudden Stratospheric Warmings and an earlier transition from winter to summer circulations. Relatively small flux amplitudes will result in a more stable polar vortex and will extend the winter circulation well into the Spring.” NOAA

Here you can see that 10 day Averaged Eddy Heat Flux Towards The North Pole At 100mb is near a record daily maximum as it was in early January when the earlier weak vortex event began:

In terms of Planetary Waves “a vortex displacement event is associated with anomalously high wave number-1 planetary wave activity entering the stratosphere and is characterized by a vortex with a comma-like shape that is shifting equatorward. Often this shifting occurs ‘‘top down’’ and the vortex has a baroclinic structure. Subsequently the Aleutian high, a weak anti- cyclone, encroaches over the pole and is especially dominant at lower levels.”

“A vortex splitting event is associated with anomalously high wavenumber-2 planetary wave activity entering the stratosphere. During such an event the vortex barotropically splits into two ‘‘daughter’’ vortices that tend to align along the 90°E – 90°W axis, with one centered over Siberia and the other centered over northeastern Canada (Matthewman et al. 2009, hereafter M09).” Mitchell et al. 2011

Planetary Wave 1 activity can be see on this Zonal Wave #1 Amplitude Jan, Feb, March Time Series showing strong Wave 1 activity in January;

but in February we are seeing more Planetary Wave 2 activity:

So if the Northern Stratospheric Polar Vortex splits and does not break up, and if  Mitchell et al. 2012 are correct, we should begin see “positive temperature anomalies over eastern North America of more than 1.5 K” and “negative anomalies over Eurasia of up to −3 K” in the coming weeks. We shall see.

For an array of real time Northern Stratospheric Polar Vortex graphs and graphics please visit the WUWT Northern Polar Vortex Reference Page.