Stratosphere - Polar vortex
In this first post, I will directly link the stratosphere analyses, from the FU Berlin, Institute of Meteorology. These analyses are sourced by ECMWF 12z runs.
Up in the stratosphere, there is a phenomenon called the Polar Vortex. It affects the Troposphere and the weather patterns and circulation over the entire North Hemisphere. The same phenomenon is also present on the south pole. Some say that the AO can be a indicator of the polar vortex, but that is not really true, since the main activity happens much higher than where AO is based (1000hpa). AO can basically reflect the changes, but the polar vortex changes cant be forecasted by just looking at the AO.
The most important things to monitor, are the geopotential height and the temperature. Stratosphere is very "tall", so we must look at several different layers.
This is just an analyse post. I will write something about the polar vortex in the next days.
As I promised, here is something about the polar vortex.
It is basically a large scale cyclone, situated over both poles. It is "based" in the upper troposphere and the entire stratosphere. it is the strongest in winter (unless a Sudden Stratospheric Warming occurs), when the temperature gradients are also the strongest.
This is the official definition:
"This is a phenomenon that occurs during the polar winter in which stratospheric air moves in a circular motion, with an area of relatively still air in its centre. The temperature in the vortex is approximately -80°C, which assists in the formation of polar stratospheric clouds. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America. It is important to note that the polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometers)."
I've put the words in bold, to point out, that AO (Arctic Oscillation) cannot be used for referring to Polar Vortex. Changes in the Polar Vortex do reflect in the AO (with quite some lag), till some extent of course, but that is about it.
Here is a Earth circulation schematic. Polar vortex affects basically the entire hemisphere. It is not really the same as a normal cyclone, because the air in the centre is descending. Kinda like a Hurricane.
For the polar vortex to "function" or spin normally, there must be allot of cold present. Warmth ( I mean higher than normal temperatures, not actual warmth), and of course consequently higher geopotential heights, disrupt the Polar vortex and can stop the circulation or even reverse it.
The warming and disruption is currently occuring at higher levels of stratosphere. It can lead to a SSW, but things are not certain yet about this top stratosphere warming. But, as the warm air surrounds the vortex, and penetrates into the core, the vortex can weaken dramatically at that level, and even stop or reverse.
We are going to witness a large "heat" wave 1 induced warming, that will disrupt the stratosphere for the first time since last winter. These waves usually occurs from blockings and heat flux, or a High in the upper stratosphere.
This wave that will break into the upper stratosphere, will be large enough to give a forecast reversal of mean zonal mean winds at the upper levels of the stratosphere. The vortex will probably not break up completely. It will be on the edge of existence, but I think that we will need another wave and subsequent warming to break the vortex.
The warming will propagate down to the 10 hPa level, but it is insufficient to give us a SSW at the moment.
But, the forecasts are very encouraging, because a strong Pacific High (ridge) is forecasted, and will have some effects on the vortex positioning in the troposphere. It is possible, that this Pacific ridge builds into a block over Pacific, which would be a good condition for another heat flux wave, from the troposphere into stratosphere, for an eventual SSW. That would be a real game changer, and a very decent winter could happen if the SSW occurs. The problem is still the time and the Lag effect (delay). It takes some time for these large scale processes to reflect down to the lower levels of atmosphere, troposphere.
This is all for now. Somewhere in the next few days, I will post something about the Sudden Stratospheric Warming (SSW), and its effects on the overall patterns, connection with the Quasi-Biennial Oscillation (QBO), and so on,... So I will develop this topic in sequences and not all at once, because that would be too much data and information at once. :)
A year has gone by and I guess its time for some more info on the current state of the stratosphere and the polar vortex.
The polar vortex is going to split up, and I am optimistic, that this year we could see a bit more intense "SSW" action than in the 11/12 season. Mainly because of the overall setup, which looks a bit different than last year.
First I am going to make the reanalysis of the 500mb pattern composite from October 1. to November 10., just to show how we stand against last year.
I don't think any comment is needed, because its clearly obvious that this year the overall pattern looks almost reversed, when compared to last year. What I like the most, is the strong Aleutian blocking and the warm AMO ridging.
Now I think we all know there are also significant changes in the ocean oscillations compared to last year.
Most obvious are the "neutral warm" ENSO regions, with ENSO4 a bit more intense than the rest. Also worth pointing out is the well defined warm AMO area, to support Atlantic ridging and hopefully some heat fluxes. :D And the "warm cored" cold PDO area (as funny as it may sound).
The AO and NAO have been responsive to all that.
Last but not least, QBO.
As usual, QBO propagated downward. Lost a bit of its intensity, but still in the game, at least in my eyes.
This is just an intro. I'm going to post more info and some forecasts on the stratosphere in the near future. :)
The forecasts for the stratosphere at mid and higher levels, look really amazing. I have never seen anything like it, not even in the archives.
Warming events like this, would really disrupt the polar vortex, and if this would really happen and downwell into the troposphere, it could be a real game changer. Depending where the leftovers of the polar vortex would fall, we could have either summer in January and February, or a mini ice age.
While models always seem to show snow scenarios and then removing them or just keeping them in the future forever, the stratosphere is a different story. The general idea with the warming and the polar vortex disposition remains more all less the same, with minimal "away moving". The models are quite accurate for stratosphere, basically because it is on a "flat" calculation surface, and the features are on a very large scale, so it is easier for the model to forecast activities in the mid and upper stratosphere. The accuracy of models for stratosphere in the 336-384 hours, is the same as the 500mb forecasts in the 120-144 hours.
The latest runs are basically already carving out the "R.I.P Polar Vortex" tombstone, or at least for its influence, hinting at a possible "official" SSW event around January 10th.
ECMWF is also picking up the warming in the mid and upper strat. and will possibly continue to do so as the "main events" move closer to the 240h time frame. And with the wave1 picking up the pace again throughout the whole 0-10 day timeframe, the vortex already began its displacement mode.
The forecasted sounding for the point where EC has Tmax at 10mb at 192h. You can see the the warm air intrusion fairly good in the mid and more the upper strat.
With the High strengthening and putting more pressure on the Polar Vortex, along with the warm air intrusion, the wind forecasted beyond 300h, shows a zonal reversal over the pole.
So basically, quite a clash forming above our heads.
It is still too far away to start speculating on possible effects on the troposphere and the winter in Europe.
Ok. An update on the effects of the SSW.
The first cold weather we had, around the 14th, was SSW connected. That was the result of the SSW polar vortex disruption, and the wave 2 vortex split. The wave 2 was coming strong from the Atlantic, so when the split occurred, it was more or less instantaneous throughout the troposphere and stratosphere. Basically geopotential height rises near and over the pole.
A few graphics.
And just prior to the cold/cool shot:
As for this second cold shot, it was amongst all, also an SSW response. I say amongst all, because the troposphere has its own dynamics and forcings besides the stratosphere, so SSW is not the main factor, but it plays an important role. In blue, I marked the height rises connected with the first split. And in green, we actually have the real SSW effect infiltrating into the troposphere, with respect to the SSW response time-lag climatology.
This SSW had a decent downwelling.
And on this graph, we can see a bit more obviously, how the SSW induced effect downwelled.
And a few graphics:
I write on many different forums (some of you maybe know me as Recretos on the Netweather forum), and on most of them if not all, people were seeing this SSW as a 100% guarantee for a severe winter or cold period. But that is not how stuff works. At least on NW forum, me and some others were trying to point out the fact that even tho an SSW statistically increases chances for EU cold shots, it cannot 100% guarantee its occurrence and especially not its longevity, because of the constant tropospheric dynamics and forcings constantly mixing in this year, especially the MJO.
So the bottom line is, both cold shots had a "connection" with the stratosphere and the SSW, especially the last one.
This is probably one of my last post on the stratosphere in this winter, because I kinda shifted my focus from the stratosphere on the 2013 storm season. I am already doing reanalysis, and preparing analog forecasts and comparisons of the 2013 storm season with past seasons and years. :)
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