Stratosphere and Polar Vortex Watch

[Blessed Weather]

If the GEFS forecasts are correct I wonder if we might be about to enter a phase where the Strat Vortex shrugs off the early season knocks and starts to ramp up in strength? If I'm reading the charts correctly, current forecast Wave 1 activity peaks by the 23rd Dec and this coincides with the Zonal mean u-wind speed at 10hPa 60N reaching a low point of 15 m/s (today's figure - 16th Dec - is 23 m/s; note the record for this date is 53 m/s in 2015). By 31st Dec GEFS are predicting the Zonal u-wind speed will reach 43 m/s.

And I've noted that the number of members forecasting a stronger-than-average Vortex by the end of the forecast period has been creeping up over the last few runs, to now reach 12 out of 21 members. And the Vortex at 10 hPa regains shape and moves closer to the Pole as the influence of the Alaskan ridge wanes.

I'm hoping one of the Strat experts will tell me I'm worrying over nothing!

Geopotential Wave 1 forecasts:

17th  25th  30th

Geopotential Height, Temp and Zonal Wind forecasts:

16th  23rd   31st

Edited December 16, 2017 by Blessed Weather
Correction to min zonal wind figure

[bluearmy]

Depends whether a strengthening upper vortex is worth worrying about BW - the displaced chap hasn’t allowed the upper strat imprint to make it to the trop re Canadian heights so perhaps if it gets closer to the pole there is a chance it could be split by some wave 2 which we could see as the Atlantic ramps up and no doubt collides at some point with the Russian ridge. Knowing our luck a split would leave something nasty  in a bad place upstream but you never know ! 

Edited December 16, 2017 by bluearmy

[Singularity]

Worth noting that we're moving into a pattern which GFS/GEFS is known to perform particularly badly; a mean trough in the N. Atlantic but with ridge building trend signals both across the Arctic and across NW. Eurasia. This being in large part due to how critical the intensity and broadness of the N. Atlantic trough is - two characteristics that the model (more than most; but others do have similar faults) tends to take too far at more than 6 or 7 day's range. Much of the remaining error relates to the rate of change in Arctic heights and their orientation, which is a great bother for all of the models.

With this in mind, I suspect wave-2 potential is being underplayed. How much, I'm not at liberty to speculate .

 

[Dennis]

its a very interesting setting......

[KyleHenry]

29 minutes ago, Dennis said:

its a very interesting setting......

Definitely interesting with the wind speeds reaching a record low from previous season’s data.

There is that dual issue arising. PV running at 20 m/sec but a lack of energy to complete the reversal. 

Pacific being so dormant and not providing the necessary energy into the wave. 

It’s all about the thermodynamics and the super El Niño of (2015) that released so much heat/energy.

PV displaced poleward, U winds down to approximately 5 m/sec is the best call I could make. 

As I said before we have the Troposphere to create colder interludes for the start of January 2018, but I’d need to see a SSW to visualise an extended period of cold for NW Europe. 

Nothing is set in stone and depending on fruition of the forecasted twin Pacific storms then models may change. If not mid January  2018 for the next window of opportunity. 

 

Edited December 16, 2017 by KyleHenry

[comet]

Trying to make sense of these two composite charts below. The first is from Dec 9 and the second Dec 13. The composite anomaly charts seem to show geopotential heights increasing with time over northern most Canada while decreasing over northern Europe and Russia yet when you look at the scale on the right this seems contradictory. Am I missing something here.

[KyleHenry]

31 minutes ago, comet said:

Trying to make sense of these two composite charts below. The first is from Dec 9 and the second Dec 13. The composite anomaly charts seem to show geopotential heights increasing with time over northern most Canada while decreasing over northern Europe and Russia yet when you look at the scale on the right this seems contradictory. Am I missing something here.

PV reforming/condensing with the predicted wave activity. Wave creating a tilt so Canadian sector heights increase.

Stratopause lowers in height from mesosphere level. Hence the differential in the two heights.

Edited December 16, 2017 by KyleHenry

[KyleHenry]

We witnessed the opposite during the Atlantic Hurricane season. 

Then the Tropopause heights increased upwards into the stratosphere. Allowed more convection and increased ACE index plus landfall storms. 

[comet]

 So the scale refers to differential between the two heights, thanks.

[Interitus]

12 hours ago, comet said:

Trying to make sense of these two composite charts below. The first is from Dec 9 and the second Dec 13. The composite anomaly charts seem to show geopotential heights increasing with time over northern most Canada while decreasing over northern Europe and Russia yet when you look at the scale on the right this seems contradictory. Am I missing something here.

The ESRL PSD composites automatically create the scale which can make direct comparisons difficult, but the scale can be overridden to make the plots consistent -

This shows the heights have fallen over Eurasia, but the height rises over Canada is not quite as intense. Comparison should be made with the mean charts which show that at first the vortex was being perturbed by a large eastward and poleward displacement of the Aleutian anticyclone causing a large wave break, but then the anticyclone weakens southwards allowing the vortex to recover -

[bluearmy]

Zonal flow on ecm is picking up at 60N as the focus of the strongest flow right up top shifts north from 40N where it's been for a while. Of course that's going to increase the U wind numbers that we generally see on graphs - how far down the increased flow gets at 60/70N is the question. The ecm is working the upper vortex clockwise from east Asia towards n scandi by day 10, getting a little closer to the pole with time. Evidence that a trop wave two is coming in around then days time, courtesy of the Pacific ridge and developing Russian,  downstream of the strong Atlantic jet running into w Europe.  

[comet]

3 hours ago, Interitus said:

The ESRL PSD composites automatically create the scale which can make direct comparisons difficult, but the scale can be overridden to make the plots consistent -

This shows the heights have fallen over Eurasia, but the height rises over Canada is not quite as intense. Comparison should be made with the mean charts which show that at first the vortex was being perturbed by a large eastward and poleward displacement of the Aleutian anticyclone causing a large wave break, but then the anticyclone weakens southwards allowing the vortex to recover -

 Would be interested to know how to override the scale. As you say the plots look more consistent and less confusing. Didn't think to check the mean my bad. Thanks.

This wave breaking will hopefully provide some fun and games when it works its way down into the trop.

[Interitus]

3 hours ago, comet said:

 Would be interested to know how to override the scale. As you say the plots look more consistent and less confusing. Didn't think to check the mean my bad. Thanks.

This wave breaking will hopefully provide some fun and games when it works its way down into the trop.

 

[comet]

5 hours ago, Interitus said:

 

Thanks again.

[Blessed Weather]

On 12/16/2017 at 14:51, Blessed Weather said:

(Snipped)

And I've noted that the number of members forecasting a stronger-than-average Vortex by the end of the forecast period has been creeping up over the last few runs, to now reach 12 out of 21 members.

Unfortunately (for those of us seeking cold) the trend continues today - based on the 12z run the number of GEFS members forecasting a stronger-than-average Stratospheric Vortex at 10hPa 60N by the end of the forecast period (01-01-18) has now increased to 15 out of 21.

[Dennis]

something realy to watch at strato lvls - a quick change noticed

[Blessed Weather]

A very interesting tweet by Dr Cohen based on GEFS 0z run but I must say I'm slightly puzzled by the Wave 1 observations because looking at the GEFS charts I see on Weatheriscool.com, based on the 12z run yesterday and today, the Wave 1 activity fades by the start of Jan 2018. My other observation is that if there is downward propagation, with the Strat Vortex u-wind speed forecast to rise to 49 m/s (which is above average) by 01/01/18, it suggests not such good news for favourable mid-latitude cold patterns in the trop?? As a 'learner' in this thread I'd again be interested in the views of others.

Here's the tweet. Then the Weatheriscool GEFS Wave 1 charts from 12z today.

Geopotential Wave 1 forecasts at 10 hPa 60N:

24th Dec  1st Jan

 

[bluearmy]

I assume he is pointing out that the strat currently has a ridge in the west of n America whereas the trop does not.

In ten days, the trop has a big n Pacific ridge. If you work the strat pattern west then you see that pattern in the trop with the ridge in the aleutians and the mobile flow across the Atlantic. it's still a wave one pattern although there are isgns of some wave two appearing 

[Mattias]

1 hour ago, Blessed Weather said:

A very interesting tweet by Dr Cohen based on GEFS 0z run but I must say I'm slightly puzzled by the Wave 1 observations because looking at the GEFS charts I see on Weatheriscool.com, based on the 12z run yesterday and today, the Wave 1 activity fades by the start of Jan 2018. My other observation is that if there is downward propagation, with the Strat Vortex u-wind speed forecast to rise to 49 m/s (which is above average) by 01/01/18, it suggests not such good news for favourable mid-latitude cold patterns in the trop?? As a 'learner' in this thread I'd again be interested in the views of others.

Here's the tweet. Then the Weatheriscool GEFS Wave 1 charts from 12z today.

There is a lag between the stratosphere and the troposphere so I think the downward propagation now suggested in the models will be of the negative zonal wind anomalies seen in the stratosphere during the next couple of days, se image below.

My interpretation of this situation is that the current disturbance of the stratospheric vortex is driven by upward wave activity flux and the wave 1 pattern with negative zonal wind anomalies in the stratosphere culminates in a couple of days from now. Then the models suggest downward propagation of the pattern back to the troposphere, with a deceleration of the zonal winds in the troposphere in the end of December and begining of January . This downward propagation is also related to downward wave activity flux in the lower part of the stratosphere which means that the stratospheric vortex can gain in strength from above when the upward wave activity flux fades. Its not fun to see the uptick in the zonal winds in the stratosphere but in this case it might be a price we have to pay to get something tropospherically out of the current disturbance of the stratospheric vortex, but that's just my thoughts.   

[Mattias]

Just want to add that I dont think that this possible downward propagation directly will result in a favorable pattern for cold weather in Europe since the rising geopotential heights (according to the easterly phase tilt by height during downward propagation) probably will be in the "wrong" location, at least initially. But if the rising geopotential heights can spread across the arctic area as in GFS 12z it would of course be a great thing.  

[Blessed Weather]

Thank you @bluearmy and @Mattias for your comments.

Hot off the press this evening is the latest blog from Dr Cohen with his thoughts on how things have developed so far this season and where we might end up going forward:

Taking a broader view and looking at the latest polar cap geopotential height (PCH) and vertical Wave Activity Flux (WAFz) plots, a troposphere-stratosphere-troposphere (T-S-T; Figure i) coupling event is underway (Cohen et al. 2007).  The first stage was the tropospheric precursor with warm PCHs in the troposphere only during the first half of December and ended this weekend.  During this stage cold temperatures dominated many regions of the NH including the Eastern US, Western Europe and East Asia.  This is followed this week by active energy transfer from the troposphere to the stratosphere all this week and peaking this weekend.  PCHs are predicted to turn warm in the stratosphere while turning colder in the troposphere.  The warming of stratospheric PCHS is related to a PV disruption and displacement towards Eurasia with a strengthening anticyclone or high pressure near Alaska and northwest Canada.  This stage is dominated by a turn to milder temperatures in the Eastern US, Europe with the possible exception of East Asia where temperatures are predicted to warm but could still remain below normal.  The final stage is the downward propagation of the circulation anomalies from the stratosphere to the troposphere.  This includes northerly flow across much of North America favoring relatively cold temperatures but westerly flow across Eurasia favoring relatively mild temperatures.

This latest predicted PV disruption looks to be minor.  Further, with the predicted weakening of Ural blocking subsequent WAFz or poleward heat transport will likely be weaker than the activity predicted for this week but something to watch.  Given that the PV disruption looks to be minor, I expect that the subsequent related tropospheric circulation will last on the order of three weeks or about half that following a major disruption.  The important question is what to expect after this T-S-T coupling event ends. I think three different scenarios are most likely.  One, the stratospheric PV rapidly strengthens favoring a mild pattern for much of the NH from mid-January on.  Two, a more robust PV disruption takes place favoring an overall cold mid- to late- winter across large parts of the NH. And third, additional relatively minor PV disruptions occur that favor more regional cold.  Given that the greatest sea ice anomalies this fall were in the Chukchi and Bering Seas, most likely favors North America for cold air outbreaks.  I continue to favor the second and third scenarios for the same reasons that I have been discussing since October. And based on the trends that I am seeing in the stratospheric PV, for now I favor the third scenario the most with the winter of 2013/14 being the closest analog.  I have been hesitant to jump on the 2013/14 bandwagon based on an easterly quasi-biennial oscillation (QBO) but so far there are no signs of a sizeable and prolonged disruption of the stratospheric PV.

Full blog: http://www.aer.com/science-research/climate-weather/arctic-oscillation

[Blessed Weather]

Last Thurs 14th Dec I posted a tweet from Dr Butler with a forecast of a promising spike in heat flux. The latest tweet (Mon 18th) now showing a forecast of the opposite - a drop off in heat flux.

 

[Interitus]

As suggested above this does look like another possible wave reflection event - the GEOS forecast in the MERRA2 data shows more negative vertical wind shear (or at least isotachic) in the coming days -

which is followed shortly after by Arctic high pressure in close proximity to the stratospheric high - watching the Hannah Attard 500/10mb animation shows how the tropospheric high seems drawn towards its stratospheric counterpart - http://www.atmos.albany.edu/student/hattard/realtime/dt_500/DT_500.php

Unsurprisingly Cohen is relating events to his own work which is based on the propagation of zonal anomalies. This typically occurs through wave-mean flow interaction, though a number processes have been suggested and it is open to debate. According to Perlwitz & Harnik this dominates over the longer monthly to seasonal time scales but like the use of many longer term teleconnections, it tends to be all a bit 'hand-wavy' and generalising.

A further possible drawback is highlighted by Plumb & Semeniuk who show that while wave-mean flow interaction can transfer zonal anomalies downwards without the action of wave reflection or 'downward control' - induced meridional circulations (eg BDO), rather like the QBO - "downward migration observed in the Arctic oscillation should not be taken to indicate any controlling influence of the stratosphere on the troposphere"

Reflection of zonal wave 1 however dominates up to around 12 days and as such is potentially visible in forecast charts.

 

[Interitus]

3 hours ago, Blessed Weather said:

Last Thurs 14th Dec I posted a tweet from Dr Butler with a forecast of a promising spike in heat flux. The latest tweet (Mon 18th) now showing a forecast of the opposite - a drop off in heat flux.

 

Problem is that using daily forecast values is prone to the fluctuations that afflict the troposphere more than a few days ahead. The choice of 100hPa eddy heat flux is not by accident, the low strat recognised as significant by many eg by Polvani & Waugh in their paper "Upward Wave Activity Flux as a Precursor to Extreme Stratospheric Events and Subsequent Anomalous Surface Weather Regimes" show empirically that 100hPa heat flux is strongly anti-correlated (-0.8) with 10hPa AO; Jucker et al quote an even stronger correlation for SSW frequency and heat flux at 97hPa as determined by a global climate model.

However, typically this correlation is found when the flux is averaged over a period of time - Polvani & Waugh used the 45-75° 100hPa flux over 40 days - handily the NASA MERRA2 chart above has a version averaged over 45 days which shows that while it has been increasing recently it has been generally quite modest this season -

This would seem to be at odds with the recent and forecast weakness of the vortex - for the GEOS forecast on 26/12 the averaged flux is 17.5 K m/s for a zonal wind of 20.22 m/s. The chart below shows December averaged heat flux vs 7 day minimum 10mb zonal wind. The correlation is a high end moderate -0.57 but there is a fair amount of scatter. The average 7 day zonal wind for a flux between 17.4 and 17.6 is 27.7 m/s but ranges from 54.81 m/s down to -11.38 m/s.

Like when considering wave heights for example, there is a general trend but the state of the vortex at the time and it's prior preconditioning would appear to be critical.

Edited December 19, 2017 by Interitus

[Singularity]

If there is a genuine downwelling of anomalies from the stratosphere in the works for next week, I can see why this is seriously messing with the models, as the alignment of tropospheric anomalies that this encourage is offset from those encouraged by tropical forcing.

Any over or underestimation of either of these two processes will have major ramifications going forward. Today the broad trend appears to have been toward more tropical and less stratospheric, but this could well reverse without much warning.

There are also knock-on effects in terms of further vortex perturbation, as more tropical forcing encourages a Pacific-Siberia ridge linkup that maintains WAFz when otherwise it subsides.