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Stratosphere Temperature Watch 2014/2015


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With winter soon approaching it is time for a new thread. This is the sixth winter that the strat thread will be running!

 

As ever, the first post will become both a reference thread and basic learning thread for those wanting to understand how the stratosphere may affect the winter tropospheric pattern. And then I will have a look at how we may expect the stratosphere to behave this year.

 

The stratosphere is the layer of the atmosphere situated between 10km and 50km above the earth. It is situated directly above the troposphere, the first layer of the atmosphere that is directly responsible for the weather that we receive. The boundary between the stratosphere and the troposphere is known as the tropopause. The air pressure ranges from around 100hPa at the lower levels of the stratosphere to around 1hPa at the upper levels. The middle stratosphere is often considered to be around the 10-30hPa level.

 

post-4523-0-14411900-1413045000_thumb.jp

 

Every winter the stratosphere cools down dramatically as less solar UV radiation is absorbed by the ozone content in the stratosphere. The difference in the temperature between the North Pole and the latitudes further south creates a strong vortex – the wintertime stratospheric polar vortex. The colder the stratosphere, the stronger this vortex becomes. The stratospheric vortex has a strong relationship with the tropospheric vortex below. The stronger the stratospheric vortex, the stronger the tropospheric vortex will be.

 

The strength and position of the tropospheric vortex influences the type of weather that we are likely to experience. A strong polar vortex is more likely to herald a positive AO with the resultant jet stream track bringing warmer wet southwesterly winds. A weaker polar vortex can contribute to a negative AO with the resultant mild wet weather tracking further south and a more blocked pattern the result. A negative AO will lead to a greater chance of colder air spreading to latitudes further south such as the UK. So cold lovers will look out for a warmer than average polar stratosphere.

 

 

post-4523-0-16995200-1413044984_thumb.jp

 

 

The stratosphere is a far more stable environment then the troposphere below it. However, there are certain influences that can bring about changes - the stratospheric ozone content, the phase of the solar cycle, the Quasi Biennial Oscillation ( the QBO), wave breaking events from the troposphere and the autumnal Eurasion/Siberian snow cover to name but a few.

 

The ozone content in the polar stratosphere has been shown to be destroyed by CFC's permeating to the stratosphere from the troposphere but there can be other influences as well. Ozone is important because it absorbs UV radiation which creates warming of the stratosphere. The Ozone is formed in the tropical stratosphere and transported to the polar stratosphere by a system known as the Brewer-Dobson –Circulation (the BDC). The strength of this circulation varies from year to year and can in turn be dictated by other influences.

 

One of these influences is the QBO. This is a tropical stratospheric wind that descends in an easterly then westerly direction over a period of around 28 months. This can have a direct influence on the strength of the polar vortex in itself. The easterly (negative ) phase is though to contribute to a weakening of the stratospheric polar vortex, whilst a westerly (positive) phase is thought to increase the strength of the stratospheric vortex. However, in reality the exact timing and positioning of the QBO is not precise and the timing of the descending wave is critical throughout the winter.

 

The direction of the QBO when combined with the level of solar flux has been shown to influence the BDC. When the QBO is in a west phase during solar maximum there are more warming events (increased strength BDC) in the stratosphere as there is also during an easterly phase QBO during solar minimum.( http://strat-www.met...-et-al-2006.pdf) (http://onlinelibrary....50424/abstract)

 

The QBO is measured at 30 hPa and has entered an easterly phase for this winter. As mentioned warming events are more likely during solar minimum – solar flux below 110 units. Currently, we have just experienced a weak solar maximum and the solar flux heading into winter is slightly above 110 units. This doesn’t rule out warming events, but they will not be as likely unless the solar flux continues to drop prior to winter.

 

post-4523-0-88542200-1413045006_thumb.gi

 

 

 

One warming event that can occur in the stratospheric winter is a Sudden Stratospheric Warming (SSW) or also known as a Major Midwinter Warming (MMW). This as the name suggests is a rather dramatic event. Normally the polar night jet at the boundary of the polar vortex demarcates the boundary between warmer tropical and cooler polar stratospheric air (and ozone levels) and is very difficult to penetrate. SSWs can be caused by large-scale planetary waves being deflected up into the stratosphere and towards the North Pole, often after a strong mountain torque event. These waves can introduce warmer temperatures into the polar stratosphere and can seriously disrupt the stratospheric vortex, leading to a slowing or even reversal of the vortex. This year if the solar flux drops below 110 units then the chances of a SSW increase - as can be seen by the following chart.

 

post-4523-0-46395900-1413045176_thumb.pn

 

Any SSW will be triggered by the preceding tropospheric pattern - in fact the preceding troposheric pattern is important in disturbing the stratospheric vortex even without creating a SSW.  Consider a tropospheric pattern where the flow is very zonal - rather like the positive AO phase in the diagram above. There has to be a mechanism to achieve a more negative AO or meridional pattern from this scenario and there is but it is not straightforward.  It just doesn't occur without some type of driving mechanism. Yes, we need to look at the stratosphere - but if the stratosphere is already cold and a strong polar vortex established, then we need to look back into the troposphere. In some years the stratosphere will be more receptive to tropospheric interactions than others (such as the eQBO this year) but we will still need a kickstart from the troposphere to feedback into the stratosphere. This kickstart will often come from the tropics in the form of pulses of convection interacting with long wave undulations in the polar vortex which influence the positions of the sub tropical jet stream and polar jet streams respectively. The exact positioning of the large scale undulations (or Rossby waves) will be influenced by (amongst other things) the pulses of tropical convection (aka the phase of the MJO) and that is why we monitor that so closely. These waves will interact with land masses and mountain ranges which can absorb or deflect the Rossby waves disrupting the wave pattern further - and this interaction and feedback between the tropical and polar systems is the basis of how the Global Wind Oscillation influences the global patterns. The ENSO state will influence the GWO base state

 

If the deflection of the Rossby Wave is great enough then the wave can be deflected into the stratosphere. This occurs a number of times during a typical winter and is more pronounced in the Northern Hemisphere due to the greater land mass area. Most wave deflections into the stratosphere do change the stratospheric vortex flow pattern - this will be greater if the stratosphere is more receptive to these wave breaks (and if they are substantial enough, then a SSW can occur). The change in the stratospheric flow pattern can then start to feedback into the troposphere - changing the zonal flow pattern into something with more undulations and perhaps ultimately to a very meridional flow pattern especially if a SSW occurs - but not always. If the wave breaking occurs in one place then we see a wave 1 type displacement of the stratospheric vortex, and if the wave breaking occurs in two place then we will see a wave 2 type disturbance of the vortex which could ultimately squeeze the vortex on half and split it – a split vortex SSW. The SSW is defined by a reversal of mean zonal winds from westerly to easterly at 60ºN and 10hPa. This definition is under review as there have been suggestions that other warmings of the stratosphere that cause severe disruption to the vortex could and should be included.  http://birner.atmos.colostate.edu/papers/Butleretal_BAMS2014_submit.pdf

 

The effects of a SSW can be transmitted into the troposphere as the propagation of the SSW occurs and this can have a number of consequences. There is a higher incidence of northern blocking after SSW’s but we are all aware that not every SSW leads to northern blocking. Any northern blocking can lead to cold air from the tropospheric Arctic flooding south and colder conditions to latitudes further south can ensue. There is often thought to be a time lag between a SSW and northern blocking from any downward propagation of negative mean zonal winds from the stratosphere. This has been quoted as up to 6 weeks though it can be a lot quicker if the polar vortex is ripped in two following a split SSW.

 

One noticeable aspect of the recent previous winters is how the stratosphere has been susceptible to wave breaking from the troposphere through the lower reaches of the polar stratosphere - not over the top as seen in the SSWs. This has led to periods of sustained tropospheric high latitude blocking and repeated lower disruption of the stratospheric polar vortex. This has coincided with a warmer stratosphere where the mean zonal winds have been reduced and has led to some of the most potent winter spells witnessed in recent years.

 

We have also seen in recent years following Cohen's work the importance of the rate of Eurasian snow gain and coverage during October at latitudes below 60ºN. If this is above average then there is enhanced feedback from the troposphere into the stratosphere through the Rossby wave breaking pattern described above and diagrammatically below.

 

post-4523-0-62629500-1413045023_thumb.pn

 

 

And it appears that the reduction in Arctic sea ice may be contributing to this mechanism and this should be factored in to any forecast.  http://web.mit.edu/jlcohen/www/papers/Cohenetal_NGeo14.pdf

 

 

So that leaves us to the try and forecast what will happen in the stratosphere this year. Out of the many variables what we do know at the moment is that the QBO is descending easterly and that we are probably entering El Nino conditions – although weak presently. And as mentioned earlier, the level of solar flux is slightly above conditions that are favourable for SSW’s. Despite this, conditions are favourable enough to suggest that we will see a warmer than average stratosphere this year. Evidence of this may already be suggested by an enhanced BDC in the Southern Hemisphere leading to a possible early final warming.

 

 If we look at 500hPa analogue composites for comparable easterly QBO/ El Nino 'lite' years (holding off on solar flux analogues just yet) then we see that the suggestions are that the polar vortex will have positive anomalies in December and January, before the vortex gains strength later in the winter. I would put the likelihood of an SSW at around 80% with the peak time for this to occur around early January. http://www.columbia.edu/~lmp/paps/butler+polvani+deser-ERL-2014.pdf

 

December

 

post-4523-0-30196600-1413045055_thumb.pn

 

January

 

post-4523-0-03308700-1413045067_thumb.pn

 

February

 

post-4523-0-02366000-1413045077_thumb.pn

 

 

It’s a little too early to suggest how exactly this will effect the troposphere until we see other data - including the updated solar flux and ENSO as well as the SAI, SCE values. But all in all, if the stratosphere behaves as we expect at this point, then tropospheric northern blocking would be favoured during the winter leading to a negative AO index and mid latitude polar episodes being experienced.

 

But after last year, when the stratosphere cooled dramatically, it is best that we remain cautious and wait to see how cold the stratosphere becomes over the next 6 weeks prior to winter, and how this may subsequently affect the strength of the polar vortex.

 

As ever the best sites to monitor the stratosphere and forecasts are listed below:

 

GFShttp://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/

 

ECM/Berlin Site: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html

 

Netweather: http://www.netweather.tv/index.cgi?action=stratosphere;sess=75784a98eafe97c5977e66aa65ae7d28

 

Instant weather maps: http://www.instantweathermaps.com/GFS-php/strat.php

 

Analysis can be found here: http://acdb-ext.gsfc.nasa.gov/Data_services/met/ann_data.html

 

http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/

 

 

 

 

Previous NW stratosphere monitoring threads:

 

2013/2014 https://forum.netweather.tv/topic/78161-stratosphere-temperature-watch-20132014/

 

2012/2013 https://forum.netweather.tv/topic/74587-stratosphere-temperature-watch-20122013/

 

2011/2012 https://forum.netweather.tv/topic/71340-stratosphere-temperature-watch-20112012/

 

2010/2011 https://forum.netweather.tv/topic/64621-stratosphere-temperature-watch/?hl=%20stratosphere%20%20temperature%20%20watch

 

2009/2010 https://forum.netweather.tv/topic/57364-stratosphere-temperature-watch/

 

2008/2009 https://forum.netweather.tv/topic/50299-stratosphere-temperature-watch/

 

Here's hoping for another exciting and intriguing season.

 

Ed

 

PS I look forward to all the contributers on this thread. It has grown from strength to strength over the years which has helped increase our knowledge of this fascinating and important subject - and there have been a core of extremely knowledgeable contributers from both national and international quarters and I thank them all and ask them to keep the discussion coming!

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 Thanks for all the work you put in to this thread each year Ed and good to have you back, really looking forward to this coming winter and will follow this thread with much interest.

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rather early to be looking too closely at events in the strat but it cant do any harm to see a warming area on the asian side at the top of the strat being consistently shown at the back end of recent gfs runs. anything that might disrupt a cold organised strat vortex from forming will be welcome. i recall it was a series of small warmings early on that helped to set the building blocks for the frigid december in 2010.

 

not wanting to burst any bubbles but a quick read through the strat thread from this time last year is worth doing if only to bring a degree of pragmatism to what is inevitably, much anticipation of what the upcoming winter may have in store.

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Thanks all.

rather early to be looking too closely at events in the strat but it cant do any harm to see a warming area on the asian side at the top of the strat being consistently shown at the back end of recent gfs runs. anything that might disrupt a cold organised strat vortex from forming will be welcome. i recall it was a series of small warmings early on that helped to set the building blocks for the frigid december in 2010.

 

not wanting to burst any bubbles but a quick read through the strat thread from this time last year is worth doing if only to bring a degree of pragmatism to what is inevitably, much anticipation of what the upcoming winter may have in store.

 

It's certainly worth being pragmatic, Nick. I see that the GFS has started early with the gold at the end of the rainbow this year! The warmings at the end of the runs are almost an omnipresent feature over the last few years!

Great post Ed! Hoping to join in on the discussion this fall/winter.

Cheers from Toronto, On.

 

Great to see you back and look forward to your input.

Great opening post, going to be a fascinating season again. We will also have the GFS upgrade include better modelling of the stratosphere come November time..

I think that the upgrade isn't due until December now, Tony.

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Thanks all.

 

It's certainly worth being pragmatic, Nick. I see that the GFS has started early with the gold at the end of the rainbow this year! The warmings at the end of the runs are almost an omnipresent feature over the last few years!

 

Great to see you back and look forward to your input.

I think that the upgrade isn't due until December now, Tony.

 

not just gfs ed - i note yesterdays ecm output showed zonal winds dropping in the upper strat over the next 10 days (at a time when i suspect they should be headed the other way). looks like some wave 1 activity responsible ?

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Many thanks as usual Ed for getting the 'ball rolling', always a pleasure and interest to see the various posts and information that appear in here and to get involved at times too. After last winter I for one, along with many other individuals who always hope for an actual 'winter', will no doubt be hoping that all the usual initial signs and signals come to fruition. As you stated in your post the expectation is definitely for more northern blocking this winter and overall a much greater risk of more -ve AO and NAO conditions. Clearly whether the atmosphere and environment wish to play ball is another matter and only time will tell, but my optimism is particularly high this year, especially in comparison to this time last year and am expecting a very interesting winter ahead. We were essentially robbed of a winter last year and ended up with 6 months of autumn if you ask me, so here's hoping 'we' get some payback this winter.

 

Regards to all,

 

Matt.

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Thank you, Ed - a very comprehensive 'mode d'emploi' in your opener. :hi:

http://journal.frontiersin.org/Journal/10.3389/fphy.2014.00025/full?utm_source=newsletter&utm_medium=web&utm_campaign=Physics-w18-2014

 

Variation in solar irradiance is considered an important factor in natural climate forcing. Variations in the solar UV in particular are now regarded as a major source of decadal variability in the stratosphere, influencing surface climate through stratosphere–troposphere coupling. However, by analyzing meteorological re-analysis data we find that the magnitude of the solar controlled energetic particle forcing signal in stratospheric zonal mean zonal winds and polar temperatures is equivalent to those arising from solar irradiance variations during the Northern Hemisphere polar winter months. We find that energetic particle forcing drives warmer polar upper stratospheric temperatures from early winter leading to an anomalously strong polar night jet via modulation of the vertical temperature gradient. By midwinter the stratosphere–troposphere coupling pathway becomes analogous to the solar UV impact at high latitudes. This not only highlights the importance of the energetic particle forcing contribution to stratospheric circulation, but enables us to understand the pathways responsible for the previously reported energetic particle forcing impacts on the troposphere in terms of the coupling of solar UV forcing to dynamics in the latter part of the winter.

 

 

This article was published in the spring of this year so don't know if it has been discussed in other topics - didn't see it in the previous thread. It is looking at another aspect of solar influence on the winter polar stratosphere, namely energetic particle forcing.

 

Some have posited that sudden bursts of solar activity have scuppered forecasts - could this research offer validation of the idea?

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Always welcome this thread, its a good read for those looking at possible longer term trends and posts are always well constructed with evidence to back up thoughts - unlike the model thread which unfortunately at times can become full of posts with sensationalist or doom mongering overtones.

 

I remember this thread in Dec 2012 very well, with early signs of a SSW and alas this occurred.

 

Last year the thread was very quiet - associated with dire wintry prospects, indeed a way of monitoring cold potential is monitoring the amount of activity in this thread.

 

Reading the first post - yes some encouraging signs that this winter will not be a repeat of last year's, my only concern at the moment is solar activity, if this could go quiet in the next few weeks I'd be much happier, it does seem to be taking ages to quieten down this cycle, and I really do think low activity was a major factor in the cold conditions of the winters of 08/09 through to 10/11, the milder 11/12 winter when we saw stronger activity, is it a coincidence last winter was mild, just when solar maxima cycle peaked?

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First of all just to echo the sentiments from various people on here already, great to have the 14/15 thread up and running and thank you to Ed as ever for the wonderful introduction which will prove invaluable for both newcomers and the more seasoned members alike as a reference point.

 

A few signs starting to appear already that the BDC is a little more active this year, with the likely early final warming across the Southern Hemisphere and a slowly increasing positive O-Zone anomaly starting to show up across our half of the globe too over the past few days:

 

current_1.gifcurrent.gif

 

It is also noticeable that the more defined positive anomaly towards the Aleutians at present is aligned to 10 day forecasts at various levels of the stratosphere for a minor warming episode

 

ecmwf100f240.gifecmwf50f240.gifecmwf30f240.gif

 

Nothing too significant but to quote a troubled supermarket, every little helps at this very early stage of proceedings.

 

The GFS also shows this but highlights rather well the very minor nature of the warming:

 

post-1038-0-62832200-1413120382_thumb.gi

 

We can see this is being lead by suggested Wave 1 activity in current forecasting:

 

ecmwfzm_ha1_f240.gif

 

The other encouraging signs are the forecast "vortex" placement during the next 10 days from 30-100mb, indicating a good chance of helping the Eurasian snow cover advance further in the coming days.

 

All very early days but it is worth keeping an eye on the O-Zone concentrations and overall anomaly during the coming weeks from here:

 

http://exp-studies.tor.ec.gc.ca/e/ozone/Curr_allmap.htm

 

As Ed has already stated one would expect, judging by current events down under, a slightly more active BDC this year which will only aid attempts at keeping the Stratosphere a little more toasty than last year.

 

SK

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Cheers SK. All interesting and positive early signs. Out of interest the ECMWF and EUROSIP seasonal models will be updated mid-week this week. Neither, so far, are anywhere near blocked and signal a zonal-style winter. I'll comment on the latest update this week out of interest.

M.

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