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  1. 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. 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. 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. 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. 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. 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 January February 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: GFS: http://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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