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  1. Here we go then, already plenty of interest in the strat this year, and with a La Nina likely, perhaps a less hardcore strat than last year can be expected? @chionomaniac will be along soon to fill in his thoughts on where things may be headed this year, but in the meantime, I've copied his excellent strat guide from 2015 below. For more info you can also read his full tutorial here: https://www.netweather.tv/charts-and-data/stratosphere/tutorial Ed's opener from 2015/16 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, so forgive me for some repeat from previous years, but it is important that those new to the stratosphere have a place that they can be directed to in order to achieve a basic grasp of the subject. 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 and the layer that is directly responsible for the weather that we receive at the surface. 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 below 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 increasing 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 polar stratosphere in relation to that at mid latitudes, the stronger this vortex becomes. The stratospheric vortex has a strong relationship with the tropospheric vortex below. A strong stratospheric vortex will lead to a strong tropospheric vortex. This relationship is interdependent; conditions in the stratosphere will influence the troposphere whilst tropospheric atmospheric and wave conditions will influence the stratospheric state. At the surface 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 and 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. AO chart The stratosphere is a far more stable environment then the troposphere below it. However, the state of the stratosphere can be influenced by numerous factors – the current solar state, the Quasi Biennial Oscillation (QBO), the ozone content and distribution and transport mechanism, the snow cover and extent indices and the ENSO state to name the most significant. These factors can influence whether large tropospheric waves that can be deflected into the stratosphere can disrupt the stratospheric polar vortex to such an extent that it feeds back into the troposphere. Ozone Content in the stratosphere Ozone is important because it absorbs UV radiation in a process that warms 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. The ozone content in the polar stratosphere has been shown to be destroyed by CFC's permeating to the stratosphere from the troposphere. The overall ozone content in the polar stratosphere will help determine the underlying polar stratospheric temperature, with higher contents of ozone leading to a warmer polar stratosphere. The ozone levels can be monitored here: http://www.cpc.ncep.noaa.gov/products/stratosphere/sbuv2to/index.shtml One of the main influences on the stratospheric state 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 thought 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 can be critical throughout the winter. Diagram of the descending phases of the QBO: (with thanks from http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/qbo/index.html ) The QBO has been shown to influence the strength of the BDC, depending upon what phase it is in. The tropical upward momentum of ozone is stronger in the eQBO , whereas in the wQBO ozone transport is stronger into the lower mid latitudes, so less ozone will enter the upper tropical stratosphere to be transported to the polar stratosphere as can be seen in the following diagram. http://www.atmos-chem-phys.net/13/4563/2013/acp-13-4563-2013.pdf However, the direction of the QBO when combined with the level of solar flux has also been shown to influence the BDC. When the QBO is in a west phase during solar maximum there are more warming events in the stratosphere, as there is also during an easterly phase QBO during solar minimum, so the strength of the BDC is also affected by this – also known as the Holton Tan effect . http://strat-www.met.fu-berlin.de/labitzke/moreqbo/MZ-Labitzke-et-al-2006.pdf http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50424/abstract http://onlinelibrary.wiley.com/doi/10.1002/2013JD021352/abstract The QBO is measured at 30 hPa and has entered a westerly phase for this winter. As mentioned warming events are more likely during solar maximum when in this westerly phase – with the solar flux below 110 units. Currently, we have just experienced a weak solar maximum and the solar flux heading into winter is still around this mark. This doesn’t rule out warming events, but they will not be as likely – perhaps if the solar flux surges then the chance will increase. Latest solar flux F10.7cm: http://www.swpc.noaa.gov/products/solar-cycle-progression Sudden Stratospheric Warmings: 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 mid latitude and colder polar stratospheric air (and ozone levels) and this is very difficult to penetrate. SSWs can be caused by large-scale planetary tropospheric (Rossby) 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 which can seriously disrupt the stratospheric vortex, leading to a slowing or even reversal of the vortex. 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 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 and patterns of convection. These can help determine the position and amplitude of the long wave undulations Rossby waves - that are formed at the barrier between the tropospheric polar and Ferrel cells. The exact positioning of the Rossby waves will be influenced by (amongst other things) the pulses of tropical convection, such as the phase of the Madden Jullian Oscillation and the background ENSO state 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. If the deflection of the Rossby Wave then a wave breaking event occurs – similar to a wave breaking on a beach – except this time the break is of atmospheric air masses. Rossby wave breaks that are directed poleward can have a greater influence on the stratosphere. The Rossby wave breaks in the troposphere can be demonstrated by this diagram below – RWB diagram: https://www.jstage.jst.go.jp/article/jmsj/86/5/86_5_613/_pdf 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 places at once then we will see a wave 2 type disturbance of the vortex which could ultimately squeeze the vortex on half and split it – and if these are strong enough then we would see a displacement SSW and split SSW respectively. The SSW is defined by a reversal of mean zonal mean 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 A demonstration of the late January 2009 SSW that was witnessed in the first strat thread has been brilliantly formulated by Andrej (recretos) and can be seen below: The effects of a SSW can be transmitted into the troposphere as the downward 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. A recent paper has shown how the modelling of SSW and strong vortex conditions have been modelled over a 4 week period. This has shown that there is an increase in accuracy following weak or strong vortex events – though the one area that the ECM overestimates blocking events following an SSW at week 4 is over Northwestern Eurasia. http://iopscience.iop.org/article/10.1088/1748-9326/10/10/104007 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. Six stage Cohen Process: The effect of warming of the Arctic ocean leading to colder continents with anomalous wave activity penetrating the stratosphere has also been postulated http://www.tos.org/oceanography/archive/26-4_cohen.pdf As ever, I will supply links to various stratospheric websites were forecasts and data can be retrieved and hope for another fascinating year of monitoring the stratosphere. 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: https://www.netweather.tv/charts-and-data/stratosphere Instant weather maps: http://www.instantweathermaps.com/GFS-php/strat.php NASA Merra site: http://acdb-ext.gsfc.nasa.gov/Data_services/met/ann_data.html Previous stratosphere monitoring threads: 2016/17 https://www.netweather.tv/forum/topic/86485-stratosphere-temperature-watch-201617/ 2015/16 https://www.netweather.tv/forum/topic/84231-stratosphere-temperature-watch-20152016/ 2014/2015 https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/ 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/ 2010/2012 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/
  2. With a potential cold end to winter and start to spring on the horizon, here's a thread to discuss the ins and outs of that, how the latest forecasts are looking and so on. There's obviously a lot of chat in the model thread about this currently, and you can also find info about the SSW over in the strat thread. Nick has also blogged about the SSW here: Sudden Stratospheric Warming This Weekend, But What Is It & How Will It Affect Our Weather? And about the model mayhem currently being caused by it here: Sudden Stratospheric Warming Brings Weather Model Mayhem It's fair to say that confidence in the exact weather we're going to see from mid-next week onward is currently very low, but the Met Office are confident enough in the likelihood of cold weather that they've recently put out a press release: https://www.metoffice.gov.uk/news/releases/2018/a-sudden-stratospheric-warming-and-potential-impacts-on-uk Winter could be set to go out with a bang it seems, but it's not nailed on, yet.....
  3. The goal of this thread is to create a valuable learning thread about long range forecasting. First, the concept of long range forecasting will be explained in short. Thereafter, we will have a global look at the GWO (Global wind oscillation) and how it affects our weather. Long range forecasting Long range forecasting (10+ days out) has proven to be a very difficult subject over the past several years. It is a timeframe where global models lose their deterministic value, although they can still be used as a guide for trends. It is also a timeframe where the presence or absence of tropical convection at a given place near the equator can change the complete midlatitude synoptic setting (this is showing some resemblance to the so-called butterfly effect). Fortunately, this is how far the bad news goes. Even though small details can change whole patterns, these details can be predicted to quite some extent and can even show a kind of cyclical pattern. This is, for example, the case for tropical convection activity anomalies (e.g. the MJO). That means that knowing how these patterns will develop makes one able to tell something about the weather at the midlatitudes, mainly through analogues of previous years which have seen a same kind of pattern. To make this recognition of patterns somewhat easier, teleconnections have been developed. Think of the GWO (Global Wind Oscillation, a recently developed index), MJO (Madden-Julian oscillation) and ENSO (contains and explains El Nino and La Nina) to name but a few. Aside from the indices listed above, a fairly new subject is stratospheric meteorology, which also has predictive value for forecasting, for example, the likehood of blocking developing at the midlatitudes. A separate thread can be found on this forum about this subject. The interesting, yet complicated, part comes when one tries to interpret one teleconnection separately. This is not possible, because all the teleconnections are interrelated. For example, ENSO has an effect on the convective anomalies in the tropics (which is, in very simple terms, where the MJO relies on). Therefore, if one wants to make a very good long range forecast, all factors need to be incorporated in one view. Glacier Point, an old member of this forum, is a master on this subject. For most of us, though, there is much that can still be learned about this. It would be nice to get as much input as possible on these teleconnections in order to make this a valuable thread in terms of long range forecasting all year round! GWO One of the several interesting teleconnections is the GWO (global wind oscillation). The part below may help in grasping the concept of this. Basics of the concept The GWO is an index which tells something about the amount and latitudinal localization of AAM in the atmosphere. Atmospheric Angular Momentum is a conserved quantity in the atmosphere. It is defined from the Earth' axis of rotation (so from the north pole through the Earth’ core up to the South Pole). We will regard the wind speed relative to the Earth’ rotation (so the wind speed we can measure). The image below gives a good representation of how this should be visualized. Visualization of AAM as it could be seen from viewing the Earth. Courtesy: COMET. AAM is, in terms of the atmosphere, equal to the velocity of an air parcel times the distance it is away from the Earth’ axis. For example, at the Equator, the distance of an air parcel to the Earth’ axis is very large. Therefore, it has a relatively low velocity. When the air parcel is being carried away from the Equator, its distance relative to the Earth’ axis decreases. That means the velocity needs to increase in order to maintain conservation of AAM. As a result, the parcel will accelerate. This is all under the assumption that the parcel does not exchange AAM with the surface or other air parcels. Near the equator, the wind is from west to east relative to the Earth. This, paradoxically, means the air is still moving from east to west, but at a slower speed than the Earth rotates itself. This all results in AAM being added to the atmosphere from the surface. At the midlatitudes, this situation is reversed. Winds tend to flow quickly from east to west at this latitude relative to the rotation Earth. This means that the air flows from east to west even faster than the Earth rotates itself. As a result, AAM is being lost to the surface due to this imbalance. The above yields a surplus of AAM at the equator and a shortage of AAM at the midlatitudes. This in turn creates a “flow†of AAM from the equator to the midlatitudes. The image above illustrates this well. Mountains (courtesy to Tamara for contributing in this part) Mountains can add and reduce AAM via torques (in terms of friction). This process is quite complicated, but it is an important factor for the GWO. Basically, this event can be thought of some kind of weather event colliding with a large mountain range (Rockies, Himalaya etc.). This torque mechanism can add or remove AAM from the atmosphere. Such mountain torque events can send Rossby waves into the stratosphere in a certain part of the Northern Hemisphere. The net effect of this is to create a disturbance to the polar vortex and a jet stream amplification which feeds downstream. In layman’s terms a mountain torque can affect the amount of amplification that happens downstream. If, for example, the Pacific jetstream collides at the Rockies, it may via complicated mechanisms (aka the Rossby waves mentioned above) cause amplification in the flow toward Europe, causing blocking to form. GWO orbit explained The GWO has a cyclical nature. This means that the GWO undergoes a kind of repetitive pattern, which can be explained by a circle diagram. Analogous to the MJO, the GWO has been divided in 8 phases, each with its own characteristics. All these phases are basically a follow-up of the phase before. The GWO orbit can be best seen as a measure for the total amount of AAM in the atmosphere. Below is the GWO orbit diagram with a brief explanation of what happens at every phase. Visualization of the GWO orbit In phase 1, negative mountain torque removes AAM from the atmosphere. The longer the GWO stays there, the lower the amount of AAM becomes in the atmosphere. This can be thought of a Jetstream colliding at a large mountain range Phase 2 and 3 generally describe low AAM values in the atmosphere (which is on average also occurring according to the conceptual model described above). In phase 4 and 5, positive mountain torque adds AAM to the atmosphere. The longer the GWO remains in that position, the higher the amount of AAM becomes in the atmosphere. Finally, phase 6 and 7 indicate high levels of AAM in the atmosphere. Concluding remarks There is much more that can be told about the GWO (and many other parameters), but that is for a later time! Any help or corrections in the explanation are greatly appreciated. Also, I hope many people will be willing to contribute to this thread! Here’s hoping that this will become a fruitful thread and a learning place for many! Useful links In the end, a list of links which could help for teleconnections are given here: GWO forecast: http://www.atmos.albany.edu/student/nschiral/gwo.html GWO composites: http://www.atmos.albany.edu/student/nschiral/comp.html MJO forecasts: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml MJO composites: http://www.americanwx.com/raleighwx/MJO/MJO.html Update on tropical weather (expert assessment on tropical convection, including the MJO, great link): http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/ ECMWF stratosphere forecast: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/ Stratosphere updates: https://forum.netweather.tv/topic/81567-stratosphere-temperature-watch-20142015/ GWO further reading: http://www.esrl.noaa.gov/psd/map/clim/gwo.htm Sources: https://www.meted.ucar.edu/ http://www.esrl.noaa.gov/psd/map/clim/test_maproom.html
  4. 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!
  5. Around this time of year I often search out new papers to assist in winter forecasting. However, quite often I lose the links to these papers by the time winter arrives. So, I think it makes sense to have a drop off zone for these type of papers that I and others come across. Please post in here any abstracts or PDF links that you may find of interest. A brief description of the paper would be most welcome. ( No climate change papers please)
  6. Hey Everyone, Been following this forum since the early days, and it really helped me in ''finding my feet'' with everything weather. Now I'm in my final year of studying Meteorology at University. The stratosphere thread really introduced me into that fascinating research area, and now I have chosen to base my dissertation on just that. I will be looking into the troposphere-stratosphere part of Cohen's research on the October snow advance in Eurasia. I will be using a GCM to (hopefully) simulate the results of this tropospheric precursor. I've come to you guys to ask for your help with calculating the eddy heat flux for perturbed snow extents, and really anything related to increasing my understanding of eddy heat fluxes! I've tried to play around with the ECMWF ERA Interim, but failed... Many thanks, Tom. P.s. While researching up-to-date journals, this might be of interest to some: http://onlinelibrary.wiley.com/doi/10.1002/joc.3968/full (Relationships of the wintertime AO with the October circulation anomaly over the Taymyr Peninsula) ''The main new finding of the study is an involvement of the processes spanning the whole depth of the troposphere in October in exciting of the wintertime AO. These processes are the cause of the October anomalies of snow extent and snow extent increase rate rather than their consequences''
  7. With winter fast approaching it is time for a new thread. It seems that the demand for a new strat thread gets earlier every year and this is the fifth 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 becomes. 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 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 during an easterly phase QBO during solar minimum.( http://strat-www.met.fu-berlin.de/labitzke/moreqbo/MZ-Labitzke-et-al-2006.pdf) (http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50424/abstract) The QBO is measured at 30 hPa and has entered a westerly phase for this winter. 10 out of 11 warming events that have occurred during a wQBO have occurred at a solar maximum. Even though we are seeing a quiet maximum one would expect a slightly increased chance of a warming event this 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 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 can occur by the vortex being displaced off the pole – a displacement SSW, or by the vortex being split in two – a splitting SSW. The 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) but we will still need a kick start from the troposphere to feedback into the stratosphere. The kick start often will come from the tropics in the form of pulses of convection interacting with slight 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 can affect this - giving us the underling atmospheric 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 NH 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. 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 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 it appears that the reduction in Arctic sea ice may be contributing to this mechanism and this should be factored in to any forecast. So what are we likely to expect this year. Well, so far we know some of the factors. There is a wQBO combined with probable ENSO neutral conditions and even though we are at a possible solar maximum, this is about half the maximum seen in recent times. However, I think that there is still a strong possibility that we will see a SSW - possibly late January or early February this winter. Using similar years analogues we can help predict the likely stratospheric conditions for this year. Unfortunately, there is a shortage of comparable years that meet the above data - but at least we can get an idea. And with another low Arctic sea ice year and early snow gain already then perhaps by the end of the month confidence can increase. Here is the predicted stratospheric height anomaly pattern for this winter at 30 hPa. November Here we see a strong central vortex beginning to build that is well established by Dec December However the height anomaly at latitudes south of the vortex suggest to me that there is some wave propagation from the troposphere - and this builds through January January before overwhelming the vortex and weakening it during February ( remember that the anomaly's are for the whole month and the vortex will have recovered somewhat from any warming episode) February - Split type SSW? The suggested pattern of warming at 30 hPa for January and February are as follows: January Feb So, during the coming months we will need to keep an eye on all the stratospheric data. Here are the links: Forecasts: ECM http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html GFS http://www.cpc.ncep.noaa.gov/products/stratosphere/strat_a_f/ and here: http://www.netweather.tv/index.cgi?action=stratosphere;sess=75784a98eafe97c5977e66aa65ae7d28 http://www.instantweathermaps.com/GFS-php/strat.php Analysis here: http://acdb-ext.gsfc.nasa.gov/Data_services/met/ann_data.htmland here: http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/ Many papers for further reading found here: http://forum.netweather.tv/topic/73911-technical-teleconnective-papers/ Fingers crossed for another exciting and productive stratospheric watch this winter. c
  8. Welcome to the new season stratosphere thread for the 2012/2013 stratospheric NH winter. With the excitement building and expectations high for the coming winter, the role of the polar stratosphere will play an important part in determining what type of winter we shall have. As ever for those new to the stratospheric input I will include in this post a basic guide to how the stratosphere may influence tropospherical weather systems before looking at what we can expect this winter. 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 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 becomes. 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. 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 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 during an easterly phase QBO during solar minimum. ( http://strat-www.met...-et-al-2006.pdf ) The QBO is measured at 30 hPa and has been in an easterly phase since August 2011 (http://www.esrl.noaa...lation/qbo.data). The easterly phase is likely to come to an end at 30 hPa over the coming winter, however, even after this we are likely to see easterly winds descend the stratosphere spreading polewards for some time yet. The easterly QBO winds can be demonstrated on the following zonal wind stratospheric profile chart: 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 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 can occur by the vortex being displaced off the pole – a displacement SSW, or by the vortex being split in two – a splitting SSW. 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. So the question to be asked is are we able to predict how the stratosphere is likely to behave this year. The real answer is not yet, though there are some aspects that we can use as a guide looking at previous years. The most useful of these is the easterly descending QBO. We know that the stratospheric polar vortex is a lot weaker in easterly years and more susceptible to disruption. Combine this with what is in effect a low solar maximum then this may enhance this effect. See post from GP in last thread regarding analogue year 1968-69 I have been collecting relavent papers regarding the role of the stratosphere and other influences and they can be found here - http://forum.netweat...nective-papers/ I am starting this thread earlier than normal because of the increased importance that has been placed on the role of the stratosphere since I first monitored a few years back. Rather than being viewed as a small piece in the jigsaw, it is being realised that the state of the stratosphere can/may overrule all other teleconnective pieces. Last years cold stratosphere demonstrated this only too well. So it is eyes down (or up!) in the coming weeks to monitor how the polar stratosphere cools and what affect this has on the strength of the stratospheric polar vortex. There are a number of sites that provide information regarding this. Firstly, the two important sites that can be used to look at the temperature profiles are: http://acdb-ext.gsfc...t/ann_data.html and http://www.cpc.ncep....re/temperature/ Graphically and previous years information : http://www.cpc.ncep....ere/strat-trop/ and http://www.geo.fu-be...pole/index.html Forecasts: http://wekuw.met.fu-.../wdiag/diag.php and http://www.cpc.ncep....here/strat_a_f/ and not forgetting! http://www.netweathe...atosphere;sess= (more available on nw extra!) So, as ever, we have a lot to keep an eye on. Early indications suggest that the polar stratosphere is cooling pretty much as expected. I am happy to report that there are already signs that this cooling is not uniform, with a Canadian warming a possibility this autumn. It's early days to see a slightly warmer area, but the GFS forecast does suggest this- An early heartener! Happy strat watching fellow strat watchers! c
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