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Welcome to the latest stratospheric temperature watch thread.
A bit later this year with a new thread â€“ but better late than never! It is now the 7th winter stratospheric temperature watch thread on netweather, and how much have we learnt in the past years!
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 â€“ 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.
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 â€“ 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:
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.
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 â€“ also known as the Holton Tan effect .
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 â€“ 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 doesnâ€™t rule out warming events, but they will not be as likely â€“ perhaps if the solar flux surges then the chance will increase.
Latest solar flux F10.7cm:
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 â€“ 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 â€“ 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 â€“ similar to a wave breaking on a beach â€“ 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 â€“
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 â€“ 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 60ÂºN 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 SSWâ€™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 â€“ though the one area that the ECM overestimates blocking events following an SSW at week 4 is over Northwestern Eurasia.
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 60ÂºN. 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
Last year we saw a large snow gain but unfortunately tropospheric atmospheric patterns prevented the full potential of these being unleashed on the stratosphere â€“ hence no SSW, but this winter could be different, but we will have to wait until the end of October.
One of the main influences in the global atmospheric state this winter will be the upcoming El Nino, and that is forecast to be the strongest since 1997. Studies have shown that SSWâ€™s are more likely during strong ENSO events ( http://www.columbia.edu/~lmp/paps/butler+polvani-GRL-2011.pdf)
but also that there is a particular pattern of upward propagating waves. During El Nino events wave formation is suppressed over the Indian Ocean Basin whilst it is enhanced over the Pacific Ocean
The ENSO pathway taken may be all critical this year as can be demonstrated by this paper http://www.columbia.edu/~lmp/paps/butler+polvani+deser-ERL-2014.pdf
This can lead us to suggest that a rather distinctive wave 1 pattern is likely this winter with the trigger zone likely to be over the north Pacific in the form of a quasi stationary enhanced wave 1 â€“ a traditional Aleutian low SSW trigger pattern is suggested by Garfinkel et al ( http://www.columbia.edu/~lmp/paps/garfinkel+etal-JGR-2012.pdf ) and this should be expected at some point this winter.
The reported incidence of SSW in EL Nino years is roughly around 60% - which is more than ENSO neutral years. A big question remains however, whether the ENSO wave 1 pattern will override the negative HT effect that the wQBO with the reducing solar ouput link brings. And even if it does, and we do achieve a displacement SSW, the next question is how will this affect the Atlantic sector of the Northern Hemisphere? My suspicion is that even if we do achieve a SSW this winter it will be in the second half, and also any subsequent blocking may not be quite right for the UK and, that if we were to achieve a â€“ve NAO, any block will be nearer Canada than Iceland, leaving the Atlantic door ajar. It is still too early this winter to be making any definitive forecasts â€“ the next 6 weeks are very important stratospherically, determining in what vein winter will start. Already we are seeing a forecast of weak wave activity disrupting the growing vortex and it will be interesting to see if this is repeated during November.
And it will be especially interesting to see what occurs in November and what is forecast for December before winter starts because typical strong El nino wQBO stratospheric composite analogues tell an opposite story. They suggest that the stratospheric vortex will be disrupted and weaker early in the winter before gaining in strength by February.
The mean zonal winds are already forecast to be below average so perhaps an early disrupted vortex is more likely this year!
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.
ECM/Berlin Site: http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html
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:
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!
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 Earthâ€™ core up to the South Pole). We will regard the wind speed relative to the Earthâ€™ 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 Earthâ€™ axis. For example, at the Equator, the distance of an air parcel to the Earthâ€™ 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 Earthâ€™ 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 â€œflowâ€ 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 laymanâ€™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.
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! Hereâ€™s hoping that this will become a fruitful thread and a learning place for many!
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
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