Major Stratwarm to force tropospheric cooling over N America, setting stage for bitter arctic attack mid/late Jan
I am sure other mets will start drawing your attention to this. ( stratwarms in mid winter associated with major cold)
And while on the topic of cold and snow, the GFS and ECMWF models show significant stratospheric warming across the North Pole and Canada towards the middle of January. With the NAO expected to fall rapidly at that time, and with the AO expected to rise rapidly at that time, it’s about to get very cold for many across the United States. The split & temporary collapse of the polar vortex we’ve seen in recent years has been interesting.. Considering it tends to happen after solstice, one of these years it’s not going to set back up for the rest of winter.
Interesting to note the tropospheric polar vortex at d16 shifts from Siberia into Sapporo!
Insane conditions over northern Japan !
Polar Vortex Collapse, Arctic Freeze Imminent
By Andrew at 9:24 AM The model forecasts above both show big high pressure centers interrupting the polar vortex and splitting it into two pieces. Such a split exemplifies the traditional weakening of the polar vortex. When that polar vortex is weakened, the cold air locked up in said polar vortex flows south, possibly into North America, maybe into the US. This is a very good sign coming from the models at this point in time to go along with the stratospheric observations.
Continuing with watching the models, we turn to a charted forecast of the ECMWF model. I want you to look at the bottom image, called the EP Flux. In short, the EP flux shows the direction and strength (shown by length of arrow) that air moves into the stratosphere. In events where warm air enters the stratosphere, long, extended arrows are commonly seen. The forecast into the days before New Year’s Day show a batch of extended arrow motions, suggesting a large motion of warming may be incoming into the stratosphere. This enhanced EP Flux would theoretically enhance warming potentials in the Arctic, thus further weakening the Polar Vortex. I find it likely that the aforementioned 50mb wave over East Asia would be to blame for this rise in EP Flux values…. http://theweathercentre.blogspot.de/2012/12/polar-vortex-collapse-arctic-freeze.html?showComment=1356230240926 Posted 30 September 2012 – 00:42
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:
Graphically and previous years information :
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!
Sunday, December 30, 2012Significant Cold Ahead For January-February
By Andrew at 11:58 AM
Significant cold is ahead for the months of January and February, especially late January into February. I’ll explain what could cause this.
This chart of observed stratospheric temperatures for the years of 2011 and 2012 shows tat we have had two recent warming events, as shown in the red temperature line spikes on the far right of the graph. When these spikes occur, they are called Sudden Stratospheric Warmings, or SSW events. When an SSW occurs, warm air from lower levels of the atmosphere is forced up into the stratosphere. Because of the warm air being forced up, cold air originally in the stratosphere is displaced and forced down to fill the void left by the warm air now being pushed into the stratosphere. This is why we want SSW’s to happen in the winter, so cold air can come down to the surface.
This ECMWF multi-panel graph forecast shows temperature forecasts for different latitudes. We see that the top 3 panels show significant and sudden warming of the upper stratosphere, including the 10mb and 30mb layers. This is a direct result of the sudden stratospheric warmings, and if these forecasts happen, it is certainly plausible that significant amounts of cold air will be displaced down into the troposphere (where we live). Even better news is how the two stratospheric layers do not directly cool down after the warming.
Even better news for cold air prospects is how the polar vortex is getting torn apart. For those unfamiliar with the polar vortex, it is the driving force behind the Arctic Oscillation and varies in its strengths. When it is weaker than normal, the polar vortex releases cold air down into the troposphere, much like the air displacement example shown above with the sudden stratospheric warming.
These two forecasts above of different isentropic layers show how decimated the polar vortex will be. The colors depict potential vorticity values, much like how high vorticity values signify low pressure systems at the tropospheric 500 millibar level. The two forecasts above show virtually no polar vortex to be found- the potential vorticity that IS present doesn’t resemble a polar vortex in either of these two layers. This is exactly what I have been warning about- a polar vortex collapse. It seems to be happening in the upper stratosphere. If it propagates down to lower layers of the stratosphere, there’s no telling what could happen.
Now, here’s the big thing that will kick off the cold for mid-late January: CROSS POLAR FLOW.
The above image is for the lowest level of the stratosphere- the 100mb layer. I have outlined my best guess as to where the jet stream is. The plus sign signifies the North Pole. Using the outlined area, we see that the jet stream starts in Siberia, shoots north into the North Pole, and crashes south into the Plains, Great Lakes, Midwest, Ohio Valley and Northeast. The jet stream crossing the Polar region and then fleeing south into the US is called a cross polar flow and is commonly identified with the strongest cold air outbreaks in the US.
GGEM Ensembles showing Cross-Polar Flow in very long range
GFS Ensembles showing Cross-Polar Flow in very long range
Both the GFS ensembles and GGEM ensembles are showing cross-polar flow regimes setting up in the long range. This cross-polar flow is shown by high pressure pressing toward the North Pole, with low pressure then appearing across Canada and the US. Now, the GGEM Ensembles are indeed showing this cross-polar flow much better than the GFS ensembles. It is a very good sign to see two major ensemble systems showing cross-polar flow evolving, as well as the stratospheric ECMWF. If this happened, cold would invade the Lower 48 and would not be easily pushed out.
This is the zonal mean temperature forecast from the ECMWF model. I inserted an arrow into the 1mb – 5mb layers to signify what will happen now that this sudden stratospheric warming event is ongoing. That depression of light blue colors signifies cold air being forced down through the stratosphere in response to the sudden stratospheric warmings. Now, this air will warm as it pushes down through the stratosphere, but nonetheless, it is stratospheric air, meaning it will naturally still be colder than normal.
For the long range model forecasts, the CFS v2 monthly forecasts show a very cold January and February: