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WhiteFox

SSW and Average Winters - Links?

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So, as the usual "Winter's over" posts start to appear in the model discussion thread (despite snow lying outside!), I thought it might be a good time to start a debate on what drives the winter in the Northern Hemisphere.

Much of the talk this year has been about SSW of course, which has led to many predictions of a new ice age and severe cold outbreaks lasting for lengthy periods (Note: this may still happen of course; as of writing it's still only January 21st).

We all know that the period 1988 - 2008 was, barring a couple of exceptions (1991 and 1996 spring to mind) a period of mild winters with generally less snow than average. Given that the concept of SSW is a fairly new one, do we know if this was caused by less SSW events than we would normally see? Going back to Chino's chart from a while ago (I really don't have the time to plough through all the posts and find it, sorry!), there is a theory that greater than average snow cover over Siberia early in the season feeds back into the atmosphere and drives SSW which in turn leads to disruption of the Polar Vortex. In support of this theory, we have the evidence that the Polar Vortex generally breaks down towards February when the snowcover over Siberia is extensive even in the warmer years.

If, theoretically, we were to remove all snowcover from Siberia, would the Polar Vortex remain dominant throughout February as well?

Second, were the warmer winters characterised by below average snowcover over SIberia early in the season?

Even if the answer to the second question is no, this only questions the link between Siberian snowcover and SSW. So, the third question is, were the winters between 1988 and 2008 characterised by less SSW events than we would expect to see?

My theory is that SSW events have always happened and that they are a pre-requisite for delivering cold spells of any lasting duration, by which I mean lying snow and 850 temperatures of less than -5 or thereabouts for a period of seven days or more. The requirement for snow and cold in this country is a disruption of the zonal track of the jetstream and associated low pressure systems along with a displacement or dissipation of the Polar Vortex. WIthout SSW, we get zonal months with short outbreaks of cold characterised by northerly topplers or surface cold from anticyclonic conditions over the UK. With SSW, we see the increased probability of blocking forming somewhere over the North Atlantic/Greenland/Scandinavia delivering deeper cold for longer periods.

Of course, if there is evidence to show that this winter's SSW is really a rare phenomenon then my theory is blown out of the water and de-iced immediately! Are there figures available for Stratosphere temperature profiles in winters past? Obviously, if my theory is correct (or even constitutes a large element of the requirements for sustained colder spells) then the period 1988-2008 is likely to show less events like this year so this SSW event would stand out as being exceptional. However, if we have any data for the 1980s then we may get more of an idea.

Anybody have any theories for discussion, or want to simply say that I'm wrong? I'm perfectly happy to accept that I may ahve misunderstood the whole thing, but it is something that has been gnawing at me for several weeks!

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Myself and bobbydog started a thread back before winter called "The Hunt For White December", In it, correlations and other bits of analysis were done on how different teleconnection may or may not influence the winter weather in the UK and Ireland. Here's a link if you're interested http://forum.netweat...white december

Unfortunately I couldn't keep it going once I was back in Uni, but you might find something of interest in there.

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Some great stuff there BornFromTheVoid! Do you know if there are any sources for Statosphere warming events? I don't know if it's something that has just started being measured.

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Some great stuff there BornFromTheVoid! Do you know if there are any sources for Statosphere warming events? I don't know if it's something that has just started being measured.

In the stratosphere thread, I think Interitus, was discussing a site that had listed out the dates for most known stratospheric warming events.

EDIT: Here it is http://curriculum.pm...ssw-animations/

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In the stratosphere thread, I think Interitus, was discussing a site that had listed out the dates for most known stratospheric warming events.

EDIT: Here it is http://curriculum.pm...ssw-animations/

Now i must confess to not knowing a great deal technically about SSW effects..but judging by the dates of the warmings shown..many were the precursor to milder weather as much to colder weather..now if there is lag involved as well then some dont fit the cold spell that followed as they were already underway or started a few days after...my philosophy is that SSW encourage pressure to rise in northern latitudes but where that happens can be hit or miss and can result in mild or cold weather..dont forget where its gets colder somewhere else in the northern hemisphere will get milder.

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Now i must confess to not knowing a great deal technically about SSW effects..but judging by the dates of the warmings shown..many were the precursor to milder weather as much to colder weather..now if there is lag involved as well then some dont fit the cold spell that followed as they were already underway or started a few days after...my philosophy is that SSW encourage pressure to rise in northern latitudes but where that happens can be hit or miss and can result in mild or cold weather..dont forget where its gets colder somewhere else in the northern hemisphere will get milder.

I agree. Which is why I think Interitus' analysis showed little significant temperature change after SSW events, the mild balances out the cold. What might be interesting is to ignore whether you get a positive or negative temperature change after a SSW event, so you can just see the magnitude of change

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Only going on my memory of the last few years, so purely anecdotal. My hunch is that there is some correlation between SSW in winter and a cold spell for the British Isles, but (1) SSW isn't an essential precursor to a cold spell and (2) some - I think a minority of - SSW events don't precipitate a cold spell here at all. Was it the 2006 or 2008 event when there was a load of excitement about a SSW and the resultant cold stayed determinedly to the east of us? Whereas the 2009 event and the one we've just had seem to be clearly linked to a cold spell.

I guess SSW and the associated disruption to the polar vortex tend to be associated with blocking in northern latitudes, but the exact position of the blocking isn't set in stone, therefore it doesn't necessarily produce a cold spell for us.

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We all know that the period 1988 - 2008 was, barring a couple of exceptions (1991 and 1996 spring to mind) a period of mild winters with generally less snow than average. Given that the concept of SSW is a fairly new one, do we know if this was caused by less SSW events than we would normally see?

So, the third question is, were the winters between 1988 and 2008 characterised by less SSW events than we would expect to see?

My theory is that SSW events have always happened and that they are a pre-requisite for delivering cold spells of any lasting duration, by which I mean lying snow and 850 temperatures of less than -5 or thereabouts for a period of seven days or more.

Lots of good questions WhiteFox, just to look at a couple of points, SSW are not a new concept having been known about and observed from at least the 1960s and 70s but what might have made the difference recently is having the NCEP and ERA-40 reanalysis data and general circulation computer models to examine them more thoroughly.

For the time period 1988-2008 only the years 1990, 1993, 1996 (cold notwithstanding) and 1997 don't feature in my list of SSW and it's possible that there were minor warmings in those years too or stratospheric disturbances which may have affected the troposphere. The observation is that the vast majority - >75% of winters have an SSW so should be considered a normal winter event with all that entails for the average weather in the UK and the CET.

However, the impact on the circulation also depends on the pre-existing conditions and during the winters 1988-2008 it can be seen that the arctic oscillation was in a positive phase http://www.cpc.ncep....ason.JFM.ao.gif

The actual extent to which SSW affected this and vice versa is unclear.

They are not a prerequisite for cold with long term average temperatures before and after an SSW being almost identical with similar numbers of anomalously cold days.

There was a link to a video by the BBC which stated something like SSW led to cold two-thirds of the time, and computer simulations led to a negative arctic oscillation in over 70% of samples (Gerber, Orbe & Polvani, 2009). However in reality it may be less than this with Nakagawa & Yamazaki (2006) observing propagation from the stratosphere to troposphere on 55% of occasions, which is in accord to my figures of roughly 56% of days following the SSW having an anomalously low AO.

There are trends though as can be seen in the graph below (45 first in season SSW), with 60% of day 14 after SSW having a negative CET anomaly. Then typically follows a recovery in temperature before a second dip around days 39-40 after SSW which puts it this year in the third week of February. Valentines Day blizzard perhaps?

Some of the trends before the warming are just as persistent (also to be seen in NAO & AO plots), which may give some forecasting potential.

post-2779-0-52421200-1359040107_thumb.gi

I agree. Which is why I think Interitus' analysis showed little significant temperature change after SSW events, the mild balances out the cold. What might be interesting is to ignore whether you get a positive or negative temperature change after a SSW event, so you can just see the magnitude of change

I did do this in an earlier post on the subject. Obviously the graph above shows that temperature differences are dependent upon the choice of comparison dates.

But for example using the 20-days before, compared with the week of days 10-16 afterwards matching the first temperature response, from 45 first in season SSW there were 22 coolings, average CET anomaly -2.50°C, and 23 warmings average CET anomaly 1.46°C. The average temperature swings are more dramatic -3.46° and 2.42° with a slight tendency for larger anomalies prior to SSW to have larger swings.

Only going on my memory of the last few years, so purely anecdotal. My hunch is that there is some correlation between SSW in winter and a cold spell for the British Isles, but (1) SSW isn't an essential precursor to a cold spell and (2) some - I think a minority of - SSW events don't precipitate a cold spell here at all. Was it the 2006 or 2008 event when there was a load of excitement about a SSW and the resultant cold stayed determinedly to the east of us? Whereas the 2009 event and the one we've just had seem to be clearly linked to a cold spell.

2009 is interesting because the cold start to February does follow an SSW on 24/01 but the winter was cold in general with spells as cold or colder in December and January, and longer term temperatures were above average from mid-February onwards with not much further cold.

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