Winter 2008/9

[Glacier Point]

So here we are. No strong ENSO warm or cold and sustained period of average to slightly below avergae months behind us presenting us with a totally different and challenging set of variables to get to grips with as we head into the winter season.

I've based my forecast on several key variables as I see them heading from the Autumn into Winter:

1) QBO and zonal wind anomalies

2) Ocean - Atmosphere base state

3) Tropical forcing

4) The NAO

QBO and zonal wind anomalies

The Quasi Bienniel Oscillation (QBO) is getting its deserved attention as a key seasonal forecast tool and I cannot emphasize enough the importance of it in determing the likely pattern of zonal wind anomalies (and therefore a strong lead on the AO) and the depth and extent of tropical convection (possibly some influence on the location also).

First up, the forecast for the QBO which follows a couple of really good analogs which have proven themselves over the last two years - 1961/2, 1973/4, 1980/81, 2006/7. These predict the westerly QBO to remain in force until April, with values between +5 and +10m/s for a substantial part of the winter, certainly the part that is important in influencing the state of the polar field in December and January.

Composite reanalysis of these anlogs and the way the upper atmosphere is behaving since Spring has verified well. October's anlogs and observed were very good and November's has been very good. Compare here the analogs and the observed for November thus far. Good but not totally unexpected I guess.

Moving on, follow the trend, December, January and February's anlogs suggest a gradual fall over the equator, a slump of -ve zonal wind anomalies in the tropics and increasing +ve anomalies in the mid and high latitudes, peaking 60N-65N during late December into January.

This is important because it drops a big hint that tropical convection will be enhanced just north of equator, supressed somewhat elsewhere (see http://hydro.jpl.nasa.gov/other/ams.collimore.pdf ), with a rising anomaly over the tropopause around the NAO domain. Consider this linear relationship between the AO and 30mb zonal wind anomaly.

About 0.65 may not be everyone's idea of perfect correlation, but to the seasonal forecaster, those are odds you would like to have on your side. Given the lag effect of 30mb to troposphere connect - 20 to 30 days - I would suggest that we have a window of opportunity in December as these anomalies are picking up before they impact at the surface in January. An anomaly of around +6 to +10 m/s is not in iteslf enough to support a strongly organised stratospheric jet. However, consider the last few years' temperature profile of the stratosphere and any increase in zonal winds is more likely to promote cooling rather than not.

Whilst we're on stratospheric temperatures, current values are pretty close to climatic norms. A good sign with some prospect of slight warming into early December given the state of the atmosphere and recent mountain torques. However, although the increase in seasonal snowcover has been rapid, we still sit around or below the average of the last few years.

http://moe.met.fsu.edu/snow/

I think this will help to moderate future cooling of the stratosphere but not head it off completely so I see some grounds for cooling and increase in upper level zonal winds mid December impacting the lower levels into 2nd week January.

Putting these factors together supports the notion of early cold as zonal wind anomalies throughout the layers will be nominal in December and conducive to polar outbreaks before the polar westerlies stir into January. February is interesing as the anlogs suggest a slackening off of the +ve zonals at upper levels which might be useful to consider later on.

Still on the subject of the QBO. I'm keen to explore the idea that the state (east - west) will have an influence on not just the exent and depth but also the location of tropical convection given that it has an impact on the height of the tropopause and this may vary with longitude. For similar QBO years with a similar SSTA profile, composites suggest the west Pacific and far eastern Indian Ocean as the convective core with a centre somewhere around Indonesia / Philippines.

Ocean - Atmosphere base state

Quite a complex one this. El Nino Southern Oscillation (ENSO) numbers suggest neutral. So does most dynamic and statistical model guidance. Yet the atmosphere is still in what we might term a low angular momentum or Nina-like base state with Globally Averaged Angular Momentum (GLAAM) values still averaging below normal although there has been some tendency for this to trend away towards neutral. We also have to consider a strongly negative Pacific Decadal Oscillation (PDO) for at least the first part of the Winter [warm body of water in the NW Pacific].

Key points here:

1) Sub-surface anomalies off the South and Central American Coasts are below normal to the tune of -3 to -4C and the overall arangement of SSTAs is indicative of a residual Nina with easterly wind flows to the western Pacific. These anomalies have begun to work their way to the surface but at a slow rate and there is no guarantee that they will manifest themselves in a full blown Nina before the turn of the Year.

2) There is a large subsurface cold pool 8N 140W which is likely to continue to feed easterly wind anomalies in the SE northern Pacific. Allied to the warm pool to the north-west Pacific consistent with the cold phase PDO, we have good coupling for a large anticyclone (or twins) driving a largely low angular momentum base state which is only likley to change very slowly given the depth of subsurface anomalies.

http://www.ecmwf.int/products/forecasts/d/...al_time/yzmaps/

3) The warmest sea temperatures on the planet exist in the far western Pacific and eastern Indian Ocean. These are in excess of 29C and are likely to sustain an easterly trade wind flow given the gradient between warm and cold pools.

4) Against this, angular momentum has been increasing since the peak of La Nina in the Spring of 2008.

This trend is continuing on the back of fading cold surface anomalies in the central and eastern Pacific and increased westerly wind flow generated across the tropics. I think this is likely to continue into December given the current SSTA although the reduction in tropical convection tied into the QBO and possible rejuvenation of Nina timed for January would signal an end to this trend.

Thus a possible picture of continued increase in GLAAM through December before the atmosphere responds to an emergent Nina and reduced westerly wind flow in January emerges. In terms of the Global Wind Oscillation (GWO) [measure of the net balance of winds and their interation with the oceans and continents], I would see this manifesting itself in quasi stationary states around phases 4-5 for the early part of the winter and then shifting back towards 0-3 from mid January onwards. GWO composites for these states suggest December to have a negative North Atlantic Oscillation (NAO) pattern before the ridges get shifted and a more Nina type pattern takes over for January before an early December pattern takes hold in February with a mid Alantic ridge once more a feature.

I must add here that this is my weakest angle on the forecast and I could easily bust here if Nina fails to arrive and the atmosphere continues to show an increase in angular momentum.

Also worth pointing out the strong early-season hook up between the -PDO and warm anomaly in the mid Atlantic with a long-wave patternn that fits perfectly the two ridges in the Pacific and Atlantic with fast flow over the top. I'm thinking that the Pacific wave lengths will shift enough to allow this pattern to be broken somewhat. Current SSTAs are hinting this but again, strengthening -PDO = bust on the idea of support for the early -NAO. However, I'm guided most by the GWO and early season zonals on this one.

Tropical forcing

Current plots suggest the warmest sea temperature anomalies in the western Pacific and eastern Indian Ocean. These anomalies extend to depth in these areas and continued elevated temperatures are likely to provide the fuel for convective flare ups give favourable surface instability. Analysis of the likely windfield responses to the hemispheric arrangement of SSTA suggests that there will be a convergence zone centred on Indonesia/Philippines, rigtht where the warmest SSTAs are. Factor in support from the QBO for deep convection in this area and I think there is a very persuasive case for the core of tropical convection here.

Composite reanalysis for years with tropical convection centred in this area...

..and the H5 field response...

December is notably cooler and therefore another signal for early cold but the general theme is for ridging and warmth. Also, worth noting the Atlantic ridge. Factor in the SSTA with that warm pool off Newfoundland, and you have a strong case for fast flow over the top of this Atlantic ridge which would also trash any hope of high latitude blocking January onwards. February might however offer some limited oppotunities for transient disruption of the flow as the polar field begins to extend equatorwards following the bottling up January cold.

The current SSTA, with weak or non-existing anomalies in the central / western Pacific opens up the possibilty for more coherent Madden Julien Oscillation (MJO) [tropical convection waves] activity through phases 3-6 and possibly 7 through December and more particulary early January for the latter phase. This would assist with height rises over the polar field.

The NAO

I consider the NAO a product of much more than just the North Atlantic. Comparison of the ten most positive and negative NAO winters in terms of Ooutgoing Longwave Radiation (OLR) suggest the following fields of importance where there are significant differences.

Based on where SSTAs will likely be, and centres for tropical convection, I would assess the NAO for the winter (DJF) as being neutral or slightly positive with a negative December (GWO / SSTA driven), a slightly positive January and neutral or slightly negative-east based NAO in February associated with a strong mid Atlantic ridge.

So, putting these factors together, I'm struck by the overlaps for a developing -NAO in December with average or colder conditions, the ridges to get shifted in January with a very strong signal for a large anticyclone to dominate the UK and NW Europe for a large part of January into early February and return to a mid-Atlantic ridge into February. The forecast suggests temperatures to be average or slightly above average for the most part with a stronger signal for average or below average December and milder conditions in January (although a large anticyclone could result in an inversion and lower surface temperatures) with the chance of a return to less mild conditions in February as the ridge in the Atlantic re-establishes raising the possibility of Arctic incursions. The winter is expected to be much drier than average.

Forecasts as follows (500hPa geopential height anomalies and surface temps)

December:

January:

February:

Winter:

Enjoy !

Edited November 30, 2008 by Glacier Point

[johnholmes]

tks Stewart, first class analysis as usual and well argued for your viewpoint on the upcoming winter. Thanks also for putting full explanations of the terms.

[chionomaniac]

Thanks GP for your teleconnection analysis and forecast for the coming winter. On the video thread I asked the following question(s) and would be grateful for your thoughts as it is relevant to your winter forecast.

On the video you showed analogues for previous westerly QBO years for zonal wind anomalies at the 30hPa level. I think you suggested that on average this would add on 6m/s for Dec and 8m/s for Jan. When I look at previous SSWs, I see that the zonal wind (not anomaly) at 30hPa level only just about reverses ( 0 to -10m/s), whereas at a 10hPa level the reversal of zonal winds is often greater (over -10m/s). So firstly, do analogues for the 10hPa level for previous westerly QBO years, have greater positive wind anomalies and should we think of ruling out a SSW for this winter?

Also, if the stratosphere continues to warm and stay above average for the next couple of weeks would you reconsider your late winter forecast from mid January, where to quote you worry that things may go 'pear shaped'.

Many Thanks (in anticipation).

c

[Tom Montalbano]

Hi GP,

Stewart, tremendous amount of work put in there and a great read as ever. I really enjoyed the video dicussions as well. You seemed to have backed away from a very mild February now, what has happened in the last couple of weeks for you to have a rethink about Feb.

Also, are you still confident that we can record a colder than average December given that this weeks cold spell seems a little uncertain given todays model outlook.

Regards,

Tom.

[wysiwyg]

Great analysis GP! Many thanks. This type of in depth analysis (particularly when all the acronyms are expanded the first time they are used) is invaluable for a novice weather enthusiast like myself!

[Glacier Point]

Thanks GP for your teleconnection analysis and forecast for the coming winter. On the video thread I asked the following question(s) and would be grateful for your thoughts as it is relevant to your winter forecast.

The 10 hPa layer may be used as an indicator to what might happen at 30hPa - but it is not a reliable guide by any means. The analogues are a bit all over the place to be honest w/r/t 10hPa layer and do not provide a coherent pattern. I'm therefore minded to stick with this as the principal guidance in so far as the AO.

Do I think we will see a SSW ? Probably not given the westerly QBO and solar minima. However, a more southwards displaced jet and fast build up of snowcover allied to limited convective activity should ensure that there is no sustained severe cold phase in the stratosphere this year, just below average would be the call.

Am I minded to change mid to late January on the basis of what's happening now ? No. The warming should drop off in the next few days although the atmosphere is primed to take advantage of -ve zonal wind anomalies in the coming few weeks, regardless of what the current very inconsistent long range model guidance suggests. I am therefore still keen on average or colder for December.

What has influenced me w/r/t February ? Two things. The -PDO acting in tandem with the Atlantic SSTA (that warm pool off Newfoundland) will be a powerful forcing factor once the peak of zonal winds and any lag impact on the polar westerlies has been reached. It still will have an influence in January but the ridge will literally get blown eastwards. Into February and I can see the ridge re-establishing. Plus, you'll note that the signal from the tropical forcing in the far west Pacific is also for this arrangement - a VERY strong and harnessed signal. A ridge in the Atlantic is prone to transient northerly attacks as we are seeing now.

Hope that helps.

[chionomaniac]

Thanks GP, hopefully we will get something out of the next few weeks. No '62/'63 likely this winter then though. I'll be awaiting your updates with interest as always.

c

[Tom Montalbano]

Thanks for your reply GP. O/T but hope you're finding a few winners. I dont suppose you saw the conditions at Fairyhouse yesterday, certainly the worst conditions I've ever seen for a meeting to take place in, thick frost on the track and the fog wasnt much better. Dont think it would've passed an inspection this side of the Irish Sea. Mind you those Irish jockeys are tough so and so's, just look at how many times Ruby Walsh et al have come back from bad falls. :lol:

Regards,

Tom.

[BLAST FROM THE PAST]

Hi Stew

Thanks for that and worth waiting for. Will be interesting come January because I believe that the polar westerly flow will continue to be disrupted/slowed by current solar pattern and so we have distinct difference of opinion re latter Jan in particular, otherwise Dec and Feb have good similarities.

good luck mate

Fred

[summer blizzard]

Very good forecast, unfortunately, i do not have the time for a detailed forecast of my own, but there are my predictions based on the data i have interpreted..

Dec: 4.1C

Jan: 6.2C

Feb: 2.7C

(Please not, i may not use these predictions in the CET threads).

[Glacier Point]

December verification:

z500 uppers

Forecast>

Observed>

Surface temperatures

Forecast>

Observed>

The broad arc of higher pressure across the northern and eastern parts of Europe well depicted by the forecast, as was the theme for low pressure over the near continent although magntitude was too much in the forecast.

Forecast temperatures broadly reflected this pressure pattern although the UK and the majority of western Europe was colder than forecast, largely as a result of a stronger northerly component c/o that well defined pool of warm water off Newfoundland drawing a stronger northerly airstream.

Into January, I like my forecast for the high pressure anomaly centred over the UK. Probably to the northern half to start being forced steadily southwards towards the month end. Very dry throughout seems a strong probability. The cold start may mean I'm underestimating the cold anomaly. Also worth noting where the forecast predicted the core of tropical convection was and now is. Also, the forecast strengthening of La Nina and second surge in the QBO (increased in December) and increasing zonal wind anomalies place the forecast in very good stead.

[chionomaniac]

I think one should say how well your forecast has verified so far GP. I have just reread it and cannot see why January won't come in as forecast either, with the large anticyclone forecast to be in position for the foreseeable future. All the updates that you give in the model thread have come pretty close as well, so respect is due, so far you are having a terrific season. Well done.

[summer blizzard]

Glacier Point, after looking at the model runs and available data, i'm thinking that we may have another chance at cold in mid-January (would you agree with this).

This is because our current failed easterly was brought about by a battle between the Atlantic and the Pacific, because when the Polar Vortex split, it left two vortexes, one over western Canada, and the other over western Russia, which as you know, are conflicting atmospheric signals. Now, we have seen moddled the demise of the Greenland High as the Canadian Polar Vortex won out and we now see a strong northern Jet Stream modeled,but perhaps after all, this is not a bad thing?

During our current cold spell, we were always up against a -PNA, which in winter is always going to encourage a +AO/NAO and thus prevent a real cold shot, however with the Vortex now shifted over Greenland, perhaps this will allow a more faviourable Pacific signal over time, with the moddeling of a high over western Canada indicating the formation of a +PNA pattern over the USA, which in turn should encourage a -AO/NAO signal.

What are the thoughts of Steve Murr and Glacier Point on this one??