In Depth Model Discussion, Analysis and Summaries

[damianslaw]

Our impressive trade wind surge in late August / early September has begun to flux negative (easterly) mean zonal wind anomalies polewards repeating a similar drop in late June which dropped the global wind oscillation into a La Nina pattern. This time round, tropical forcing following on looks to be quite weak so a persistence of a low angular momentum base state throughout the autumn looks assured.

Relative angular momentum is impressively low driving the global wind oscillation towards phase 1:

http://www.esrl.noaa.gov/psd/map/images/gcm/gwo_40d.gif

Composites for phases 1-2-3 are somewhat variable (there is for example some difference between MJO and GWO plots) although my interpretation would generally be for an anomalous ridge into Alaska and tendency for blocking in the mid to high latitudes over the Atlantic.

Noticeable once again (I count this about 4-5 times now) whenever the models see a pulse of westerly winds over the sub-tropical ridges they programme a poleward shift in the jet in the extended range before realising that the Atlantic circulation is much more different with changes in the t72 -t96 period, usually manifested in the trough over the UK or Scandinavia being much more stubborn.

Interesting comments suggesting the remainder of the autumn in the main will see a continuation of the pattern which was set last december i.e. stubborn troughing over the country or Scandanvia disrupting any sustained westerly pulse of trade winds sending the jet on a more southerly course aided by strong heights to the north or north west - good news for those who do not like sustained south westerly airstreams.

[BLAST FROM THE PAST]

Noticeable once again (I count this about 4-5 times now) whenever the models see a pulse of westerly winds over the sub-tropical ridges they programme a poleward shift in the jet in the extended range before realising that the Atlantic circulation is much more different with changes in the t72 -t96 period, usually manifested in the trough over the UK or Scandinavia being much more stubborn.

This sounds promising, it would seem models have a default bias, jetstream to remain on more southerly track

Stew thanks for reply and keep up the excellent work

BFTP

[Glacier Point]

Global angular momentum has now fallen to very low levels, heading towards the lowest levels so far in this current Nina phase.

The poleward fluxing of negative (easterly) zonal wind anomalies has continued, no doubt a reason for the ongoing problems with models resolving extra-tropical patterns. This will continue for the next 7-10 days although there are now indications of the Pacific pattern settling down somewhat and moving us towards a pattern that will set for the rest of the autumn and potentially well into a core of the winter.

Surface wind data is suggesting the development of twin anomalous anticyclones in the Pacific across 30N, which is consistent with a west-based La Nina.

http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/winds/archive/wavetrkP-1.html

The Global Wind Oscillation is very consistent with the low angular base state centred in phase 1-2 high amplitude.

http://www.esrl.noaa.gov/psd/map/images/gcm/gwo_40d.gif

As per last posting, there has been very little tropical convection and any increments in the global wind budget are only likely to come from torques (which are unlikely to move the global wind budget signifacantly upwards). This means that we are locked in a low angular momentum base state (suggest phases 2-3 high amplitude) for the forseeable future.

Composites for these phases taking us into October:

Notice the twin anomalous ridges in the Pacific wich are very consistent with the pattern that is evolving there.

As far as the UK's long term outlook, the indications are solidly for a mean ridge to our west and north-west giving dry and average to below average temperatures, possibly more inclined to below average as I suspect these anologues are under-doing the presence of trouging into Scandinavia. Weatherwise, probably calm and autumnal best sums it up for late September and into October.

[johnholmes]

thanks for the usual well argued post Stewart and charts backing it up.

I was going to post my thoughts on lrf into October later today but see little point, less technical than you post but the overall pattern was going to be along the same lines.

My view is that once the current wavering in the synoptic models have cleared, thanks to the hurricane situation, then the longer term pattern from just about every teleconnection is for blocking rather than mobility. Your comment about the Pacific being particularly pertinent.

So yes to more anticyclonic weather type rather than cyclonic type through much of October. I am still not quite so sure that there may not be a temporary hiccup in this with the probability of more hurricane influence for a time. This though may manifest itself as northern blocking with lower latitude mobility, interesting thought that?

[phil nw.]

Thanks Stewart and John for your valued contributions as always.

Certainly no sign of West to East Autmnal mobility as yet with continued signals for blocking to our West/North West.

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking/real_time_nh/mrf2.nh.shtml

[The PIT]

Interesting GP. How will the ex hurricanes effect this idea???

[Iceberg]

Firstly I am not a long range forecaster, All I am really doing is looking at some of the trend see so far.

Firstly using the CPC stormyness diagrams, when they have happened storms have been very much a SW to NE affair as can be seen in this diagram. This seems to be supported by the NW/SE split seen over a large portion of summer.

Looking at the tracks it would suggest that stormyness is probably down generally from what it should be and probably at a generally gentler NW track, so instead of over Greenland and iceland it's more Scotland.

Compared to the last two summers though the storms are nowhere near as south as they have been, hence the South and East has faired much better.

Moving onto the NAO what's notable when looked at over a longer timeframe is how persistantly negative it has been this year . If we were to want the NAO to continue to be negative over winter then looking in the past it's only 68-69 and 62-63 which saw anything this big last for over 12 months.

It might happen, but it's a very big ask.

It's a similar story for the AO, however that has turned positive recently, really for the first time since the first half of 2009. A positive AO obviously doesn't prevent blocking but does hint that that blocking is likely to be mid latitude based rather than arctic. Again this is unlike the begining of the year which had a strong negative AO.

Finally the PNA.

We've continued the run of a much more positive PNA than normal starting really since 2007, possibly 2005. I would weakly suggest a neutralor positive PNA is probably the most conclusive in reaching a cold UK winter, as that allows the AO/NAO signals to really play out. It has recently started to increase again, however it has again not been negative since Q2 2009 so to see a continued positive PNA would be something not seen since 83-84 or 60-61.

All in All the chances of having all the variable in place like last winter seem to loaded against, however the chances of some of them coming up trumps looks pretty good.

I know that weather forecasting does not react to chance or pattern matching, but a rough guide using the above would seem to suggest a more "normal" winter than last year, however still with the distinct possibility of a cold snowy more blocked bias...

[johnholmes]

Thanks for that nicely argued post Ice. Its going to be an interesting lead into late autumn-early winter and what actually turns out.

I do tend to favour an average to perhaps slightly colder than average late autumn/early winter but beyond that I have even less idea.

[chionomaniac]

All in All the chances of having all the variable in place like last winter seem to loaded against, however the chances of some of them coming up trumps looks pretty good.

I know that weather forecasting does not react to chance or pattern matching, but a rough guide using the above would seem to suggest a more "normal" winter than last year, however still with the distinct possibility of a cold snowy more blocked bias...

I agree with your thoughts, Iceberg. Come November I think that we will have a better idea.

One of the main drivers of the -ve NAO and AO last winter was the exceptional stratospheric state during November that propagated into the troposphere during December and January that allowed tropospheric vortex disruption to occur and reoccur.

This is illustrated nicely by the mean zonal wind anomaly charts where we can see the negative anomalies break through the tropopause in December.

This year the Brewer Dobson circulation is much reduced with a net result of far less ozone being transported to the North Pole which will lead to a greater stratospheric temperature contrast between the NP and latitudes further south. We also see evidence of the westerly QBO phase picking up momentum in the equatorial stratosphere with negative anomalies being squeezed out.

The equatorial stratosphere is already showing signs that it is warmer than the last few years though it is still below average.

My guess for this winter's AO, would be for it to start off neutral to weekly positive, but then slowly becoming more positive as the season progresses. There is a hope that a SSW could knock this off course but this does not guarantee cold on our shores.

With a neutral AO there is still chance that a negative NAO could occur (though this would likely be weak) and this would depend on other factors being favourable.

c

[BLAST FROM THE PAST]

C

I thought a strong la nina coupled with a -ve PDO, transitioning westerly QBO would lead to heights to our NW and trough over UK / Scandi with a pronounced northern blocking epsiode as winter progressed with strong -ve NAO in place. All is very much open but Icebergs contribution and yours are good and interesting insights to the potential players down the line.

BFTP

Edited September 20, 2010 by BLAST FROM THE PAST

[chionomaniac]

C

I thought a strong la nina coupled with a -ve PDO, transitioning westerly QBO would lead to heights to our NW and trough over UK / Scandi with a pronounced northern blocking epsiode as winter progressed with strong -ve NAO in place. All is very much open but Icebergs contribution and yours are good and interesting insights to the potential players down the line.

BFTP

Yes, it is how these players interact that will dictate how winter pans out. There are so many variables that it is another difficult one to call. I was never of the opinion that El Nino would ruin last winter as was being advocated in some quarters (mostly Ian B's) and likewise this winter has too many variables to confidently predict anything at present.

[cooling climate]

Using the site below it looks to me as though ozone levels look to be similar to last year.

Canadian Ozone

• Network

• Today's obs

• Ozone+UV plots

• Forecasts

• Data

<A style="TEXT-DECORATION: none" href="http://exp-studies.tor.ec.gc.ca/e/ozone/uv.htm">UV Index (UVI)

• Definition

• Climatology

• UVI Today

• Year-to-date UV

• Year-to-date max UV

• UVI graph archive

• Data

• UV Index Calculator

World ozone maps

• Climate NH SH Globe

• Recent NH SH Globe

• Individual sources NH SH Globe

• Forecasts NH SH Globe

• Map archive

• Data Site Map

Select one or two map set(s) by type, date and hemisphere, from the archive:

First map set

Total Ozone

Deviation from normal Start date

1-end1-1011-2021-end12345678910111213141516171819202122232425262728293031

JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember

19701971197219731974197519761977197819791980198119821983198419851986198719881989199019911992199319941995199619971998199920002001200220032004200520062007200820092010 How many

1234567891011121314151617181920 Interval

1234567891011121314

Day10-dayMonthYear Hemi-

sphere

North

South

Globe Second map set

No map

Total Ozone

Deviation from normal Start date

1-end1-1011-2021-end12345678910111213141516171819202122232425262728293031

JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember

19701971197219731974197519761977197819791980198119821983198419851986198719881989199019911992199319941995199619971998199920002001200220032004200520062007200820092010 How many

1234567891011121314151617181920 Interval

1234567891011121314

Day10-dayMonthYear Hemi-

sphere

North

South

Globe Map size? 100% 75% 50% 33%

(Note: 100% is 1000 pixels wide by 726 high)

What indecies are used to measure the BDC. Is it ozone levels, atmospheric angular momentum ect.

From what I have read up on the QBO during the summer (although a lot of it goes straight over my head,

no pun intended) we probably will not see anything like the rosby and planetary wave breaking into the

upper stratosphere that we saw last winter due to the waves being deflected towards the equator by the

QBO positive westerly zonal winds instead of being refracted poleward.

A colder equatorial stratosphere would help offset this somewhat so that is worth closely monitoring

as we go through Autumn.

If an above average snowpack and cold forms again over Russia and Siberia later this Autumn helping

to produce another very large and strong high cell this could very well play a much bigger role than it did

last year as I think one of the more dominate players last year was the PNA ridging.

Atlantic sea surface temperatures seem to be improving all the time and a relatively warm Davis strait

may not be a bad thing at all.

All in all a lot to be encouraged about in my opinion, big question though is how strong will the polar vortex

get and where will the two main vortices's bed down for the winter, plus how cold will the high latitude

stratosphere become in relation to the equatorial stratosphere(westerly zonal winds).

[cooling climate]

I will try to post the ozone monitoring and mapping site again.

This site shows current ozone levels and the deviation from normal.

There is also an archive section that goes back to 1970.

http://exp-studies.tor.ec.gc.ca/e/ozone/ozoneworld.htm

[jethro]

Thought some of you here may find this interesting:

http://www.newscientist.com/article/mg20727793.100-the-sun-joins-the-climate-club.html

For anyone who wishes to discuss the climate change angle, please can you swap over to here:

[Guest FireStorm]

Thought some of you here may find this interesting:

http://www.newscient...imate-club.html

I may not be the most technical but reading this particular part of the forum is interesting - even if i may not fully understand. That link was a brilliant read though - thank you for sharing

[chionomaniac]

Composites for these phases taking us into October:

Notice the twin anomalous ridges in the Pacific wich are very consistent with the pattern that is evolving there.

As far as the UK's long term outlook, the indications are solidly for a mean ridge to our west and north-west giving dry and average to below average temperatures, possibly more inclined to below average as I suspect these anologues are under-doing the presence of trouging into Scandinavia. Weatherwise, probably calm and autumnal best sums it up for late September and into October.

Hi Stewart,

With October almost upon us are sticking with these thoughts or have things changed significantly?

I am seeing things presently, more along the lines of Iceland troughing which is more in line with a mixture of MJO phases1/2 for October, and I wonder how this pans out with the GWO forecasts and analogues?

Cheers

Ed

Edited September 27, 2010 by chionomaniac

[Glacier Point]

Yes, it's really interesting Ed as the atmosphere is behaving more like an El Nino with a positive tendency in angular momentum developing as westerly winds are being introduced across large parts of the circulation.

Tendency in angular momentum:

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/gltend.sig.90day.gif

... which I think has been well advertised by the models if we look at phase 3 GWO composites for El Nino phases:

I can see the GWO hovering around phase 3 for a few days as this forward intertia develops. However, and the models came a cropper here in the summer by overdoing the westerlies (or underestimated the easterly tendency) and we may well see the GWO swing back towards phase 2 in the next 10 days which favours a return to the ridging in the Atlantic.

What is perhaps taking on board is the use of tendency of angular momentum to sift samples for composite use.

[chionomaniac]

Yes, it's really interesting Ed as the atmosphere is behaving more like an El Nino with a positive tendency in angular momentum developing as westerly winds are being introduced across large parts of the circulation.

Tendency in angular momentum:

http://www.esrl.noaa.gov/psd/map/images/reanalysis/aam_total/gltend.sig.90day.gif

... which I think has been well advertised by the models if we look at phase 3 GWO composites for El Nino phases:

I can see the GWO hovering around phase 3 for a few days as this forward intertia develops. However, and the models came a cropper here in the summer by overdoing the westerlies (or underestimated the easterly tendency) and we may well see the GWO swing back towards phase 2 in the next 10 days which favours a return to the ridging in the Atlantic.

What is perhaps taking on board is the use of tendency of angular momentum to sift samples for composite use.

Yes, I can see how the tendency for angular momentum can be different from the actual position of the GWO when working out which composites are most relevent, so thanks for explaining that. I guess that is why the trough is forecast to be positioned to our west. It will be interesting to see if this reverses when the westerly input diminshes and the tendency for angular momentum drops again.

[Glacier Point]

In instances such as this where we have such a notable differences in pattern (in this case strongly positive NAO within the context of a sustained -NAO phase), we have a golden opportunity to analyse the impacts of tropical forcing.

The strong pulse sent through the atmosphere from a trade wind surge and deepening of La Nina in the last few weeks (visualise pebbles thrown into a pond) has worked its way poleward. On the back of it we have probably seen a westerly wind pulse which as manifested itself in a vigorated jetstream. Tendency in angular momentum plots show this increase in westerlies nicely across 50N.

As can be seen, this increase in westerlies is being snaffled by our dominant low angular momentum base state. What this means is that the jet pulse will be transient and within the 10 day timeframe, the residual impacts should have dissapated and we should see that persistent mid Atlantic ridge / negative NAO pattern re-emerge.

Extended long range model tools would appear to be going this way. A lesson perhaps to take forward into the winter though. 2-3 weeks after Nina surges we may see jet pulses which could well be interesting in terms of setting up periods of Atantic troughs and, as these decline, eddies in the jet and Scandinavian ridges.

[damianslaw]

In instances such as this where we have such a notable differences in pattern (in this case strongly positive NAO within the context of a sustained -NAO phase), we have a golden opportunity to analyse the impacts of tropical forcing.

The strong pulse sent through the atmosphere from a trade wind surge and deepening of La Nina in the last few weeks (visualise pebbles thrown into a pond) has worked its way poleward. On the back of it we have probably seen a westerly wind pulse which as manifested itself in a vigorated jetstream. Tendency in angular momentum plots show this increase in westerlies nicely across 50N.

As can be seen, this increase in westerlies is being snaffled by our dominant low angular momentum base state. What this means is that the jet pulse will be transient and within the 10 day timeframe, the residual impacts should have dissapated and we should see that persistent mid Atlantic ridge / negative NAO pattern re-emerge.

Extended long range model tools would appear to be going this way. A lesson perhaps to take forward into the winter though. 2-3 weeks after Nina surges we may see jet pulses which could well be interesting in terms of setting up periods of Atantic troughs and, as these decline, eddies in the jet and Scandinavian ridges.

Thanks for another very informative post. Suggests the upcoming surge in westerlies is likely to be a temporary affair in a background where atlantic ridging is still the default position, quite like the thoughts of such patterns occuring through the winter, whilst it would kill the chance of a sustained long cold period it conversely would offer a great deal of variety with colder weather still more the dominant force interspersed with temporary atlantic assaults with the real risk of major frontal snow events when the milder air moves in and also on the back edge of mild air.

Your thoughts would seem to support the long term forecasts of colder weather being more in evidence than milder at least as is being suggested for late autumn/early winter.

[weather eater]

Thanks for another very informative post. Suggests the upcoming surge in westerlies is likely to be a temporary affair in a background where atlantic ridging is still the default position, quite like the thoughts of such patterns occuring through the winter, whilst it would kill the chance of a sustained long cold period it conversely would offer a great deal of variety with colder weather still more the dominant force interspersed with temporary atlantic assaults with the real risk of major frontal snow events when the milder air moves in and also on the back edge of mild air.

Your thoughts would seem to support the long term forecasts of colder weather being more in evidence than milder at least as is being suggested for late autumn/early winter.

That is indeed interesting, certainly the GFS has repeatedly dropped hints at a return to a -NAO state for around mid October. As for the winter ahead and the prospect of sustained colder spells, it could depend on where any -NAO positions itself, the more west based the better, as a sustained cold block over western Europe with Atlantic depressions butting up against it could make things very interesting for us.

[damianslaw]

That is indeed interesting, certainly the GFS has repeatedly dropped hints at a return to a -NAO state for around mid October. As for the winter ahead and the prospect of sustained colder spells, it could depend on where any -NAO positions itself, the more west based the better, as a sustained cold block over western Europe with Atlantic depressions butting up against it could make things very interesting for us.

Under the right conditions mid atlantic ridging can leave us very exposed to cold attacks from both the north and the east and can transcend eventually into major blocking over Greenland when there are no short wave developments to our NW and when most of the energy is in the southern arm of the jet..

equally it can leave us under fairly quiet average conditions with modified tropical maritime air alternating with polar maritime air brought around its north eastern flank bringing lots of low cloud and weak frontal attacks, this tends to happen when we have a stronger NAO and energy is stronger in the northern arm of the jet.

[pyrotech]

May be i am missing something here, but with us moving into la Nina and conditions ripe for a positive NAO over the next few days, will that not make the difference to the projected GWO phase.

I see the Westerlies continue well into mid October and the jet being kept South, influenced by deep low pressure towards Greenland. I can not see a sustained ridging in the Atlantic making its presence felt here in the UK or moving us moving towards phase 2 due to this.

I am not as experienced in this area as some on here, so i expect to be corrected, but i have posted this so that i can learn so when correcting please add explanation as to why this line of thinking is wrong, so i can learn from it and anyone else who needs further understanding of this subject.

[Iceberg]

There are definately changes afoot in the middle timeframe.

Zone's 1,2 and 3 have experienced a marked uplift in ENSO temperatures in the last week along with a subtle shift in the tropics.

It's too early to see whether this is a longer term 3 month change but will need watching over the next few weeks. What we have also seen is the SOI 90day avg hitting 20+ and the 30day avg hitting 25+

In the last decade whenever the 90day avg has gone above 20 it has always been followed by a sharp fall to at least 10, it looks like some almost in built mechanism.

My prediction is that October will be a month of variability and shifting in the longer range predictors.

[Typhoon John]

There are definately changes afoot in the middle timeframe.

Zone's 1,2 and 3 have experienced a marked uplift in ENSO temperatures in the last week along with a subtle shift in the tropics.

It's too early to see whether this is a longer term 3 month change but will need watching over the next few weeks. What we have also seen is the SOI 90day avg hitting 20+ and the 30day avg hitting 25+

In the last decade whenever the 90day avg has gone above 20 it has always been followed by a sharp fall to at least 10, it looks like some almost in built mechanism.

My prediction is that October will be a month of variability and shifting in the longer range predictors.

I think there's a strong chance that SOI will get above +30 over the next few months with more favourable conditions and the La Nina really taking hold.

Perhaps back to neutral sometime next year.