76 replies to this topic

[johnholmes]

Stewart a question
we all know what did not happen with the Stratosphere warming. The fact that at 30mb, its almost back to normal?
Is there any link with this and what you are reporting and now forecasting in your view?
If so what is your view on why we got no real effect, well most of the northern hemisphere got no real effect from this huge warming event?

answers in simple English please, none of your exotic science if possible for me as a simple soul!

many thanks

Edited by johnholmes, 03 March 2009 - 09:07 .

[johnholmes]

any chance of a reply Stewart?

[Glacier Point]

Before we can qualify what if any impacts the MMW has had, we must I think contextualise this event within a west QBO under low solar minima. There has never been an early breakdown of the polar vortex during such conditions and we can see this I think with the polar vortex remaining intact or attempting to reform.

There have I think been several impacts of this warming:

1) angular momentum at the upper levels has dropped since mid January. This can be seen in angular momentum plots:

http://www.cdc.noaa....g.qbo.90day.gif

2) the Artic Oscillation has shown 2 large negative phases over February. Nothing special in that unless you consider the west QBO / solar minima. I speculate that had this been an east QBO, the AO would have come up with a value in excess of -4 SD.

http://www.cpc.ncep....ex/ao.sprd2.gif

note the third drop coming up.

3) the storm track across the NH has been shifted anomalously southwards. The largest positive zonal wind anomalies for February are evident across North Africa.

So whilst the spectacular blocking synoptics have yet to materialise, the MMW has resulted in measurable impacts across the NH, perhaps more diffuse but non the less significant. These continue as the anomalously warm lower stratosphere persists.

http://www.cpc.ncep....JFM_NH_2009.gif

Bar far the most discernible impact from the MMW for me is its reinforcement of a low angular (Nina like) base state. That relates directly to my expectations for the next few weeks. Low angular momentum, phase 2/3 of the GWO = blocking during March when shortening wavelengths allow for the release of colder air, even within the context of the west QBO.

[johnholmes]

thanks Stewart

I think!
Taking your points 1), 2) and 3).

If I summed it up thus for numpty's like me would my overall idea be correct?

Low GWO means that northern blocking in winter is more likely than an active Atlantic?

and this in spite of the QBO having a westerly component?

but with a low solar minimum?

[Glacier Point]

 johnholmes, on 3 Mar 2009, 02:41 PM, said:

thanks Stewart

I think!
Taking your points 1), 2) and 3).

If I summed it up thus for numpty's like me would my overall idea be correct?

Low GWO means that northern blocking in winter is more likely than an active Atlantic?

and this in spite of the QBO having a westerly component?

but with a low solar minimum?

Low angular momentum during the winter makes blocking less likely at high latitude and the overall flow pattern zonal (there are exceptions in terms of specific months and GWO phases).

Both this and the west QBO emphasise to me how well we have faired, all things considered. The factors that have been on our side have been the solar minima which has beefed up the polar cell and the hemispheric arrangement of SSTAs.

Edited by Glacier Point, 03 March 2009 - 15:17 .

[johnholmes]

I have to confess I am rather 'lost' Stewart so rather than bore people I'll pm you

cheers
John

[BrickFielder]

My take would be that Low angular momentum means less energy going into the jet stream. This would suggest to me that during a low angular momentum period the jet stream tends to stay south and not arch northwards leading to high latitude anticyclones. This means that low angular momentum is not good for a particular type of blocking. Typically we get cold periods due to being north of the jetstream, but the Atlantic storm track is rarely disrupted.

A strong la nina period which is associated with low angular momentum however means trade winds tend to be strong across the Pacific. This leads to a stationary anticyclone north of the equator which by nature of the fact it does not move can cause extra tropical blocking to its north. Unfortunately this type of blocked pattern is not often conducive to cold weather in the UK with I think high pressure in the north pacific, low pressure in north west Canada and high pressure again in north east Canada. All this does depend on the season and number of Rossby waves around the globe, hence why Glacier Point uses re- analysis to show the effect during any particular month.

You might think that a strong el nino period would give you the opposite effect, but what you get is a strong jet blasting across the Atlantic with the UK ending up with a largely wet winter. You will get periods where the jet arches northwards but any anticyclone tends to get undercut or blown away by the strong jet. In contrast to the fixed anticyclone north of the equator in the Pacific you tend to get Thunderstorm activity (MJO) moving west to east across the equator, again not giving a very fixed pattern.

The point about the QBO and its direction is that stratospheric anomalies tend to be transmitted down to the troposphere during a easterly QBO rather more than a westerly one. This is not well explained why in literature but seems to reflect the fact that north pole stratospheric winds (stratospheric vortex) may be pulling in the opposite direction to equatorial stratospheric winds (QBO). I also think the actual nature of the Stratospheric disturbance plays a part. Split vortexes tend to lead to a lot of blocking, displace vortexes lead to less blocking and completely disrupted vortexes give very little blocking, but tend to reinforce low angular momentum. We have actually gone through all three of these phases with different results.

[johnholmes]

thanks Brick - very well explained for me at any rate

[chionomaniac]

Yes thanks to both GP anb BF.

This explains very well why (apart from the initial split) the MMW has not led to the blocking some of us would have preferred. The low angular momentum (perhaps from a more diffuse downwelling of stratospheric negative wind anomalies due to the persistant westerly QBO) coinciding with a weak La Nina state have contrived to prevent northern blocking patterns that every mid range forecast even seemed to suggest.

This to me makes perfect sense with what we have seen on the ground and is another illustration of why we need all the troposheric fastors to be favourable to achieve maximum benefit from a SSW.

c

[BrickFielder]

One of the key things about Spring and Early summer is that the jet stream begins to shift North. Typically in the region of the UK there is a northern arm and a southern arm to the jet and where ever they occur you often get wet and windy weather. Due to the low global angular momentum (la nina like) state of the climate then the jet stream is currently further south than it would normally be. This is born out by the ITCZ being relatively far south across Africa for this time of year.

I tend to think that we get two periods of about a month of wet weather as the jet stream moves north. During la nina those months might be April and June with August and October being wet as the jet comes back south. During el nino those months may be earlier for instance March and April. Ideally you would want a transition from la nina to el nino during spring which would push the jet north quickly, limiting the about of wet weather.
One of the best teams for predicting la nina and el nino are the Australians who tend to look at the SOI (Southern Ocean Oscillation Index) as a pre cursor to the state of equatorial water temperatures in the Pacific (which is what la nina and el nino are). During the last month we have seen a significant change and although this was enhanced by some strong activity through phase eight of the MJO it should be noted that the recently dominate pacific trade winds have been over come.


I will take time for the MJO signal to travel around the world again and to some extent you might expect some change at this time of year, but I still see some significance that may not yet be reflected in the models.Warm water volumes should be declining somewhat now so a full on el nino seems unlikely but the changes seem to be significant even though global angular momentum is on the low side.

Since the stratospheric vortex has never really recovered from the mid winter disruption we have had an unusually spring like late winter season with very little pattern blocking. Significantly the North Atlantic storm channel has been open for over a month and this has had an impact on ice formation to our north especially in the Barents sea area (yes I do know all the caveats surrounding the following chart but it goves an indicator).

This is perhaps not good news for snow lovers for next winter in the UK although the snow fields of Asia may still be quite late clearing leaving us watching for a beast from the east as a source of cold next winter. Coming back to the storm channel being open and how weather patterns seem to be to be limited to a two month period then the current pattern should not have that much more to run. I put this down to the rossby wave lengths (number of undulations of the jetstream around the world) tending to change with time and season.

In conclusion I am quite hopeful for a drier spring season than usual with temperatures beginning to catch with the norm from here on in. I am also rather hopeful that with plenty of cold air to the north due to the low angular momentum conditions and with the jetstream beginning to come north more quickly we may have an interesting storm season ahead (although it may be shorter than usual?). We might also see a Hot month mid summer instead of barely escaping the clutches of the jet stream last year. I reserve the right to completely change my mind though if the SOI changes, or the stratospheric vortex reforms, or any one of a number of variables change.

[chionomaniac]

Thanks for the interesting update BF.

I don't know how this may affect things, but I reckon that the stratospheric vortex has more than reformed since the severe disruption in January. Looking at the mean zonal winds we are now running above average for this time of year throughout the whole height of the stratosphere. Does have any bearing on your thoughts?





c

[BrickFielder]

I am not sure I really agree about the vortex at lower levels. Certainly there is some cold air and a deep circulation at 30hPa but it all looks pretty weak at 100hPa. Vorticity at the tropopause is very messy indicative of a weak vortex.

Analysis out of Japan also shows a very weak vortex.


Heat flux also looks to be fairly flat line.


The thing about the average of zonal winds at this time of the year is that it is the average of years when both the vortex is still active and when it has already broken up. You would expect it to be above or below the average with average indicating a weak vortex at lower levels. There are indications though that it will strengthen and we might be in for a late break up of the vortex.
gif]

We wait to see which way it goes.

[Glacier Point]

For the first time since early February, NWP forecast tools are suggesting a northern block to develop across the Barents Sea to Greenland. How much credance can we give this ?

During March and April, complex dynamical processes (torques and eddies) in the atmosphere have worked to add westerly winds to the overall global circulation indicative of a shift in the synoptical pattern from La Nina to neutral ENSO. The net impact of these processes has seen the Global Wind Oscillation ladder up in phase space 3-4-5 (Nino like).

http://www.cdc.noaa....gcm/gwo_40d.gif

However, since late April / early May the westerly wind input has temporarily declined. As nature abhors a vacuum, easterly winds have begun to progressively replace the westerlies and significant easterly wind tendency has developed in the circulatory pattern. This has largely been driven by frictional torques:

http://www.cdc.noaa....ltauf.90day.gif

Mountain torques are following the lead:

http://www.cdc.noaa....ltaum.90day.gif

and tendency in earth angular momentum is sharply down:

http://www.cdc.noaa....teaam.90day.gif

The MJO after almost completing a full orbit has beat the retreat in phase 7 - a sure sign of negative (easterly) tendency in the atmopshere:

http://www.bom.gov.a...Last40days.html

Where the GWO goes in the next 10 days will have a large influence on the global weather patterns. Given that both frictional and mountain torques are negative and likley to remain so for several days (some suggestion of frictional torques heading upwards) and lack of tropical intertia c/o a lack of convective activity, I think we will see the GWO orbit in medium amplitude phase 0 and 1.

Composite reanalysis for phase 0 at 500hPa identify a large negative anomaly in the eastern North Atlantic and height rises further north and north-west which is consistent with the NWP outputs.



Phase 1 reanalysis suggests the low to become stationary and fill with a height rise over continental Europe - something of a progressive pattern which should result in warmer, drier weather developing from the south and pushing northwards towards day 10 (mid month onwards) - note the trough persisting over Scandinavia.



Today's ECM 00z (08/05/09) has a similar evolution t144 onwards and I think this is very plausible. Whilst it looks very likely to become unsettled mid next week onwards, this should only last a few days in the south and indications to me that we should see conditions improve from next weekend onwards provided that we do not see the GWO stall in phase 0.

Longer term, even though the atmosphere is becoming more Nina like, I think this is only temporary state and the overall building of warmer SSTAs across large parts of the Indian and Pacific Ocean are likely contribute to increasing westerly (Nino) tendency in the pattern as we head into June, as is the state of the polar atmosphere which remains more conducive to sustained warm and dry periods June through August.

Edited by Glacier Point, 09 May 2009 - 08:38 .

[anvilhead]

Have you guys ever pondered putting on an afternoon lesson somewhere where the rest of us could come along and attempt to be shown some of the bigger picture ? .. I think theres a lot of people who'd pay good money for this... just a sugestion.

sam

[johnholmes]

 Glacier Point, on 09 May 2009 - 08:35 , said:

hi Stewart
just tried to pm you-unable to receive any more it says!
just wondered if your last 2 comments with June are meant to be July

regards
John

[john w]

 anvilhead, on 27 July 2009 - 15:34 , said:

Have you guys ever pondered putting on an afternoon lesson somewhere where the rest of us could come along and attempt to be shown some of the bigger picture ? .. I think theres a lot of people who'd pay good money for this... just a sugestion.

sam

I agree, I have a Bsc in Phys Geo and spent most of that doing meteorology and atmospheric science and some of this os over the bonce.....some well written guides would come in really really handy!

[johnholmes]

the idea of some kind of meet up, mind you it would need paying for, has been discussed on the open forum and elsewhere, both for synoptic meteorology and the teleconnections side.
There are enough quite well qualified people to make it feasible.
Best you pm Paul and see if it could be put up to test reaction.
I would be happy to do day to day synoptic meteorology.