In the Arctic Ice thread Beng, today, mentioned GLAAM - a term I'd not come across before. Googling, the first description I came up with was this - (Dated May 25)
With a La Nina beginning to unfold, one index we need to watch is the globally averaged atmospheric angular momentum (GLAAM). All of the other major indices that are critical to ENSO are in a La Nina state now, but the GLAAM continues to be in the positve El Nino state. IMO if this index goes persistently negative we will have a significant cold ENSO event.The GLAAM measures the overall momentum of the entire atmosphere of the Earth. Negative GLAAM indicates an easterly anomaly which is favorable for enhanced trade winds in the tropics. If the GLAAM remains positive, I would have to guess the ENSO will settle into a cold neutral or maybe weak La Nina state.from -http://westernusawx.info/forums/index.php?showtopic=29946
This describes pretty well what I was trying to describe. The next question then, is, how does the strength of the trade winds including GLAAM, respond to the position of the jetstreams? - any experts out there?
Atmospheric Angular Momentum is like the drag the atmosphere exerts on the rotation of the Earth through its interaction with mountains, trees and higher elevations. Most of the wind on the planet blow west-east opposite to the direction of the rotation (something like inertia of the atmosphere) (This you can see in the animation)
The rotation can literally slow down if there are strong enough winds blowing west-east. It is only a millisecond per day slow-down but it has been measured carefully since it was first discovered in the 1950s.
It is only a plus/minus measure and the atmopshere can never really change the ultimate rotation of the whole Earth which is millions of times bigger than the atmosphere. The rotation will eventually overcome some extra wind drag and force the wind to slow down instead.
The Trade Winds, however, blow east-west, in the direction of the rotation and are generally less strong than the mid-latitude winds which are the main driver the GLAAM changing. The Trade Winds are a balance to the stronger mid-latitude winds. It just the way the atmosphere has organized itself. (You can see this in the animation as well).
When GLAAM is high, the mid-latitude winds are high and the rotation slows ever so slightly and the Trade Winds slow down as well. Slow Trade Winds help an El Nino develop. High GLAAM might also help the Pacific Ocean to slosh backards a little from the Pacific Warm Pool area to the Nino regions (which are cooler and have less sea height than the PWP).
When the GLAAM is low or negative, the mid-latitude winds are slow, the Trade Winds speed up and this can help a La Nina along.
All these things need to be sustained over several months to help along a an El Nino or a La Nina.
The article noted that GLAAM has to go negative in order for their to be a switch to La Nina. Well it has gone strongly negative over the past month - reaching some of the lowest numbers recorded. It is not quite so negative right now.
Global Atmospheric Angular Momentum versus the ENSO (updated for June's GLAAM number and a guess for where the Nino 3.4 index ended up at which around -0.500).
Edited by user 04 July 2010 21:17:25(UTC)
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To Gavin P
This La Nina will be the biggest one yet - lower than -2.5C
I was browsing this page http://oceanworld.tamu.edu/resources/oceanography-book/teleconnections.htm and one graphic really stood out, the one showing Atlantic hurricane tracks during El Nino and La Nina years. As we know, the standard El Nino (EPW) supresses hurricane activity and La Nina (EPC) enhances it.
BUT a Central Pacific El Nino event (CPW) enhances hurricane activity even more! I didn't know that, but I guess it must be because the dispacement of the Walker cell is different, leading to greatly reduced wind shear over the Atlantic.
Speaking of teleconnections, don't count on this causing here
It's my understanding that both strong El Nino and strong La Nina conditions generally coincide with enhanced zonal flow over NW Europe, and neutral (or weak/moderate either way) is best for a cold UK winter.
This graph shows what will happen to UK temps over the months ahead:
Nino 3.4 was about -0.85C yesterday (it looks like the warm water that was left-over from the last El Nino in the north equatorial counter-current (5N) has almost been cooled off now by the La Nina waters at the equator).
Nino 1+2 has cooled rapidly in the past week to about -1.7C yesterday as the Peru-Humbolt current has also joined in now in pushing cool water into the ENSO equatorial surface current system. There are some 1.5X1.5 degree boxes that are at -5.0C in this current.
Edited by user 13 July 2010 14:29:38(UTC)
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Love those eddies on the edge of the cold anomaly.
Those are what GCMs are pretty bad at modelling, the resolution isn't quite good enough to pick up the structure of that particular heat transport mechanism.
These animations show the eddies / currents at the highest resolution available right now.
30day anomaly forecast (not excited about the colour scheme here - white is +/-0.4C and the blues don't match the reds at the right scale but nonetheless).
Thats a real La Nina signal now, isn't it? Very, very early in the year for such a well defined signature. This La Nina really does mean buisness, IMO.
At this point in 2007 when we we're at the beginning of our last La Nina the signal looked like this;
Edited by user 13 July 2010 16:11:13(UTC)
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Good so maybe this one deliver a global cold spell and a proper snowy winter in the UK
YD, strong La Nina's tend to favour zonal winters in the UK.
That said, if the Atlantic is set for a negative NAO, cold winters can still occur with a strong La Nina going on. 1917, 1955 and 1956 tell us this.
I would say strong La Nina + low solar activity = strong southerly tracking jet
Difficult to say how strong this La Nina will be but it looks like a fairly important one at least. The latest spread of models is on page 27 below:
The NASA GMAO (dynamical) model predicts the La Nina of the century, stronger than anything since at least 1950. Others are more clustered in the range -0.5 to -1.5, with the biggest cluster (mainly of statistical models) sitting between -0.5 and -1.0.
Tim - I remember you saying last year that the models have a period where their accuracy for Enso forecasting decreases - when would you think we ought to see better agreement on the likely strength? My own thinking (guess!) is that we'll peak around -1.5 on the tri-monthlies.
Sorry O/T but just on the NAO state for the coming winter CFS is being quite consistent now on a strong negative NAO - more especially in the early part of the winter so might be worth a flutter on the white Christmas this year. The CFS blocking charts pretty much nailed last winter by the way IMO - darn climate models - always said they're hopeless
I don't have evidence to back this up, but here's what I'd expect; if the jets are pushed closer towards the equator, then more energy is trapped around the tropics which ought to lead to a stronger pressure gradient and stronger trade winds. I guess the implication of this is that if a weak solar signal does indeed produce increased blocking in polar regions, pushing the jets south, then it could produce stronger trades in the tropics, increased trade winds and increased upwelling of water off the coasts of South America and Africa.
Of course it's not that simple though since you'd also then expect some kind of negative feedback to kick in due to reduced tropical heat as a result of the original cold upwelling. Perhaps the overall result would be stronger la nina events and weaker el ninos? - but total speculation on my part.
What you describe, sounds to me, a good rough explanation. The tropical feedback cooling, could well compliment the original cause of the Jets moving towards the equator? Interesting.
Edited by user 15 July 2010 16:10:43(UTC)
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Yes, we're still in the "spring predictability barrier" at the moment but we come out of it during boreal summer. ENSO tends to transition over the summer months and once the pattern is set there is greater persistence through boreal autumn and winter, so the models are likely to cluster more over the next couple of months.
The state of the South Asian monsoon is thought to be one factor driving unpredictability in early summer. It's such a huge atmospheric event that it can overwhelm other tropical influences on the Walker circulation.
It's interesting that the CFS models show blocking in the NH as the ENSO signal is supposedly opposite to this, but then I think the ENSO effect is more pronounced in the late winter so the early winter pattern may be related to mid-latitude features such as N Atlantic SSTs.
Thanks Tim - and yes in terms of the predicted blocking - if you look at the three month chart for Dec-Feb - it's indicative of a positive AO (as you'd expect from the negative Enso), but also still significant blocking in mid atlantic - presumably due to the Atlantic sea Temps.
I'm not sure how that'll work out for the UK - depends how on where the jet digs South around the blocking and whether that's to our East and how much energy goes in the northern arm (presumably not that much with the enso signal). At the moment, I'd bet on a colder than average winter for Eastern Europe - and early season cold in the UK. The other detail is how much will sea temps warm over the next couple of months off the US Eastern Seaboard with the US SE Ridge likely to setup - I think this spoilt any negative NAO signal in 2007.
3.4 now down to -1 - which is slightly below the corrected CFS forecast http://www.cpc.noaa.gov/products/people/wwang/cfs_fcst/images3/PDFcr_nino34SSTMon.gif
but a little above the non corrected CFS forecast (although the time scale is quite hard to pin point accurately).The corrected output now peaks the event at around -1.7 although some of the recent 'uncorrected' model output is extreme taking 3.4 down to -3 at the peak of the event.
Only a small drop in 3.4 this week - down to ~-1.1
It's interesting to compare this coming la nina event with the last one which started in 2007:
It certainly does look like this even is shaping up to be considerably stronger at this stage. We may see some temporary sub surface warning soon (September mainly I'm thinking) though with the reduction in cloud cover as the sun heads towards the equator - i.e. negative feedback.
CFS by the way is still sticking to its guns with blocking across the atlantic for this coming winter - the Jan chart is looking impressive.
It should get much colder pretty fast going by all the indicators especially this equatorial cross-section which is a model output but based on the newest data from the Argo bouys. There are some -7.0C's in there.
But then the forecasts show that the La Nina is not going to peak until 4 months from now - November.
One issue is that there is still some warm water on the north side of the equator which has to be cooled off in step and then pushed west before a La Nina can really get going. There is a particularly warm spot at 5N, 110W 100 metres depth which will continue infiltrating the Nino regions and offsetting some of the cooler water. it is going to take some time for this anomaly to reach the temperatures of a La Nina. This is the cross-section at 110W from the TAO Triton bouys.
Edited by user 28 July 2010 01:08:06(UTC)
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