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  1. For the charts featured in this tweet, I checked out the 500 hPa height patterns for the most analogous years to a very weak polar vortex and neutral or negative Nov-Dec mean GLAAM. At first, one might assume below-average temps being indicated across NW Europe especially, but actually, the negative 500 hPa anomalies are entirely driven by below-normal MSLP; I have the actual temp means to hand and they're near-average for the W. half of Europe and above average for the E. half (same's true for both netural & negative GLAAM, intriguingly). The eastern North Atlantic storm track is shifted south, but only by enough to throw an unusual number of LPs straight at the UK and (a bit less frequently) NW. Europe. Only when I look at the most analogous years with positive period-mean GLAAM do I see a shift of the mean storm track right down level with Iberia, with a negative temp anomaly signal for the N. half of Europe. This demonstrates the critical role of GLAAM - the 'Mini-ENSO Cycle', when it comes to how much and in what manner perturbations to the polar vortex affect our weather patterns, that Tamara has been producing superb posts on in recent weeks. It remains to be seen whether the polar vortex becomes particularly weak in the first place; NWP projections have been trending away from that in the past few days, toward something near-average for late Nov. Judging by my composites, it doesn't appear we'll miss out on much as a result. This, of course, is assuming GLAAM doesn't shift into a mean positive state for Dec. I've yet to see any clear reason to expect that it will do so, but the atmosphere isn't fully 'at peace' with its current state, so I'm being kept on my toes.
  2. Thanks for the positive new updates David. The Arctic's really benefiting from the Nina-style low-AAM state, with suppressed climatological ridges i.e. reduced poleward transport of momentum, related to which are surges of relative warmth and moisture into the Arctic regions. We're a long way from 2016's high AAM situation forced by that year's intense El Nino event. This doesn't cover all the helpful pattern behaviour though; reduced poleward transport allows for a strong stratospheric polar vortex to develop, which is very much the case as of early Nov 2019. If that was connected fully to the troposphere, deep storms would be stirring up the areas of newly formed ice as and when they cross them, interrupting the thickening process quite widely - albeit with some counteracting ridging of ice in places, where the flow is toward rather than away from young ice that's up against land or thicker, older ice. So, the curious disconnect of Oct into Nov 2019 is also helping things out a good deal, IMO. So far, the resulting refreeze has largely been of the 'easy' regions with the shallowest, freshest waters. Recently, it's set upon the 'moderate' Laptev region (a bit deeper and more saline), while the roughly equivalent Barents has been worked on for a fair while now (as per David's reports). Beaufort hasn't been going well at all, but judging by the SSTs now, it ought to start to give way sooner rather than later. Then will come the assault on the 'hard' regions - the Chukchi and Bering seas. Should patterns remain very supportive of sea ice growth (I'm not as confident as David about this; there are some not-inconsiderable perturbations starting to manifest in the NWP modelling for both the troposphere and stratosphere, which need watching in case they become significant i.e. capable of forcing a more than brief pattern shift), I'll be very interested in seeing how those two seas respond - it will say much regarding how well natural variability can interrupt the 'oceanification' process on that side. This year has demonstrated that with enough favourable wind patterns, the volatile Atlantic side can catch a welcome break and build up some better defences to take on the next melting season with. Unfortunately, it's not easy to have such wind patterns on both sides of the Arctic at once for more than a short period at a time - usually, one side or the other has to take the hits. Volume/thickness is also going to be of extra-special interest this refreeze; how well can helpful atmospheric patterns make up for an extremely delayed refreeze in so many of the peripheral seas? We may well learn the answer, unless there's a big pattern shift, as I cautioned earlier (there are few more wary than I! I'll admit, it can take some of the fun out of things...).
  3. Finally had a good read of David's latest omega-post. I really like the analogy to the NAO in the North Atlantic; the signs oppose with respect to circulation strength, but in terms of direction, they align (as the climatology supports high pressure in the areas of interest, but + phase equates to clockwise spin regardless of hemisphere, which leads to stronger HP in the N. Hemisphere but weaker HP in the S. Hemisphere). During significant +NAO episodes, the Atlantic's subtropical high is anomalously strong , driving stronger trades across the tropical region which leads to lowering of SSTs - an important factor in the hurricane season due to its strong impact on the Main Development Region. So, we see parallels to the -SPO phase. This gets me wondering whether the solar cycle has any impact on the SPO, given that there is some reasonable evidence of impacts on the NAO. The stronger, more durable nature of the S. Hemisphere polar vortex on average (but not this year, funnily enough - it's been strongly disrupted Sep-Oct!) may be a barrier to that, but then again, maybe not - research desired! Briefly on the predictability of the S. Hemisphere weather patterns compared to N. - I expect this is due to the much smaller amount of land interaction, especially by mountainous terrain. Fewer complications in the flow to resolve. Imagine of the U.S. didn't have the Rockies... the weather wouldn't be nearly as dramatically-inclined!
  4. Well, it sure took its time, but at last the expansion of Arctic sea ice coverage has started to make up some ground on the rest of the pack. Over 200,000 km of increase in extent was measured for yesterday, following several days of near-average increase overall. The huge delay has occurred despite relatively (compared to most of the recent years) little in the way of major intrusions of a anomalous atmospheric warmth, or particularly strong wind events. I expect that’s largely down to the anomalously warm peripheral seas, and perhaps regionally some exceptional concentrations of atmospheric methane. It remains to be seen whether the uptick in pace of refreeze will sustain, but I can’t see much in the weather patterns or salinity observations to suggest it won’t at least stay near average overall - and arguably, above is naturally favoured by the deficit on other years. Well, at least a a basin-wise average; the most anomalously warm parts of the Pacific peripheral seas may be exceptions. I realise that the unprecedented lateness of refreeze onset for most of the peripheral seas is likely to hurt in terms of ice thickness and volume build this season, but at least we’re not following the 2016 trajectory of weather patterns. We may also see the best CAB region seasonal gains for many years, as it fared relatively well this past melt season and has seen a quick (by 21st Century standards) refreeze this October. I’d not be surprised if next March we find ourselves looking at the greatest CAB to non-CAB contrast on record. That’d make for a very interesting melt season 2020, with how much weather patterns attack the CAB (2019’s main ‘shortfall’) being even more important than usual.
  5. True excellence from Tamara. To those who question the extraordinary consensus of long-range modelling on an extremely +AO/+NAO winter, well, what they show is highly plausible should there be no disruption to the low-AAM cycle. So, some comfort can be taken from the fact that the models aren't likely to resolve the 'seeds' for such disruption very well - if at all. Sadly (for cold/snow seekers away from the high latitudes), however, due to the dependency of that on very particular events (e.g. active MJO making it fully eastward to the Central Pacific) that co-operate with one another, the scenario depicted by those long-range models has to be taken as the most probable until if & when we see good evidence that we're going to see an event capable of forcing a marked +ve shift in AAM.
  6. As an autumnal run of weather sets in for the UK in a dramatic fashion (deep low, widespread wind gusts 25-40 mph Fri, gale force winds for the far-south on Sat), it's easy to feel a sense of summer having packed itself up and gone on indefinite leave... but that'd be a classic case of failing to see the wood for the trees. If the tropical Pacific sported below-normal sea surface temps right across from the East Pacific to the Central Pacific, then sure, it wouldn't be unreasonable to expect little recovery to summer-like weather across the UK within the foreseeable future. This being based on the atmosphere being trapped in a Nina-like state, with equatorial tropical waves (e.g. MJO) struggling to make it east of the West Pacific and break it free. As it is, though, we have a not just warm, but very warm Central Pacific compared to even the 1981-2010 average. This provides a very attractive environment for uplifting motion with low-level convergence of winds in that region, which is supportive of equatorial tropical waves, and their associated westerly wind bursts, propagating eastward beyond the West Pacific. There are now signs of this appearing in the GFS output when looking at the 850 hPa zonal wind anomalies. It represents a dramatic flip from the powerful trade wind surge currently underway. This will serve to halt and reverse the negative tendency to global atmospheric angular momentum (GLAAM), and the associated westward retraction of subtropical ridging that's currently allowing areas of low pressure to track right through the UK instead of being held to the west or diverted to the north. Extension of subtropical ridging across N. Europe, including the UK, should be capable of establishing by sometime between 7 and 14 days from now. Likely bringing a return to warm weather in the UK, and perhaps some of the driest of the summer season for the northern half / two-thirds (south of that, there's been plenty of dry spells to enjoy last week of June through first week of August). Perhaps also the annual false claims of an 'Indian Summer' which technically is a spell of summer-like warmth occurring after the first widespread frosts of the autumn .
  7. An example of high-frequency variability, or the 'Mini-ENSO cycle' as Tamara aptly calls it, changing the global atmospheric momentum budget via poleward propagation of +AAM from the tropics. I believe there's a good chance that this forms the starting point of a succession of Nino-supportive tropical waves, culminating in the next amplified MJO propagation across the Pacific from the Indian Ocean or Indonesia (which also has high statistical probability while the Nino standing wave fights, as Tamara pointed out in that superb round-up last week).
  8. Well then. The sea ice has taken one hell of a battering in the past few months, with the warmest May followed by what looks to also be the warmest June on record leaving its mark, helped along by extraordinarily strong and persistent high pressure systems, these tied into an unusually warm lower stratosphere - the legacy of the strangely vigorous final warming event in late April. Yet while a lot of attention focuses on the dramatic loss of sea ice coverage (most measures indicate record low extent, area or both, and there a signs volume may now be record low again too), there's another assailant attempting to flank us, but for the watchful eyes of a gradually increasing collection of scientists and enthusiasts. The attention on permafrost-covered methane (and nitrous oxide) deposits has stepped up a gear this month, due to the second half of June having seen a focus of the anomalous heat across the East Siberian Sea, where some of the largest known deposits are known to exist. There's recently been scattered reports of locations right by the Arctic Ocean experiencing temperatures into the 20s Celsius! This has inevitably led to some particularly large wildfires breaking out: There are signs of something much cooler with some rain around establishing within the next few days there, while the anomalous heat focus shifts to the N. American and Atlantic sides of the Arctic, but the fear is, critical damage may already have been done this season (massive permafrost melt-out). Sudden regional release of methane and nitrous oxide may occur at short notice, any time in the next 6 weeks or so, kicking atmospheric temperatures up as part of a vicious feedback that could melt further permafrost or delay the ground-freezing later this year. So while ice melting focus looks to shift away from the ESS in the near future, attention on the less visible climate threat of gaseous release should remain on that region.
  9. As far as I can see, the El Nino standing wave is fighting on (to an extent that the models are struggling to handle), continuing to weight the FTs a little toward +ve (deviations tend to be weak in the summer season), so propping up the global AAM anomaly. How much this promotes spells of high pressure over the UK and W. Europe will depend on how much poleward transport of this +AAM occurs. It's proven a stuttering affair of late, and it continues to be difficult to anticipate the variation of this in the coming weeks. The dissipation of those strong Pacific trades is a sign, though, that poleward transport will increase as July gets underway. Or so I currently understand it. As we can see in that EC monthly for 3rd week of July, unusually strong and persistent Arctic blocking continues to muddle the pattern too. Note, however, that the presence of below-normal heights to the west of Europe is close to what's typical with some El Nino forcing in play. This is where a tendency for very warm or hot spells punctuated by thundery breakdowns comes from. I have some concern that this week may not be the only one this summer to feature high-end heatwave conditions Central & Western Europe.
  10. I've been seeing a fair bit of social media activity predicting that the +ENSO state won't survive into the summer, but as far as I can tell, these are generally based on American model data, and I'm not convinced it's handling the tropical wave propagation very well at the moment. Admittedly I'm only basing that on the historical tendency for large Kelvin waves and associated WWBs such as we saw mid-May to be followed by at least one more eastward propagating wave within the following month. So I'm not dismissing GEFS/GFS outright, by any means. Whichever way it goes, I sense a traditional El Nino may prove hard to come by this summer - rather a more C. Pacific focused affair... which doesn't tend to suppress Atlantic hurricane activity so much. Something to bear in mind.
  11. I wish we had an AAM plot that showed both the tendency and the total AAM anomaly values together. Then it would be very clear how AAM tendency inevitably oscillates between positive and negative as the atmosphere seeks balance, with large-scale forcing phenomena such as the warm and cool ENSO phases determining how close to true neutrality (i.e. zero total AAM anomaly) the total AAM anomaly can reach; the warm phase (El Nino) tending to keep it above the line and the cool phase (La Nina) below the line. This oscillation on top of the base forcing is what makes it impossible - at least until we have many decades more historical data - to effectively classify historical months by AAM behaviour; there's a multitude of different factors to that behaviour that are important, such as the number of complete oscillations and how many stayed in line with the base state suggested by the ENSO region SST anomalies versus how many contradicted it. Perhaps a useful approach would be a rigorous classification of historically observed AAM tendency oscillation alongside total AAM anomaly into cycles of different types... hmm, where have I seen something like that before...? Ah yes, the GSDM . Though from personal experience, it's not as simple as going by the phase numbers - it also matters a great deal how large the tendency and total anomaly values are. At which point you start finding that your classes contain only a few years at best, and so we come back to the 'need more data' problem. It's a shame GSDM wasn't monitored pre-1970s, but given the lack of reliable satellite data, it's hardly surprising that it wasn't .
  12. Spotted this from MV showing the EPS version of the 850 hPa zonal wind anomalies plot. Comparing with the GEFS one, we can see the latter's typical positive trade wind strength bias that forms part of the negative AAM bias we see in the mid- to longer-range (though less obviously these days what with the AAM plots for that model having glitched out). What EPS instead shows is the low amplitude Nino standing wave that promising UK summer fortunes are built upon, albeit perhaps not as coherent as is most desirable. No hiding the fact that the WWB has been seriously stunted in duration, but it still looks to have done just about enough to shift the atmospheric wave guide. I'm nodding at any longer-term modelling that has the Arctic blocking becoming increasingly 'Sceuro' based with subtropical ridging first interacting with it and then taking over as the predominant mode. CFSv2 has been doing this on many of its recent runs for final third of May through much or all of June, sometimes July too but other times with a dramatic collapse - which I figure are most likely to be runs where no further CCKWs or MJO cycles manage to occur this side of mid-July.
  13. The culprit here is the models suddenly latching onto a strong CCKW/MJO wave emerging in the Indian Ocean in a location that destructively interferes with the Nino base state. Sadly, the modelling of these waves has been particularly poor in the past year, for reasons unknown. It's making it near impossible to avoid occasional hiccups such as with the pattern evolution this coming Wednesday through into the following week. The eastward propagation should replace destructive with constructive interference by mid-May, with AAM recovering and the GWO resuming positive orbits, so an improving prospect during the first half of the month, especially as the effects from the final warming subside as well. The unfortunate coinciding of that FW with the CCKW/MJO awakening has served to further dramatise the collapse of fortunes starting Wednesday. This sort of variation is why the GSDM is a broad analysis and guidance tool rather than one to use for detailed forecasting. For that we need to rely on the NWP and (to some extent...) long-range models and, crucially, applying experienced judgement to them (for which the GSDM is usually very handy). Edit - a requested addition from Tamara: "We are better relying on the GSDM analysis than NWP interpretation of wind-flows as represented by the GSDM. Whole suites are prone to inaccurate reading of these signals and so any convincing apparent consensus for a solution can easily be consensus for the wrong solution." "That has already happened in the last week alone and these days I am just as cautious with ECM handling of the tropics and extra tropics as the GFS. The ECM has shown a lot of fallibility not just last winter, but through this Spring as well". Back to me again: We can see evidence of this from CFSv2 currently; it's initialised AAM is a bit too positive.
  14. Hi Matt, some good observations there and well constructed . True what you say regarding the AAM changes and increased westerly flow across the N Atlantic next week - but now, the falling phase is starting to look too brief for the Atlantic trough to push right across to the UK longitudes (thankfully, I’d you prefer it warm) before AAM cycles back around and the ridge builds back in. With the Pacific El Niño event underway, even though it’s weak, its reasonable to assume further positive AAM cycles going forward. The main uncertainty is the time spent in each phase, as the case for next week has shown. Got to rush off now - I’ll see if I can find time to expand on this later.
  15. A very insightful and at times somewhat shocking video. What hits me hardest (edging out the part about what happens after the latent heat of fusion stops being required) is the sea ice thickness plot for 8th March shown at 8:58. Compared with last year's already record low mid-March thickness, the ice away from the Canadian coasts is another 0.5 to 2.0 m thinner this year - placing a lot of it in the sub-2 m thickness category which typically fails to survive even a modest melting season. Last year with the widely 1.5 to 3.0 m thick ice, I was thinking it could be a bad one if the weather's either very warm and clear or very stormy - in the end it was neither so we lucked out. This year, I believe it'll take an unusually cloudy and calm summer to save the sub-2 m ice. Not a common combination for obvious reasons. So it seems to me that there's a high probability of going below the 2012 minimum this year - but one can't be too presumptuous given all the complex feedback mechanisms in place so I'll reserve any firm judgements until we've seen what the next few months bring. If and when we see a 'blue ocean' event with 1 mn or less square km of sea ice at minimum, we'll enter a period of fascinating but, sadly, most likely very disruptive climatic responses. Even where we're at currently, I'm seeing evidence that the Hadley Cell has strengthened with a greater incidence of unusually strong and persistent ridges at the mid-latitudes in recent years. Such as the Ridiculously Resilient Ridge that brought extreme drought to California, and the multiple runs of 2-3 weeks of HP-dominated weather across the UK during the past year or so (including the one that culminated in the record-high Feb temps for large swathes of the UK & Europe), of which there appears to be yet another in the works (starting on Monday; duration not yet certain but the trend is longer and longer!). Seasons featuring a Blue Ocean event may see the Hadley Cell strengthen even further, bringing even more resilient ridges to the mid-latitudes. There could (I must stress the speculation here; this is all based on theoretical modelling) even be a persistent poleward shift of the subtropical high pressure belt, which would shift the climate zones north with it; e.g. Mediterranean turns more like desert, and temperate (such as the UK) turns more like Mediterranean. I'm not sure how long this would take though; could be within one decade or some half a dozen. I know that in the late 90s there was a lot of buzz in the UK over climate change bringing a trend toward Mediterranean-style summers by the late 21st Century but I feel that the rates of change (due to overlooked feedbacks) were being seriously underestimated back then.
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