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  1. 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 .
  2. 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).
  3. 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.
  4. 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.
  5. 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.
  6. 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 .
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. Getting close to the true melting season now (after s false start a week ago), and the time to review the state of affairs with a eye to what vulnerabilities are greater or less compared to recent years. I need to allocate some time to look in detail but my initial impressions are a stronger ice situation on the Atlantic side but mostly in areas that should melt out regardless, and weaker ice on the Asian and Pacific sides - even compared to 2018 for the latter which is really saying something; there was some crazy opening of waters last week before some recovery but with the ice of course being very thin and fragile now. Given the weak Asian side ice, it’s concerning to see snow cover extent running some 1 SD below normal and with modelling suggesting little in the way of cold spells to try and turn things around within the next few weeks. Could be an exceptionally or even record early spring warmup for quite a lot of the high altitudes of Asia unless the modelling is wide of the mark.
  12. Here in the UK and also across a large part of Europe, something truly extraordinary seems to be going on now and until further notice. Seeing that magnitude of 850 hPa temperature anomaly twice in the space of a week and three times in the space of 11 days is just staggering. The 500 mb height anomalies are also very impressive, especially later next week. All assuming the 12z ECM (with the 12z GFS similar but the 06z having been more so) that forms the basis for these charts is along the right lines - but the signal is proving very strong in the modelling now that the chance of some cross-polar ridging has largely been dropped (yet again this winter). Looking for the causes of this, well it's going to be an unusual combination and I have a theory that firstly, the propagating anomalies from the strong MJO passage across the tropical Pacific are being distorted by the negative AAM anomaly at 30*N (a Nina-like feature; as discussed by Tom in recent days) that as far as I'm aware continues to resist change even in the face of an extremely strong WWB across the Central Pacific (perhaps due to the anomalies from that being focused S of the equator as Snowy Hibbo proposed), and secondly, the lack of a strong lower stratospheric vortex (it's near the LTA which is a good deal lower than it is for Dec & Jan) is providing little in the way of zonal push on the blocking high, meaning that Atlantic troughs only serve to pump it up further via warm air advection aloft. If it wasn't for the continued Nina-like interference, I expect the high-latitudes would be permitting the height rises needed to set up a powerful retrograding HLB from Scandinavia to Greenland, as is typical following MJO propagation through phases 7 & 8 with a Nino background (though given the weakness of the Nino we'd probably have seen the blocking head too far west for the UK (from a cold perspective) pretty soon - like what happened to start March last year, but probably less dramatic!). One factor I'm unsure of the role of is the QBO; does a westerly QBO at 40 mb and above have a direct ability to keep the AO more positive? I don't expect so based on what I currently know (it impacts the stratosphere's interactions with the troposphere, not the troposphere's interactions with itself) - but I could be missing some crucial information. I'm planning to read into this good and proper if and when I can make the time, but if anyone can provide a short answer to this particular question, that'd be greatly appreciated .
  13. What's this, the GEM being wheeled out...?! Ha - for a change, it shows something useful beyond a few day's range. We see the ridge make an advance northwest as a strong ridge joins forces from the Pacific, and over this way, it becomes a matter of waiting for the Euro heights to subside southward with the jet cutting across somewhere in the vicinity of the UK to meet the deep trough retrograding toward N. Europe. For the U.S. the main interest lies in what happens shortly after that, as the blocking high in theory sets up over Greenland and then retrogrades gradually further west with time. The usual nor'easter potential arises with this during 1st half of March. The ECM 00z was only a little more sharpening of the Atlantic trough away from reaching this point by D10. Meanwhile, GFS continues to be GFS and to an extent I struggle to reason with. For example, at just 5 day's range its 12z has the European high centred nearly 1000 miles south of the 12z UKMO, 12z GEM and 00z ECM runs. Curiously, it's treatment of this has barely changed despite the RRM projections for the MJO having undergone a sudden massive adjustment to fall in line with ECM/EPS. The observed MJO is very odd today though; almost identical to yesterday's. What's up with that? I can only imagine! p.s. Interesting points @Snowy Hibboregarding the positioning of the WWB - I look forward to reading your thoughts when you do share them. My existing understanding is that a WWB across the dateline is as good as it gets for producing +FTs where they can significantly affect the broader-scale patterns via weakening the subtropical highs etc, but which side of the equator the WWB is mostly on hasn't come into my analysis before .
  14. @MattHugo, There was evidence in favour of the E to W transitional QBO stage supporting SSWs occurring, but not necessarily propagating all the way downward. I believe for that we needed a predominantly wave-2 driven vortex split event, instead of the mainly wave-1 driven displacement-then-split event that we saw. Reason being that the former drives a much faster response that would likely have outpaced the descending W QBO and established major easterly flow anomalies in the troposphere. The much slower 'drip-down' nature of the event we've seen instead has allowed the W QBO to become a blockade that's kept most of the negative zonal wind anomalies in the stratosphere. I have a feeling that the manifestation for a time of a very unusual three-way vortex split right after the reversal initiation may further have impeded the downwelling by distributing the negative zonal anomalies more widely, essentially making them more diffuse. At the time of the SSW, part of the response was a cooling of the upper troposphere above the tropics, enhancing convection even where the base state doesn't support it. Alongside the negative zonal anomalies, the SSW-driven positive temperature anomalies have also been held in the lower stratosphere, keeping the upper tropical troposphere cooler than usual and allowing the convection to continue to behave in ways that don't tie in with the Nino base state (interference with the patterns the Nino base state otherwise drive being the scrambling of the tropical-extratropical processes that Tamara refers to). Climate change may have taken this even further than it would otherwise have gone via increased overall oceanic heat content, but that's a matter of debate - to me the stratospheric mishaps seem more important for this particular winter's turnout. I think we can see a good example of the interference this coming week; the neutral ENSO MJO composites (top row) fit the ECM 00z much more than the Nino ones, despite AAM and GWO observations indicating that it would usually fit more to the positive ENSO composites; GFS fits mostly the neutral P6 composite before seemingly jumping straight to something akin to the P8 neutral composite (see below, left) with little P7 response - probably due to how it stalls the MJO in P7 for a while; continued eastward propagation is needed to bring about the typical response to activity within a given phase of the RMM plots. The negative NAO-related UK cold potential for late this month and into March depends on how well we can break free of the Nina-like interference and so bring about an MJO P8 response more typical of the Nino base state. So more toward the right-hand composite than the left-hand one. Note how I'm saying 'more toward' here; the composites should never be taken too literally (ideally we'd have different ones for each possible combination of MJO phase, GWO phase and AAM tendency, but there simply aren't enough historical years of obs data to do this meaningfully!). As you can see - the temperature regime across the UK will be extremely sensitive to how much the interference gives way.
  15. Really important to bear in mind that the GSDM guides us with respect to the probability of certain patterns manifesting, which means no guarantees should there be unusual developments such as the MJO (which is one of many contributors to the GSDM) not behaving like historical guidance indicates it should. Numerous times this season, the GWO analysis has revealed us to be in a position from which a typical Nino-like MJO behaviour would serve as the launchpad for significant high-latitude blocking development, only for there to instead be Nina-like MJO behaviour, and to an extent that I believe to be unprecedented for the background state otherwise being observed. I dread the thought, but this may be a symptom of climate change; a distorting of the climate responses to certain forcing setups. We need a number more years of evidence before drawing more definite conclusions, though. As if those multiple instances of MJO disobedience weren't enough, the one time that it did behave more Nino-like, the downward 'flushing' of zonal winds as a result of the major SSW initiation served to blockade the Arctic latitudes against the strong poleward ridging. I honestly consider that to be the most unfortunate (for those who predicted and/or sought after major cold weather outbreaks) destructive interference of broad-scale weather pattern drivers that I've ever observed. Right now, we're seeing evidence that the MJO will finally be able to behave in a more Nino-like fashion, at a time when the GWO analysis puts us in a decent position to build some HLB if it does, but the models are being extremely difficult with their MJO projections; Here we see GEFS stubbornly continuing to predict that the MJO will go through another stuttering period before moving into P8 around 20th Feb. It's been shifting the stall back in time for a few days now as the observation data has instead revealed eastward propagation, so I'm sceptical that this new stuttering interlude will unfold. ECM/EPS sees no stuttering interlude so seems more credible in that respect, but I'm not sure I buy there being so much loss of amplitude in P8; there are some unusually low SSTs off the S. American coast at the moment but they only have a narrow span so I'd be surprised to see such a strong restraining effect on the tropical convection. I'm hoping to see something close to the ECM/EPS eastward propagation, with at lowest a halfway house between that model and GEFS for the amplitude. That'd be better for HLB development across the NAO region than what either of the model means currently project so we'll likely see a significant negative correction to the NAO if it comes to pass... anyone feeling lucky?
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