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Singularity

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Singularity last won the day on April 17 2019

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  1. Thanks for the compliments Catacol, but I'm afraid I must contradict your points on the AAM/MJO patterns last winter. The MJO was actively propagating all of winter 2018-19, and as the plots in my previous post showed - albeit not as clearly as they should due to a lack of month name in the titles, my bad! - AAM increased overall between Jan & Feb. Also, as my plots for Dec show below, Jan was itself higher-AAM than Dec overall. Dec, however, was not as high as Nov. The key here is, it's not just the eastward movement that matters, but how much time it spends in each phase and at what amplitude. It just so happened to manage to spend Dec outside the phases that promote big AAM rises until the final few days of the month. An active crossing of the IO & Indonesia served to suppress the AAM downward after an active Pacific crossing in the 2nd half of Nov raised it to moderately high levels for late Nov. Remember how promising Nov 2018 was, before Dec gradually became less so in the troposphere, while the stratosphere became exciting? The exciting stratospheric events were largely a consequence of the Nov AAM rise, helped on by the continued cycling of an active MJO in Dec and perhaps some boost from the greater energy within the Arctic troposphere owing to anomalously open waters. You raise a good point regarding Atlantic SSTs - they can have a significant impact on cyclogenesis tendency, both frequency and position, during the 1st half of winter especially. The GSDM is the most useful way I know of to monitor to what extent tropical-extratropical momentum transports are occurring that have the potential to promote high-lat blocking and wave-breaking events within the next week or so, should other factors permit. Generally, using it is about looking at what's going on and using that to modify expectations accordingly with respect what numerical weather prediction models are suggesting. I don't use it to predict specific outcomes as it wasn't designed for such things. I take it as a high-ranking guidance tool among many for judging the likelihood of certain broad-scale patterns - a component to forecasts rather than the whole basis. Dec should be very interesting scientifically speaking - I think the NWP models are generally right to suggest some wintry potential on-and-off for NW Europe including the UK, without anything notably sustained. However, it's not clear what the MJO will get up to after its time in the Indian Ocean, or even how long it will be active there to begin with - here NWP modelling really falls flat on its face. So, I'm not assuming much regarding late Dec - let alone Jan-Feb.
  2. Concerns over lack of Pacific crossing only proved accurate for Dec of last winter; the MJO had little trouble propagating eastward last Jan-Feb and until late Feb was most active away from the Indian Ocean, as one may anticipate with a weak El Nino in place. This added plenty of AAM - though I seem to recall that more of it than usual was in the southern hemisphere. Even so, it was reasonable to anticipate some move toward higher-latitude ridge patterns IF the stratosphere played ball. To the displeasure of many, that proved to be the ultimate 'if', with the stratosphere very much not playing ball. We had that powerful wave-1 driven SSW which, having initially been diagnosed as a split type event, we hoped would propagate down by late Jan / early Feb at the very latest. Instead, it behaved like a displacement event, with a slow propagation down, taking until mid-Feb to even begin to effect the troposphere much, with peak impacts late Feb through March. So it was, that the more poleward ridge attempts driven by the very high AAM in Jan and early Feb were, in a sense ,'beheaded' by a strong +AO/+NAM. Then - and this was the real kick in the teeth - we saw the MJO become impressively strong across the Indian Ocean, sending AAM on a downward trajectory with the GWO moving through phases 7-8 while the reduction in N. Atlantic zonal winds took place. Those phases can support a -NAO, but with far more subtropical ridge presence across Europe than is conducive to a cold outcome for the UK - especially when the MJO is so strong over the Indian Ocean. So, we had the Arctic blocking features and -NAO, but in a configuration that supported record-breaking warmth across NW Europe, rather than cold weather with snow chances. I've not enjoyed recalling the events of last winter as it felt like we were very hard done-by, but there's no denying, it was an extremely educational experience. It taught me to pay more respect to the state of the QBO in the lower half of the stratosphere, along with which manner of wave breaking was responsible for the SSW initiation.
  3. In my reading of other posts on this forum and also research articles, I've come across at least one reason, possibly two. The first and most likely to be true (as opposed to coincidence) is the strongly positive IOD event of late. This encourages slower, more amplified MJO events across the IO and generally serves to add Nina-like standing wave characteristics to the global wind oscillations. Thinking about how regional anomalous ocean warmth interacts with the atmosphere, this connection makes good intuitive sense. The second, more uncertain connection was recently published: https://www.nature.com/articles/s41586-019-1764-4 Sadly, I don't have full access, but from the abstract, the results seem well-founded. It actually conflicts with the first a bit under 'typical' circumstances, as it works to encourage shorter MJO transits of the Indian Ocean - but during a strong +IOD event, the impact may well be much less or even negated. What won't be reduced or negated, however, is the impact of the increased West Pacific warmth relative to Central & East Pacific. Potentially, this is at least part of the reason why we've seen so many MJO waves struggle to make it fully across to the Central Pacific in the past couple of years. We really need to see a strong MJO into the Central Pacific, or at least one strong convectively coupled Kelvin wave, to sustainably set up a Nino-like base state. As in, for more than a week or two. Punching in and out of it can still produce interesting results, mind. Especially when those punches are of decent size as we're starting to see (+East Asian MT event looking promising). Usually more for N. America than the UK, but all it takes is a sliding low on the right trajectory during a Nino-like fluctuation and snow can be afoot. 'All it takes' - ugh, when do I think this is, the 20th Century? Anyway, the point is that while calls of long-lived -NAO spells are hard to justify for 1st half Dec, GSDM analysis is by no means shouting for a persistent +NAO either.
  4. There's a nice upward kick in the frictional torques (FTs) at the moment following a modest westerly wind burst in the central Pacific, which may lead a +MT event within the next week. With that, comes the potential for a period of reduced zonal flow across the N. Atlantic and the UK early-mid Dec. It'll likely struggle to sustain for very long, however, as the tropical profile is expected to shift markedly, trade winds increasing across the C. Pacific as a more Nina-like situation returns. As has been stated many times in the past week on various threads, the signals point toward an atmosphere that's fluctuated away from a Nina-like configuration, but not performed a sustainable transition. This also makes it questionable whether vertical wave activity on the polar vortex can amount to enough to produce a minor SSW (let alone a major one), though the current +FT and possible +MT within the next week do give us a better shot than we could have had.
  5. What can I say, @Tamara, @Isotherm and @Bring Back 1962-63 have given us a brilliant view of the base state forest in which the tree of fluctuation resides. Or something like that! Contemplating the warm IOD serving as a low-frequency version of a active MJO located in phases 1-2 of the Indian Ocean and producing Nina-like forcing on the global atmosphere, it may be of interest that the latest daily SST anomalies show relaxation of the warmth off the east-African coast compared to the weekly mean. It remains to be seen whether this is sustained, let alone forms part of a broader weakening of the +IOD. If it proves to be so, then that could be a factor favouring more & longer Nino-like fluctuations by the 2nd half of winter. Natural seasonal tendencies in the MJO cycles may also come in handy. The behaviour of the Pacific SSTs will be important too, of course - but modelling generally stays near-neutral up to the 'spring predictability-barrier', meaning we have to make do with weak signals there.
  6. 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.
  7. 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...).
  8. 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!
  9. 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.
  10. 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.
  11. 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 .
  12. 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).
  13. https://www.livescience.com/63380-arctic-lakes-melt-deep-permafrost.html 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.
  14. 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.
  15. 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.
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