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About Interitus

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  1. It depends on how a final warming is defined, but only 3 times in the MERRA data since 1979 has the 10mb 60°N wind become easterly in March and remained so onwards for the rest of the summer. Last year was the earliest on 05/03, a whole two weeks earlier than 19/03/86, then 24/03/85. Further to the previous post that the average 10mb 60°N wind speed becomes easterly on 12th April, the actual average date is 17th April. The latest are 09/05/09 (after record SSW in Jan), 10/05/01 and finally 13/05/81. Looking at the analogues, the suspicion is that this year it will probably be at least a month away, towards the end of April.
  2. Is it possible to embed a full twitter conversation made up of a number of tweets with images, as displayed on twitter itself when just the using the link?
  3. The average date of 10mb wind reversal in the MERRA-2 reanalysis data is 12th April. Research shows that sometimes the final warming occurs first in the mid-stratosphere eg 10mb and other years in the upper stratosphere and working down. Hardiman et al. categorise the FW by which level they occur at first - 1mb or 10mb, and suggest that this has implications for tropospheric effects and forecasting -
  4. Yes, possibly. Where it does become interesting is in the future when composites and research make use of SSW events, usually little thought is given to the actual details, just whether it was a 60°N 10mb reversal.
  5. Following the 10mb 60°N zonal wind reversal criteria, there was an SSW on 1st February according to the ECM analysis at the time (?? m/s) and the MERRA2 reanalysis (-0.63 m/s), but wind speed just fell short on NCEP/NCAR reanalysis, with reversal at 62.5°N (-0.89 m/s) but not 60°N (+1.48 m/s). This was forecast reasonably well. A follow-up 2nd wind reversal or separate SSW - arbitrary criteria based typically on being at least 20 days apart - has been repeatedly forecast but hasn't happened. However, not sure that this is all down to poor modelling or just a bit of luck. With the event in the image above, on the 26th February the ECM analysis was 0.53 m/s, MERRA2 was 0.72 m/s and NCEP/NCAR 1.87 m/s. Fine margins.
  6. But the next day - Hard to make any meaningful predictions with such variable output, but it also raises questions about what would be assumed to be downward propagation. How does this manifest itself, and is that what is being witnessed in the second image or is it just coincident with tropospheric variability giving polar height rises for the trop and lower strat?
  7. No, both wave 1+2 warmings or displacements/splits as they typically refer to can produce colder conditions for the UK, or they can produce warmer conditions, or indeed little change at all! As to wave-1 re-consolidating vortex over NE Canada/Greenland, this might be observed but is it any different from having no warming at all? The lagged composite response is actually for height rises in that area, but as with UK conditions, it varies between each event. Here is a good opportunity to link to the SSW compendium where the effects of individual SSW can be examined in more detail - It is a little disappointing that they don't include charts for a shorter time range after the SSW eg. 0-14 days - quick response effects are often seen, perhaps inked to wave reflection by the critical line created by the zonal wind reversal for example, which fades away as the wind returns westerly in the 0-30 or 0-45 day timescales which have been chosen for anomaly propagation. Don't know about 1946/7, but there was an SSW at the end of Jan 63 which was classed as a split, and it is a good example of the points made above regarding the UK effects. It was a bit of a slow burner which shows how tricky it can be to get the central date right for comparison with other events or creating composites. From the ERA40 reanalysis it was 28/1/63, from JRA-55 30/1/63. The NCEP/NCAR reanalysis chart below from 29/1/63 shows that it only nipped the end off the vortex leaving it mostly intact, more like a large wave break - And a few days later it looks like the cut-off daughter vortex rejoined - Not very impressive and looks like recovering in the next couple of days. However, an analysis of minimum NAM by Martineau instead of day of wind reversal gives a central date of 11/2/63 at which time it looked like this - That's a bit more like it! Though soon after it reformed to become more like a displacement again - the distinction between these is not always clear and a displacement can often feature a split and vice versa. But here is the thing, whichever date chosen, the CET afterwards was still very low initially, but not quite as cold as before the SSW - More interestingly, from some early composites I made from an assortment of 54 other SSW/minor warmings/final warmings, the 7-day running mean temperature trends after 11/02/63 match the composite CET anomaly response very well - (Note the 1963 values are 7day running mean of CET seen in the previous graph, but then scaled for comparison). So the SSW might been partially responsible for ending this legendary cold spell. Here is the original post from 2013 -
  8. It's the latter. Not sure I follow your reasoning. The warmings in the temperature charts show up on the Siberian side but this is the norm - the predominant forcing is planetary wave 1 which more often than not gives stratospheric anticyclone on Pacific/American side pushing vortex to Eurasia/Atlantic side. On these occasions (i.e wave 1), warming will usually coincide with the eastern edge of the vortex and then progress along the northern edge, with the coldest temperatures from the centre to the western flank. The warming in the temperature charts over Asia doesn't show that the strat vortex is pushed to NE Canada/Greenland - though as the forcing varies with height giving a baroclinic vortex with the centre rotated westwards it isn't unusual for, say, the upper levels eg at 1mb to be located there, 10mb north of Scandinavia and 100mb over Siberia for example. But then the location of warming at each level also moves westwards accordingly. How this is then reflected in tropospheric low pressure systems over Canada/Greenland is not directly or linearly related. In a nutshell, usually wherever the strongest warming occurs at a particular pressure level, the vortex is adjacent to it.
  9. Three days later..... Keeping options open!
  10. Not a full SSW and it was generally in late November anyway - this was when the minimum wind speed was recorded and although 10mb temperatures peaked in the first week of December, the vortex was already cooling and strengthening lower down. edit: He's been taken to task to explain it in subsequent tweets and apparently "it was kind of a remnant of more robust warming earlier in November". Hmm.
  11. Just a minor quibble, while the temperature observations were generally correct, the comment relating to the jet stream was slightly awry. The temperature contrasts helped create a powerful nor'easter, the formation of which was widely described as "bombogenesis" (the weather bomb is popular at the moment). This gave blizzards for the northeastern States, described in typically fine fashion at wunderground - In association with this depression, the contrasts went on to generate a powerful jet streak over the northwest Atlantic - It just wasn't aimed in our direction, instead with the extreme amplification it headed into the Arctic, with the subtropical originated air giving Iceland a provisional new record February maximum temperature of 19.1°C. I had meant to mention this a while ago, but much has been made of the jet stream in relation to 'Doris' this week which gives the opportunity to post this excellent cross section by Hannah Attard of the jet streak shown above - The right hand side shows the mild Atlantic air with a dynamic tropopause height defined by 1.5 potential vorticity units at or above 200 mb, and roughly between 300 and 500 mb in the cold Arctic air on the left. In the centre is a deep tropopause fold bringing stratospheric air down to a ridiculously low 700-850 mb, beneath a jet core in excess of 90 m/s or over 200 mph. The blue shading highlights the strong horizontal potential temperature gradients involved. edit: Just to add it shows a classic example of an upper-tropospheric / lower-stratospheric front system with the jet sandwiched in between, which has been a focus of Attard at the Lang research group, compare with chart below, more details and explanation -
  12. Last year was a large major SSW, but because the winds didn't return to westerly it doesn't count as a major midwinter warming but as a final warming instead. Not sure what a 'normal' event is, but this forecast 2nd SSW of the winter is dragging on for ages and is even maybe not a certainty having vanished within 10-days on yesterday's ECM, and not on last night's GFS 0z as plotted on the weatheriscool site. Bear in mind that the GFS at one point had a reversal with a split vortex for as early as the 14th February to generally by the 18/19th for a number of runs.