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Winter 2017-18: Discussion
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If one was to make a long range prediction based long term averages alone, I wonder if one could make maybe make fairly reasonable long-range forecast? though of course, weather rarely follows a set pattern on a year by year basis.
However, this chart I generated showing the 1981-2010 MSLP pattern (based on a running 7 day period) for the country as a whole for the winter season, does show a fairly interesting pattern, with 3 major 'dips', occurring at specific intervals, namely: end of December, mid-late Jan and mid Feb, which would indicate that, on average, the most unsettled spells of the Winter season are likely to occur at these times. Same would with peaks, which would suggest 3 or 4 relatively settled periods to occur at fairly specific intervals in the season.
Data from the ECMWF.
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As far as the December pattern shown above, the longer-term data have a similar appearance although the amplitude (as you might expect taking more data) is about half or perhaps a little more than half of this recent period (it is a higher fraction if you average the data on the assumption of a gradual forward shift in time). It may look larger than that but keep in mind that I am using unsmoothed data and Oneiric3 was using a 7d running mean which will tend to make for smaller amplitudes (timing would be less affected).
I have lined up with Oneiric's graph (visually as close as I can) the actual lowest pressure readings after 21 December (to 2 Jan) in the following graph of 20-year averages starting 1851-70 and ending with 1991-2010. Times are from the four-daily series. Pressures are in each case 1000 mb plus amount shown. This series strayed over into 1 Jan during 1931-50 then back into late December.
1851-70 ____________ 09.4 _ 26th 00z
1871-90 _____________09.4 _ 28th 18z
1891-1910 ___________ 04.9_ 29th 12z
1911-30 ______________02.3_ 30th 12z
1931-50 ______________ 08.5 _ 1st 06z
1951-70 ______________07.7 _ 30th 06z
1971-90 ___________06.7_ 20th 12z
1991-2010 ____________08.1 _ 30th 06z
The phenomenon came a bit early 1971-90, which half overlaps Oneiric3's period of 1981-2010 so I generated a separate 30-year filter for Dec 1981-2010 and found that the lowest pressure in that was
1981-2010 ___________09.5 _ 25th 06z
This is quite similar to what Oneiric3 found and is 9 mb lower than the earlier peak (12th 00z).
Meanwhile, for the entire period of record this is the late December pressure minimum.
1851-2010 ____________09.4 _ 30th 12z
The late December troughs of pressure are in some cases not the lowest in December altogether (as was the case in 1981-2010). But there is usually a peak in pressure around mid-December too, as in 1981-2010, those values (maximum for the second half of the month, except 1971-1990 where the effect was noted earlier than other periods and a higher pressure occurred towards the end of the month) are as follows ... this graphic also shows the drop in pressure towards the end of the month:
1851-70 ______ 14.2 (17th 18z) ______ -4.8 mb (8.25 d)
1871-90 __________ 15.7 (26th 12z) __ -6.3 mb (2.5 d)
1891-1910 ______ 18.5 (21st 12z) ____-13.6 mb (8.0 d)
1911-30 ______ 17.4 (17th 12z) ______-15.1 mb (13.0 d)
1931-50 __________ 20.1 (23rd 12z) __-11.6 mb (8.75 d)
1951-70 _________16.9 (21st 12z) ____ -9.2 mb (8.75 d)
1971-90 ______ 11.0 (16th 00z) ______ -4.3 mb (4.5 d)
1991-2010____ 21.1 (15th 12z) ______-13.0 mb (14.75d)
and for comparison
1981-2010 ____16.4 (15th 12z) _______ -6.9 mb (9.75d)
___ note higher before 15th by 2-3 mb __
for the entire period of record (1851-2010)
1851-2010 _______13.4 (21st 12z) ____ -4.0 mb (9.0 d)
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ANALYSIS:
The first thing worth noting is that my grid point which is slightly northeast of Oneiric3's data grid has not led to any obvious discrepancies in the analysis, the 1981-2010 data look very similar. I did not run a 7-day smoothing function on my data so we're looking at actual timing of max and min values.
It appears that this pressure wave has been a constant except for some period in the 1970s which was an interval of frequent blocking patterns. It was particularly strong from about 1891 to 1950 (perhaps a longer overlap of other intervals) and seems to be getting stronger again recently.
One possible cause that fits the slight lateward drift in the 20-year intervals would be a Sun-galactic centre resonance effect. It is possible that this amplifies lunar tides as well around late December. So the effect may be related to ocean circulations and might be discernible in other climate regions. It would make sense to find opposite pressure signals in different climate regions too, as it must balance out somehow.
The Sun is crossing the galactic equator a bit later each century, for example in the first century the Sun crossed the galactic equator around 30 Nov rather than 21-22 Dec as nowadays (this is due to the long cycle (26,400 y) of precession of the earth's axis) although the drift shown for the pressure signal here is slightly larger than the drift in dates implied by precession, which is about 2d per century. I may get a better insight into this by studying the time drift shown by any other pressure oscillations. Another factor that would speed up the lag effect (making it go faster with relation to the calendar) would be any connection to the earth's magnetic field since the North Magnetic Pole has been moving more westward in recent decades, so if anything is generated within that grid it would arrive later downstream, differentials in the 19th century would be smaller as the NMP was then either nearly stationary or moving slowly northward. Having the field stronger and focused further south could also be factors that would generate an earlier arrival. My working idea (early stages yet) would be some sort of effect generated over eastern North America or the western Atlantic around the time of solar-galactic alignment then sent downstream by travelling systems that would normally take 3-5 days to reach Ireland. I am aware of a similar effect in Toronto temperatures indicating a slight second-order peak around the time of this effect (without the 3-5 day lag).
But the main point is that the effect that Oneiric3 discovered in the data was not confined to the interval 1981-2010, and may have been stronger at times in the past, as we might expect in a stronger magnetic field with greater air mass contrasts.
I will take a similar look at the pressure wave that he found for 1981-2010 in January in the next few days. A caveat is of course that this is a background phenomenon, a 10-15 mb pressure wave while quite significant is the equivalent of a weak frontal wave, a stronger low will create a pressure wave of 30 to 50 mbs at least. But if we could find a cause and effect explanation for any of this, it would advance our understanding considerably.0 -
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M.T. Cranium wrote: »and since I have the entire year available, can say there's a few other seasonal variations .
Interesting M.T. I have the entire year available on my dataset also (which only goes back to 1957 admittedly) so would be interesting to compare and contrast sometime.
I still have to get around doing up a comparison chart with the 1981-2010 average against 1961-90 and 1971-00. (and other 30 year means within this period perhaps), but just so bloody wearied out in the evenings lately. Getting auld!
New Moon
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I remember buying a big snow shovel in Lidl at the beginning of January 2011, still waiting to use it!Squeaksoutloud wrote: »
Its somewhere in the shed covered in dust and cobwebs. 0 -
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Interesting M.T. I have the entire year available on my dataset also (which only goes back to 1957 admittedly) so would be interesting to compare and contrast sometime.
I still have to get around doing up a comparison chart with the 1981-2010 average against 1961-90 and 1971-00. (and other 30 year means within this period perhaps), but just so bloody wearied out in the evenings lately. Getting auld!
You're not auld, I am.
So, I had a look at your January pressure wave and saw some evidence for it longer term but there's a better and more time focused fit long term for the February mid-month trough and late month pressure rise that you demonstrate for 1981-2010. I will post the details on that later.
As to the rest of the year, the annual pressure cycle is basically low in winter and higher in summer but there's an interesting trough in August between one peak around late June and another one in mid-September. I haven't had time to look at how those evolve through the 160 years of data.
There is also a period of lower pressure near the end of October compared with most of November. Some days in late October get down pretty close to the winter minimum values.
Were you aware of any of those from your 1981-2010 work?0 -
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