Reason for slow-moving low pressure systems

compsys

I'd always thought that, in general, low pressure systems were always fairly quick moving, and high presure systems, when they eventually came around, far more slow moving.

However, over the past few weeks it seems that any low pressure systems that are moving over Ireland and the UK are staying around for days.

If you run the attached BBC pressure chart you can see that our next low is going to spend the guts of the next fours days swirling around the British isles.

I'm just wondering what, if any, is the reason for this?

http://www.bbc.co.uk/weather/2643743

vicwatson

Great question. This is the third Thursday/Friday low pressure system in a row. Well this weeks begun Wednesday evening.

MiNdGaM3

This may play a part, though it's always going to be difficult to narrow the causes of these things down to even a handful of factors.

http://www.deas.harvard.edu/climate/seminars/pdfs/FrancisVavrus2012.pdf

Arctic amplification (AA) – the observed enhanced
warming in high northern latitudes relative to the northern
hemisphere – is evident in lower-tropospheric temperatures
and in 1000-to-500 hPa thicknesses. Daily fields of 500 hPa
heights from the National Centers for Environmental Prediction
Reanalysis are analyzed over N. America and the
N. Atlantic to assess changes in north-south (Rossby) wave
characteristics associated with AA and the relaxation of poleward
thickness gradients. Two effects are identified that
each contribute to a slower eastward progression of Rossby
waves in the upper-level flow: 1) weakened zonal winds,
and 2) increased wave amplitude. These effects are particularly
evident in autumn and winter consistent with sea-ice
loss, but are also apparent in summer, possibly related to
earlier snow melt on high-latitude land. Slower progression
of upper-level waves would cause associated weather patterns
in mid-latitudes to be more persistent, which may lead
to an increased probability of extreme weather events that
result from prolonged conditions, such as drought, flooding,
cold spells, and heat waves.

Deep Easterly

MiNdGaM3 wrote: »

http://www.deas.harvard.edu/climate/seminars/pdfs/FrancisVavrus2012.pdf

Thanks for posting that link MG, looks like a very interesting read. Certainly true that we seem to be stuck under slow moving upper troughs this summer so far though I would disagree with Compsy's a little about the speed of the actual surface lows as they seem to be zipping by a little faster than usual for the time of year, although with an alarming frequency!

I remember posting a few months ago that the first 3 months of this year saw the longest run of higher than normal MSLP values over the country since 1953. I have to say it really looks as if we are paying for this now - almost as if nature is redressing this imbalance.

eskimocat

I'll bet its that pesky Jet Stream, I know that it sometimes can stall a system in place, sorry can't remember which site you can see the jet stream location from but if you check the sticky on links I am sure its there somewhere. ( no time myself to find it for you, and awful at linking things from other websites )

delw

Have to say i would of thought it was that pesky Jet Stream as eskimocat said but i suppose that's only one variable in weather

Tae laidir

I spent some time in West Bengal in the summer. All my fellow tourist had gone! Summer = Monsoon, and the weather in India is beginning to resemble ours!
Nice warm & dry Spring.
Cool & wet summers.
And when the monsoon arrives, the temperature drops, the sky turns grey, the winds often abate and the heavens open.
Some say the 'Little European Monsoon' is getting Bigger!
Some describe it as equivalent to a Monsoon Depression.
See what you think:
http://www.weather.com/blog/weather/8_12415.html

MiNdGaM3

Deep Easterly wrote: »

Thanks for posting that link MG, looks like a very interesting read. Certainly true that we seem to be stuck under slow moving upper troughs this summer so far though I would disagree with Compsy's a little about the speed of the actual surface lows as they seem to be zipping by a little faster than usual for the time of year, although with an alarming frequency!

I remember posting a few months ago that the first 3 months of this year saw the longest run of higher than normal MSLP values over the country since 1953. I have to say it really looks as if we are paying for this now - almost as if nature is redressing this imbalance.

It mentions that it can also effect summer conditions through anomalous snow cover loss, and seen as the northern hemisphere is currently 2nd lowest on record, it may well be having some effect.


I remember that slp stuff, was surprised at the lack of attention that got. Maybe we'll get another long spell of high pressure soon!?

Su Campu

These low pressure systems are slow moving due non-pregoressive upper wave pattern of late, which has shown around 4 waves around the globe, with persistent Greenland blocking and an associated trough and hence southward-shifted jetstream over the eastern Atlantic. This trough has been the source of the surface low systems, which have more rotated around the axis of the trough.



There may be the signs of an increase in wave numbers and hence a more progressive shift in the pattern by the end of the month, which, coupled with anomalously cold SSTs in the eastern Atlantic, may lead to a more positive NAO and a northeastward extension of the Azores high.

http://weather.unisys.com/archive/sst/sst_anom_loop.gif

Deep Easterly

MiNdGaM3 wrote: »

It mentions that it can also effect summer conditions through anomalous snow cover loss, and seen as the northern hemisphere is currently 2nd lowest on record, it may well be having some effect.

Do you know if it is possible to get the source of that data in text form Mindgame? It would be interesting to do a quick analysis to see if NH seasonal snow cover anomalies coincide with any distortion from the normal seasonal pressure patterns here in Ireland over the last few decades.

MiNdGaM3

Deep Easterly wrote: »

Do you know if it is possible to get the source of that data in text form Mindgame? It would be interesting to do a quick analysis to see if NH seasonal snow cover anomalies coincide with any distortion from the normal seasonal pressure patterns here in Ireland over the last few decades.

Yep, this page has the data as weekly or monthly snow extent.

http://climate.rutgers.edu/snowcover/table_area.php?ui_set=2

I might start doing a little analysis with it myself...

Deep Easterly

MiNdGaM3 wrote: »

Yep, this page has the data as weekly or monthly snow extent.

http://climate.rutgers.edu/snowcover/table_area.php?ui_set=2

I might start doing a little analysis with it myself...

Thanks MG, that is a great link. Just using the NH snow cover anomalies from the data provided on the link for the summer months for years between 1972 up to 2011 and comparing them to the 'IMT' MSLP summer means for the same period, there seems to be an interesting alignment of trend between both over the 40 year period.



Data from the NOAA-CIRES Reanalysis & Rutgers University
Axis of Northern Hemisphere snow cover extent centered around the 1972-2001 summer mean .

It is interesting that the lowest 5 year NH summer snow cover mean extent (2007-2011) of 3973348.1 km2 coincides with the lowest 5 year summer IMT MSLP mean (2007-2011) since at least 1961. 5 year mean extent of cover over this 40 year period peaked at 7606174.1 km2 between 1977-1981, which is close enough to the highest IMT 5 year (1979-1984) mean summer MSLP since at least 1961 also.

I am not sure does this contradict the idea in the paper above that the so called 'Arctic Amplification' has increased the incidences of 500 hPa ridging in the eastern Atlantic during the summer months as it would seem that troughing over the region has been more the norm over the last few summers, though I am well open to contradiction on this.

PS, Just to point out a mistake in above chart title, should read 'period from 1972-2011' and not '1972-2012'

MiNdGaM3

Deep Easterly wrote: »

Thanks MG, that is a great link. Just using the NH snow cover anomalies from the data provided on the link for the summer months for years between 1972 up to 2011 and comparing them to the 'IMT' MSLP summer means for the same period, there seems to be an interesting alignment of trend between both over the 40 year period.



Data from the NOAA-CIRES Reanalysis & Rutgers University
Axis of Northern Hemisphere snow cover extent centered around the 1972-2001 summer mean .

It is interesting that the lowest 5 year NH summer snow cover mean extent (2007-2011) of 3973348.1 km2 coincides with the lowest 5 year summer IMT MSLP mean (2007-2011) since at least 1961. 5 year mean extent of cover over this 40 year period peaked at 7606174.1 km2 between 1977-1981, which is close enough to the highest IMT 5 year (1979-1984) mean summer MSLP since at least 1961 also.

I am not sure does this contradict the idea in the paper above that the so called 'Arctic Amplification' has increased the incidences of 500 hPa ridging in the eastern Atlantic during the summer months as it would seem that troughing over the region has been more the norm over the last few summers, though I am well open to contradiction on this.

PS, Just to point out a mistake in above chart title, should read 'period from 1972-2011' and not '1972-2012'

Very nice work!

The paper though mentions an increase in wave amplitude, not just in ridging. I think we've just been unlucky to find ourselves caught in a mean upper trough in recent summers, while Greenland has been hogging the ridging!
The 500hPa geopotential height anomalies of the last 5 summers



EDIT: Just to add, the first 20 days of this summer is following a very similar upper pattern to the last 5.

Deep Easterly

MiNdGaM3 wrote: »

Just to add, the first 20 days of this summer is following a very similar upper pattern to the last 5.

Today's 00z analysis showed an almost scarily similar pressure pattern around Ireland to that of the 00z analysis this day last year, and also the 00z analysis on this day 4 years ago.



Seems that a trend is emerging alright over the last 5 years.


PS, Mindgame, something I have wondered about more than once over the last few years and doing that chart sort of prompted it again, but do you think there is any possibility that the Chernobyl disaster back in 1986 could have had any sort of impact on the short or even long-term climate? I am not sure if there are any studies on this, or if it even warrants one, but looking at the NH snow cover extent, it really seems take a dip from 1986/1987 onwards.

Calibos

Studies have shown there hasn't even been much of an impact on cancer rates in the short never mind long term. ie. The only fatalities that can be laid at the door of the disaster are some of the firefighters at the plant itself.

http://www.who.int/ionizing_radiation/chernobyl/backgrounder/en/index.html

It was a only a disaster in terms of the radiation and massive evacuation of population (250,000), not in terms of explosive force and weather affecting particulate fallout like from a Volcano. etc In terms of climate it was no worse than an explosion and fire at a factory. ie. no effect whatsoever.

MiNdGaM3

Deep Easterly wrote: »

Today's 00z analysis showed an almost scarily similar pressure pattern around Ireland to that of the 00z analysis this day last year, and also the 00z analysis on this day 4 years ago.



Seems that a trend is emerging alright over the last 5 years.


PS, Mindgame, something I have wondered about more than once over the last few years and doing that chart sort of prompted it again, but do you think there is any possibility that the Chernobyl disaster back in 1986 could have had any sort of impact on the short or even long-term climate? I am not sure if there are any studies on this, or if it even warrants one, but looking at the NH snow cover extent, it really seems take a dip from 1986/1987 onwards.

The 8-10 day 500hPa mean comparison offers little hope for any change in the pattern, with strong ridging up through Greenland once more and the upper trough over us or just to our south west.


As for the change after 86, it seems to me that the trend had already become well established by then, with the drop from 86 not really standing out compared to the trend. I'd say the change from the late 70s through to the early 80s might be worth looking at though. A brief glance at some different climate indices shows that there was quite a step change in that time, with both ENSO and the PDO moving to strong positive phases and solar cycle 21 reaching it's peak around 1980.

ZX7R

Watched a program on edan last night on about Chernobyl,one of the scientists said he tought there has to be affects on the climate after it.

Redsunset

ZX7R wrote: »

Watched a program on edan last night on about Chernobyl,one of the scientists said he tought there has to be affects on the climate after it.

What made him come to that conclusion?
Seems to me, there just inventing excuses at this stage. He has a long queue to get behind

ZX7R

redsunset wrote: »

What made him come to that conclusion?
Seems to me, there just inventing excuses at this stage. He has a long queue to get behind

I dont know what made him come to that conclusion, maby it was just his views, anyhow my wife translated what he said in his native ukraine tongue and was totaly different to what the subtitles said he said on the program,he said ;"as well as the affects the raidation had on nature ,i am sure the localised weather has been afected also"

Su Campu

In 1995 I did a 3rd year project in Germany on the effects of rainfall on soil radiation levels from Chernobyl fallout. It involved collecting both shallow and deep layer soil samples from much of southern Germany and measuring the difference in Cs137 gamma radiation between sites.

This had initially been done in 1986 (3 weeks after the explosion), and a plot of the results showed elevated levels in the eastern area, where rainfall had occured in the days after the disaster, and low levels further west, where there was less rainfall. The shallow samples then contained the bulk of the radiation as at that stage it had not washed down to the deeper soil.

When I repeated the experiment 9 years later it was interesting to see how much the deep soil had picked up and maintained the radiation, with the shallow soil showing very little residual radiation. The overall levels had reduced, but it still showed that the effects were obvious a decade later.

This doesn't deal with whether or not the radiation had an effect on the weather, as it was in fact the other way around. I would imagine that any possible effects would have been on a short timescale (days-weeks) and could involve increased cloudiness (increased CCN levels), but that was unfortunately not part of the experiment at the time.

Calibos

Why was my post deleted?

Redsunset

Calibos wrote: »

Why was my post deleted?

Your post is on page one. Got accidentally misplaced for a time when some spam post was shown the door.

Deep Easterly

MiNdGaM3 wrote: »

As for the change after 86, it seems to me that the trend had already become well established by then, with the drop from 86 not really standing out compared to the trend. I'd say the change from the late 70s through to the early 80s might be worth looking at though. A brief glance at some different climate indices shows that there was quite a step change in that time, with both ENSO and the PDO moving to strong positive phases and solar cycle 21 reaching it's peak around 1980.

Yep, certainly a marked change around that time alright, though there does seem to be a notable sharp steepening within the annual temperature trend towards the late 80's in northern Europe at least. Quick chart below shows the 5 year annual mean temperature anomaly at Oaksoey FYR in southern Norway and our own Valentia since 1965 along with the annual NHSC 5 year mean area anomaly since 1972 (annual area cover before then too inconsistent to include). Both axis are centered around the the 81-2010 normal.


Not sure if this late 80's 'kink' in trend is reflected in the northern hemisphere or even global temperature trend though.

Edit: data from the ECA&D / Rutgers University.

MiNdGaM3

The graph isn't appearing on my screen DE?

Deep Easterly

MiNdGaM3 wrote: »

The graph isn't appearing on my screen DE?

Nor mine now :mad: these boards.ie attachments are crap!! will try and upload again.

Edit, chart uploaded again, so hopefully will show up this time.

MiNdGaM3

Working now!

I think with coastal stations in the Atlantic, you'd need to be able to differentiate the trend caused by the AMO, which appears to follow the temperature trends quite closely.

Here's the AMO 5 year mean since 65

Deep Easterly

MiNdGaM3 wrote: »

Working now!

I think with coastal stations in the Atlantic, you'd need to be able to differentiate the trend caused by the AMO, which appears to follow the temperature trends quite closely.

I'm not even going to pretend that I understand what the AMO is fully. Very basic grasp at best!

Plotting the AMO 5 yearly anoms into the chart I did earlier, it seems that though the overall the trend is pretty much aligned with that of the trend in temperature, the rising trend still seems disproportionately slack around the period in question relative to the rather sharp rise in temperature / decline in NHSC, which seem to occur in and around the same time. The AMO 5 year mean trend does take a sharp jump upwards, but not really until the mid-late 90's, or at least it seems that way from just a quick eye over. (Apologies in advance if chart seems cluttered)

MiNdGaM3

Seems we remain stuck in this pattern still, while Arctic sea ice extent and area are 2nd and 1st lowest on record respectively (by most counts) and June just had the lowest northern hemisphere snow cover on record.
The upper pattern this summer is remaining broadly similar to the last 5, with an anomalous upper ridge extending into Greenland tending to dominate, leaving us stuck in the trough. The pattern, if anything, seems stronger this year.
Perhaps the coming El Nino can change things for the 2nd half of summer, or even just August?

Birdnuts

MiNdGaM3 wrote: »

Seems we remain stuck in this pattern still, while Arctic sea ice extent and area are 2nd and 1st lowest on record respectively (by most counts) and June just had the lowest northern hemisphere snow cover on record.
The upper pattern this summer is remaining broadly similar to the last 5, with an anomalous upper ridge extending into Greenland tending to dominate, leaving us stuck in the trough. The pattern, if anything, seems stronger this year.
Perhaps the coming El Nino can change things for the 2nd half of summer, or even just August?

Thats a good summary of why recent summers have sucked - I think only August can save us now going on tonights outputs, even then I wouldn't be that confident given how poor that month has been in the past decade(the last good one I can remeber was 2003):(

20silkcut

August 2006 was good. The last extended period of heat we got in mid summer in this country.

Iancar29

ONly just seeing this thread now... great read indeed with great insight. Makes the weather forum on here stand out above the rest.