autumn Autumn 2016

[knocker]

Rumours of an arctic plunge have been greatly exaggerated

[Summer Sun]

The warm air may not be done with yet

[markyo]

47 minutes ago, Summer Sun said:

The warm air may not be done with yet

Highly unlikely,to far out to be credible at this stage but something a tad less easterly may have a short lived appearance. Certainly no Arctic plunge on the horizon though.

[Ross Andrew Hemphill]

The average daytime temperature here in the first week of October is 15.7C, which is a whole 1.8C ABOVE the average of 13.9C. Now that's weird considering the easterly flow we've all had.

Edited October 8, 2016 by Ross Andrew Hemphill

[Ross Andrew Hemphill]

September's daytime average here was 17C, which is 1.1C above the average of 15.9C. So currently, October's anomaly is greater than September's... What the hell!!

[I remember Atlantic 252]

2 hours ago, Summer Sun said:

The warm air may not be done with yet

looks good SS, who knows delay it a day and again another warm spell timed for Tuesday, trend of the summer

[iapennell]

5 hours ago, knocker said:

Rumours of an arctic plunge have been greatly exaggerated

Yes they have. I am certainly not predicting one for the foreseeable future, sea surface temperatures well above normal north of Britain, Arctic sea-ice extent below normal for the season are not good breeding-grounds for strong blocking-highs in the vicinity of Greenland/Iceland that would encourage the flow of Arctic air towards the United Kingdom. With sea-surface temperatures north of Britain widely 3 to 4C warmer than normal for October Arctic air would in any case be much moderated before reaching northern Scotland, let alone the rest of the country.

Furthermore, the up-tick of hurricane activity in the tropics i.e. Hurricane Matthew that has recently decimated Haiti (combined with westerly QBO) would serve to strengthen the higher-latitude Westerlies, particularly as these are displaced further north than normal, because of the increase in westerly AAM in the Northern Hemisphere circulation as a whole. I am not expecting the block over Scandinavia to persist beyond the 20th October and as this crumples away southwards warm south-westerlies will return to Britain as a consequence of strengthening NE Trade Winds (blowing off cooling North Africa and Southern Asia) further adding westerly AAM to the general circulation and increasing the need for a corresponding sink for it elsewhere.

My money is still very much on a warmer-than-usual October, wet in the latter half (though the SE of England may remain relatively dry). Even where I live in the North Pennines I am not expecting the first air frost until November!

[Methuselah]

I wonder what'll happen should the PV fail to form? Would weaker westerlies allow discrete blobs of deep cold to form, away from the Arctic; over parts of northern Europe and southern Alaska/Canada, perhaps? Could be interesting, I think/hope?

[lassie23]

No fog yet in October

[Emz by the Thames]

5 minutes ago, lassie23 said:

No fog yet in October

Fog twice here already and I'm not far away!

 

[Yarmy]

32 minutes ago, iapennell said:

Yes they have. I am certainly not predicting one for the foreseeable future, sea surface temperatures well above normal north of Britain, Arctic sea-ice extent below normal for the season are not good breeding-grounds for strong blocking-highs in the vicinity of Greenland/Iceland that would encourage the flow of Arctic air towards the United Kingdom. With sea-surface temperatures north of Britain widely 3 to 4C warmer than normal for October Arctic air would in any case be much moderated before reaching northern Scotland, let alone the rest of the country.

Furthermore, the up-tick of hurricane activity in the tropics i.e. Hurricane Matthew that has recently decimated Haiti (combined with westerly QBO) would serve to strengthen the higher-latitude Westerlies, particularly as these are displaced further north than normal, because of the increase in westerly AAM in the Northern Hemisphere circulation as a whole. I am not expecting the block over Scandinavia to persist beyond the 20th October and as this crumples away southwards warm south-westerlies will return to Britain as a consequence of strengthening NE Trade Winds (blowing off cooling North Africa and Southern Asia) further adding westerly AAM to the general circulation and increasing the need for a corresponding sink for it elsewhere.

My money is still very much on a warmer-than-usual October, wet in the latter half (though the SE of England may remain relatively dry). Even where I live in the North Pennines I am not expecting the first air frost until November!

Yet here we have one next week:

 

Why has the anomalously warm water to our N and low Arctic sea ice extent not prevented this?

 

[markyo]

1 hour ago, I remember Atlantic 252 said:

looks good SS, who knows delay it a day and again another warm spell timed for Tuesday, trend of the summer

Suppose it's like folk looking for snow in April,every chart that brings a little hope gets their attention,interesting stuff!

[Matthew.]

59 minutes ago, Yarmy said:

Yet here we have one next week:

 

Why has the anomalously warm water to our N and low Arctic sea ice extent not prevented this?

 

I thought low sea-ice was more favourable for blocking highs? Maybe longer term Siberian High in the Barents-Kara sea. Can understand how warmer SST anomalies would hinder  cold air progression to England though. Maybe the weak PV in stratosphere, troposphere is adding fuel.

Edited October 8, 2016 by Matthew Wilson

[Methuselah]

1 hour ago, Yarmy said:

Yet here we have one next week:

 

Why has the anomalously warm water to our N and low Arctic sea ice extent not prevented this?

 

Well it certainly ought to reduce the N-S temperature gradient, up in that part of the world? Then there's the current Solar nothingness? That might be two factors that favour a meandering PFJ?

[iapennell]

9 minutes ago, Ed Stone said:

I wonder what'll happen should the PV fail to form? Would weaker westerlies allow discrete blobs of deep cold to form, away from the Arctic; over parts of northern Europe and southern Alaska/Canada, perhaps? Could be interesting, I think/hope?

@Ed Stone, It would take something pretty serious to ensure the Polar Vortex fails to form. The fact that the Earth rotates and the NE Trade Winds that result between subtropical highs and the Intertropical Convergence Zone (ITCZ) means that there is always a need for a sink for the Westerly Atmospheric Angular Momentum elsewhere. The frictional impact of NE Trade Winds on the underlying surface over the tropics and subtropics leads to increased AAM in the upper atmosphere- air rising in the ITCZ has lost much of its "easterly" momentum through frictional impact with the underlying surface and as this air moves polewards aloft it retains it's absolute eastward momentum courtesy of the speed of the Earth's rotation at the Equator. By the time it reaches the subtropics (where the Earth's surface is rotating at a lower speed) the upper air retains the absolute west-to-east (i.e. Westerly) momentum it had when it first rose in the ITCZ. Thus the upper winds at 30 to 35N blow very strongly from west to east- this is the subtropical jet-stream. Further north and in the absence of interaction with the underlying surface the tendency of air to move aloft towards the cold polar regions tend towards creating further very strong winds from west to east- and always will. This largely explains the existence of the Circumpolar Vortex, so I cannot foresee any situation with the current arrangement of sea-surface temperatures, polar ice-cover and steep atmospheric temperature gradients in winter at high altitude on the fringes of the Arctic would ensure that the Polar Vortex fails to form going into winter. 

There is only two or three ways in which the situation you described could happen.

Firstly, a massive volcanic eruption near the Equator cools the surface in the tropics and leads to slight warming aloft: This would greatly weaken the ITCZ, the NE Trade Winds and with it the flow of air polewards at high levels. This in turn would greatly reduce the creation of large volumes of strong westerlies over the subtropics. There would still be a pole to sub-tropical temperature gradient aloft but the weak subtropical jet would weaken the subtropical high and there would be increased north-south mixing of air aloft weakening the temperature gradients; thus the circumpolar Vortex would also weaken dramatically. With a weak Hadley Circulation caused by our tropical volcano the weak NE Trades would not create much need for a sink for westerly AAM at higher altitudes and in that situation there would be lots more scope for persistent blocking in high-latitudes.

Secondly, there is the situation where the upper-atmosphere over the Arctic warms dramatically- as in a Sudden Stratospheric Warming event. This could result from a volcano injecting lots of fie dust into the stratosphere where it would directly absorb heat from the low-elevation sunshine and warm (at the expense of the surface below which would be cut off from insolation). This process of warming the Arctic stratosphere could not, alas, happen in winter when the Sun does not rise at all in the Arctic Interior! Alternatively the strong upper Westerlies further south impact with a high mountain range (such as the Rockies) in such a way that warmer air is pushed up over the Arctic, and this is something that does happen from time to time. In that situation there would be weak westerlies or even easterlies aloft but the Circumpolar Vortex is not destroyed but merely pushed out to lower latitudes; that is because the resulting higher-latitude easterlies and, of course, the NE Trade Winds continue to add Westerly AAM to the atmosphere (probably to a greater extent than before) so before long the corresponding need for surface Westerlies results in depressions moving east at 45N instead of, say, 60 to 65N and so westerly winds, rain and wind plague the Med and parts of the Middle East instead of Britain (which is often what happens in such circumstances). The southwards displacement of the circumpolar Vortex, and attendant westerlies, in this situation means that the surface Westerlies blow further from the axis of the Earth's rotation and therefore do not need to blow as hard to satisfy Conservation of Angular Momentum laws, and thus strong blocks of high-pressure will extend south over North America and Asia will then occur but with stronger westerlies over the North Atlantic and Pacific near 40N. But it would not be correct to state that the Circumpolar Vortex disappears or does not form. What tends to happen in the weeks after a SSW over the Arctic is that the Arctic stratosphere again cools and, unless the northern seas have been cooled by several degrees (unlikely after just a few weeks) the Circumpolar Vortex and upper Westerlies return to something closer to their normal positions. 

In the summer months, you will be aware that the Circumpolar Vortex is weak anyway because the Arctic is not bitterly cold and the weak NE Trade Winds (thanks to heating over subtropical continents weakening the subtropical high) require less of a sink for the westerly AAM they create through being slowed by frictional interaction with the underlying surface requires less of a sink for it at higher latitudes. Anything that weakens the NE Trade Winds and reduces the pole to tropic temperature gradients in autumn and winter would achieve this purpose, such as if the subtropical oceans are unusually hot following a warm summer and a volcano leads to the Arctic stratosphere being warmed as the dust absorbs sunlight.

A third major possibility is the displacement of the Intertropical Convergence Zone (ITCZ) to well south of the Equator (say to 15S)  in winter thanks to, perhaps, a very hot summer in the Southern Hemisphere tropics. In this situation there would be NE Trade Winds north of the Equator but the changed sign of the Coriolis Effect on crossing the Equator would lead to NW Trade Winds between the Equator and 15S. Large numbers of tropical depressions could then form over the hot seas in the Southern Hemisphere tropics and move westwards along the ITCZ; the strong westerlies on their northern flanks would enter the circulation of the Northern Hemisphere and these NW trade Winds and strong westerlies associated with tropical depressions would remove rather than add Westerly AAM through frictional impact with the underlying surface. This could be enough to counter all of the NE Trade Winds imparting westerly AAM to the Northern Hemisphere General Circulation with the result being a collapse in the subtropical jet and the absence of a need for a sink for Westerly AAM in higher northern latitudes would lead to the Circumpolar Vortex collapsing (or at least becoming really weak). The bitter cold over the Arctic would still encourage some  westerlies aloft in this situation with an extreme southward-displacement of the ITCZ with NW Trade Winds between the Equator and 15S but the modest west winds against the high Rockies and the Pamirs of Central Asia would then be sufficient to counter bitter north-east winds over Europe and North America. This latter scenario with an extreme southward-displaced ITCZ could happen should global warming continue un-checked over coming decades.

You are of course right about a weaker Circumpolar Vortex allowing cold air to push south and get established over extensive regions of mid-latitudes in winter: The weaker the Vortex is the amplitudes of the northward ridges and southwards troughs increases. This is something that is more likely to happen with surface cooling over the Arctic (or more warming of the Arctic stratosphere- both, incidentally, would be assisted by a well-timed and well-placed powerful volcanic eruption) as that would encourage a strong Polar High and a southwards displacement of the Polar Vortex. The Polar Vortex (and attendant surface westerlies) can be weaker if they blow in somewhat lower latitudes, i.e. further from the axis of the Earth's rotation because there they don't have to blow as hard to remove excess Westerly AAM (arising from high and low-latitude easterlies) from the Northern Hemisphere General Circulation. Cold air would dig further south at the surface under and just west of the upper troughs and with a weaker Circumpolar Vortex displaced south liable to exhibit a four-wave (ridge-trough) pattern (with a ridge (likely) anchored to the Rocky Mountains that places Western Europe under a trough allowing bitterly cold Arctic air to flood southwards. If the tracks of depressions moves into lower mid-latitudes that would result in locations further north being in polar air much more frequently in any case. 

Unfortunately I do not see how any of the above could play out this coming season. The only significant straws to clutch at are the fact that we have now a weak La Niná in the Equatorial Pacific whilst the subtropical North Pacific is very warm- this would weaken the NE Trade Winds), the Sun going quiet and the patch of cold water in the mid-North Atlantic that could increase the chances of cold weather in January-February 2017. Arrayed against that we have the warmth of high-latitude oceans whilst Greenland and the Arctic still get very cold, increased hurricane activity in the tropical North Atlantic (though this is unlikely to persist beyond November) and the QBO remaining Westerly, all of which are conducive to mild, wet weather (at least during the early part of the winter). True, there has been increased snow-cover over Siberia but that will mean nothing if a strengthening Circumpolar Vortex and westerlies off the Atlantic keep any strong Siberian High that results away to the east.     

[Methuselah]

6 minutes ago, iapennell said:

@Ed Stone, It would take something pretty serious to ensure the Polar Vortex fails to form. The fact that the Earth rotates and the NE Trade Winds that result between subtropical highs and the Intertropical Convergence Zone (ITCZ) means that there is always a need for a sink for the Westerly Atmospheric Angular Momentum elsewhere. The frictional impact of NE Trade Winds on the underlying surface over the tropics and subtropics leads to increased AAM in the upper atmosphere- air rising in the ITCZ has lost much of its "easterly" momentum through frictional impact with the underlying surface and as this air moves polewards aloft it retains it's absolute eastward momentum courtesy of the speed of the Earth's rotation at the Equator. By the time it reaches the subtropics (where the Earth's surface is rotating at a lower speed) the upper air retains the absolute west-to-east (i.e. Westerly) momentum it had when it first rose in the ITCZ. Thus the upper winds at 30 to 35N blow very strongly from west to east- this is the subtropical jet-stream. Further north and in the absence of interaction with the underlying surface the tendency of air to move aloft towards the cold polar regions tend towards creating further very strong winds from west to east- and always will. This largely explains the existence of the Circumpolar Vortex, so I cannot foresee any situation with the current arrangement of sea-surface temperatures, polar ice-cover and steep atmospheric temperature gradients in winter at high altitude on the fringes of the Arctic would ensure that the Polar Vortex fails to form going into winter. 

There is only two or three ways in which the situation you described could happen.

Firstly, a massive volcanic eruption near the Equator cools the surface in the tropics and leads to slight warming aloft: This would greatly weaken the ITCZ, the NE Trade Winds and with it the flow of air polewards at high levels. This in turn would greatly reduce the creation of large volumes of strong westerlies over the subtropics. There would still be a pole to sub-tropical temperature gradient aloft but the weak subtropical jet would weaken the subtropical high and there would be increased north-south mixing of air aloft weakening the temperature gradients; thus the circumpolar Vortex would also weaken dramatically. With a weak Hadley Circulation caused by our tropical volcano the weak NE Trades would not create much need for a sink for westerly AAM at higher altitudes and in that situation there would be lots more scope for persistent blocking in high-latitudes.

Secondly, there is the situation where the upper-atmosphere over the Arctic warms dramatically- as in a Sudden Stratospheric Warming event. This could result from a volcano injecting lots of fie dust into the stratosphere where it would directly absorb heat from the low-elevation sunshine and warm (at the expense of the surface below which would be cut off from insolation). This process of warming the Arctic stratosphere could not, alas, happen in winter when the Sun does not rise at all in the Arctic Interior! Alternatively the strong upper Westerlies further south impact with a high mountain range (such as the Rockies) in such a way that warmer air is pushed up over the Arctic, and this is something that does happen from time to time. In that situation there would be weak westerlies or even easterlies aloft but the Circumpolar Vortex is not destroyed but merely pushed out to lower latitudes; that is because the resulting higher-latitude easterlies and, of course, the NE Trade Winds continue to add Westerly AAM to the atmosphere (probably to a greater extent than before) so before long the corresponding need for surface Westerlies results in depressions moving east at 45N instead of, say, 60 to 65N and so westerly winds, rain and wind plague the Med and parts of the Middle East instead of Britain (which is often what happens in such circumstances). The southwards displacement of the circumpolar Vortex, and attendant westerlies, in this situation means that the surface Westerlies blow further from the axis of the Earth's rotation and therefore do not need to blow as hard to satisfy Conservation of Angular Momentum laws, and thus strong blocks of high-pressure will extend south over North America and Asia will then occur but with stronger westerlies over the North Atlantic and Pacific near 40N. But it would not be correct to state that the Circumpolar Vortex disappears or does not form. What tends to happen in the weeks after a SSW over the Arctic is that the Arctic stratosphere again cools and, unless the northern seas have been cooled by several degrees (unlikely after just a few weeks) the Circumpolar Vortex and upper Westerlies return to something closer to their normal positions. 

In the summer months, you will be aware that the Circumpolar Vortex is weak anyway because the Arctic is not bitterly cold and the weak NE Trade Winds (thanks to heating over subtropical continents weakening the subtropical high) require less of a sink for the westerly AAM they create through being slowed by frictional interaction with the underlying surface requires less of a sink for it at higher latitudes. Anything that weakens the NE Trade Winds and reduces the pole to tropic temperature gradients in autumn and winter would achieve this purpose, such as if the subtropical oceans are unusually hot following a warm summer and a volcano leads to the Arctic stratosphere being warmed as the dust absorbs sunlight.

A third major possibility is the displacement of the Intertropical Convergence Zone (ITCZ) to well south of the Equator (say to 15S)  in winter thanks to, perhaps, a very hot summer in the Southern Hemisphere tropics. In this situation there would be NE Trade Winds north of the Equator but the changed sign of the Coriolis Effect on crossing the Equator would lead to NW Trade Winds between the Equator and 15S. Large numbers of tropical depressions could then form over the hot seas in the Southern Hemisphere tropics and move westwards along the ITCZ; the strong westerlies on their northern flanks would enter the circulation of the Northern Hemisphere and these NW trade Winds and strong westerlies associated with tropical depressions would remove rather than add Westerly AAM through frictional impact with the underlying surface. This could be enough to counter all of the NE Trade Winds imparting westerly AAM to the Northern Hemisphere General Circulation with the result being a collapse in the subtropical jet and the absence of a need for a sink for Westerly AAM in higher northern latitudes would lead to the Circumpolar Vortex collapsing (or at least becoming really weak). The bitter cold over the Arctic would still encourage some  westerlies aloft in this situation with an extreme southward-displacement of the ITCZ with NW Trade Winds between the Equator and 15S but the modest west winds against the high Rockies and the Pamirs of Central Asia would then be sufficient to counter bitter north-east winds over Europe and North America. This latter scenario with an extreme southward-displaced ITCZ could happen should global warming continue un-checked over coming decades.

You are of course right about a weaker Circumpolar Vortex allowing cold air to push south and get established over extensive regions of mid-latitudes in winter: The weaker the Vortex is the amplitudes of the northward ridges and southwards troughs increases. This is something that is more likely to happen with surface cooling over the Arctic (or more warming of the Arctic stratosphere- both, incidentally, would be assisted by a well-timed and well-placed powerful volcanic eruption) as that would encourage a strong Polar High and a southwards displacement of the Polar Vortex. The Polar Vortex (and attendant surface westerlies) can be weaker if they blow in somewhat lower latitudes, i.e. further from the axis of the Earth's rotation because there they don't have to blow as hard to remove excess Westerly AAM (arising from high and low-latitude easterlies) from the Northern Hemisphere General Circulation. Cold air would dig further south at the surface under and just west of the upper troughs and with a weaker Circumpolar Vortex displaced south liable to exhibit a four-wave (ridge-trough) pattern (with a ridge (likely) anchored to the Rocky Mountains that places Western Europe under a trough allowing bitterly cold Arctic air to flood southwards. If the tracks of depressions moves into lower mid-latitudes that would result in locations further north being in polar air much more frequently in any case. 

Unfortunately I do not see how any of the above could play out this coming season. The only significant straws to clutch at are the fact that we have now a weak La Niná in the Equatorial Pacific whilst the subtropical North Pacific is very warm- this would weaken the NE Trade Winds), the Sun going quiet and the patch of cold water in the mid-North Atlantic that could increase the chances of cold weather in January-February 2017. Arrayed against that we have the warmth of high-latitude oceans whilst Greenland and the Arctic still get very cold, increased hurricane activity in the tropical North Atlantic (though this is unlikely to persist beyond November) and the QBO remaining Westerly, all of which are conducive to mild, wet weather (at least during the early part of the winter). True, there has been increased snow-cover over Siberia but that will mean nothing if a strengthening Circumpolar Vortex and westerlies off the Atlantic keep any strong Siberian High that results away to the east.     

But it doesn't always maintain a perfect circle (I should have said 'form as tight a circle as it's done so often recently', so my bad) does it? Though, it is true that any amount of deep cold over Europe can be blown away in a very short time: late January 1982 is testament to that...

[BLAST FROM THE PAST]

8 hours ago, knocker said:

Rumours of an arctic plunge have been greatly exaggerated

Indeed Knocker, I've gone for an October cet of 13.2c. With some date records going.....Models now hinting at southerlies coming

 

BFTP

[lassie23]

2 hours ago, Team Squirrel said:

Fog twice here already and I'm not far away!

 

Two snow falls for you and counting

[Ǝssex Ǝasterly]

2 fogs here too

[lassie23]

38 minutes ago, Essex Easterly said:

2 fogs here too

no wonder your dog is grinning

[Ravelin]

4 hours ago, iapennell said:

It would take something pretty serious to ensure the Polar Vortex fails to form.

<snip>

So to sum up, in the very unlikely event that it did fail to form we'd be talking a "Day After Tomorrow" type event (otherwise known as your perfect winter from your post the other day) . 

Joking of course! 

[The Post-modern Winter]

From Ian Ferguson in the ENSO thread - in reference to the ECM monthly . . 

22 minutes ago, fergieweather said:

Indeed it did. Essentially it runs an consistent story through N-D-J-F of +ve GPH/MSLP anomalies to NW/N of UK; -ve ones to S/SW. 

 

Edited October 8, 2016 by The Post-modern Winter

[cheese]

I don't understand why people on the MOD thread are getting so arsey over suggestions that the weather might be mild? Its only October you loons! Far too early to be searching for bloody cold and all of the nonsense that comes with it - and this comes from a person who actually does like cold and snow - but you know, in winter, where it belongs.

Until we get to the time of year where sustained cold and snow is a possibility (late-November onward for me), no point in easterlies or northerlies now that will deliver nothing to 99% of us. I think it's just below average for the sake of it in some people's eyes.

Anyway, now that's out of the way - we have showers moving in from the North Sea tonight. If it was winter we'd probably be getting snow showers instead.

Edited October 9, 2016 by cheese

[markyo]

4 hours ago, cheese said:

I don't understand why people on the MOD thread are getting so arsey over suggestions that the weather might be mild? Its only October you loons! Far too early to be searching for bloody cold and all of the nonsense that comes with it - and this comes from a person who actually does like cold and snow - but you know, in winter, where it belongs.

Until we get to the time of year where sustained cold and snow is a possibility (late-November onward for me), no point in easterlies or northerlies now that will deliver nothing to 99% of us. I think it's just below average for the sake of it in some people's eyes.

Anyway, now that's out of the way - we have showers moving in from the North Sea tonight. If it was winter we'd probably be getting snow showers instead.

Fully agree,i think folk falsely get their hopes raised by every chart that shows a cold synoptic,only to have them dashed by the next few outputs. Its the trend that you look for,that's much harder to find than people think. Exactly the same happens in April/May,warmth hunters do the same,jumping on any chart showing what they want to see only to be let down time after time. Winters not even started yet so what's the point in even looking for that holy grail of cold yet? Plenty of time for that in the coming months!

[Summer Sun]