Autumn 2015

[Cheshire Freeze]

I have a question which may seem very stupid because I am a novice when it comes to forecasting so go easy

I have read reports that the North Atlantic is five degrees below average for this time of year.

Surely that would mean a very cold autumn and freezing winter? 

Or is this not the case?

 

If that anomaly was to last through until winter (which I can't really see it doing!) you would expect a weaker N Atlantic temperature gradient, a weaker, more southerly tracking jet and perhaps higher than average pressure sitting in the N Atlantic. This is weather though....you can't be sure of anything!

[summer blizzard]

If that anomaly was to last through until winter (which I can't really see it doing!) you would expect a weaker N Atlantic temperature gradient, a weaker, more southerly tracking jet and perhaps higher than average pressure sitting in the N Atlantic. This is weather though....you can't be sure of anything!

The opposite I'd say. Colder air getting further south would produce an enhanced thermal gradient. Not a problem if we get a -AO however in a +AO it could lead to storms passing over rather than north of the UK.

[snowy weather]

If that anomaly was to last through until winter (which I can't really see it doing!) you would expect a weaker N Atlantic temperature gradient, a weaker, more southerly tracking jet and perhaps higher than average pressure sitting in the N Atlantic. This is weather though....you can't be sure of anything!

What would this mean for the weather?

The opposite I'd say. Colder air getting further south would produce an enhanced thermal gradient. Not a problem if we get a -AO however in a +AO it could lead to storms passing over rather than north of the UK.

What does an enhanced thermal gradient mean?

[BornFromTheVoid]

What would this mean for the weather?

What does an enhanced thermal gradient mean?

 

It's the difference in temperature from one point to another. In winter when there is a strong gradient between the Arctic (extremely cold) and the mid latitudes (cool/mild), the jet stream is typically stronger. In summer when the gradient weakens (cool Arctic and warm mid latitudes) the jet stream is usually weaker, meaning less storms and less westerly type winds.

 

The argument is whether or not the cold pool out in the Atlantic, which you can see on the image below, in positioned in such a way as to enhance or reduce the temperature gradient, and how this will alter the jet stream locally.

 

 

 

In winter 2013/2014, there was some very cold air across the eastern side of north America, coupled with a mild north Atlantic, which produced a stronger than average jet stream across our part of the world and lots of storms.

 

As for this year, I'm unsure as to how it will play out. If the cold ocean temps last through to Autumn and Winter, it would make a typical westerly winds quite a bit cooler, and increase the chances of cold night time temps. The effect on the jet stream is hard to say.

It's not a direct cold connection to the Arctic due to the warmer than average SSTs to our north (unlike the cold over the US in 2013/2014, which was connected to the Arctic) so it's unlikely to strengthen the jet stream the same way as winter 2013/2014.

On the other hand, a Met Office study a few years ago suggested that warm north Atlantic SSTs (in the region where it's currently cold) promotes a -ve NAO and increases our chances of a cold winter, which would suggest that the cold SSTs might work against the chances of a cold winter.

 

So it's a really tough call. Throw into the mix a possible strong or even super El Nino and things get even more uncertain!

Edited August 3, 2015 by BornFromTheVoid

[Yarmy]

It's a brave man who would call the position and strength of the polar jet this autumn and beyond. As mentioned, it's possible (assuming the current negative anomaly in the North Atlantic persists) that the jet could be weaker and track further south as the temp gradient will not be so steep. This also assumes the SST anomaly is mirrored aloft. 

 

I suppose there are some analogue years with a coldish North Atlantic/El Nino combination so it might be worth looking at the mean jet position and strength for them. But there have been few, if any, El Ninos like the current one, so we are in undiscovered country. Experience tells us that cold and blocked is always the outsider though.

[Ravelin]

Are there any insights around as to why the North Atlantic has seen this persistent 'cold pool' this summer?

 

Difficult to say what effect it will have on the coming winter, especially since we don't know whether it will persist. Over the next few months the SST could return to normal i.e. the anomaly disappears, or it could even flip to a warm anomaly.

 

If there's one thing you can say for certain about the Earths climate, is that we still understand far too little about the Earths climate!

[BLAST FROM THE PAST]

This cold anomaly has been persistent and we are in about the right time of AMO going into negative phase.  I'm not massive fan of SST anomalies playing that big a part in synoptic patterns but for sure it will make pM air somewhat more potent. 

I was camping last week and with a general NW'ly flow the nights turned darned cold for July, now that I think would have been largely influenced by the current Atlantic SST pattern.

 

BFTP

[Yarmy]

Are there any insights around as to why the North Atlantic has seen this persistent 'cold pool' this summer?

 

Difficult to say what effect it will have on the coming winter, especially since we don't know whether it will persist. Over the next few months the SST could return to normal i.e. the anomaly disappears, or it could even flip to a warm anomaly.

 

If there's one thing you can say for certain about the Earths climate, is that we still understand far too little about the Earths climate!

 

It might be indicative of a switch to the negative phase of the AMO. This oceanographer thinks so:

 

http://theconversation.com/the-atlantic-is-entering-a-cool-phase-that-will-change-the-worlds-weather-42497

 

Too early to say in my opinion, and Knocker posted a CFS (I know, I know) SST chart on another thread showing a return to normal or slightly positive anomalies in future months. So who knows.

[BornFromTheVoid]

Are there any insights around as to why the North Atlantic has seen this persistent 'cold pool' this summer?

 

Difficult to say what effect it will have on the coming winter, especially since we don't know whether it will persist. Over the next few months the SST could return to normal i.e. the anomaly disappears, or it could even flip to a warm anomaly.

 

If there's one thing you can say for certain about the Earths climate, is that we still understand far too little about the Earths climate!

 

In addition to what others have said, a few studies recently have linked it to a slow down in the gulf stream. The first used satellite data, the second used purpose built ocean instruments as part of the RAPID array.

 

Research Article:Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation

Review: http://www.realclimate.org/index.php/archives/2015/03/whats-going-on-in-the-north-atlantic/

 

 

Research Article: Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises

Review: http://www.carbonbrief.org/blog/2015/06/the-atlantic-conveyor-belt-and-climate-10-years-of-the-rapid-project/

Edited August 3, 2015 by BornFromTheVoid

[Yarmy]

In addition to what others have said, a few studies recently have linked it to a slow down in the gulf stream. The first used satellite data, the second used purpose built ocean instruments as part of the RAPID array.

 

Research Article:Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation

Review: http://www.realclimate.org/index.php/archives/2015/03/whats-going-on-in-the-north-atlantic/

 

 

Research Article: Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises

Review: http://www.carbonbrief.org/blog/2015/06/the-atlantic-conveyor-belt-and-climate-10-years-of-the-rapid-project/

 

The pedant in me is aching to point out to the latter's authors that something cannot be both inevitable and a surprise.

 

Anyway, the sudden drop in Sverdrups (my new favourite unit) during 2009/10 was coincident with a cold N Europe winter:

 

 

 I don't suppose up to date figures can be found anywhere online? 

Edited August 3, 2015 by Yarmy

[BornFromTheVoid]

The pedant in me is aching to point out to the latter's authors that something cannot be both inevitable and a surprise.

 

Ha, that caught my eye too! Perhaps it was inevitable that there would be surprises?

[snowy weather]

It's the difference in temperature from one point to another. In winter when there is a strong gradient between the Arctic (extremely cold) and the mid latitudes (cool/mild), the jet stream is typically stronger. In summer when the gradient weakens (cool Arctic and warm mid latitudes) the jet stream is usually weaker, meaning less storms and less westerly type winds.

 

The argument is whether or not the cold pool out in the Atlantic, which you can see on the image below, in positioned in such a way as to enhance or reduce the temperature gradient, and how this will alter the jet stream locally.

 

 

 

In winter 2013/2014, there was some very cold air across the eastern side of north America, coupled with a mild north Atlantic, which produced a stronger than average jet stream across our part of the world and lots of storms.

 

As for this year, I'm unsure as to how it will play out. If the cold ocean temps last through to Autumn and Winter, it would make a typical westerly winds quite a bit cooler, and increase the chances of cold night time temps. The effect on the jet stream is hard to say.

It's not a direct cold connection to the Arctic due to the warmer than average SSTs to our north (unlike the cold over the US in 2013/2014, which was connected to the Arctic) so it's unlikely to strengthen the jet stream the same way as winter 2013/2014.

On the other hand, a Met Office study a few years ago suggested that warm north Atlantic SSTs (in the region where it's currently cold) promotes a -ve NAO and increases our chances of a cold winter, which would suggest that the cold SSTs might work against the chances of a cold winter.

 

So it's a really tough call. Throw into the mix a possible strong or even super El Nino and things get even more uncertain!

Thank you so much for this detailed post.

Just goes to show how complex the weather is. Newspapers would have you believe that a cold Atlantic leads directly to a cold winter, 

[Ravelin]

Anyway, the sudden drop in Sverdrups (my new favourite unit) during 2009/10 was coincident with a cold N Europe winter:

 

Kind of suggests though that the reduction in flow was at least partially caused by the -ve NAO that year rather than the reduction in flow causing the cold winter. As I said before though, quite how all these factors interact i.e. what are the causes and what are the effects is far from clear.

Edited August 3, 2015 by Ravelin

[Yarmy]

Kind of suggests though that the reduction in flow was at least partially caused by the -ve NAO that year rather than the reduction in flow causing the cold winter. As I said before though, quite how all these factors interact i.e. what are the causes and what are the effects is far from clear.

 

Hard to say: there will be a lot of non-linearity. The NAO was negative for the entirety of 2010, but the AMOC strength increases rapidly and then falls rapidly during that year on the chart above. 

 

Some more information here:

 

http://noc.ac.uk/news/ten-years-ocean-monitoring-uncovers-secrets-changing-uk-winters

[Nouska]

There are some more graphs on the RAPID-MOC website that detail the different depths they are monitoring. Can also see more clearly the difference throughout 2009/2010.

 

 

http://www.rapid.ac.uk/rapidmoc/

Edited August 3, 2015 by Nouska

[Mother Goose]

This is getting too technical for me! But what do people think with regards to the El Nino impacting on our weather in the Autumn?

[cheeky_monkey]

This is getting too technical for me! But what do people think with regards to the El Nino impacting on our weather in the Autumn?

IMO I think autumn will start relatively dry and warm..but gradually decend into a wet fest..with dry interludes few and far between.

[summer blizzard]

This is getting too technical for me! But what do people think with regards to the El Nino impacting on our weather in the Autumn?

Without strong forcing from heights over the Arctic/Greenland (there are exceptions but the AO is typically close to neutral during EL Nino Autumns) my opinion is that the SST profile in the Atlantic will enhance the thermal gradient in the Atlantic and lead to a cyclonic and probably wet Autumn.

[Yarmy]

Without strong forcing from heights over the Arctic/Greenland (there are exceptions but the AO is typically close to neutral during EL Nino Autumns) my opinion is that the SST profile in the Atlantic will enhance the thermal gradient in the Atlantic and lead to a cyclonic and probably wet Autumn.

 

The NAO was generally (modestly) negative during the super El Nino of 97/98:

 

1997 9 0.74

1997 10 -1.34

1997 11 -1.08

1997 12 -0.98

1998 1 -0.05

1998 2 -0.57

1998 3 0.51

 

Would the thermal gradient be enhanced? In my simple mental image, the polar jet is fuelled by the sharp contrast between the warm North Atlantic and cold polar air masses aloft. If the North Atlantic air is cooler than average then the temperature (and therefore pressure) contrast is reduced, no? Or at least the boundary will be further south.

 

That said, wet and cyclonic is normal Autumnal fare.

[summer blizzard]

The NAO was generally (modestly) negative during the super El Nino of 97/98:

 

1997 9 0.74

1997 10 -1.34

1997 11 -1.08

1997 12 -0.98

1998 1 -0.05

1998 2 -0.57

1998 3 0.51

 

Would the thermal gradient be enhanced? In my simple mental image, the polar jet is fuelled by the sharp contrast between the warm North Atlantic and cold polar air masses aloft. If the North Atlantic air is cooler than average then the temperature (and therefore pressure) contrast is reduced, no? Or at least the boundary will be further south.

 

That said, wet and cyclonic is normal Autumnal fare.

El Nino typically strengthens the sub-tropical jet which does support the boundary being further south however without heights over Greenland this simply means that we get a washout rather than a N/S split. Cold water over warm water to me certainly indicates a strong cyclonic gradient.

[Guest]

With the current washout Summer, I am praying to the Spaghetti Monster this year so we can get a nice warm September and a cold, snowy Winter. 

[Turnedoutniceagain]

Lords and Ladies in the garden - a welcome sight to late Summer. Nearly Autumn !!

[Mokidugway]

I get them ,quite colourful ...and poisonous berries too !!.

Edited August 7, 2015 by Mokidugway

[Jax]

With the current washout Summer, I am praying to the Spaghetti Monster this year so we can get a nice warm September and a cold, snowy Winter. 

As a Pastafarian I suggest you keep wearing a colander on your head also to please the monster.

[lassie23]

Autumn looks like starting settled and sunny before turning very stormy come November, yup winter 2015-2016 is already over  It is looking like a mild, stormy winter, if this guy is on the ball.     http://www.weatherweb.net/wxwebtv2.php