Sunday 18 December 2011

Cold Winters: The Snow Advance Index.

Recently (well, back in November) Dr Judah Cohen published a new paper on the linkage between Siberian snow cover and Northern Hemisphere winters. The paper By Judah Cohen and Justin Jones is "A new index for more accurate winter predictions." There's a link to it at the end of this post. I've previously commented on the possibility of increased incidence of cold winters due to low solar activity and Arctic sea-ice. I've seen those as biassing for colder winters, with Cohen's previous work on Siberian Snowfall showing a trigger for specific cold winter events. However Cohen and Jones' new index makes me wonder what room there is for the other factors.

The major factor dominating Northern Hemisphere in winter is the Arctic Oscillation (AO). For us in the UK (and Europe) the useful rule of thumb is that when the AO is negative winter weather is typified by blocking high conditions, clear skies and colder than average weather. When the AO is positive the UK's weather is typified by low pressure systems from the Atlantic. We've seen this recently; in the last few weeks the UK has been battered by low pressure systems, strong winds, and days marred by wintery showers. That's typical high index AO weather, and the AO has been high (NOAA). In contrast, during the winter of 2009/10 the AO index dropped to record lows in February and was low throughout the winter.

Cohen & Jones follow up previous work investigating the correlation between October Siberian snowfall and the state of the AO in December, January and February. However in this paper they use a new index of Siberian snowfall: The Snow Advance Index (SAI). What Cohen & Jones do is they move from using Siberian snow extent to the rate of change of Siberian snowcover. They use snowcover in Eurasia south of 60degN and by moving to the rate of change of snow cover find a better correlation with the winter AO. Their figure 2c shows the best correlation using the highest quality data (daily data are only available for this period).

Figure 2c of Cohen & Jones. October SAI - blue. Winter (Dec-Feb) AO - black.

Here the correlation is 0.844, using weekly snow cover extent and October extent the correlations are lower. It would be more convenient if we had more events like 2009/10 in the series, however in my opinion Cohen has already previously demonstrated a role for Siberian snowfall in the winter using NCEP/NCAR reanalysis to examine what was going on in the atmosphere. As the SAI and AO index (NOAA) used in Cohen and Jones are indepenent of the NCAR/NCEP reanalysis the two lines of evidence give independent corroboration.

What I find challenging about Cohen and Jones is this:

Previously I've discussed how Lockwood et al found a link between low solar activity and cold European winters using the Central England Temperature series (first link in this post). How then am I to resolve Cohen and Jones with this line of evidence? Lockwood et al leave open the issue of mechanism, although they do discuss candidate mechanisms. Cohen & Jones and Cohen et al 2010 suggest that the winter of 2009/10 was instigated at the surface, could the reduced solar activity linkage be through impacts on clouds in line with Svensmark's hypothesis? It would be good to see if the SAI/AO correlation held in the Little Ice Age, but I can't think of a proxy that would work for this purpose.

Furthermore as previously discussed (second link this post) James Overland has been arguing that the winter of 2009/10 was probably a consequence of the loss of Arctic sea-ice. I've found the idea of reduced sea-ice causing impacts on winter weather persuasive, particularly due to the observation by Francis et al of persistent impacts to the Atlantic geopotential height thickness. Am I now to assume that this actually has a near negligible impact on European weather? If loss of sea-ice were having an impact I'd expect to see a reduction in correlation in Cohen & Jones as time moves on. However on balance their figure 2 doesn't suggest the sort of deviation I had initially expected might be there. Even with the longer timeseries used in the paper there doesn't seem to be a perceptible divergence between SAI or SCE and winter AO, so I'm left wondering if the Arctic sea-ice - cold winters paradigm has been overstated.

If Cohen and Jones are correct and the correlation is as high as 0.84 (84 71% of variance) then these other factors acting directly are left with 16%. There is of course the indirect effect, such as increased atmospheric humidity due to open water in the Arctic. However even this indirect effect doesn't seem to be supported as Rutger's Snowlab doesn't suggest a trend in October or November anomaly.

I think Cohen and Jones are onto something, and as a result being able to predict such winters as 2009/10 as early as November will be a substantial benefit allowing preparation to reduce the impact of such winters.

But as for the Cold Winters-Solar/Arctic linkages, at present I don't know. I remain unwilling to throw the baby out with the bathwater and dismiss these ideas. However I'm less persuaded than I was as to the magnitude and mechanism of the effect. Lockwood (et al) does seem to have found a real link between low solar activity and cold winters in the Central England Temperature series (ref), and this seems to be associated with increased incidence of blocking highs during winter at such times (ref), also the long term behaviour of Arctic sea-ice (ref) seems to suggest a support for Lockwood's theory. In terms of the impact of reduced sea-ice there is no doubt that this is having an impact upon the Arctic atmosphere: The Arctic Dipole Anomaly has risen to dominance (ref, ref), and through it spring sea-ice concentration has impacts on summer rain in China (ref), and may have wider impacts in the Northern Hemisphere (ref). Amidst this evidence there still seems reason to expect a winter impact of reduced solar activity and reduced Arctic sea-ice.

Cohen & Jones, 2011, "A new index for more accurate winter predictions."


Kevin O'Neill said...

Chris, is there a linkage between low sea ice extent and Siberian snow cover? If there is, it would explain both Screen and Cohen's results.

Low SIE => Increased Siberian Snow cover => Colder UK winters.

Chris Reynolds said...

When I wrote that post I didn't know, but now I think you're thinking along the right lines. Last night Dr Cohen was good enough to let me know of a new paper that might interest me (Cohen et al 2011). It's the top paper on his publications page:

Looks like it'll bear reading in tandem with Cohen et al 2009. To my regret I find that's been in my ever-expanding 'to be read' folder for a while.

With regards Cohen et al 2011, I'll be reading it and suspect it'll be worth a post. I've only read the abstract and checked out the graphs so far. So what follows has a BIG caution.

Check out figure 2. Follow it as a string of cause and effect.
A) Arctic Temperature increase - bit of feedback here as sea-ice loss is a large part of Arctic amplification.
B) September sea ice decline.
C) Increase in mean lower tropo moisture.
D) Increase in Oct Eurasian snow cover.
E) Decrease in DJF AO index - which leads to cooling pattern in late winter NH.

I suspect I now know why Dr Cohen pointed me to that paper after reading this post. But that's a suspicion based on one graph, where I've not read the full report... And as Eli The Erudite Bunny says - RTFR! ;)

Actually, as I've got this far, I'll try to fit Cohen et al into my bus-commute time for the remainder of this week. Anyway the paper's there if you want to check it out.

Kevin O'Neill said...

Chris, if you haven't read Jeff Masters' article on reduced May snow cover leading to changes in the jet stream, it points to some interesting research. Summer though, not winter :)

Chris Reynolds said...

Thanks Kevin I'll read that.

I did read the Cohen et al (2011) paper on the bus - not as big as I'd thought. It'll probably warrant a fresh post, but I'm working long days trying to clear work before the Xmas shutdown and I need a clear head to do this matter justice.

Kevin O'Neill said...

Chris, I know the overwhelmed feeling of too much work (of the wage paying kind) and too much to read. And just to make the unread pile a little larger I just discovered SOLA - Scientific Online Letters on the Atmosphere

I quickly read Impact of the Assimilation of Sea Ice Concentration Data on an Atmosphere-Ocean-Sea Ice Coupled Simulation of the Arctic Ocean Climate and it confirmed what I'd already believed - better modelling of sea ice leads to model results that better reflect reality. The paper basically says that accounting for Sea Ice Concentration greatly improves the results, but other feedbacks still need to be taken into account (modeled) to get realistic results. They point to the Beaufort Gyre and ice-cloud feedbacks as remaining significant deficiencies in the model.

Chris Reynolds said...

Hi Kevin,

Sorry for not getting back to you. My phone and internet went down and the engineers have only just sorted the problem out. Now I have a stinking cold which is really affecting my concentration. I'm having a jinxed Xmas period.

I wrote the following reply but didn't get round to posting it.

Thanks for putting me onto the Toyoda paper about Model/Data assimilation and Arctic Sea-Ice. Sorry I've not replied sooner, my internet and phone line have been out of action.

The paper shows that after initialisation with observations of sea-ice extent the improvement in skill persists for up to four years, and that enhanced heat storage plays a role in this improvement, i.e. the un-initialised model loses too much heat. This seems relevant to Tietsche et al, which I've used in a strand of reasoning to support my view that the Arctic won't be sea-ice free in summer this decade. Tietsche et al didn't use MIROC (they used ECHAM), as an aside MIROC was one of the models chosen by Wang and Overland as giving a good reproduction of the seasonal cycle - therefore a candidate to produce projections of future changes, in that study the MIROC model gives an ice-free summer in 2075.

However with regards Tietsche et al and the possibility of tipping points - I'm having doubts about my conservative stance on a rapid transition because of a recent paper I've been grapling with. More in a post to follow.

What also interests me is that Toyoda et al find a vestigial effect similar to the Arctic dipole, although they don't find the dipole anomaly to be a robust feature. They find a significant impact of ice-albedo because their sea-ice is specified. But they don't find cloud feedback, probably because the time window considered (1990s) is too short.

Esop said...

Did Cohen make a prediction for this winter? AO and NAO have been strongly positive so far, with temperatures way above average for Northern Europe and the US Eastcoast.

Chris Reynolds said...

Hi Esop,

Yes there was a prediction, I got it by email late November (28/11/11). I've been asked not to give all the details, however I am OK to confirm that the prediction was for a warm winter for the UK and East Coast US.

Kevin O'Neill said...

Chris, I see Cohen's research has hit Science Daily. I read the following quote and immediately realized, 'Damn, I'm an idiot.'

"In my mind there is no doubt that the globe is getting warmer and this will favour warmer temperatures in all seasons and in all locations; however, I do think that the increasing trend in snow cover has led to regional cooling as discussed in the paper and I see no reason why this won't continue into the near future. Also if it continues to get much warmer in the fall, precipitation that currently falls as snow will fall as rain instead, eliminating the winter cooling."

It never dawned on me that the snow could turn to rain. Doh!


Esop said...

Thanks Chris.
That is very interesting. That means that both Cohens and the MET office forecasts are spot on so far. (Can't say the same for Piers Corbyns forecast... he lucked out last year and was celebrated as a hero by the MSM, but he gets off easy (unlike the MET) when he is wrong (ie.most of the time)). Double standards by the MSM as usual.

What I find curious are Cohens claim of overall cooling since 1988. In most of Scandinavia and large parts of the US the exact opposite is the case, except for winters 09/10 and 10/11.

Chris Reynolds said...


Corbyn made a complete hash of last year, I've got his forecast somwehere, he blathered on about a severe winter for the UK, after December it wasn't overall. He seems to have been rather reticent this year.

It may seem odd but the lack of warming is clear in the data. Cohen et al (unpublished), as discussed in the post following this one, uses Hadley/CRU data. I've used GISS in my reply to OJ on that thread, see here. It may seem odd but the data doesn't lie, there is indeed a regional, seasonal, lack of warming.

Bear in mind this is regional and seasonal. The warming continues globally on an annual basis (e.g. Foster & Rahmstorf).