the Air Vent

Because the world needs another opinion

Arctic Winds

Posted by Jeff Id on April 4, 2010

Often here, I’ve made the point that the 06-07-08 sea ice reductions in the Arctic were created by winds and water flow rather than temperature.    These reductions look pretty dramatic in video, but the key is that the wind patterns are very much visible during the summer melt season – especially in 07.

It’s like the southern portion of the Arctic sea ice shown above, is attacked by September winds.  Now in my previous post, I made a point WRT a paper on Arctic winds which was critical of certain claims – without even reading the paper.  Frank left a message criticizing my interpretation on the thread especially considering I hadn’t read the paper.  A reasonable point for sure.  After all, there is the possibility that I have surmised incorrectly.

The paper claimed that 1/3 of the minimal Arctic melting trend was due to winds and the recent 07 losses were 50 percent due to winds, leaving the rest for interpretation.  Now from the video, I would estimate any differences from normal (whatever that is) in 07 to be 99% wind and water currents but that’s just a thumbnail.

Here’s what I wrote:

Of course the 1/3 or 1/2 numbers in the quoted paragraph are completely bogus in my opinion. I’m not saying fraudulent, there is a difference, but I’m extraordinarily skeptical of anyone who claims to know what percentage of sea ice was melted due to one effect or another.  It is almost guaranteed to be nothing but handwaiving with some loosely defined regressions for evidence.  Most of us are familiar with that trend in climatology – (some pun).

This is what reader Frank wrote in reply.

This isn’t a reason to suggest that information in a science paper is bogus. The paper, since you obviously haven’t read it, can be found at: http://www.jamstec.go.jp/frsgc/research/d2/masayo.ogi/2009GL042356-pip.pdf

In Figure 1, there are some very nice maps showing the anomaly in arctic summer and winter wind patterns multiplied by the anomaly in sea ice extent. The maps clearly show that reduced sea ice extent is associated with stronger winds blowing towards the North Atlantic. These maps would be worth posting on your site. The regressions show that about 50% of the variation in sea ice extent can be explained by how closely winds reflect the pattern in Figure 1.  It isn’t clear to me how one can perform a “loosely defined regression” – the numbers must come from a defined location. In this case, they consider winds over the ocean everywhere north of 65 degrees. If they had considered winds from the most relevant areas in Figure 1, perhaps the correlation would have been stronger. However, with wind from an infinite combination of grid cells to choose from and only 30 years of ice data, it would be easy to overfit the data.

Now I’m not at all being critical of Franks response, his point was clear and he generously provided a link to the paper.  However, the paper is exactly what I predicted from just a few short sentences.  The authors, took raw wind data and turned it into an index value and regressed that directly against sea ice extent.  This means we have the standard assumption of linearity, which makes the math simpler but in this case cannot be true.  Here is a quote from the paper, my bold.

First, the 925-hPa wind fields are regressed on September SIE to determine the seasonally-varying wind pattern that is linearly related to the subsequent September SIE.

Now they have taken a complex wind pattern (see figures at the end of the above link) and decided it’s linearly related to extent.  It’s right in the assumption.  However, there will be a big difference between a wind coming straight in from North Russia, vs coming from the Baring Strait.  All we have to do is imagine wind across land (Russia) vs pushing a bit of slightly warmer water – a powerful ice melter – right up the strait.    The energy transfer from a bit of wind driven water flow is many times greater than a bit of warmer southern air.

Another quote from the paper:

We then use the standardized time series of these winter and summer wind indices as predictors in a linear model to predict difference in September SIE from one year to the next (ΔSIE).

Now the standardized time series are just a means of representing the wind in a linear quantity.  There is no method to account for the expected melting of the Arctic ice  when the wind comes from different directions, nor is there an attempt to correct for the flow of the ocean.  Again, it’s easy to understand that these effects are in no way linear.

However, that does not make this paper a bad one.  The authors clearly found a relationship between wind and ice minima.  Where they go to far (as often happens in climatology) is to claim that a certain percentage of the change can be attributed to wind.  From this work, we have absolutely no idea how much the sea ice is affected by wind, we only know that it IS.  — No real surprise there.

So in using the wind field directly, we are regressing what are multiple effects against sea ice extents.  Thus we have a loosely defined yet reasonable relationship and what I termed- a loosely defined linear regression.  The result of which cannot tell us the true amount of affect wind patterns had on sea ice.

You might ask, how can we do it better?  I would say, you could include ever more effects in more complex regressions but the main improvement I would suggest, without resorting to an expanded study, would be to change the conclusion.  Perhaps to say:  These results indicate a substantial relationship between wind field and ice melt.  Through linear regressions against wind indices, we have been able to explain 50% of the long term variance of Arctic sea ice indicating a strong relationship.  However, inclusion of non-linear effects in complex wind patterns and geography could lead to a substantial improvement in the determination of  the wind’s total influence on sea ice extent.

—-

Now how did I know the paper would be like this without reading it?  Because, it’s common in climatology to regress one factor against another to define relationships.  It’s common to assume linearity and it’s common to overconclude that some % of the natural effect is then explained.   I already know sea ice is very complex as it basically acts a fluid with non-Newtonian properties (a bit of stiffness).  Currents flow and change through the Arctic based on weather patterns and on top of that there has been a warming trend.

Anyway, in my opinion, claims of 1/3 or 1/2 of ice loss being due to winds are spurious at best.  Claims that these authors have demonstrated that winds have a substantial effect on sea ice are appropriate tho.

6 Responses to “Arctic Winds”

  1. Morley Sutter said

    Jeff,
    Congratulations on your analysis. You have brought out two very important points that bedevil science in general and, as you point out, climatology in particular: 1.correlation never proves causation and 2.one should never “over-conclude” (conclude more than the experimental results allow).

    In respect of point 1., no matter how sophisticated the analysis (linear or non-linear regression, etc.), conclusions based on the mathematics of correlation derived from observations and not from interventional experiments, can not define causality. Therefore we do not know how and to what degree winds affect arctic ice formation. Similarly it is not known to what extent CO2 affects global warming.

    In respect of point 2., “Over-concluding” from a set of observations is the action of an enthusiast, not a scientist. I was involved in the education of potential scientists and both points 1. and 2. were difficult to teach, particularly when dealing with the person’s own research.

  2. Eric Steig said

    Your points on sea ice are spot on. But reader Morley Suttter is off base when he says:

    “Similarly it is not known to what extent CO2 affects global warming.”

    This is a totally different situation. While there are certainly papers out there that say ‘XX % of the warming of the last century can be attributed to CO2″, this is really a measure of how big other impacts (aerosols, etc.) might be. The fact is that our understading about CO2 and global warming aren’t based on correlations, but on physics. That is, we know what the W/m^2 is in this case. We might not know how big the the W/m^2 for everything else is, but we sure as heck know the CO2 part!.

  3. Morley Sutter said

    Eric Steig:
    How accurate and complete a model of climate does your certainty based on physics encompass? It is one thing to say that physics predicts that a certain amount of CO2 should give us a certain value of heating but do you know how much occurs under the circumstances present at any given time in the real world? After all, historically, the rise in temperature does not exactly parallel the rise in CO2. Therefore other ill-understood factors must be operating. Models can be shown to be accurate only by the future behaviour of the system in question. After all, it took thousands of years to understand the solar system and tides to the degree that solar eclipses and tidal cycles could be predicted with any degree of accuracy. I suspect that climate is more complicated than either of these.

  4. curious said

    2 – Eric Steig – I read your 2009 paper and followed Jeff, Ryan and other’s work on it. I remain baffled by the conclusion you drew that “This warming trend is difficult to explain without the radiative forcing associated with increasing greenhouse-gas concentrations.”

    Please can you give a reference for the W/m2 forcing you associate with CO2?

    Please can you also comment on what you would expect “global climate” to do if all the forcings summed to zero? Would this be different to the case where each individual forcing were zero? Would you expect every global temp anomaly for (say) Jan 1st be the same as every other? If not what bounds would you put on it and over what timescales?

    Thanks

  5. crazy bill said

    Great Blog Science Jeff. I especially like that you completely diss their attempt to quantify the various contributions to ice cover by means of (simple) mathematical modelling, yet in your own analysis you eyeball “Now from the video, I would estimate any differences from normal (whatever that is) in 07 to be 99% wind and water currents”. Fantastic stuff!

  6. jeff Id said

    #5, It’s supposed to be funny b/c you can see the wind blowing away the ice. I was waiting for someone to say something. Note Dr. Steig’s take in #2 tho. Basically, the change in energy from wind driven waterflow can so vastly overpower a couple C of warming there is no comparison.

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