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The NAO is the dominant mode of atmospheric variability in the north Atlantic throughout the year and particularly prominent in the winter.
Nomad, when people talk about "dominant mode of variability" in this context, is it much more than a statistically derived principal component of the system? In other words, is it just descriptive or is there a real understanding of its dynamics and role?

Of all the ways of organizing banking, the worst is the one we have today — Mervyn King, 25 October 2010
by Migeru (migeru at eurotrib dot com) on Thu Dec 9th, 2010 at 06:17:36 AM EST
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Hard for me to say - NAO is largely a meteorological phenomenon, which is not my strongest suit. It isn't even sure, I believe, whether the NAO has a coupling effect on oceanic currents. And experts still seem to quibble how to exactly measure it, or define it. So I'm not even sure if it can fully cover "descriptive", as the NAO may include traits that haven't been properly associated to it...

However, if I were to wager, I'd say that the science on NAO is somewhere halfway between describing it and mapping out the dynamics involved.

Does that help?

by Nomad on Thu Dec 9th, 2010 at 07:06:03 PM EST
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I just remember a turbulence workshop where some people presented a Principal Orthogonal Decomposition of atmospheric data and waxed lyrical about it. "POD modes" are a great way to reduce the hell that is fluid dynamics to a finite-dimensional system of equations, but if you impose the POD modes you obtain for describing past history onto a model to try to predict future behaviour you cannot predict deviations from past history...

Of all the ways of organizing banking, the worst is the one we have today — Mervyn King, 25 October 2010
by Migeru (migeru at eurotrib dot com) on Fri Dec 10th, 2010 at 04:11:45 AM EST
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In layman's terms - if you try to predict the future based on (complex equations derived from) the past, all you can do is extrapolate the past, not predict new or deviations from past trends?  

But can you not factor in new data - e.g. increased CO2 levels - leading to significantly different results?

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by Frank Schnittger (mail Frankschnittger at hot male dotty communists) on Fri Dec 10th, 2010 at 05:09:57 AM EST
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You can find different amplitudes, speeds, or signs of the oscillations, but if you write something like the NAO into your coordinates you may be unable to see the NAO being replaced by a substantively different pattern of variability.

The problem with dimension reduction (otherwise a very useful technique) is precisely that.

Of all the ways of organizing banking, the worst is the one we have today — Mervyn King, 25 October 2010

by Migeru (migeru at eurotrib dot com) on Fri Dec 10th, 2010 at 05:16:50 AM EST
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