date: Fri, 05 Mar 1999 15:22:07 +0000
from: Mark Rodwell <mjrodwell@meto.gov.uk>
subject: NAO response to HadAM2b, GISST3.0 runs
to: t.osborn@uea.ac.uk

Dear Tim,

Thanks for your email. Sorry I didn't reply to you sooner. I was ill at the beginning
of the week and have been trying to catch up with work since then.

The short answer is: Method 1. However...

As with everything, the problem gets less clear the closer you get to it! There are
obviously many ways to analyse the NAO and it depends on what questions you want to
address. For example, do you want to find out the predictability of the NAO index in
a particular model or do you want to get an idea of the ultimate predictability of
the NAO?

A big question is: Can the observed timeseries be considered as simply another
realisation of the model? This requires the model and real world to have (eg) the same
sensitivity to SSTs, the same amount of internal variability and to have no relative
bias. This is not the case.

By counting the number of zero crossings, it was clear that every ensemble member has
more high-frequency variability than the real world. The use of the ensemble mean removes
this 'erroneous' variability and gives a timeseries with more similar variability to the
observations. This is one reason for us using the ensemble mean in the way we do.

In addition, by correlating the Iceland pressure series with that of the Azores, it was
clear that the model failed to fully capture the strength of the negative correlation
seen in nature. This may be one of the reasons for why the observed NAO index has a
significantly larger standard deviation than it does in the model.

These problems are beyond the scope of the short paper in Nature but we hope to address
them in future with a longer paper.

It is important to point out that our model *does* include anthropogenic forcing that
comes via the prescribed, observed SSTs.

Best wishes, Mark.

________________________________________________________________

   Mark Rodwell                Tel:   (+44/0) 1344 856751
   Hadley Centre               Fax:   (+44/0) 1344 854898
   London Road                 Email: mjrodwell@meto.gov.uk
   Bracknell
   RG12 2SY
   UK
________________________________________________________________
