date: Fri, 05 Mar 1999 15:22:07 +0000 from: Mark Rodwell 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 ________________________________________________________________