Email 4997, Phil Jones on NAO, 2004: "Also putting this in a box doesn't mean we know what is causing it. My view is that we will never know. Is the influence stronger now because of anthro effects - who knows."

Email 4997

the [NAO] influence is clearly weaker between 1920-60 - just at the same time as there was a weaker influence of ENSO and less strong and maybe fewer El Ninos. So there is a lot of variability and just saying it is decadal variability doesn't mean that we can forget about it ! Also putting this in a box doesn't mean we know what is causing it. My view is that we will never know. Is the influence stronger now because of anthro effects - who knows, it is a possibility but no more than that. I also think that there is too much emphasis on the NAO (and SOI) and other factors become more important at times. There is some modelling work with volcanoes which says that the aerosols impact the stratosphere which then feeds back on the troposphere causing the NAO to be more positive in the winters after eruptions. Maybe the 3 large eruptions in recent decades and the lack of eruptions between 1920 and 1960 is a factor. Again who knows. I don't think of David Stephenson as the leading authority on the NAO in the UK. I don't think we have one - we all know bits. There seems nothing special about recent trends in the instrumental record. There does if the paleo stuff is used - but here you come back to changes in the influence. Finally, you can explain much more of CET, EWP if you develop an index based on Plymouth and Lerwick. The NAO is a fundamental mode of variability, but is it nothing more than a measure of westerly wind strength? Cheers Phil