cc: "Wallace, John" , "Jones, Phil" , "Kennedy, John" date: Fri, 09 Nov 2007 11:38:42 +0000 from: David Parker subject: Re: A discontinuity in surface temperature observations to: David Thompson Dave, Mike Thanks. I somewhat misinterpreted your original email as referring to a one-month "blip" in August 1945, as I didn't receive the diagrams. Now it is clear that you meant a sudden "cooling" to a sustained (for several years) lower temperature. It might be worth including John Kennedy as a co-author of your paper on 20th century climate variability. He has made an attempt to correct uncorrected biases in HadSST2 during the 2nd World War by splitting the data into individual countries and then calculating climatologies for 1940-1970 for each country, thus removing any relative biases (which are strongly related to recruiting country of the ship) over the climatology period. He then recombined the different countries into a single 'corrected' HadSST2. The effect was to reduce the height of the 2nd World War peak and also to reduce the depth of the subsequent fall in 1945. Like Phil, however, we note that during this period, there are fewer observations so the uncertainties are likely to be larger. John notes that the problems are particularly marked in the southern hemisphere: http://hadobs.metoffice.com/hadsst2/diagnostics/hemispheric/southern/ It looks like the rapid changes are due to rapid changes in the mix of countries available in ICOADS. We have had over a million new data for the 2nd World War period digitised in the project Phil mentioned, and these data do not show such a big peak in the early 1940s or such a big fall in 1945. However these newly-digitised data are not yet incorporated in HadSST2 and their inclusion in our analyses is only likely to be done in the next financial year (April 2008-March 2009). Marine air temperatures also have problems in the 2nd World War so may not help; we are giving priority to sea surface temperature. So whatever John may be able to contribute to your paper won't be the final word! Regards David On Fri, 2007-11-09 at 08:27 +1100, David Thompson wrote: > Phil, David, > > > Thanks so much for the quick responses. Some quick comments: > > > 1. the temperature drop in 1945 is evident in NMAT data, earlier > HadSST products and the MoHSST products. > 2. the drop is also apparent if you freeze the domain to only those > grid boxes available 41-44. > 3. if you look at, for example, the FAR Summary for Policymakers, you > can really see the 0.3 K drop reflected in the key global mean time > series. > > > I'll do some more investigating and will send another email next week > summarizing things in more detail. > > > Thanks again for quick thoughts... > > > -Dave > -- David Parker Met Office Hadley Centre FitzRoy Road EXETER EX1 3PB UK E-mail: david.parker@metoffice.gov.uk Tel: +44-1392-886649 Fax: +44-1392-885681 http:www.metoffice.gov.uk