date: Fri Dec 10 16:37:37 2004 from: Phil Jones subject: Re: Land-sea differences to: "Tett, Simon" , Harry Harris , "Kennedy, John" , "Brohan, Philip" , "Folland, Chris" Simon, There is a lot of spatial consistency in your map with coloured crosses and a few diamonds. Just selecting those stations near sea level is easy to do, but is there a simple way of selecting those sites that are nearest the sea. This is likely much harder. All the UK stations fell off the <5m list (even Plymouth, which we know is almost next to the sea). Just wondering if there is another way we can get coastal stations. 0.1K is similar to 0.125 though which suggests that however you subsample the full set you will get a number about 0.1K. David Parker has produce a Table for AR4 which gives trends over 1979-2003 and other periods. Here NH land warms at 0.353K/dec and SST at 0.177K/dec. For the SH, the figures are 0.164 and 0.093 K/dec. The differences can be estimated, but not quite for your period. I guess, I'm saying that unless you get really coastal stations can you really say anything? Land is warming faster, so coastal land should probably be warming slightly faster than coastal SST. Your map is differences, but does this manifest itself as a trend? One final thing, can you calculate for the same SST boxes, the differences between SST and NMAT. Is this zero, or does it approach 0.1K over the 21 years? The blue crosses over the Great Lakes and Canadian Maritimes struck me. I'm presuming there are no issues with 10% missing data with sea ice. If there were this would make the difference larger than 0.1? I would like to believe that 2) is the likely answer. I ruled out 3) ! Cheers Phil At 10:29 10/12/2004, Tett, Simon wrote: All, I've managed to compute differences between station temperatures and SST. I have done this by computing anomalous temperatures for 1981-2000 from Phil/Harry's new land records. For SST I used MOHSST (but I've also done the calculation for the new ICOADS). There appears to be a systematic difference between the two. Attached is a PDF file. Page 1 shows anomalous SST and station temperatures. Page 2 shows difference between stn mean temp and SST (both ~ 20 year averages). Diamonds are where land/SST agree to +/- 0.05K. I have been strict -- I only make the comparison where I have < 10% missing data (monthly averages). The simple avg over all stn - SST is about 0.1K. Looking at the 140 stations with a height below 5m ("low lying" -- see attached) suggests there is a systematic difference for those stations. The mean difference for "low lying" stations is 0.125 with a 2 SE of 0.055. The plots don't suggest that there are systematic differences that vary as you get further away from the ocean. (Thus the look at low lying stations). So I think we may be seeing something else! There seems to be three possibilities: 1) We have changing bias in the land record for the period 1980-2000 leading to an over-estimate of the warming 2) The ocean warming has been underestimated (due to a change in the observing system) -- John K. has some evidence that this is the case. 3) My code is in error.... If 2) is correct note we will be increasing global warming over the last 20 years by about 0.05K/decade or so. If 1) is correct then we will be reducing global-warming by 0.025K/decade (less land than ocean). If 3) then I just look silly. Simon Dr Simon Tett Managing Scientist, Data development and applications. Met Office Hadley Centre (Reading Unit) Meteorology Building, University of Reading Reading RG6 6BB Tel: +44 (0)118 378 5614 Fax +44 (0)118 378 5615 Mobex: +44-(0)1392 886886 E-mail: simon.tett@metoffice.gov.uk [1]http://www.metoffice.gov.uk Global climate data sets are available from [2]http://www.hadobs.org Prof. Phil Jones Climatic Research Unit Telephone +44 (0) 1603 592090 School of Environmental Sciences Fax +44 (0) 1603 507784 University of East Anglia Norwich Email p.jones@uea.ac.uk NR4 7TJ UK ----------------------------------------------------------------------------