date: Fri Dec 10 16:37:37 2004
from: Phil Jones <p.jones@uea.ac.uk>
subject: Re: Land-sea differences
to: "Tett, Simon" <simon.tett@metoffice.gov.uk>, Harry Harris <i.harris@uea.ac.uk>, "Kennedy, John" <john.kennedy@metoffice.gov.uk>, "Brohan, Philip" <philip.brohan@metoffice.gov.uk>, "Folland, Chris" <chris.folland@metoffice.gov.uk>

    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
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