Wednesday, June 20, 2012

5336.txt

cc: "Rayner, Nick" <nick.rayneratXYZxyzoffice.gov.uk>, Wallace@metoffice.gov.uk, John <wallaceatXYZxyzos.washington.edu>, Jones@metoffice.gov.uk, Phil <p.jonesatXYZxyz.ac.uk>, Kennedy@metoffice.gov.uk, John <john.kennedyatXYZxyzoffice.gov.uk>
date: Wed, 14 Nov 2007 08:40:02 +0000
from: David Parker <david.parkeratXYZxyzoffice.gov.uk>
subject: Re: A discontinuity in surface temperature observations
to: David Thompson <davetatXYZxyzos.colostate.edu>

Dave

Thanks. I'm familiar with COWL from your published papers. The plots are
very interesting and I hope John Kennedy, who is working on marine data
biases, will be able to contribute in due course regarding the 1945 dip.

Regards

David


On Wed, 2007-11-14 at 17:44 +1100, David Thompson wrote:
> Dear David, Phil and John,
>
>
> (This is a bit of a long email, so you might want to grab a cup of
> coffee - or tea - before reading on...)
>
>
> Thanks again for the quick and helpful responses last week.
>
>
> Mike and I would be happy to include John as a coauthor on our paper.
> And David, Phil: we understand if you are too busy to join another
> project. But if you are interested in joining the paper, too, that
> would be great. The goal of the paper is to clarify some key aspects
> of 20th century temperature variability, and the study would certainly
> benefit from your expertise.
>
>
> Before I get too far ahead of myself, let me review the main points of
> the paper as it currently stands. I've attached 3 pages of figures
> (the figures will evolve as the writing evolves, but as of now it
> appears the paper will end up being short and punchy)
>
>
> Figure 1 includes 2 panels. The top time series in the top panel shows
> the global-mean temperature time series. The next two time series show
> the linear fit of ENSO and the COWL (cold-ocean/warm-land) time series
> to the global-mean. The ENSO time series is found as a damped thermal
> response to variations in the cold-tongue (this gives a marked
> improvement in the representation of ENSO in global-mean
> temperatures). The COWL time series represents the effects of random
> fluctuations in climate acting on the different heat capacities of the
> ocean and land (eg: periods of warm advection over land/cold advection
> over the ocean lead to warmer than normal global-mean temperatures by
> virtue of the fact that the continents have a lower heat capacity).
>
>
> I'll provide more details of the ENSO and COWL methodologies in a
> future email, but the main point is that a lot of the high frequency
> 'noise' in global-mean temperatures can be accounted for on the basis
> of two simple, physically based time series.
>
>
> The bottom panel in Fig. 1 includes a reproduction of the global-mean
> time series (top) and also shows the residual global mean time series,
> which is found by removing the ENSO and COWL time series from the
> global-mean time series.
>
>
> The bottom panel of Fig. 1 is the key figure in the paper. We think
> it's remarkable how well the fitting 'cleans up' the global-mean time
> series. If you look closely, you'll see that the major volcanoes of
> the past century (marked by solid vertical lines) are much, much
> clearer in the residual time series. But the fitting not only isolates
> the volcanoes, it also isolates the very large drop in Aug 1945. The
> Aug 1945 drop is about 0.3 K, almost twice as large as the response to
> Pinatubo.
>
>
> The residual time series also suggests a slightly different view of
> 20th century temperature variability. The canonical view is that
> temperatures warmed in the 20s, settled from the 40s-70s, and warmed
> from the 70s-the present. But if you stare at the residual time
> series, you get the impression that global-mean temperatures have
> actually risen steadily over the past century, but that the warming
> has been disturbed by several discrete and abrupt drops in
> temperature.
>
>
> As for the volcanos:
> In figure 2 we're exploiting the fitting procedure to provide a
> 'cleaned up' version of the volcanic response in surface temperatures.
> The figure shows the composite temperature response for the 4 largest
> tropical volcanoes of the 20th century. The composite is done such
> that the 10 year period before each volcano has a trend of zero and
> mean of zero. (If you don't remove the trend then the long term
> warming biases the composite). The response in the residual data is
> surprisingly coherent (much more so than in raw data). But we think
> what's most striking is that surface temperatures do not appear to
> fully recover for up to a decade after each volcano (the composite is
> limited to 9 years after the eruptions since the eruption of Pinatubo
> occurred 9 years after the eruption of El Chichon).
>
>
> You can see the long timescale of the volcanoes in the residual
> global-mean time series: if you follow the temperature time series
> before, say, Agung or Pinatubo, you can see that it takes a long, long
> time for global-mean temperatures to catch up to where they would have
> been, assuming they would have continued to warm...
>
>
> And as for the dip in 1945:
> Fig 2 shows the residual (ie with the COWL or ENSO time series
> removed) global-mean land and ocean time series. The point here is
> that the large dip in Aug 1945 does not show up in the land data.
>
>
> My impression from your emails last week is that the dip is almost
> certainly due to changes in instrumentation during the war. But it's
> also my impression that the specific reasons for the dip are not yet
> known. It's possible that the dip is offset by spurious rises in SSTs
> at the start of the war. But this isn't certain. And even so, there is
> a large drop in SSTs between the period before the war (1939) and the
> period after the war (1945). To my eye, the residual ocean time series
> in Fig. 3 suggests temperatures ratcheted downwards spuriously in
> 1945.
>
>
> In our view, the dip in Aug 1945 is very important and warrants being
> highlighted in the literature. In fact, once you know it's there, it's
> hard to view any time series of global-mean temperatures and not
> wonder how different it would appear if the dip was not there. For
> example, Fig. 4 shows the raw and residual global-mean temperature
> time series assuming the 0.3 K drop in Aug 1945 is spurious. I realize
> the figure is crude, and it might not make it into the paper. But the
> point is that you would get a very, very different impression of 20th
> century temperature trends if the dip proves to be an artifact of the
> end of the war.
>
>
> So our main point regarding the drop in Aug 1945 is this: if we assume
> the dip is spurious, then the global-warming of the past century would
> be at least ~0.3 K larger than currently thought, and global-mean
> temperatures would have risen steadily throughout the past century.
>
>
> That's enough for now. I'm working on the writing and our goal is to
> submit something by Xmas. Please let me know what you think, and if
> you are interested in continuing to interact with Mike and I on the
> paper.
>
>
> And again: thanks for your time and interest.
>
>
> -Dave
>
>
>
> --------------------------------------------------------------------
> --------------------------------------------------------------------
> David W. J. Thompson
> www.atmos.colostate.edu/~davet
>
>
> Dept of Atmospheric Science
> Colorado State University
> Fort Collins, CO 80523
> USA
>
>
> Phone: 970-491-3338
> Fax: 970-491-8449
>
>
>
--
David Parker Met Office Hadley Centre FitzRoy Road EXETER EX1 3PB UK
E-mail: david.parkeratXYZxyzoffice.gov.uk
Tel: +44-1392-886649 Fax: +44-1392-885681 http:www.metoffice.gov.uk

No comments:

Post a Comment