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I have done some simple analyses of historical surface temperature from the NOAA NCAR/NCEP reanalysis (downloaded from ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/surface/air.sig995.*.nc). It consists 66 years of daily temperature values on a 73*144 point grid.

The analyses I have done range from doing a simple historical average for each grid point to trying various matrix decomposition methods such as SVD, NMF & ICA to try identify interesting patterns and applying simple image processing filters to view the results.

One of the recurring features showing up in different analyses are wavelike pattern most prominent approximately along the Humboldt, Equatorial, Gulf Stream currents. These patterns appear to have a wave length of ~550 km. This is not something I would expect to see in temperature distribution data. The natural thought is that they are some kind of artifact, but they appear across range of algorithms and parameter settings. They also appear in analyses of subsets of the data. This leads me to think that these patterns really are in the data, though they could be some kind of artifact of the data collection process or reanalysis.

Here is a short discussion of these patterns. I would like to find out:

  • if similar patterns have been observed in other analyses
  • suggestions for convincingly determining if these patterns are physical or some kind of artifact
  • suggestions of plausible explanations if these patterns are physical

Here are some processed images that show the patterns:

historical median and local rank filter thereof

The image on the right is a local rank filter applied to the historical median at each grid point (left image).

ICA components

10 ICA components of the date by grid point matrix for the Pacific region.

The patterns are strongest in the second row images and the left image on the third row. They appear to emanate from the coast of Chile NW towards the central pacific. The images are histogram equalized to improve contrast.

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    $\begingroup$ A first though is that it may be an artefact because temperature measurements at sea are fairly sparse compared with data density on land. You should look at how and what data were assimilated into the reanalysis. $\endgroup$
    – Peter Jansson
    Commented Jul 13, 2014 at 11:19
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    $\begingroup$ It seems to be a data artifact. Have you tried to see if it shows up in any other databases? From the ocean side, maybe you could look at metoffice.gov.uk/hadobs/en4 $\endgroup$
    – arkaia
    Commented Jul 14, 2014 at 17:30
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    $\begingroup$ ENSO is the El Nino Southern Oscillation. The oscillation referred to in the name is of the mean temperature across a large area of the Pacific, not these wave-like structures. $\endgroup$
    – Andy Clifton
    Commented Jul 30, 2014 at 15:41
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    $\begingroup$ How about artifact arising from your image processing filter? $\endgroup$
    – Balinus
    Commented Aug 8, 2014 at 15:49
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    $\begingroup$ I just edited the question to make it clearer that this is a reanalysis dataset, not direct observations. Not sure if you were aware of that. Doesn't really affect the meat of the question though. Also, there are at least two lists of known problems with this dataset (here and here), but neither appear to include this problem. I'd say it's definitely worth contacting the authors and alerting them to this post - they might even be able to answer the question. $\endgroup$
    – naught101
    Commented Oct 10, 2014 at 5:28

1 Answer 1

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It looks like a Gibbs oscillation, a basic feature of spectral analysis. In a spectral atmospheric model this means that near high orography artificial surface waves exist which may reflect in different variables. They are usually not considered to affect meteorology, however.

onlinelibrary.wiley.com/doi/10.1029/2010JC006927/full http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281994%29007%3C1169%3AROTGOI%3E2.0.CO%3B2

This is from Bacmeister, Julio T. "Weather Prediction Models." Climate Change Modeling Methodology. Springer New York, 2012. 89-114.

enter image description here

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    $\begingroup$ Citation needed. $\endgroup$
    – milancurcic
    Commented Aug 26, 2015 at 15:00
  • $\begingroup$ Yes, of course. This paper says that Gibbs-like phenomena is a problem in NCEP (onlinelibrary.wiley.com/doi/10.1029/2010JC006927/full) and this is description of the phenomena in atmospheric models journals.ametsoc.org/doi/abs/10.1175/… Do not take these references as a "proof" but just an explanation of why I think the images in original question are related to Gibbs phenomena. $\endgroup$
    – kakk11
    Commented Aug 26, 2015 at 16:20
  • $\begingroup$ @kakk11 Welcome to the site! Could you please add those references into your answer, so that everything is in one place? $\endgroup$
    – gerrit ♦
    Commented Aug 26, 2015 at 16:27

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