<p>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.</p>

<p>onlinelibrary.wiley.com/doi/10.1029/2010JC006927/full
<a href="http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281994%29007%3C1169%3AROTGOI%3E2.0.CO%3B2" rel="nofollow noreferrer">http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281994%29007%3C1169%3AROTGOI%3E2.0.CO%3B2</a></p>

<hr>

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

<p><a href="https://i.sstatic.net/YhGTH.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/YhGTH.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2015-08-26T15:00:29.133Z
Citation needed.
milancurcic
CC BY-SA 3.0
2015-08-26T16:20:02.613Z
Yes, of course. This paper says that Gibbs-like phenomena is a problem in NCEP (http://onlinelibrary.wiley.com/doi/10.1029/2010JC006927/full) and this is description of the phenomena in atmospheric models http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281994%29007%3C1169%3AROTGOI%3E2.0.CO%3B2
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.
kakk11
CC BY-SA 3.0
2015-08-26T16:27:57.300Z
@kakk11 Welcome to the site!  Could you please add those references into your answer, so that everything is in one place?
gerrit
CC BY-SA 3.0
2015-08-26T14:35:16.277Z
2015-08-26T19:36:45.703Z
2015-08-26T19:36:45.703Z
kakk11
<p>I have done some simple analyses of historical surface temperature
from the NOAA <a href="http://www.cpc.ncep.noaa.gov/products/wesley/reanalysis.html" rel="nofollow noreferrer">NCAR/NCEP reanalysis</a> (downloaded from <a href="ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/surface/" rel="nofollow noreferrer">ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/surface/air.sig995.*.nc</a>).
It consists 66 years of daily temperature values on a 73*144 point grid. </p>

<p>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 &amp; ICA to try identify interesting patterns and applying simple image processing filters to view the results.</p>

<p>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.</p>

<p><a href="http://azimuth.mathforge.org/discussion/1387/odd-patterns-in-temperature-data" rel="nofollow noreferrer">Here</a> is a short discussion of these patterns.
I would like to find out:</p>

<ul>
<li>if similar patterns have been observed in other analyses</li>
<li>suggestions for convincingly determining if these patterns are physical or some kind of artifact</li>
<li>suggestions of plausible explanations if these patterns are physical</li>
</ul>

<p>Here are some processed images that show the patterns:</p>

<p><img src="https://i.sstatic.net/QzKKC.png" alt="historical median and local rank filter thereof"></p>

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

<p><img src="https://i.sstatic.net/icBtF.png" alt="ICA components"></p>

<p>10 ICA components of the date by grid point matrix for the Pacific region.</p>

<p>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.</p>

CC BY-SA 3.0
2014-07-13T11:19:58.227Z
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.
Peter Jansson
CC BY-SA 3.0
2014-07-14T17:30:34.537Z
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 http://www.metoffice.gov.uk/hadobs/en4/
arkaia
CC BY-SA 3.0
2014-07-17T08:02:03.553Z
Isn't this just ENSO? http://en.wikipedia.org/wiki/El_Ni%C3%B1o
Dikran Marsupial
CC BY-SA 3.0
2014-07-30T15:41:09.807Z
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.
Andy Clifton
CC BY-SA 3.0
2014-08-08T15:49:39.893Z
How about artifact arising from your image processing filter?
Balinus
CC BY-SA 3.0
2014-08-08T19:09:26.477Z
@Balinus That was my firs thought, but the same pattern turns up in a variety analyses. The pattern is faintly visible even if you just average the historical temperatures at each point without further enhancement. This makes me think it is really in the data. That does not mean it is physical. It could still be an artifact of the way the data was created, perhaps how it was discretized to the grid. This is what I am trying to determine.
Daniel Mahler
CC BY-SA 3.0
2014-08-08T19:21:43.527Z
@aretxabaleta Thanks for the pointer. Cross checking against comparable datasets is the best test I can think of.
Daniel Mahler
CC BY-SA 3.0
2014-10-10T04:30:47.360Z
Interesting find. Have you tried contacting the authors of the reanalysis?
naught101
CC BY-SA 3.0
2014-10-10T05:28:45.467Z
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](http://www.cpc.ncep.noaa.gov/products/wesley/reanalysis.html#problem) and [here](http://www.esrl.noaa.gov/psd/data/reanalysis/problems.shtml)), 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.
naught101
CC BY-SA 3.0
2014-10-18T14:42:53.430Z
This is most likely an artifact of the processing. The other question you should go after is, if there isnt a newer dataset out  there. This website hasnt been updated since 2002. In climate science this is really old. I would be very, very, very surprised, if there isn*t a newer dataset somewhere.
mace
CC BY-SA 3.0
2015-08-21T18:48:55.450Z
The grid resolution in both the horizontal and vertical direction is around 280km, if we take the earth's circumference and half the earth's circumference as the domain length and height, respectively.Then, given that the wavelength is about 550km (as OP mentions), 280km is about the distance from crest to trough on those waves. This is a very strong indication of a data artifact.
Isopycnal Oscillation
<climate><temperature><ocean-currents><ncep><reanalysis>
Odd structures in 66 year NCEP daily temperature reanalysis
Daniel Mahler
# Odd structures in 66 year NCEP daily temperature reanalysis

(Asked by: username_0 on 2014-07-13)

I have done some simple analyses of historical surface temperature
from the NOAA [NCAR/NCEP reanalysis](http://www.cpc.ncep.noaa.gov/products/wesley/reanalysis.html) (downloaded from [ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/surface/air.sig995.\*.nc](ftp://ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/surface/)).
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](http://azimuth.mathforge.org/discussion/1387/odd-patterns-in-temperature-data) 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](https://i.sstatic.net/QzKKC.png)

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

![ICA components](https://i.sstatic.net/icBtF.png)

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.

**Tags:** climate, temperature, ocean-currents, ncep, reanalysis

> **Comments:**
> **username_1**: 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.  
> **username_2**: 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 http://www.metoffice.gov.uk/hadobs/en4/  
> **username_3**: Isn't this just ENSO? http://en.wikipedia.org/wiki/El_Ni%C3%B1o  
> **username_4**: ENSO is the El Nino Southern username_8. The username_8 referred to in the name is of the mean temperature across a large area of the Pacific, not these wave-like structures.  
> **username_5**: How about artifact arising from your image processing filter?  
> **username_0**: @username_5 That was my firs thought, but the same pattern turns up in a variety analyses. The pattern is faintly visible even if you just average the historical temperatures at each point without further enhancement. This makes me think it is really in the data. That does not mean it is physical. It could still be an artifact of the way the data was created, perhaps how it was discretized to the grid. This is what I am trying to determine.  
> **username_0**: @aretxabaleta Thanks for the pointer. Cross checking against comparable datasets is the best test I can think of.  
> **username_6**: Interesting find. Have you tried contacting the authors of the reanalysis?  
> **username_6**: 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](http://www.cpc.ncep.noaa.gov/products/wesley/reanalysis.html#problem) and [here](http://www.esrl.noaa.gov/psd/data/reanalysis/problems.shtml)), 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.  
> **username_7**: This is most likely an artifact of the processing. The other question you should go after is, if there isnt a newer dataset out  there. This website hasnt been updated since 2002. In climate science this is really old. I would be very, very, very surprised, if there isn*t a newer dataset somewhere.  
> **username_8**: The grid resolution in both the horizontal and vertical direction is around 280km, if we take the earth's circumference and half the earth's circumference as the domain length and height, respectively.Then, given that the wavelength is about 550km (as OP mentions), 280km is about the distance from crest to trough on those waves. This is a very strong indication of a data artifact.  

---

## 1 answers

### Answer by username_9 on 2015-08-26. Score: 6

It looks like a Gibbs username_8, 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](https://i.sstatic.net/YhGTH.png)](https://i.sstatic.net/YhGTH.png)

> **Comments:**
> **username_10**: Citation needed.  
> **username_9**: Yes, of course. This paper says that Gibbs-like phenomena is a problem in NCEP (http://onlinelibrary.wiley.com/doi/10.1029/2010JC006927/full) and this is description of the phenomena in atmospheric models http://journals.ametsoc.org/doi/abs/10.1175/1520-0442%281994%29007%3C1169%3AROTGOI%3E2.0.CO%3B2 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.  
> **username_11**: @username_9 Welcome to the site!  Could you please add those references into your answer, so that everything is in one place?  

---

