Wednesday, June 17, 2015

Checking on May Temperatures

For the first time since I have been tracking both NCEP/NCAR and surface temperatures (TempLS), there has been a substantial disagreement. The TempLS Mesh result showed May about the same temperature as April, and this was backed up by GISS. But NCEP/NCAR suggested a warmer month, comparable to February.

There was a new factor in the mix - GHCN has a new version 3.3. But its changes are mainly methodological, and made little difference to previous months. Both TempLS and GISS use that version. There is also a new version of ERSST, v4, but neither TempLS nor GISS (AFAIK) use that. TempLS will soon.

The difference does seem not very robust. TempLS Grid did show a significant rise, of about 0.05°C, which does indeed match February for that measure. This is working from the same data but with different (grid-based) weighting with lesser coverage. Since NOAA and HADCRUT share that characteristic, I think they might show May somewhat warmer.

HADSST3 also showed a rise in SST. So I wondered, if so much is rising, there must be something one could identify in going from April to May which would particularly affect the GISS/TLS indices. So, below the fold, I'll show a map and table

Here is an active WebGL plot of the differences, May anomaly - April, by stations (red dots) with readings in both months. The spacing between is linearly interpolated and shaded. The color scale is highly non-linear, with the extremes being quite large.

The cool spots are in the Arctic, Antarctic, and particularly NE Siberia. The ocean warms by a small, fairly uniform amount. The colors don't give a particularly good quantification of the temperature differences at the extremes. Here is a table of the top 10 cooler GHCN locations, by anomaly difference:

May-Apr   Latitude Longitude Name
-8.1980.6258.05GMO IM.E.T.
-8.0879.576.98OSTROV VIZE
-6.2681.6-16.67NORD ADS
-5.8277.72104.3GMO IM.E.K. F
-4.3836.32-119.64HANFORD 1 S
-4.2473.580.4OSTROV DIKSON

It's heavily weighted toward N and NE Siberia. I've added a facility, which I'll formalise, to the NCEP/NCAR map. The table on the right allows you to choose a day, by mousing over and tracking what it says just NE of the globe - click when you have what you want. It now allows you to choose the 0'th on the month, for Jan-May 2015, and you get the map of the monthly average anomaly. Selecting 2015-5-0, I get the May average map. Here's a plot of the Arctic region:

The reanalysis certainly shows cold anomaly in the Arctic, but nothing special in NE Siberia, and it was cold there in April too. That seems to be the main difference between GHCN and reanalysis.

Update. Here's another interesting table. This time I've weighted the anomaly differences to show their contribution to the TempLS sum. So when it says Qiqihar -0.00149, that means Q contributed that amount to the May-April anomaly difference. The weighted order is different to the absolute order, and the amounts are fairly small. The Antarctic now look more significant than the Siberian. The discrepancy between NCEP and GISS/TLS is of order 0.05°C, so none of these if changed would make more than 1/30 of that.

May-Apr weighted Latitude Longitude Name
-0.0012379.576.98OSTROV VIZE


  1. Good work!
    The behaviour of Gistemp May is still odd and defies the law of simple addition. The Gistemp map SST only says that May is up 0.07 from April, Land only 1200 km is up 0.02, but the global value is down 0.02.
    Norway has still not reported May data to GHCN. The norwegian Arctic Island have not been cool according to other sources. The May anomaly for Svalbard airport is +1.9 for example. That would take a big chunk out of the cold blue that emanates from Greenland on the Gistemp map, and replace it with warm orange.

  2. I think it would be a good idea to have also the NCEP/NCAR May - April difference as an active WebGL plot to compare.

    @Olof: The GISS based on meteorological stations only is not a land only temperature estimate, but an attempt of a global temperature estimate using only meteorological stations. It is not additive with the oceans data nor comparable to (other) land only temperature estimates. You would need to put a land mask on it.

    1. I think that is a good idea to. It's getting late here, though. Maybe tomorrow.

      That's an interesting point about GISS Ts. It actually went up, from .84 to .87, based on land only data. All very odd.

    2. Anon., The anomaly change from April with land 250 km is the same, 0.02, but I chose 1200 km so it should extrapolate over all land and sea ice. It is possible to put a land mask on the data in KNMI Climate explorer, but they have not yet uploaded Giss May data....
      Comparing anomalies between months, different data sets, and base periods is not always simple because the seasonality of anomalies can change with the base period. If we set the base period of Gistemp Loti to 1994-2013 (same as Nicks NCEP/NCAR), May will be 0.03 warmer than April..

    3. Olof,
      Very good point about the base period. That actually explains a large part of the discrepancy (0.035°C).