Mind the Gap!

Re-analysis data are results from atmospheric models where observed data have been fed into the models and used to correct the simulation in order to try to get a best possible description of the real atmosphere. But it’s important to note that the NCEP re-analysis and other re-analyses (e.g. ERA40) are not regarded as being appropriate for trend studies due to changes in observational systems (new satellites coming in etc). Nevertheless, a comparison between the re-analyses and observations can highlight differences, which may suggest where to look for problems.

Mean temperature difference between the periods  2004-2008 and 1999-2003

The animated figure shows the temperature difference between the two 5-year periods 1999-2003 and 2004-2008. Such results do not show the long-term trends, but it’s a fact that there have been high temperatures in the Arctic during the recent years.

The recent Arctic warming is visible in the animated plot on the right showing the NCEP re-analysis mean temperature difference between the periods 2004-2008 and 1999-2003.

The NOAA report card on the Arctic was based on the CRUTEM 3v data set (see figure below) which excludes temperatures over the ocean – thus showing an even less complete picture of the Arctic temperatures. The numbers I get suggest that more than 80% of the grid-boxes north of 60N contain missing values over the most recent decade.

Temperature measurements over the Arctic: CRU data and the NCEP re-analysis

Figure caption: The difference between Nov. 2007 – Oct. 2008 temperature average and the 1961-1990 mean temperature for CRUTEM 3v (upper left) and NCEP re-analysis (upper right). Below is a comparison between the 12-month 60N-90N mean temperature evolution. (click on figures for PDF-version)

The funny thing, however, is that the last decade of the Arctic CRUTEM 3v temperatures are closer to the corresponding estimates from NCEP re-analysis than the more complete HadCRUT 3v data. This may be a coincidence. The re-analyses use additional data to fill in the voids – e.g. satellite measurements and predictions based on the laws of physics. Thus, the temperature in areas with no observations is in principle physically consistent with surrounding temperatures and the state of the atmosphere (circulation).

Below is a figure showing a similar comparison between HadCRUT 3v and GISTEMP (from NASA/GISS). The latter provides a more complete representation of the Arctic by taking spatial correlation into account through an extrapolating/interpolating in space. But GISTEMP does not really have a better empirical basis in the Arctic, but the effect from the extrapolation (the filling in of values where there is missing data) gives the recent high Arctic temperatures more weight.

GISS-CRU warming difference over 1996-2004

Figure caption: The 2007 mean temperature anomaly wrt to 1961-90: (upper left) HadCRUT 3V, (upper right) GISTEMP, and (lower) temperature evolution for the Arctic (red=GISTEMP, black = HadCRUT 3v).

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