The Step Change in HADSST Data After the 1997/98 El Nino

I’ve moved to WordPress.  This post can now be found at The Step Change in HADSST Data After the 1997/98 El Nino

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When compared to the ERSST.v2, ERSST.v3b, and OI.v2 SST data sets (Figures 1, 2, and 3), there is a significant step in the HADSST global SST anomalies after the 1997/98 El Nino event. I’ve noted this in comments on numerous blogs over the past year, but have chosen not to offer an explanation for what appears to be the reason for the difference.

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Figure 1
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Figure 2
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Figure 3

COULD THIS BE IT?

The Hadley Centre changed data sources for SST at 1998. [Note: That date was originally posted by me as 1978. I've corrected it.] This quote is from the Met Office’s Hadley Centre about the HADSST2 data set:
http://hadobs.metoffice.com/hadsst2/

“Brief description of the data
“The SST data are taken from the International Comprehensive Ocean-Atmosphere Data Set, ICOADS, from 1850 to 1997 and from the NCEP-GTS from 1998 to the present.”

The following ICOADS link will bring you to the following explanation:
http://icoads.noaa.gov/products.html

“ICOADS Data
“The total period of record is currently 1784-May 2007 (Release 2.4), such that the observations and products are drawn from two separate archives (Project Status). ICOADS is supplemented by NCEP Real-time data (1991-date; limited products, NOT FULLY CONSISTENT WITH ICOADS).” [Emphasis added.]

The change of data set also helps explain why HADCRUT3 Global, Northern Hemisphere, and Southern Hemisphere data sets consistently run high since the 1997/98 El Nino when compared to other land and sea surface temperature data sets.

SOURCES

HADSST2GL data is available from the CRU website:
http://www.cru.uea.ac.uk/cru/data/temperature/

Extended Reconstructed SST Sea Surface Temperature Data (ERSST.v2) and the Optimally Interpolated Sea Surface Temperature Data (OI.v2 SST) are available through the NOAA National Operational Model Archive & Distribution System (NOMADS).
http://nomads.ncdc.noaa.gov/#climatencdc

The recently updated version of the Extended Reconstructed SST (ERSST.v3b), along with land surface temperature and combined (land + ocean) surface temperatures, are available in various latitudinal bands at:
ftp://eclipse.ncdc.noaa.gov/pub/ersstv3b/pdo
Don’t let the PDO in the address confuse you. There’s much more there. The overview for the update is here:http://www.ncdc.noaa.gov/oa/climate/research/sst/ersstv3.php