AMIP SST/sea ice forcing datasets

The CVCWG creates and distributes SST/sea ice datasets used as boundary conditions in AMIP simulations. From 2008-2023 datasets were produced following the protocols set forth in Hurrell et al. 2008. These datasets were produced using HadISST1.1 sst/sea ice data from 187001-198110 and NOAA OISSTv2.0 sst/sea ice data from 198111-present day. NOAA deprecated the OISSTv2.0 data in 2023, ending the production of the original version of these products. In 2025 the CVCWG identified the COBE-SST2 dataset as a suitable replacement to the HadISST1.1/OISSTv2.0 datasets, and new versions of the AMIP SST/sea ice forcing dataset running from 185001-present day have been created.  

Note: These datasets will not be used for boundary forcings for the upcoming CESM3 CMIP7 AMIP simulations. Those simulations will use datasets provided by CMIP. 

Process used to form COBE-SST2 forcing datasets

Monthly COBE-SST2 data was downloaded from MRI's website.  The following changes were applied to the data:

  • For the sea ice data, all land areas are set to 0 with the exception of Antarctica where it is set to 1. SST data is interpolated over land in the longitude direction first and then the latitude direction using xarray's interpolate_na function.
  • All values of SST less than -1.8C are set to -1.8C.
  • Change the units of sea ice to percentage (ranging from 0-100), set all values of sea ice less than 0 to 0, and all values greater than 100 to 100.
  • Where sea ice > 90%, set the SST to -1.8C.
  • Where sea ice < 15%, set sea ice to 0.
  • Where SST > 5C, set sea ice = 0%.

(The latter five changes match the protocols in Hurrell et al. 2008.)

After the applied changes are applied, the data are then passed through NCAR's diddling routines (available in CESM codebases under components/cam/tools/icesst), which regrid the data to the desired resolution and statistically alter the data so that the monthly averaged SST/sea ice data coming out of the model matches the SST/sea ice data passed into the diddling routines. This version of the data is referred to as the diddled data. The data that is passed into the diddling routines is referred to as the prediddled data. Within the data files output from the diddling routines one will find the diddled SST data (variable name = SST_cpl), the diddled sea ice data (ice_cov), the prediddled SST data (SST_cpl_prediddle), and the prediddled sea ice data (ice_cov_prediddle). Note that CESM will only read the SST_cpl and ice_cov arrays. Also note that even though there are values over land for SST/sea ice, CESM ignores those values. 

Data availability

Data (that has been diddled; see above) is available on a number of model resolutions on NCAR's CISL systems at /glade/campaign/cesm/cesmdata/cseg/inputdata/atm/cam/sst, or from the CESM inputdata ftp site.  The diddled data files run from 185001-202501.

This dataset will be updated at least once a year. The next expected update is in February 2026.

Other AMIP SST/sea ice forcing datasets
  • A high resolution dataset based on the daily AVHRR 0.25 degree product covering the satellite period is also available. This dataset runs from 198101-present day. More information will be posted about this dataset when it becomes available. If you are interested in obtaining this product and/or learning more about it, please post to this DiscussCESM thread.

  • PCMDI also creates and distributes AMIP forcing datasets. 
Contact Information

Questions about these data should be posted to the CAM sub-forum on the DiscussCESM website.

Acknowledgements

The CESM CVCWG thanks Dr. Yukiko Imada (University of Tokyo) and Dr. Masao Ishii (Meteorological Institute of Research at the Japan Meteorological Agency) for their assistance and for providing COBE-SST2 data.