Impact of the loss of SSMIS data on the OSI SAF Sea Ice Index

As previously announced, U.S. SSMIS data will no longer be available after July 31st, 2025. This service message describes the impact on our Sea Ice Index (OSI-420, https://osi-saf.eumetsat.int/products/osi-420).

Until now, our Sea Ice Index v2.2 (OSI-420) has been based on the Sea Ice Concentration CDR (OSI-450-a1) and its ICDR (OSI-430-a), both of which are derived from SSMIS data. With the loss of SSMIS data, the ICDR OSI-430-a will stop on July 31st. A new Sea Ice Concentration ICDR OSI-438, based on the alternative sensor AMSR2, was implemented in early July and provides daily concentration data (see our Service Message #3315, July 4, 2025). From August 1, 2025, the OSI-420 Sea Ice Index will be updated with values derived from OSI-438. The version of the upgraded OSI-420 product will increase to v2.3. However, the filename and file format of the Sea Ice Index will remain unchanged, and the data files will continue to be available at the same locations, ensuring a straightforward transition. Sea Ice Index values up until July 31, 2025, will not be impacted and will be based on SSMIS only. It will be clearly stated in the data whether the data is based on OSI-430 or OSI-438.

The AMSR2 instrument channels have finer field-of-views compared to the equivalent SSMIS channels, and OSI-438, hence, shows finer details in the sea-ice concentration fields and at times different ice mapping near the coast. This leads to differences in the computed values of sea-ice extent and area. Throughout the summer, the OSI SAF Sea Ice team has tested several methods to ensure climate consistency when introducing AMSR2 data from August 2025. The method we finally selected is based on spline curves and calibrates the AMSR2-based index onto the SSMIS-based index. More details and information about the spline-based calibration and its performance will be published shortly after the summer.

We invite our users to carefully assess and monitor the sea-ice index values after August 1st in their applications and to inform their users of the change in data source. The values after August 1st should be considered temporary, as they may be updated in the next few months if we further refine our calibration method. We will keep you updated on any changes or upgrades via these service messages.

More information about OSI-420 is available here:
https://osi-saf.eumetsat.int/products/osi-420

If you have any questions, please contact us by email at osisaf-manager@met.no