Felix Bunzel and Dirk Notz Polar ECV cross assessment: Assimilation of ice concentrations and sea surface temperatures into MPI-ESM Felix Bunzel and Dirk Notz Max Planck Institute for Meteorology Hamburg, Germany

Aim and experimental setup Aim of this study: Assessing the quality and consistency of ESA-CCI sea ice concentration (SIC) and sea surface temperature (SST) data Experimental setup: We performed combined SIC/SST assimilation runs with the Max Planck Institute Earth System Model (MPI-ESM) for all combinations of the following data products: Sea ice concentration: ESA-CCI NSIDC/Bootstrap Sea surface temperature: ESA-CCI ERAinterim

Sea surface temperature differences: CCI minus ERAinterim March September -1.8 1.8 -1.8 1.8 Difference between SST data products largest in northern high latitudes.

Sea ice concentration differences: CCI minus NSIDC Difference between SIC data products largest in the Arctic close to the ice edge, and in a few specific regions.

Inconsistencies between SIC and SST data products Inconsistencies between SIC and SST data products in a few specific regions, e.g. the Davis Strait and Baltic Sea.

Inconsistencies between SIC and SST data products No substantial inconsistencies in the Antarctic.

Arctic sea ice area When assimilating SIC and SST data from ESA-CCI a certain amount of sea ice is lost in the assimilation process.

Arctic sea ice concentrations: Data minus assimilation Spurious ice concentrations related to weather effects are lost when the ESA-CCI SIC product is assimilated into the model.

Antarctic sea ice area Behaviour of assimilation runs relatively similar in the Antarctic.

Antarctic sea ice concentrations: Data minus assimilation Differences between datasets and assimilation runs almost cancel each other out in the Antarctic.

Conclusions Differences and inconsistencies between SIC and SST datasets more prominent in the Arctic compared to the Antarctic. Quality of ESA-CCI SIC and SST products appears in general to be comparable to other data products. Differences between the ESA-CCI SIC product and other datasets originate from a different setup of the applied algorithm, e.g. weather filter. Differences between available SIC products large enough to yield a significant impact on seasonal surface temperature predictions in northern high latitudes. Seasonal climate forecasts significantly affected by observational uncertainty of Arctic sea-ice concentration Bunzel, F., D. Notz, J. Baehr, W.A. Müller, and K. Fröhlich, Geophys. Res. Lett., 2016.