Sea Surface Temperature and Sea Ice Concentration for ERA5

This report explores ways to describe Sea Surface Temperature (SST) and Sea Ice Concentration (SIC) suitable for next generation reanalyses at ECMWF. To make a long-term time series, we first evaluate two high-resolution SST/SIC products: HadISST2 and OSTIA. HadISST2 is considered for the historical period, while OSTIA would support the near-real-time reanalysis. 

The two products reveal a broad agreement in the global mean SST, suggesting that the combined time series should be consistent over time. Differences are mainly found in small scale variabilities, where OSTIA better resolves the tropical instability waves and sub-mesoscale eddies in the midlatitudes.  The high-resolution features allow OSTIA for a better fit to scatterometer surface wind observations in the 4D-Var data assimilation (DA), improving forecast scores for up to 3 days.

We also discuss the existence of other differences between the two products. Particularly important are the discrepancies found in both SIC and related SST variabilities in the polar regions. By taking into account those uncertainties in SIC as well as SST, we show that ensemble DA produces reliable spread and improves the quality of subsequent 4D-Var DA that uses the background error.

Finally, we propose a plan for the use of SST and SIC in the next ECMWF reanalysis, ERA5.