Assimilation of altimeter data in the ECMWF ocean analysis system 3

International audienceThe latest version of the ECMWF ocean analysis system was recently in- troduced into operational use. This not only provides initial conditions for the monthly and seasonal forecast systems but also creates a historical reanalysis. For the first time, altimeter data are used in the ECMWF operational ocean analysis. However, making good use of altimetric sea level information was not easy and several difficulties had to be overcome. Various strategies were tried and compared. Attempts to use mean sea level from gravimetric satellites were also tried but no satisfactory method of using this data was found. The altimetric data used show a marked rising trend which can not be directly represented in the model as the Boussinesq approximation is used. A strategy for dealing with the trend is given. Results of Observing System Experiments (OSEs) with and without altimeter data are described and results compared to illustrate the benefits from using altimetry. Although the results are positive, further developments are needed to more fully utilise the data