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The role of ARGO temperature and salinity data in the ECMWF operational ocean analysis system

A new operational ocean analysis system (system 3 or S3) has been implemented at ECMWF. It consists of two analysis streams: (i) a historical reanalysis from 01/01/1959 which is continuosly maintained up to 11 days behind real time and is used to initialize seasonal forecasts, and (ii) an early delivery ocean analysis, produced daily in real time, used to initialize the monthly forecasts. The S3 ocean analysis has several innovative features, including an on-line bias correction algorithm, the assimilation of salinity data on T-surfaces and assimilation of altimeter-derived sea level anomalies and global trends. The assimilation of data improves the mean state and variability of the ocean analysis, and it increases the skill of seasonal forecasts of SST. The S3 ocean analysis provides a record of the history of the ocean. It is continued in real time and provides a valuable tool for climate analysis. The impact ARGO on ECMWF S3 analysis system during the period 2001-2006 has been assessed by conducting a set of observing system experiments. The experiments evaluate the information content of ARGO temperature and salinity data, and its synergy with the altimeter data. The impact of ARGO is gauged in terms of its influence on the ocean state and its impact on skill of the seasonal forecasts. Results show that the salinity data from ARGO is instrumental in correcting the salinity of the S3 analysis on a global scale, in a way that could not be achieved by the assimilation of temperature data only. The spatial scale of the salinity corrections is basin wide, and the patterns suggest that ARGO data may be correcting for errors in the fresh-water fluxes, errors in the circulation, and/or errors in the water mass characteristics of the first guess. The effect of temperature data from ARGO on the upper ocean heat content, is especially noticeable in the Indian, Atlantic and Southern Oceans. In the Equatorial Pacific, where the TAO/TRITON array is active, the impact of ARGO temperature on upper ocean heat content is modest, being confined to the regions close to the South American coast. In the S3 ocean analysis, the information content of ARGO combines well with the a altimeter information, and results show that the fit to the subsurface data is best when both altimeter and ARGO are assimilated. The impact on ARGO on the seasonal forecasts of sea surface temperature has been assessed by conducting an ensemble of coupled forecasts experiments using ocean initial conditions with and without ARGO information during the period 2001-2006. Results show that the assimilation of ARGO improves the the skill of the seasonal forecasts in most of the areas, although the reduction of the error is small. In the Equatorial Atlantic the skill of the seasonal forecasts is degraded with ARGO, even when ocean initial conditions show better fit to the temperature and salinity data. Better assimilation methods are be needed to make optimal use of ARGO data in the initialization of seasonal forecasts in the Atlantic region.