Decadal climate prediction with the ECMWF coupled forecast system: Impact of ocean observations

Decadal climate prediction with the ECMWF coupled forecast system: Impact of ocean observations
Technical memorandum
Date Published
Secondary Title
ECMWF Technical Memoranda
F. Doblas-Reyes, 
Antje Weisheimer, 
T.N. Palmer, 
AbstractThe initialization of ocean-atmosphere coupled models is a fundamental problem in dynamical decadal forecasting. Three ten-year ensemble decadal forecast experiments have been run with the ECMWF coupled system using an initialization strategy where both ocean and atmosphere use the best estimate that can be obtained in a re-analysis. One of the experiments initializes the ocean model using data from an ocean simulation forced with data from an atmospheric re-analysis. The other two take the ocean initial condition from a similar ocean-only run that also assimilates subsurface observations. This is the first time that these experiments have been carried out. The coupled model drifts substantially towards the model climate. The small drift differences found between the experiments are due to differences in latent heat and outgoing top of the atmosphere radiation in the first forecast years. In the extratropics, the initial-condition differences between the experiments persist for the whole simulation, associated to a persistent difference in latent heat flux initialized with and without ocean data assimilation. In spite of the important drift, the three sets of decadal predictions show that the system is able to predict the interannual variability of the global and regional-mean air temperature up to several years in the future. No significant benefit of the assimilation of ocean observations is found over the extratropics in terms of forecast quality, although some results suggest the negative impact of the incorrect assimilation of the XBT profiles. The results illustrate the relevance of reducing the model drift, increasing the sample size typically adopted in decadal forecasting and reducing the uncertainty in the reference ocean re-analyses. These three factors contribute to prevent these experiments from extracting more definite conclusions for accurately assessing the benefit of ocean data assimilation in terms of the forecast quality of decadal predictions.