Impact of ocean observation systems on ocean analysis and seasonal forecasts

The relative merits of the TAO/TRITON and PIRATA mooring networks, the VOS XBT network, and the ARGO float network are evaluated through their impact on ocean analyses and seasonal forecast skill. An ocean analysis is performed in which all available data are assimilated. In two additional experiments the moorings and the VOS data sets are withheld from the assimilation. To estimate the impact on seasonal forecast skill, the set of ocean analyses is then used to initialise a corresponding set of coupled ocean-atmosphere model forecasts. A further set of experiments is conducted to assess the impact of the more recent ARGO array. A key parameter for seasonal forecast initialisation is the depth of the thermocline in the tropical Pacific. This depth is quite similar in all the experiments which involve data assimilation, but withdrawing the TAO data has a bigger effect than withdrawing XBT data, especially in the eastern half of the basin. The forecasts mainly indicate that the TAO/TRITON in-situ temperature observations are essential to obtain optimum forecast skill. They are best combined with XBT, however, as this results in better predictions for the West Pacific. Furthermore, the XBTs play an important role in the North Atlantic. The ocean data assimilation performs less well in the tropical Atlantic. This may be partly a result of not having adequate observations of salinity.