Development of a global ocean 4DVAR system for coupled predictions and a plan of applying it for coupled data assimilation
A global ocean data assimilation system based on a 4DVAR method is developed for improving coupled predictions in Japan Meteorological Agency (JMA). The 4DVAR analysis field with a coarse horizontal resolution (1° zonal and 0.3-0.5° meridional) is used for IAU initialization with a higher eddy-permitting resolution (0.25° zonal and meridional). Temperature and salinity (TS) observation data are assimilated through a 4DVAR method with the 10-day assimilation period in the analysis model. TS fields in the forecast model are, then, modified using the 4DVAR result of the analysis model through IAU with the 5-day assimilation period. Sea-Ice Concentration (SIC) data are also assimilated through a 3DVAR scheme. The 4DVAR system outperforms the 3DVAR system based on the same model for the analysis accuracy of near-surface temperature fields. It also reproduces a fine structure of tropical instability waves which is physically consistent. They also improve the representation of sea ice (SIC) fields in polar regions over the current operational system to which sea ice data assimilation is not applied. We also have a plan to use the 4DVAR system for updating the weakly coupled data assimilation system in JMA/MRI.
Dr. Yosuke Fujii JMA/MRI