An Assessment of JMA Serial Observation Lines in the Northwestern Pacific in OSSE Studies with the GFDL Ensemble Coupled Data Assimilation System

The representation of two major serial observational lines (137°E and 165°E, operated by Japan Meteorological Agency (JMA) in the Northwestern Pacific) for the regional circulation variability is assessed through observation system simulation experiments (OSSEs). In the OSSEs, the “truth” ocean is a historical simulation timeseries of the fully coupled model (CM2.1) developed by the Geophysical Fluid Dynamics Laboratory (GFDL), and the “control” ocean is defined as the CM2.1 simulation result from the independent initial conditions. The “observations” drawn from the “truth” based on the SST and the JMA serial lines is assimilated into the “control,” and the degree by which the assimilation recovers the “truth” in the Northwestern Pacific area is an assessment of the representation of the serial observational lines for the regional circulations and their variability.

Results show that assimilation effect of SST is maintained to deeper ocean by adjusting the interannual variability of the ocean temperature of the Northwestern Pacific. The root mean square error (RMSE) in the western area of each observational system is much lower than that of the eastern area, which is closely related to the anomalous geostrophic clockwise flow pattern strengthened throughout the assimilation process of the JMA data. We confirmed this result by using reverse bias experiment. Therefore, assimilation effect of 165°E occupying larger western areas is also stronger than 137°E, but the assimilation performances of the two observation systems were not significantly different in the long-term simulations more than 25 years in the Northwestern Pacific.