Assimilation effect of serial observation data in the East Asian Marginal Seas for long period

Despite the long-standing importance of the serial observation system since the 1960s in the East Asian Marginal Seas (EAMSs), research on the contribution of this valuable data to ocean analysis remains limited. In this study, an Observing System Simulation Experiment (OSSE) was conducted to assess the data assimilation effects of this serial observation system. The OSSE was applied to 22 serial observation lines, with different assimilation periods.

Results show that the best assimilation performance was achieved with the 2-month cycle, which matches the real observation system's period. In the surface layer, the 10-day and 1-month cycles exhibited poorer performance due to an increase in warm bias in the northern part of the East/Japan Sea. In contrast, for the deep layer below 500 m where no serial observation data is available, the 10-day and 1-month cycles showed better performance in short-term simulations for the first seven years for 2012–2018. This improvement is linked to the downward current generated in the northern East/Japan Sea.

In long-term simulations for 2019∼2041, the 2-month cycle demonstrated superior performance, likely due to signal propagation by the southward deep current, which is part of the meridional overturning circulation. These findings were also supported by results from the reverse bias experiment, although the physical mechanisms for interpreting the data assimilation process differ. This study provides valuable insights for long-term ocean prediction and highlights the significance of the serial observation system in enhancing ocean analysis.