Assessing vertical coordinate system performance in the Regional Modular Ocean Model 6 configuration for Northwest Pacific

Abstract. The Northwest Pacific (NWP) has a complex ocean circulation system and is among the regions most affected by climate change. To facilitate rapid responses to marine incidents and effectively address climate variability impacts, the Korea Institute of Ocean Science and Technology (KIOST) developed the Korea Operational Oceanographic System–Ocean Predictability Experiment for Marine Environment (KOOS-OPEM), a high-resolution regional ocean prediction system based on Modular Ocean Model version 5 (MOM5). In this study, the base model of KOOS-OPEM was upgraded to MOM6 to enhance its regional ocean modeling capabilities. A key advancement of MOM6 is its flexible vertical coordinate system enabled by a Lagrangian remapping system. Taking advantage of this feature, we evaluated the impact of vertical coordinate choices on model performance by comparing the HYBRID (z∗-isopycnal) and ZSTAR (z∗) configurations. Model outputs from the 2003–2012 period were assessed against multiple observational datasets and reanalysis products to determine their ability to reproduce key oceanographic features. The results indicated that HYBRID better preserved stratification and reduced spurious diapycnal mixing, significantly improving the representation of North Pacific Intermediate Water (NPIW). In contrast, ZSTAR exhibited excessive diapycnal mixing, resulting in a thicker isopycnal layer associated with NPIW and a salinity bias of approximately 0.2 psu. An idealized age tracer experiment further confirmed that ZSTAR facilitates excessive downward diffusion of younger surface waters, eroding the minimum salinity layer of the NPIW. In tidal simulations, HYBRID outperformed ZSTAR in reproducing M2 tidal amplitudes in the Yellow Sea, where stratification plays a key role. Conversely, ZSTAR underestimated these amplitudes due to its limitations in representing stratification. Despite its advantages, HYBRID underperformed in high-latitude regions, exhibiting larger temperature and salinity biases between 100 and 600 m depth, with temperature biases reaching approximately −1 °C. This discrepancy arose because HYBRID maintained fewer active layers in weakly stratified regions, reducing vertical resolution and leading to errors in water mass representation. To mitigate these issues and improve HYBRID's performance in high-latitude regions, adjustments to target density profiles are necessary. In addition, both configurations showed limitations in simulating winter SST, largely due to insufficient vertical resolution in the surface mixed layer. To address these issues, adopting a finer surface layer resolution (e.g., 1 m instead of 2 m) will further enhance the model's representation of mixed-layer processes.