The Role of Ice–Ocean Drag Coefficient in Bohai Sea Ice Modeling

The ice–ocean drag coefficient (Cdw) is critical yet uncertain in sea ice modeling of the Bohai Sea with limited observational constraints. Using the HAMSOM ice–ocean coupled model, we performed sensitivity experiments with Cdw over four orders of magnitude for the 2011/2012 ice season. The results reveal that Cdw directly enhances ice–ocean stress and suppresses ice velocity, whereas its influence on variables such as the mixed layer current velocity and the oceanic heat flux is indirect and cumulative. The marginal ice zone exhibits greater sensitivity than the inner ice zone. Mechanistically, a negative feedback loop (Cdw↑→τw↑→vi↓) governs ice dynamics. For thermodynamics, a primary negative feedback (Cdw↑→u*↑→Qwi↑→Rb↓ pre-existing ice basal melting) is offset by a secondary positive feedback: meltwater freshens the mixed layer, raising the freezing point, which promotes basal freezing of newly formed ice. When Cdw was increased from 164 × Cdw0 to 4×Cdw0, mean ice velocity decreased by ~0.025 m/s (80%), while mean ice thickness increased by ~1 cm (20%) because salinity-driven basal freezing slightly outweighed enhanced melting. Our findings highlight the complex, nonlinear role of Cdw in modulating ice–ocean interactions and provide an appropriate Cdw range of 10−2–10−3 for the Bohai Sea ice modeling.