Dependency of the drag coefficient on boundary layer stability beneath drifting sea ice in the central Arctic Ocean

The ice-ocean drag coefficient \(C_{w}\) $C_w$ and turning angle \(\theta_{w}\) ${\theta }_w$ are crucial parameters in ice-ocean coupled simulations, determining the transfer of momentum between the two media. These parameters are often treated as constants regardless of the static stability at the ice-ocean interface. This study investigates the variability of \(C_{w}\) $C_w$ and \(\theta_{w}\) ${\theta }_w$ based on direct observations of thermal and kinetic energy balance. The observations were conducted beneath multiyear ice packs widely across the central Arctic during a period transitioning from ablation to refreezing, indicating significant variability of \(C_{w}\) $C_w$  = 1–130 \(\times\) $\times$ 10⁻³ and \(\theta_{w}\) ${\theta }_w$  =  − 19–1° at 5 m depth. Comparing different stations, the observations suggest a pronounced dependence of \(C_{w}\) $C_w$ on the stability parameter ( \(\mu\) $\mu$ ) resulting from mechanical and buoyant forcing. \(C_{w}\) $C_w$ rapidly decays with increasing \(\mu\) $\mu$ , indicating that the ice-to-ocean momentum transfer is enhanced for neutral or unstable conditions, while it is weakened for stable conditions. In addition, observed vertical profiles of currents revealed that \(|\theta_{w}|\) $|{\theta }_w|$ tends to be smaller for unstable and larger for stable conditions. We suggest that numerical simulations using constant values could result in an underestimate of large-scale near-surface currents during the ice growing period.