Temporal and spatial scales of Near-Intertial Oscillations inferred from surface drifters

Abstract. The study investigates near-inertial oscillations (NIOs) in ocean surface currents, which are important for understanding air-sea interactions and improving satellite measurements of ocean currents, such as those planned by the ODYSEA mission. Traditional methods for measuring near-surface currents, like drifters and HF radars, have limited spatial coverage, while satellite altimetry only captures geostrophic currents, missing high-frequency components like NIOs.

Using a decade-long hourly drifter dataset (2010–2021) and outputs from the high-resolution coupled Ocean/Atmosphere model LLC2160, the study estimates the global spatial decorrelation length scales of NIOs. Drifter pairs were analyzed by latitude and distance, isolating the inertial frequency band. The LLC2160 model, which includes tidal and wind-driven forcing, was validated against the drifter data, showing good agreement in spectral characteristics and correlation scales.

Results show that NIO signals have larger decorrelation scales (~105–110 km) compared to low-frequency currents (~70–75 km), with variations by latitude (down to 50 km at high latitudes and near 20° N). These findings suggest that NIO energy is spread over larger spatial scales, linked to atmospheric forcing patterns. The LLC2160 simulation reliably reproduces these spatial characteristics and thus serves as a realistic tool for supporting ODYSEA mission planning.

The study shows that understanding NIO spatial coherency is critical for interpreting satellite Doppler measurements and mitigating aliasing effects, thereby improving ocean current observation capabilities.