Different trajectory patterns of ocean surface drifters modulated by near-inertial oscillations

Abstract

Near-inertial oscillations (NIOs) are widely observed dynamic motions in the global ocean, with a frequency related to earth’s rotation. Using a particle trajectory model, we found the combined influence of mesoscale eddies and NIOs could produce distinctive flower-like trajectories, which are a special case of near-inertial trajectories and were observed by surface drifters released within an anticyclone eddy in the South China Sea in 2021. The energy budget indicates that wind and geostrophic eddy currents are crucial in generating near-inertial energy during the flower-like trajectories. Furthermore, the particle trajectory model revealed variations in periods and widths of the near-inertial trajectory with latitudes. The width of near-inertial trajectories can exceed 8 km in the near-equatorial region and reach 3–6 km in the mid-latitude region (20°–50°). The ratios of near-inertial velocity to background velocity, defined as near-inertial trajectory shape index (NITSIs), lead to arc-shaped (0.5 < NITSI < 1.0), overlapping semi-circular (NITSI > 1.0), and near-circular trajectories (NITSI ≫ 1.0). Globally, approximately 1/3 of the drifters’ lifespan featured clear near-inertial trajectories, with a significant presence in most middle latitudes and the largest NITSI in the north Pacific westerly. These findings highlight the importance of NIOs and suggest their substantial impact on local surface matter distribution, trajectory prediction, and marine rescue operations.