GPS Surface Drifter System for Scale-dependent Horizontal Diffusivity Estimation in the Northwestern Pacific Ocean

Scale-dependent horizontal diffusivity at the sea surface is a controlling parameter for the trajectory and dilution of soluble pollutants and other tracers.A lightweight, low-cost GPS surface drifter with satellite telemetry, a durable low-profile hull, and a sustained operational lifetime of around two months has been developed, enabling cluster-style releases for pair statistics across a wide range of separation scales.Nineteen units were deployed from research vessel (R/V) Isabu during August and October 2024 at two energetic sites in the seas of the western Pacific and south of Japan.Relative dispersion from the trajectories yields scaledependent diffusivity that rises from the order of 10 1 m 2 s -1 at 10 km to 10 3 m 2 s -1 near 100 km, with values of 2.7 × 10 1 and 9.6 × 10 2 m 2 s -1 for the western Pacific site and 4.3 × 10 1 and 1.7 × 10 3 m 2 s -1 for the region south of Japan.The empirically constrained curve for horizontal diffusivity as a function of scale provides a direct constraint for subgrid-scale mixing in coarse-resolution contaminant transport models and demonstrates the field readiness of the new drifter system.