Enhanced diapycnal mixing of a subsurface anticyclonic eddy pair in the northeastern South China Sea

Mesoscale eddies, which play a vital role in transporting substances and regulating marine ecosystems, are particularly active and energetic in the northern South China Sea (SCS). However, their fine-scale structures and diapycnal diffusivity inside and outside eddies remain poorly understood. In this study, a pair of subsurface anticyclonic eddies (A1 and A2) in the northern SCS was investigated using 7 seismic reflection transects acquired in April 2009. The anticyclonic eddies consist of two closely adjacent, bowl-shaped subsurface structures, each approximately 70 km in diameter and extending to a depth of 700 m. The transition zone between the eddies is approximately 10 km wide and 500 m deep. These two eddies merge together in the upper water above 500 m. The average diffusivity across the eddy is ∼7.7 × 10⁻⁵ m² s⁻¹ quantified from seismic data. Within the eddy centers (A1 and A2), the diffusivities are relatively low, approximately 2.4 and 3.9 × 10⁻⁵ m² s⁻¹, but increase to more than 4.9 × 10⁻⁵ m² s⁻¹ in the eddy transition zone. Diffusivities in the frontal zone between the eddy and the Kuroshio reach a maximum of 13.9 × 10⁻⁵ m² s⁻¹. We suggest that the shear instability occurring near the frontal region is the primary energy source for turbulent mixing in this region. These findings provide valuable insights into the turbulent mixing associated with irregular eddies and contribute to a more comprehensive understanding of the interactions between eddies and the ocean.