Spatiotemporal Evolution of Subsurface Eddies in Northwestern South China Sea Investigated Using Seismic Oceanography Data

The vertical structure and evolutionary dynamics of subsurface eddies in the northwestern South China Sea (SCS) remain poorly constrained due to the scarcity of deep-ocean hydrographic data. This study uses numerous multichannel seismic reflection sections, combined with a reanalysis hydrographic product, to identify eddies and analyze their morphological parameters, spatial distribution, propagation, and evolution in the study area. A total of 13 eddies, identified as subsurface submesoscale eddies, exhibit horizontal scales of 12–80 km, thicknesses of 130–540 m, and center depths of 150–1000 m. Their cores are characterized by moderate-to-weak reflections and bounded by elliptical or bowl-shaped edges, suggesting that the eddies have adequately trapped the surrounding water masses. The spatiotemporal evolution of two eddies is captured by several adjacent seismic lines acquired within a few days. During the seismic acquisition period, the horizontal scale, thicknesses, and shape of the shoreward part of the eddy changed prominently, whereas the characteristics of its seaward part remained relatively stable. This contrast highlights the important role of topography in governing the spatiotemporal distribution of the subsurface eddies in the northwestern SCS. These findings provide valuable insights into the evolution of subsurface eddies in the northwestern SCS and contribute to a more comprehensive understanding of eddy–topography interactions.