Isopycnal Pathway for Small‐Particle Export in a Decaying Cyclonic Eddy Revealed by a BGC‐Argo Float

Abstract

Mesoscale eddies influence carbon export through production, subduction, mixing, and particle redistribution. In this study, a BGC‐Argo float observed a deep particle layer in the twilight zone during the decay of a cyclonic eddy in the Northwest Pacific. The layer contained pigment‐depleted particles, remained confined to a narrow density range near 26 kg m ⁻³ , and often exhibited particulate backscattering comparable to or greater than that of the overlying subsurface biomass maximum. Hydrographic and oxygen properties showed no clear signatures of recent subduction associated with the layer. An idealized model based on a friction–Coriolis balance produces outward flow below the pycnocline, which can redirect slowly sinking particles along sloping isopycnals toward deeper peripheral waters. The results suggest that eddy relaxation provides a potentially important pathway for exporting slowly sinking particles into the twilight zone—a cyclonic counterpart to the anticyclonic “wineglass effect.”