Three-dimensional Pathways of the Salinity-Driven Overturning Circulation in the Northwestern Indian Ocean

The Red Sea and Arabian Gulf sustain a salinity-driven overturning circulation linking arid marginal seas to the northwestern Indian Ocean, yet its three-dimensional structure and closure are poorly constrained. Using 3D Lagrangian particle tracking in a well-validated, eddy-resolving ocean model, we reconstruct the overturning loop and reveal a vertical partitioning of the outflow. Export splits into a rapid surface pathway and a dominant intermediate pathway (100–500 m) with multi-year transit times; the subsurface branch carries most of the salinity anomaly and acts as the reservoir transmitting marginal-sea buoyancy forcing into the Arabian Sea. Backward tracking closes the loop by identifying inverse-estuary inflows drawn from the southern Somali margin and the central Arabian Sea. These findings establish salinity-driven overturning as a depth-structured circulation with timescale separation, providing a physical framework for understanding how marginal seas regulate intermediate hydrography, ventilation, and their broader influence on the regional circulation and biogeochemical environment.