Characterizing the Central Structure of a Mesoscale Eddy-Ring Dipole in the Mozambique Channel From In Situ Observations

Abstract

During the RESILIENCE cruise aboard the R/V Marion Dufresne II (April 19–24 May 2022), a high-resolution in situ observation campaign investigated a mesoscale dipole in the Mozambique Channel, composed of a large anticyclonic ring and a cyclonic eddy. Using an innovative adaptive sampling strategy to track its movement, we employed continuous observing systems, including a Moving Vessel Profiler and Acoustic Doppler Current Profilers, to capture high-resolution vertical sections. The results revealed a distinct dipolar structure: The 250 km-wide anticyclonic ring featured low chlorophyll and homogeneous waters, while the smaller cyclonic eddy exhibited higher chlorophyll concentrations and pronounced salinity variations. These include patches, vertically stacked layers, and filaments, reflecting a mix of contrasted water masses from the southern Mozambique Channel and the Sofala Bank. A central jet between the eddies exhibited horizontal velocities up to 130 cm $s^{-1}$, facilitating significant offshore transport exceeding 10 Sverdrups in the upper 250 m and emphasizing the dipole's role in eastward water movement. Vertical velocities, derived from the Quasi-Geostrophic Omega equation, highlighted the influence of smaller-scale structures in driving vertical motions, reaching 40 m ${\text{day}}^{-1}$ at depth. Lagrangian particle trajectories revealed the dipole's spiraling structure and its connectivity to coastal waters. These findings show that Mozambique Eddy-Ring Dipoles efficiently transport properties from the continental shelf to the open ocean, enhancing regional ecosystem connectivity. This work provides new insights into their biogeochemical, biological and ecological significance, challenging traditional cyclonic/anticyclonic eddy paradigms, and setting the foundation for future studies on mesoscale dipoles in the region.