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 , 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

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.

Key Points

• First high-resolution in situ observations into a mesoscale dipole formed by a ring and a cyclonic eddy in the Mozambique Channel
• The dipole central jet swiftly transports shelf properties offshore, causing patchiness and layering in the cyclonic eddy
• Vertical velocities from the omega equation reveal the impacts of the dipole and of a smaller meander in the front

Plain Language Summary

During the RESILIENCE cruise aboard the R/V Marion Dufresne II from April 19 to 24 May 2022, we conducted detailed observations in the Mozambique Channel to study a specific ocean feature called a mesoscale dipole. It consists of two large swirling oceanic structures: an anticyclonic ring and a cyclonic eddy, with a strong current flowing to the southeast between them. Using continuous observing techniques from towed instruments, we measured ocean properties and how water moved within this dipole. We found that the anticyclonic ring extends over 250 km and had low levels of chlorophyll and uniform waters. In contrast, the cyclonic eddy, which is smaller, had higher chlorophyll levels and more varied salinity. The central current between these two features moved at speeds up to 130 cm

and is responsible for significant water transport away from the Mozambican coast. We revealed that smaller structures, rather than the dipole itself, drove most vertical water movements. The cyclonic eddy showed complex salinity patterns due to the presence of different water masses spiraling around each other. These findings help us understand how such ocean features affect water movement and will guide future research into their impact on the marine environment.