Evolution from Agulhas Ring to Mode-Water Eddy?

Agulhas Rings are anti-cyclonic warm-core eddies that originate from the interaction of the Agulhas current with the Antarctic Circumpolar Current (Agulhas Retroflection) at the southern tip of Africa. The Agulhas Rings are advected with the Benguela Current to the northwest and transport heat and salt into the South Atlantic Ocean and are thus affecting the ecosystem and the carbon cycle. The Walvis Ridge, which is located off the coast of Namibia, is a natural obstacle for the Agulhas Rings that are oftentimes unable to cross the ridge and thus remain in its vicinity until they dissolve. Due to the lack of long-term, high-resolution and sub-surface observations it is not well understood how the properties of these eddies evolve with time. We present long- and short-term observations from moorings (2022-2023) and two ocean glider campaigns (2022, 2023) as part of the SONETT I and SONETT II research expeditions near Walvis Bay. For the glider missions, automated adaptive sampling algorithms were developed to systematically improve the spatial and temporal resolution in the region of interest that is dynamically changing. Moreover, the gliders were equipped with microstructure probes for detailed energy dissipation measurements. In this study, we show how the eddy characteristics, such as temperature, salinity and oxygen, near Walvis Bay evolve with time and how these changes relate to the energy dissipation. Specifically in 2023, a deep glider (up to 1000 m) observed a unique eddy structure below the surface mixed layer that displayed subsurface eddy characteristics, but with a surface signal, indicating the characteristics of a mode-water eddy with a very distinct pattern of energy dissipation.