Energy dissipation and mixing within surface- and sub-surface intensified Agulhas Rings

Agulhas Rings are anticyclonic, warm-core eddies that play a crucial role in the exchange of water masses between the Indian and Atlantic Oceans. Formed at the Agulhas Retroflection near the southern tip of Africa, these rings constitute an essential component of the global thermohaline circulation, transporting ocean properties such as heat, salt, and energy. Their movement and property transfer significantly influence regional climate systems and large-scale ocean dynamics. It is well established that Agulhas Rings differ significantly in their characteristics. However, certain types, such as subsurface-intensified Agulhas Rings, remain remarkably understudied and are poorly represented in most ocean models. Investigations of these features often require high-resolution observational and modelling approaches. In this study, we present high-resolution glider observations from March/April 2021 and 2022 of two types of Agulhas Rings (surface- and subsurface-intensified), highlighting their distinct characteristics. Both glider campaigns utilized a microstructure probe to enable detailed observations of energy dissipation and diapycnal mixing. Our analysis reveals that major differences between the two eddy types occur near the eddy centre, where the subsurface-intensified Agulhas Ring exhibits elevated energy dissipation (log₁₀(ε)= -7 W kg^-1) and diapycnal mixing (log₁₀ (κ_ρ)= -4 m²s ^-1) beneath the surface mixed layer. Further analysis shows that mixing length scales (up to 18 km), are also elevated within the sub-surface intensified eddy, suggesting enhanced vertical and lateral distribution of ocean properties. These findings indicate a faster decay of the sub-surface intensified eddy and thus suggest a more local impact compared with the potentially longer-lived surface intensified eddy. By highlighting the distinct oceanic and energetic characteristics of surface- and subsurface-intensified Agulhas Rings, this study contributes to a better understanding of their role in influencing thermohaline structure and redistributing energy. These findings provide valuable insights that can support the development of more accurate parameterizations in future ocean models.