The ocean is a complex flow system hosting motions across a wide range of spatial scales. The forward energy cascade, which governs the transfer of kinetic energy from larger to smaller scales, provides the pathway for the energy's ultimate dissipation and is thus crucial for closing the ocean energy budget. Idealized and regional studies suggest that interactions between oceanic eddies and internal waves may be an important driver of the forward cascade. Yet the global significance of these interactions, and their key underpinning mechanisms, remain largely uncharted. Here, we analyze a realistic submesoscale eddy- and internal wave-permitting global ocean simulation to show that eddy-internal wave interactions are a significant player in the forward cascade. While the primary contributors to the forward cascade are self-type interactions (i.e., eddy-eddy interactions in winter and wave-wave interactions in summer), such self-type mechanisms can only cascade energy within their respective energy reservoir, leaving the forward cascade of the other reservoir unaccounted for. It is the cross-type eddy–internal wave interactions that fill this gap, dominating the cascade of internal wave energy in winter and of eddy energy in summer. Our results thus identify the complementary role of eddy-internal wave interactions in the forward cascade, demonstrating their importance in the dissipative branch of the global ocean's energy cycle.

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