Spectral Causal Analysis of Air-Sea Coupling Feedbacks through the Mesoscale

Ocean–atmosphere interactions play a crucial role in global climate & weather dynamics, yet our understanding of the interplay between mesoscale thermal and current air–sea feedbacks remains incomplete. The strength of this coupling influences heat and kinetic energy fluxes at different length-scales and locations across the global oceans. Eddy-parameterising climate models can resolve the large-scale energy input into the ocean, which is then transferred into eddy kinetic energy through parameterised hydrodynamic instabilities. These models, however, struggle to accurately capture the spatial patterns of energy transfer, both kinetic and thermal, back into the atmosphere from the ocean mesoscales. Here, we present insight from a mesoscale-resolving global coupled climate model that elucidates the physical mechanisms driving air–sea current and thermal feedbacks at the mesoscale, in comparison to the large-scale air–sea coupling. Spectral analysis further reveals how these feedbacks are suppressed when either the ocean or atmosphere fails to resolve a local critical coupling length-scale. Extending beyond these traditional regression-based methods, we employ a novel causal analysis framework to uncover a hybrid thermal–current mesoscale feedback which enhances kinetic energy injection directly into ocean mesoscales. This mechanism involves localised heat fluxes enhancing vertical convection and downward momentum transport within the atmospheric boundary layer, leading to increased local wind stress and consequently wind work into eddy kinetic energy. These results highlight the critical role of mesoscale air–sea coupling in accurately representing the energetic ocean mesoscales, which in turn influence the global oceanic circulation and climate.