Vertical Mode and Cyclonic Eddy Encounters Govern Internal Tide Propagation and Intermodal Cascades: High-resolution Eddy Permitting Simulations

Abstract. The interaction between internal tides (ITs) and mesoscale features plays a key role in ocean energy dissipation. Understanding how IT energy is transformed in energetic western boundary regions remains a central challenge, particularly in regions of vigorous mesoscale activity.

To this aim, we apply vertical mode decompositions to the flow from high-resolution (3 km) NEMO-AMAZON36 simulations during September-December 2015. This study shows that the IT vertical mode and the precise point of IT-eddy encounter determine whether the IT energy propagates freely, deviates, or is trapped, and how topography and coherent eddies synergistically scatter energy between baroclinic modes off the Amazon shelf.

Three representative interaction cases, each captured in a separate 25 hour snapshot, were examined: undisturbed propagation until crossing the Ceará Rise seamount, interaction with a cyclonic eddy (CE) core, and interaction with a CE eastern periphery. The principal findings establish two points.

First, an IT response (propagation, deviation or scattering) is dually controlled by its vertical mode and the mesoscale encounter properties. In the absence of a strong eddy, the Mode-1 IT propagates as a coherent beam with a long propagation range (O (1100 km)). In the presence of a strong CE, however, the IT beams are disrupted, preventing sustained long-range transmission. Within the eddy core, the Mode-1 IT is coherently refracted northward (~35° relative to its northeastward incident direction) while maintaining high energy fluxes exceeding 200 W m⁻¹. At the CE periphery, Mode-1 is diffracted into two distinct branches, with one propagating northward (∼39°) and the other eastward (∼35°). In contrast, the IT higher modes are highly susceptible to blocking and trapping: Mode-2 energy, despite initial amplitudes comparable to Mode-1, is completely arrested at the CE-seamount interface, while Mode-3 remains weak (below 200 W m⁻¹) and less propagative.

Second, intermodal energy transfer is governed by a hierarchical synergy between the seamount and CE's background flow. The seamount drives a forward energy cascade (O (10⁻⁸ W m kg⁻¹)) from the Mode-1 IT to higher modes. In contrast, the CE's strong horizontal shear triggers a competing inverse energy cascade (O (10⁻⁸ W m kg⁻¹)) from the background flow to the IT modes. This interaction is critical for the extreme damping of Mode-2 and explains the observed redistribution of energy fluxes.

These results provide mechanistic insight into the fate of IT energy in complex oceanic environments and advance understanding of multi-scale ocean dynamics.