Interactions between Internal Wave Modes and the California Current System: Differences between Internal Tides and Near-Inertial Waves

Internal waves (IWs) play a key role in the maintenance of the global ocean circulation by facilitating the downscale transfer of energy to mixing scales in the ocean interior.We investigate how IWs cause a forward energy cascade through interactions with background flows and topography in the California Current System (CCS), focusing on the differences between internal tides (ITs) and near-inertial waves (NIWs).We quantify cross-scale energy transfers by projecting the output of a realistic simulation of the CCS onto vertical modes.By employing accurate modal structures derived from buoyancy frequency profiles computed with the adiabatic leveling method, we show that including wave-background flow-topography interaction terms in the IW modal energy balance reduces the modal residual dissipation variance.We decompose the modal coupling matrices into symmetric and antisymmetric components to compute IW-background flow direct energy exchanges and IW scattering by the background flows and/or topography, respectively.After summing over all modes, background flows supply energy to both ITs and NIWs.While topographic scattering is the main driver of both the IT and NIW forward energy cascade in shallow/ rough seafloor regions, background flow-induced scattering dominates in deep/ smooth seafloor areas.In addition, a larger fraction of low-mode NIW energy input is scattered to high modes compared to ITs, implying that NIWs play the main role in transferring IW energy to small scales and ultimately to dissipation.Finally, whereas IT cross-scale energy transfers show no sensitivity to background vorticity, NIW energy cascades to smaller scales primarily within anticyclones, while NIWs gain energy predominantly from cyclonic and convergent fronts and filaments.SIGNIFICANCE STATEMENT: Driven by large-scale wind forcing and the gravitational pull of the moon and sun, internal waves are ubiquitous throughout the global ocean.Understanding what roles internal waves play in how energy is cascaded from large to small scales in the ocean has been a prominent topic.We investigate the internal wave energy budget in the California Current System to identify the mechanisms that supply energy to internal waves and drive their energy to mixing scales.We identify that the (sub)mesoscale background flows supply energy to the internal waves and facilitate their downscale transfer in the open ocean where the seafloor is relatively smooth.In contrast, in shallow and rough areas, the downscale energy transfer is mostly due to topographic scattering.