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Abstract

Thermohaline staircases (TS) are a prominent feature in the Arctic Ocean basins at mid-depths, where they overlie the warm Atlantic water mass. Internal waves (IW) able to propagate through TS might be more likely to break, enhancing vertical heat fluxes that could bring warm water to the surface. However, only large horizontal waves are likely to propagate since TS act as a filter for IWs. As wind-driven IW activity is expected to increase with the decrease of sea ice cover, it is essential to understand how TS affect IW propagation. We carried out numerical simulations to study IW transmission through a density staircase. The code used solves the two-dimensional nonlinear incompressible Boussinesq Navier-Stokes equation, evolving in time vorticity and buoyancy. The model initiates with a small amplitude vertical wave packet. We performed several simulations with varied relative wavenumbers and varying the step thickness and the step number of the ST. We discovered a novel transmission mechanism in which an incoming IW excites interfacial waves (ITW) within the TS that slowly leak energy above and below the TS. ITWs have a vertical baroclinic structure and travel along the TS. IW transmission theory predicts perfect wave transmission peaks only for IWs and ITWs in resonance. Instead, we found a smooth transition between perfect reflection to perfect transmission. Although we used IWs with short wavelengths in our simulations, long horizontal IWs might also generate ITWs within the TS after the transmission process.