Influence of Lee Wave Breaking on Far-Field Mixing in the Deep Ocean

Breaking of internal lee waves generated by flow-topography interaction is an important driving mechanism for abyssal mixing. By assuming that lee-wave-driven mixing is a local process, direct measurements in the past mainly focused on the water column over rough topography. In this study, a non-local role of lee waves on remote mixing is investigated by using mooring observations in the northwestern Philippine Basin and a series of 2-dimensional high-resolution numerical simulations. We find that the breaking of lee waves can generate near-inertial waves (NIWs) which can propagate away from their generation sites. The NIWs have strong vertical shear which can significantly elevate turbulent dissipation and mixing over remote uneven seafloor. Based on the topography perturbations numerical experiments, we find that for the remote gentle topography with inverse Froude number (Fr-1) between 0 and 0.5, the arrival of upstream NIWs can elevate the near-bottom layer (0-1,000 m above bottom) dissipation rate and vertical diffusivity by 9.2 and 10.4 times, respectively at 30-50 km away from the NIWs source. For the remote rough topography with Fr-1 between 0 and 4, the corresponding increases are 3.5 and 4.4 times, respectively. The mechanism of upstream NIWs to elevate the downstream dissipation and mixing is through strengthening the shear instability over the far-field topography, which is associated with NIWs' interactions with the topography and the generated lee waves therein. The results of this study will provide a foundation for improving the mixing parameterizations associated with current-topography interactions. Lee waves are waves inside the ocean that are generated when a strong current goes over rough seafloor. The breaking of lee waves can cause a lot of turbulence and stir up the seawater significantly (i.e., ocean mixing process) near the seafloor. Directed by linear lee wave theory, existing measurements mainly focus on the water column over rough topography, and researchers rarely address the impact of lee waves to remote areas. Here, we explore the remote impact of lee waves in the northwestern Philippine Basin using in situ observations and two-dimensional high-resolution numerical experiments. We find that when the strong background flow goes over rough seamounts, the breaking of lee waves not only causes turbulence locally but also excites a unique type of internal waves called near-inertial waves (NIWs). The NIWs have large vertical shear, which can travel away and cause a more dissipative waves field over remote rugged seafloor. Further analysis suggests that the NIWs interact with the remote seafloor and the generated lee waves therein, which strengthen the shear instability of internal waves is the reason why the remote dissipation and mixing field is enhanced. Generation of near-inertial waves (NIWs) through interaction between bottom-reaching flow and rough topography is observed The NIWs are generated by lee wave breaking which can leave the topography after being generated The NIWs have large vertical shear which can enhance mixing remotely by inducing shear instability