Near-inertial wave critical layers over sloping bathymetry

TL;DR

This paper investigates a specific critical layer mechanism for near-inertial waves over sloping bathymetry, demonstrating how wave trapping enhances mixing in coastal and potentially open ocean settings through simulations and wave energetics analysis.

Contribution

It introduces the concept of slantwise critical layers for NIWs, linking wave trapping to enhanced mixing and providing simulation evidence of this process in coastal environments.

Findings

Critical layers trap NIWs, leading to increased mixing.

Wave trapping correlates with the strength of mixing.

Simulations reproduce wave propagation and mixing enhancement.

Abstract

This study describes a specific type of critical layer for near-inertial waves (NIWs) that forms when isopycnals run parallel to sloping bathymetry. Upon entering this slantwise critical layer, the group velocity of the waves decreases to zero and the NIWs become trapped and amplified, which can enhance mixing. A realistic simulation of anticyclonic eddies on the Texas-Louisiana shelf reveals that such critical layers can form where the eddies impinge onto the sloping bottom. Velocity shear bands in the simulation indicate that wind-forced NIWs are radiated downward from the surface in the eddies, bend upward near the bottom, and enter critical layers over the continental shelf, resulting in inertially-modulated enhanced mixing. Idealized simulations designed to capture this flow reproduce the wave propagation and enhanced mixing. The link between the enhanced mixing and wave trapping in the slantwise critical layer is made using ray-tracing and an analysis of the waves' energetics in the idealized simulations. An ensemble of simulations is performed spanning the relevant parameter space that demonstrates that the strength of the mixing is correlated with the degree to which NIWs are trapped in the critical layers. While the application here is for a shallow coastal setting, the mechanisms could be active in the open ocean as well where isopycnals align with bathymetry.