Journal Article

The Influence of Vertical Resolution on Internal Tide Energetics and Subsequent Effects on Underwater Acoustic Propagation

Chassignet, E. P., Buijsman, M. C., Arbic, B. K., Raja, K. J., Coelho, E. F., Schönau, M. C., Hiron, L., Bozec, A., Solano, M. S.

2025

Overview

Internal tide generation and breaking play a primary role in the vertical transport and mixing of heat and other properties in the ocean interior, thereby influencing climate regulation. Additionally, internal tides increase sound speed variability in the ocean, consequently impacting underwater acoustic propagation. With advancements in large‐scale ocean modeling capabilities, it is essential to assess the impact of higher model resolutions (horizontal and vertical) in representing internal tides. This study investigates the influence of vertical resolution on internal tide energetics and its subsequent effects on underwater acoustic propagation in the HYbrid Coordinate Ocean Model (HYCOM). An idealized configuration with a ridge, forced only by semidiurnal tides and having 1‐km horizontal grid‐spacing, is used to test two different vertical‐grid discretizations, defined based on the zero‐crossings of horizontal velocity eigenfunctions and the merging of consecutive layers, with seven distinct numbers of isopycnal layers, ranging from 8 to 128. Analyses reveal that increasing the number of layers up to 48 increases barotropic‐to‐baroclinic tidal conversion, available potential energy, and vertical kinetic energy, converging with higher layer counts. Vertical shear exhibits a similar pattern but converges at 96 layers. Increasing the number of isopycnal layers, up to 48, increases the available potential energy contained in high (third‐to‐eighth) tidal baroclinic modes. Finally, sound speed variability and acoustic parameters differ for simulations with less than 48 layers. Therefore, the study concludes that a minimum vertical resolution (48 layers in this case) is required in isopycnal models to accurately represent internal tide properties and associated underwater acoustic propagation. Plain Language Summary Internal tides are waves that undulate along interfaces between waters of different densities inside the ocean and form when tides interact with sloping topography. Like waves at the beach, internal tides can break and mix cold, deep water with warmer surface water, helping to spread heat throughout the water column. This mixing can reduce the amount of heat at the ocean surface, affecting ocean‐atmosphere interactions and influencing the climate. Additionally, internal tides can impact acoustic propagation in the ocean interior. In the past decade, realistic numerical simulations have been able to model internal tides. However, model resolution (horizontal and vertical) may impact internal tide properties. This study uses “simplified” simulations with different vertical layers and forced only by tides to investigate the impact of the number of layers on the properties of modeled internal tides and subsequent effects on acoustic propagation. We find that increasing the number of layers up to 48 layers increases the vertical velocity and vertical shear, which have the potential to increase mixing of water and impact the way sound propagates in the ocean interior. Therefore, we conclude that at least 48 layers are required to accurately represent internal tides and associated underwater acoustic propagation. Key Points Model vertical resolution impacts internal tide‐induced kinetic energy, available potential energy, dissipation, and vertical shear Increasing the number of isopycnal layers, up to 48, increases the available potential energy contained in high (third to eighth) vertical modes At least 48 isopycnal layers are necessary to minimize variability in sound speed and acoustic propagation caused by the number of layers

• Publisher: John Wiley & Sons, Inc,American Geophysical Union (AGU)
• Subject: Acoustics / Baroclinic mode / Barotropic mode / Diurnal variations / Eigenfunctions / Energy / General circulation models / Interfaces / Internal tide generation / Internal tides / isopycnal coordinates / Kinetic energy / Ocean circulation / Ocean models / Oceans / Potential energy / Propagation / Semidiurnal tides / Simulation / Sound velocity / Tidal dynamics / Tides / Underwater / underwater acoustic propagation / Variability / Velocity / Vertical advection / vertical resolution / Vertical shear