Approximating a continuously stratified hydrostatic system by the multi-layer shallow water system

TL;DR

This paper demonstrates that multi-layer shallow water systems can effectively approximate continuously stratified hydrostatic flows, with a proven convergence rate of 1/N^2, bridging discrete models and continuous fluid dynamics.

Contribution

It establishes a rigorous connection and convergence rate between multi-layer shallow water models and continuous stratified Euler equations under hydrostatic approximation.

Findings

Proves convergence of multi-layer models to continuous flows as layers increase

Provides a convergence rate of order 1/N^2

Develops energy estimates for stability of multi-layer systems

Abstract

In this article we consider the multi-layer shallow water system for the propagation of gravity waves in density-stratified flows, with additional terms introduced by the oceanographers Gent and McWilliams in order to take into account large-scale isopycnal diffusivity induced by small-scale unresolved eddies. We establish a bridge between the multi-layer shallow water system and the corresponding system for continuously stratified flows, that is the incompressible Euler equations with eddy-induced diffusivity under the hydrostatic approximation. Specifically we prove that, under an assumption of stable stratification, sufficiently regular solutions to the incompressible Euler equations can be approximated by solutions to multi-layer shallow water systems as the number of layers, $N$, increases. Moreover, we provide a convergence rate of order $1/N^2$. A key ingredient in the proof is a stability estimate for the multi-layer system which relies on suitable energy estimates mimicking the ones recently established by Bianchini and Duch\^ene on the continuously stratified system. This requires to compile a dictionary that translates continuous operations (differentiation, integration, etc.) into corresponding discrete operations.