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

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 (Journal of Physical Oceanography 20 (1990) 150–155) 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 Duchêne (Bianchini and Duchêne (2024)) on the continuously stratified system. This requires to compile a dictionary that translates continuous operations (differentiation, integration, etc.) into corresponding discrete operations.