Quasi-geostrophic limiting dynamics and energetics of the LANS-$α$ model

The Lagrangian-Averaged Navier-Stokes-$α$ (LANS-$α$) model, a turbulence closure scheme based on energy-conserving modifications to nonlinear advection, can produce more energetic simulations than standard models, leading to improved fidelity (e.g., in ocean models). However, comprehensive understanding of the mechanism driving this energetic enhancement has proven elusive. To address this gap, we derive the fast quasi-geostrophic limit of the three-dimensional, stably-stratified LANS-$α$ equations. This provides both the slow, balanced flow and the leading-order fast wave dynamics. Analysis of these wave dynamics suggests that an explanation for the energetic enhancement lies in the dual role of the smoothing parameter itself: increasing $α$ regularizes the dynamics and simultaneously generates a robust landscape of wave-wave resonant interactions. Direct numerical simulations show that $α$ plays an analogous role to that of the Burger number ($Bu$) in governing the partition of energy between slow and fast modes -- and consequently, the timescale of geostrophic adjustment -- but with key differences. Increasing $α$, regardless of the relative strengths of rotation and stratification, extends the lifetime of wave energy by delaying the dominance of the slow modes. We find that the creation of an energy pathway only involving fast waves is a universal outcome of the regularization across all values of $Bu$, contrasting with a disruption of slow-fast interactions that is most impactful only in the $Bu=1$ case. These insights unify the LANS-$α$ model's characteristic energetic enhancement with, in some cases, its known numerical stiffness, identifying potential pathways to mitigate stability issues hindering the broader application of LANS-$α$-type models.