Diagnosing the Thickness-Weighted Averaged Eddy-Mean Flow Interaction From an Eddying North Atlantic Ensemble: The Eliassen-Palm Flux

The thickness-weighted average (TWA) framework, which treats the residual-mean flow as the prognostic variable, provides a clear theoretical formulation of the eddy feedback onto the residual-mean flow. The averaging operator involved in the TWA framework, although in theory being an ensemble mean, in practice has often been approximated by a temporal mean. Here, we analyze an ensemble of North Atlantic simulations at mesoscale-permitting resolution (1/12 degrees). We therefore recognize means and eddies in terms of ensemble means and fluctuations about those means. The ensemble dimension being orthogonal to the temporal and spatial dimensions negates the necessity for an arbitrary temporal or spatial scale in defining the eddies. Eddy-mean flow feedbacks are encapsulated in the Eliassen-Palm (E-P) flux tensor and its convergence indicates that eddy momentum fluxes dominate in the separated Gulf Stream. The eddies can be interpreted to contribute to the zonal meandering of the Gulf Stream and a northward migration of it in the meridional direction. Downstream of the separated Gulf Stream in the North Atlantic Current region, the interfacial form stress convergence becomes leading order in the E-P flux convergence.

Automated summary

The thickness-weighted average (TWA) framework provides a theoretical formulation of the eddy feedback onto the residual-mean flow by treating the residual-mean flow as the prognostic variable. The ensemble dimension in an ensemble of North Atlantic simulations allows for a more accurate understanding of means and eddies. The Eliassen-Palm flux tensor captures the eddy-mean flow feedback, with eddy momentum fluxes dominating in the separated Gulf Stream.