A new atmospheric background state to diagnose local waveguidability

Rossby waveguides constrain the propagation of Rossby waves by ducting eddy activity along paths of enhanced waveguidability in the atmosphere. Conceptually, waveguidability is the property of an eddy-free background state on which waves exist as perturbations. Because eddies are always present in the atmosphere, a procedure is required to separate the waves from the background. The choice of procedure is of practical importance when diagnosing waveguides. For example, a zonal-mean background state is easy to compute from data and often used, but does not allow for longitudinal variation of waveguidability. It has also been shown to exhibit waveguide artifacts in the presence of finite-amplitude eddies.

We introduce a new procedure to obtain an eddy-free background state for the analysis of waveguides in the atmosphere. It utilizes a redistribution (so-called zonalization) of Ertel potential vorticity on isentropes to remove eddies, including those of finite amplitude, while retaining local information. Because the procedure can be applied to instantaneous data without a need for temporal aggregation, it is suitable for causal analyses and can be applied to forecast data without lead time restrictions. Our construction is based on the "slowly evolving background state" by Nakamura and Solomon (2011) and Methven and Berrisford (2015), with additions and approximations to achieve a pragmatic compromise between theoretical grounding, usability and ease of computation.

The effectiveness of the procedure to meaningfully separate waves and the background state is illustrated with reanalysis data. Rossby waveguides are diagnosed from the background-state PV fields with a gradient-based metric. We show that our localized procedure leads to regional differences in the diagnosed waveguidability and discuss the existence of circumglobal waveguides.