Defining zonally varying, eddy-free jets to diagnose storm-track dynamics

Most of the northern-hemispheric, midlatitude cyclones develop, propagate, and decay within the storm tracks of the North Atlantic and the North Pacific. The variability within these storm tracks, along with associated blocking near their downstream end, governs a large part of the climate and weather extremes of the northern-hemispheric midlatitudes. Dynamically, the storm tracks are a coupled jet-eddy system: synoptic-scale Rossby waves and associated surface pressure systems (the ‘eddies’) propagate and evolve along the midlatitude jet. At the same time, the jet is accelerated by momentum fluxes of the eddies, hence the notion of the midlatitude jet as an ‘eddy-driven’ jet. Accurate diagnosis of this coupled storm-track dynamics remains central to understanding midlatitude variability, extremes, predictability, and climate change responses.

Traditional definitions of ‘jet’ and ‘eddies’ by spatio-temporal filtering of the underlying physical fields do not provide a clear separation between the background jet and the eddies, which limits physical interpretation on a conceptual level. More recent developments based on adiabatic re-arrangement of potential-vorticity contours, so-called zonalized background states, provide truly eddy-free background states and associated finite-amplitude wave activity to describe the eddies. Zonalized background states, however, inherently exhibit zonal symmetry, which limits their applicability to (localized) storm-track dynamics. A recent extension called ‘rolling zonalization’ allows for zonal variability of the background jet but provides only limited control over the desired separation of spatio-temporal scales.

In this contribution, we introduce a filtering method that provides full control of the spatio-temporal characteristics in the definition of an eddy-free background state. Challenges and solutions to defining finite-amplitude wave activity metrics associated with such a background are discussed. Arguably, the new framework enables a local perspective on storm-track dynamics while retaining many of the conceptual and theoretically appealing features of the zonally symmetric finite-amplitude wave-activity framework. We conclude with an illustration of first applications to describe storm-track characteristics on different time scales and muse on the prospects of the new framework to advance our understanding of storm-track dynamics.