Local Finite-amplitude Rossby Wave Activity as a Diagnostic of Anomalous Weather Regimes

We propose the use of Local Finite-amplitude Rossby Wave Activity (LWA), an extension to the Finite-amplitude Rossby Wave Activity (FAWA) proposed by Nakamura and Zhu (2010), to diagnose anomalous weather regimes.

LWA measures the meridional displacement of potential vorticity contour from its equivalent latitude at each longitude, and thus it is local (Eulerian) zonally but nonlocal (Lagrangian) meridionally. The zonal average of LWA recovers FAWA. Compared with the previously derived Eulerian Impulse-Casimir wave activity (e.g. Haynes 1988), LWA tends to be less filamentary and it emphasizes large isolated vortices.

We examine the budget of LWA for finite-amplitude Rossby waves with the barotropic vorticity equation to quantify the relative roles of radiation stress of the Rossby waves and the zonal advection of LWA. We also propose a local non-acceleration theorem in the Wentzel–Kramers–Brillouin (WKB) sense, in which the sum of the phase averaged LWA and zonal wind remains unchanged in the conservative limit, thereby implying the local deceleration of the zonal flow by the finite-amplitude wave.

We will demonstrate how this diagnostic can be applied to meteorological data by studying a blocking episode in the upper levels, using quasi-geostrophic potential vorticity as the tracer.