Characterization of Blocking Episodes in the North Atlantic and Pacific with the Finite-amplitude Local Wave Activity Formalism

Abstract:

The Finite-amplitude Local Wave Activity (LWA) is a suitable candidate of blocking index for two reasons. Firstly, it measures the degree of deviation from its zonal symmetry of the quasi-geostrophic potential vorticity (QGPV) field contours and thus highlights the location of blocking episodes on a two-dimensional map. Secondly, a large value of LWA directly implies an instantaneous local deceleration of zonal wind in the conservative limit.

We will present the climatology of LWA computed from the QGPV on the tropopause level (240hPa) obtained from ERA-Interim reanalysis data. With additional criteria in temporal- and spatial- coherences, we define a blocking index based on LWA and compare our results with the statistics obtained from existing blocking indices (e.g. Tyrlis and Hoskins (2008), Barnes et al (2012)). Specifically, we analyze the wave activity budget of individual and composite of blocking events over the North Atlantic and Pacific sectors, and partition the growth in LWA associated with the onset of blocking into advection and diabatic heating.