Using enstrophy advection as a diagnostic to identify blockingregime transition

This article proposes a method to identify blocking onset and decay by means of two stability indicators: enstrophy advection and its integral. The key to this technique is the use of local Lyapunov exponents for the barotropic vorticity equation, which can be approximated by the integral of enstrophy (IRE) over a fixed, finite region. The IRE can then be viewed as a measure of stability. However, by differentiating the IRE with respect to time, two measures of stability can be derived to assess blocking onset and decay: (i) the integral of enstrophy advection (DIRE), for which a time series is used to assess stability, and (ii) enstrophy advection, for which contours are plotted in conjunction with 500 hPa heights to locate blocking. One year’s worth of Northern Hemisphere blocking events from July 2011–July 2012 are studied to demonstrate that the integral of enstrophy advection is a useful diagnostic. In particular, time series of IRE and DIRE for four of the blocking cases are presented, while contour plots of enstrophy advection for one case are presented. In all cases studied, the diagnostics were seen to detect the instability in an incipient blocking event and in its decay.