The scale and stability analysis of an unusually prolonged and a moderately extreme blocking event occurring in Gulf of Alaska during the month of August 2004 is presented. The observed 500-hPa geopotential height is decomposed into the blocking area D, averaged planetary-scale and the synoptic-scale eddy wavelengths and the time variability of both the contributions is analyzed during the onset, the maintenance and the decay stages of the blocking event. It is found that the given blocking event scale dynamics is dominated by planetary-scale wavelengths only.

Stability analysis of the blocking events is performed, under the assumption of barotropic flow, using the following two stability indicators: i) max |„¤ƒé|, and ii) Here, ƒé is geosptrophic stream function, and ƒÇ = - „gu/ „gy, in usual meteorological notation. A comparison of the time variability of the two stability indicators over the entire life cycle of the selected blocking event with earlier works performing the synoptic and dynamics studies shows that the three stability indicators can be used as climatologically reliable stability indicators giving useful insight into the stability of the flow attending the blocking event. It is noticed that in the scale dependent flow, the scale that dominates during the mature stage of the blocking event determines the stability of the flow during the blocking, and that the blocking is relatively more stable state than the more frequent zonal flow, irrespective of which scale dominates the flow during blocking.

Interplay of both the contributions is found to be the case during the three stages of the blocking when their relative role is assessed in terms of the blocking area integrated enstrophy (stability indicator i)) and the gradient of stream function of the flow (stability indicator ii)). It is noticed that the planetary-scale environment become unstable during the onset and then stabilizes during the peak activity/mature stage of the blocking, whereas the synoptic-scale wavelengths play a dominant role in destabilizing the planetary-scale flow during peak activity/maintenance of the blocking life cycle thus initiating the blocking decay. The results obtained in this study are then compared with the results from a 3 year (2002-2004) NH climatology to provide a perspective for the scale and stability analysis presented here.