Nakamura, H., and J. M. Wallace, 1993: Synoptic behavior of baroclinic eddies during the blocking onset. Monthly Weather Review, 121(7), 1892-1903.

Abstract: Synoptic behavior of individual baroclinic eddies in the course of their interactions with amplifying blocking anticyclones is examined, based upon a 30-year record of the tropospheric circulation in the Northern Hemisphere winter. High-pass-filtered as well as unfiltered fields of geopotential height and potential vorticity were composited relative to the onset of the five different types of blocking patterns. Before the compositing, the entire sequence of the fields was slightly shifted in time and space in such a way that the strongest baroclinic eddy, during the onset of each blocking event occupies a prescribed position in the upstream storm track when it reaches its maximum intensity. Since this shifting considerably reduces the cancellation between the individual high-frequency migratory eddies, this type of compositing can present a more synoptically oriented view of those interactions than conventional compositing.

In our composite results, one or two pairs of cyclonic and anticyclonic eddies associated with baroclinic waves appear to interact with the growing blocking anticyclone within its onset period that lasts about a week. These eddies become less baroclinic as they approach the block. Each anticyclonic eddy seems to advect low potential vorticity air from lower latitudes, which becomes entrained into the block. The stronger anticyclonic, migratory eddies at the tropopause level, in association with short-wave ridges, undergo significant distortion, as the eddies interact with the preexisting ridge and finally merge with it. In contrast, during the onset of a blocking ridge located much closer to the jet-stream axis, the migratory synoptic-scale eddies tend to travel around the ridge without rapid decay or strong meridional stretching, while yielding the poleward flux of westerly momentum that acts to reinforce that amplifying ridge.