Vortex–Vortex Interactions for the Maintenance of Blocking. Part I: The Selective Absorption Mechanism and a Case Study

Abstract

A new block maintenance mechanism, the selective absorption mechanism (SAM), is proposed. According to this mechanism, which is based on vortex-vortex interactions (i.e., the interactions between a blocking anticyclone and synoptic eddies with the same polarity), a blocking anticyclone actively and selectively absorbs synoptic anticyclones (strictly, air parcels with low potential vorticity). The blocking anticyclone, which is thus supplied with low potential vorticity of the synoptic anticyclones, can subsist for a prolonged period, withstanding dissipation.The SAM was verified in a case study through trajectory analysis. Ten actual cases of blocking were examined. Trajectories were calculated by tracing parcels originating from synoptic anticyclones and cyclones located upstream of the blocking. Parcels starting from anticyclones were attracted to and absorbed by the blocking anticyclone, whereas parcels from cyclones were repelled by the blocking anticyclone and attracted to the blocking cyclone, if one was present.The results show that the SAM is effective in the maintenance of observed cases of blocking. In addition, the uniqueness and distinction of the SAM from other previously proposed maintenance mechanisms are discussed.

Importance of latent heat release in ascending air streams for atmospheric blocking

“…In agreement with classical blocking theories 12,13,14,15 , quasi-adiabatic flow contributes to blocking through the isentropic advection of air masses with mainly low PV. Of equal importance is the diabatic input of low PV entering the blocking region via ascending air streams fuelled by the release of latent heat during cloud formation.…”

“…Planetary-scale theories focus on large-scale waves and their interaction with the topography. Local theories emphasize the role of wave breaking 16 and the isentropic advection of air with low potential vorticity (PV) into the blocking region 12,13,14,15 . All these theories are essentially based on dry atmospheric dynamics, and diabatic processes have only been considered in an indirect way, e.g., via the triggering of Rossby waves by tropical convection 13 .…”

“…Kinematic backward trajectories using three-dimensional reanalysis winds 26 are calculated to reconstruct the history of these air masses and determine the sources of the low PV air. By tracing the evolution of θ and PV along more than 10 8 trajectories during the 21 years, we can discriminate between adiabatic PV advection (as described in the classical blocking theories 12,13,14,15 ) and crossisentropic transport due to in-cloud latent heating, and climatologically assess the frequency of the two mechanisms. The robustness of our results with respect to the blocking identification is evaluated through comparison with a more classical blocking index based on geopotential height 27 .…”

“…This category corresponds to the mechanism proposed in ref. 15, according to which the blocking anomaly is maintained by selectively absorbing other synoptic-scale anticyclones. The trajectories exhibit only weak vertical and a comparatively small meridional displacement ( Fig.…”

Heavy Snowfall in Kanto and on the Pacific Ocean Side of Northern Japan Associated with Western Pacific Blocking

“…PV is advantageous for analyzing blocking (e.g., Yamazaki and Itoh 2013). Figure 1b shows the PV anomalies averaged over the 4−16 February 2014 blocking event.…”