Storm swamp: A key region for concurrent blocking and heat wave events with quasi-stationary Rossby waves amplification

Atmospheric blocking often causes weather extremes in the mid-latitudes at certain preferred geographic locations. Typical mid-latitude storms are generated in the storm track where the baroclinicity is strong, which supports the rapid propagation of Rossby waves. However, concurrent blocking and extreme weather events often occur further downstream where the baroclinicity is rather weak and propagation of Rossby wave packets is largely stalled. We refer to this region as the “Storm Swamp” for its capability of slowing down and piling up storms which would ultimately lead to blocks.

During summertime, a potential cause of increased blocks and weather extremes around the storm swamp is the amplification of atmospheric planetary waves. Previous literature suggests various potential connections between waveguides, climatological basic state, and quasi-stationary Rossby waves. However, the dynamical mechanism linking summertime high-amplitude quasi-stationary Rossby waves to atmospheric blocking, especially for blocking persistence and frequency, remains unclear. This study aims to assess the role of basic states providing favorable dynamic conditions for high-amplitude RWPs on blocking statistics. To quantify the blocking frequency and persistence response to basic states providing dynamical conditions for RWPs amplification, we perform idealized simulations with the Held and Suarez dry GCM and full-physics CAM simulations with a bias correction of the climatological basic state to faithfully set up the storm swamp. We detect blocking using the local wave activity (LWA) approach and quantify the RWPs amplification by calculating the time-space phase speed and group velocity for each simulation. By focusing on the blocking statistics, we will conclude the relative role of the different basic states of the storm swamp that will lead to amplification events associated with more severe blocking events.