This is hot off the press
"Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer"
"Two mechanisms have recently been proposed that could provoke such patterns: (i) a weakening of the zonal mean jets and (ii) an amplification of quasi-stationary waves by resonance between free and forced waves in midlatitude waveguides."
"The spread in the zonal mean zonal wind reflects,in the first place, the seasonal cycle with weaker jets and hence slower wave propagation in summertime. Therefore, in boreal summer, a significant fraction of synoptic waves, notably with k = 6, have a phase speed close to zero (quasi-stationary) or are even traveling westward."
Interesting the mention of waveguides.