Surface anticyclones connected to the ridge of an upper-tropospheric
Rossby wave are the dynamical drivers of mid-latitude summer heatwaves.
It is, however, unclear to which extent an anomalously low zonal phase
speed of the wave in the upper troposphere is necessary for persistent
temperature extremes at the surface. Here, we use spectral analysis to
estimate a categorical phase speed for synoptic-scale waves. A composite
analysis of ERA5 reanalysis data reveals how a meridional shift in the
Rossby wave packet envelope associated with a change in phase speed
alters the geographically phase-locked stationary wave pattern. In both
composites for amplified low or high phase speed waves, respectively,
the ridges and troughs of these temporal-mean wave trains show enhanced
and reduced heatwave frequency. The phase speed of synoptic-scale waves
is, hence, crucial for where, but less important for whether heatwaves
occur.