Abstract

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.