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Abstract:
A persistent high-pressure ridge in the upper troposphere is a vital ingredient for mid-latitude temperature extremes. Changes in the basic state for stationary synoptic-scale waves are, therefore, commonly discussed to understand the response of dynamical heatwave drivers to global warming. However, the relationship between upper-tropospheric Rossby wave phase speed and surface extremes is not fully resolved. Combining a complex principal component analysis of reanalysis data with idealized model experiments, we relate Rossby wave phase speed to baroclinic wave amplitudes and differences in the basic state. Specifically, we find an equatorward shift for both synoptic-scale wave amplitude and the zonal-mean westerly jet during low-phase speed episodes, but an enhanced Atlantic storm track and a broadened zonal-mean jet during high-phase speed episodes. This analysis allows us to understand how the occurrence and location of large monthly-mean meridional wind anomalies and increased frequencies of surface extremes depend on the dominant synoptic-scale wave phase speed.
