Comment on ‘Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern’

The depiction of Rossby wave patterns in Hovmöller diagrams can be severely altered if low-pass filtering is applied to the data. Due to filtering, transient waves may appear stationary, and unphysical wave properties may emerge. This observation is particularly relevant in the context of the discussion about the existence of circumglobal, quasi-stationary Rossby waves related to extreme weather events. This commentary shows that the wavenumber-7 Rossby wave pattern of early Summer 2018 discussed by Kornhuber et al (2019 Environ. Res. Lett. 14 054002) (K19) emerged from a low-pass filtered version of an Hovmöller diagram, which displayed an unrealistic and incomplete representation of the hemispheric Rossby wave pattern. The use of an unfiltered Hovmöller diagram, on the other hand, does not support the claimed circumglobal extent and quasi-stationary propagation of the Rossby wave pattern claimed by K19.