Fig. 2: Different spectral and feature tracking analysis methods used to identify ETCs, WCBs and ARs.

a Climatology of wintertime ETC track density (black contours) and 10-day high-pass filtered Eddy Kinetic Energy (coloured shading) adapted from from¹⁴ (their figure 1), b Amplitude of high frequency filtered water vapour flux for January 1992 from¹⁶, c Climatology of wintertime WCB frequency adapted from from²³ and d Climatology of AR frequency from¹⁹. Figures have been centred on 0^∘ longitude and, where possible, winter hemispheres are shown. Colourbars have been deliberately removed to focus on patterns of high feature frequency rather than magnitudes which depend on individual criteria used to identify features.