Extremes of meridional energy transports in Northern Hemisphere mid-latitudes across zonal wavenumbers and dominant weather regimes

Extremes in extratropical meridional energy transports in the atmosphere are associated with the dynamics of the atmosphere at multiple spatial scales, from planetary to synoptic. This is related to the nature of amplifying baroclinic waves, that are fundamentally intermittent and sporadic, significantly affecting the net seasonal transport across latitudes. Here, we use the ERA5 reanalysis data to perform a wavenumber decomposition of meridional energy transports in the Northern Hemisphere mid-latitudes during winter and summer. Extreme transport events are linked to atmospheric circulation anomalies and dominant weather regimes, identified by clustering 500 hPa geopotential height fields. Partitioning the extreme events across zonal wavenumber highlights the different role of scales in different seasons and regions. In general, planetary-scale waves determine the strength and meridional position of the synoptic-scale baroclinic activity with their phase and amplitude. During winter, large wavenumbers (k = 2–3) are key drivers of the meridional-energy-transport extremes, and planetary- and synoptic-scale transport extremes virtually never co-occur. In summer, extremes are associated with higher wavenumbers (k = 4–6), identified as synoptic-scale motions. Focusing on recently occurred exceptionally strong summertime heat waves and wintertime cold spells, we notice that regime structures of these events are typical of extremely large poleward meridional energy transports.