The signature of extreme meridional eddy energy transports on modes of circulation, heatwaves and droughts in Northern Hemisphere mid-latitudes

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. In this work, we take advantage of a zonal wavenumber decomposition of meridional energy transports throughout the Northern Hemisphere mid-latitudinal channel to investigate how extreme transports affect the main modes of circulation, namely weather regimes identified through k-means clustering of 500 hPa geopotential height and sea-level pressure fields. We find that, in general, planetary-scale waves determine the strength and meridional position of the synoptic-scale baroclinic activity with their phase and amplitude. During winter, largest wavenumbers (k = 2–3) are key drivers of the meridional-energy-transport extremes, and planetary- and synoptic-scale transport extremes virtually never co-occur. During summer, extremes are associated with higher wavenumbers (k = 4–6), identified as synoptic-scale motions. We look further into the implications of such extreme transports by looking at heat waves and cold spells over continents in the Northern Hemisphere, as well as proxies of droughts, investigating the role of moisture transports and the moist component in static energy transport through the signature of changes into evapo-transpiration and related latent heat fluxes at the land surface. Characterizing the clusters of extreme transport events in space and time, the persistence and co-existence of heat/cold waves/spells and drought over specific regions is investigated in light of the dominant modes of atmospheric dynamics evidenced through the wavenumber decomposition.