  The extratropical meridional energy transport in the atmosphere is
fundamentally intermittent in nature, having extremes large enough to affect
the net seasonal transport. Here, we investigate how these extreme transports
are associated with the dynamics of the atmosphere at multiple spatial scales,
from planetary to synoptic. We use the ERA5 reanalysis data to perform a
wavenumber decomposition of meridional energy transport in the Northern
Hemisphere mid-latitudes during winter and summer. We then relate extreme
transport events to atmospheric circulation anomalies and dominant weather
regimes, identified by clustering 500hPa geopotential height fields. In
general, planetary-scale waves determine the strength and meridional position
of the synoptic-scale baroclinic activity with their phase and amplitude, but
important differences emerge between seasons. 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. We link these waves and the transport extremes to
recent results on exceptionally strong and persistent co-occurring summertime
heat waves across the Northern Hemisphere mid-latitudes. We show that the
weather regime structures associated with these heat wave events are typical
for extremely large poleward energy transport events.
