  The eddy-driven jet stream and storm tracks in the mid-latitude atmosphere
are known to shift in latitude on various timescales, but the physical
processes that cause these shifts are still unclear. In this study, we
introduce a minimal dynamical system derived from the classical Phillips
two-level model with the goal of elucidating the essential mechanisms
responsible for the interaction between eddies and mean flow. Specifically, we
aim to understand the link between the structure of the eddies and the shift of
the latitudinal maximum of the zonal flow. By varying the horizontal shape of
the eddies, we find three distinct dynamical regimes whose occurrence depends
on the intensity of the external baroclinic forcing: a purely zonal flow, a
barotropic eddy regime with net poleward momentum flux, and a baroclinic eddy
regime with both net poleward momentum and temperature flux. For weak
baroclinic forcing, the classical zonal flow solution with latitudinal maximum
at the centre of the beta-channel is found. For strong forcing, if eddies are
southwest-northeast tilted and zonally elongated, the system is in the
baroclinic eddy regime, resulting in a poleward shift of the jet. The
intermediate barotropic eddy regime also features a poleward shifted jet, yet
with eddies structurally distinct from the baroclinic regime. Changing the
parameters yields transitions between the regimes that can be either continuous
or discontinuous in terms of the properties of the atmosphere. The findings of
this study also provide insights into the properties of the storm track change
between the jet entrance and jet exit regions of the North Atlantic.
