Abstract/Summary

Local periodic behaviour of storm tracks is found in the North Atlantic, the North Pacific, and in an aquaplanet global circulation model. Such periodicity is suggested to be a result of baroclinic instability that is characterised by nonlinear interactions between eddy fluxes and the mean-flow structure. This periodicity is associated with different processes at the beginning and end parts of the storm track. The beginning part exhibits maxima in both storm track activity and its growth rate, both of which oscillate temporarily in intensity akin to a predator-prey relationship. A nonlinear oscillator model is proposed to describe this relationship quantitatively, yielding a good agreement with atmospheric observations. It is predicted and observed that on average stronger storm events occur less frequently but are triggered more rapidly. Examination of the associated energetics suggests additional importance of the mean overturning circulation and the transport of mean available potential energy from polar latitudes, neither of which were included in the present model. Towards the end of the storm track, the dynamics are characterised more by variability in eddy momentum fluxes and transient jets, the latter often exhibiting periodic latitudinal fluctuations. It is suggested that the above effect of cycling baroclinicity and heat flux induces changes in eddy anisotropy which are responsible for the periodic jet deflections further downstream. On average, low heat flux is associated with an equatorward deflection of the jet, and vice versa. This jetdeflecting effect is characterised by a transfer of the system to a lower-frequency variability, and a mechanism to explain the observed preferred transitions of the North Atlantic jet is proposed. The oscillations in the storm track activity, baroclinicity and eddy-driven jet are closely linked, and can be viewed as describing the spatio-temporal lifecycle of the storm track.