Marginal stability and predator–prey behaviour within storm tracks

A predator–prey relationship between storm track intensity and growth rate is revealed in reanalysis data of the North Atlantic and North Pacific, as well as in an idealized global circulation model with a zonally asymmetric heating dipole. Averaging in the phase space of these two quantities reveals that both quantities oscillate on approximately monthly time-scales. These oscillations occur due to quasi-periodic bursts in storm track activity that reduce excess baroclinicity and bring the flow back towards a state that is marginally stable to those bursts. Many detailed properties of these oscillations are reproduced well by a two-dimensional dynamical system, especially in respect of the North Atlantic storm track which is more zonally constrained than that in the North Pacific. It is predicted and observed that on average stronger storm events occur less frequently but grow on a shorter time-scale. The results suggest that nonlinearly oscillating behaviour around a state of baroclinic neutrality is a general feature of localized storm tracks, and they offer a new perspective on the study of baroclinic instability.