Abstract
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.
Highlights
In the steady state of the Northern Hemisphere, maxima in storm track activity and baroclinicity are co-located with areas of intensified meridional temperature gradients on the eastern coasts of Eurasia and North America (Hoskins and Valdes, 1990)
The localized storm tracks of the North Atlantic, the North Pacific and the idealised global circulation model (GCM) exhibit a predator–prey relationship between storm track activity and baroclinicity. This relationship is based on the competing effects of high storm track activity reducing meridional temperature gradients and a restoring forcing that enhances the baroclinicity when storm track activity is low
This relationship leads to burst-like oscillations in storm track activity and saw tooth-like oscillations in baroclinicity. The observation of this behaviour in all of the studied storm tracks, as well as for several different measures of storm track activity, suggests that this is a feature of all localized storm tracks
Summary
In the steady state of the Northern Hemisphere, maxima in storm track activity and baroclinicity are co-located with areas of intensified meridional temperature gradients on the eastern coasts of Eurasia and North America (Hoskins and Valdes, 1990). Using the two-layer quasi-geostrophic (QG) model with a small departure from a critical shear (beyond which instability occurs), Pedlosky (1970) studied the weakly nonlinear interaction between the mean flow and the primary wave This nonlinearity was shown to yield an oscillation of the perturbation amplitude due to it alternately changing the sign of its vertical phase. Once the baroclinicity is low enough to inhibit any further eddy generation, the storm track activity falls and allows the diabatic forcing to replenish the baroclinicity and the cycle repeats Such cycling agrees with the quasi-oscillatory behaviour between the mean end eddy energies observed in the Southern Hemisphere (Randel and Stanford, 1985). The remaining section discusses the results and their wider implications
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