The Role of Transient Eddies in the Poleward Tilt of Storm Tracks Using an Idealized GCM

Regions of strong jets and enhanced eddy activity such as the Pacific and Atlantic storm tracks are characterized by a downstream poleward deflection that is of major importance to the global climate. The role of the transient eddies in shaping the storm tracks is investigated using an idealized aquaplanet general circulation model (GCM) with a localized surface heating. Eddy properties are determined from the eddy flux tensor and its divergence, which describes the mean eddy shape, propagation and mean-flow feedback. In the quasi-geostrophic limit, an insightful picture emerges by considering the ‘E-vector', an extension of the Eliassen-Palm flux concept. The horizontal part of this quasi-vector, whose derivatives appear as a forcing term in the mean vorticity equation, involves only the horizontal momentum fluxes and indicates the direction of the barotropic group velocity. The vertical component, which is proportional to the poleward eddy heat flux, gives information about the vertical propagation and baroclinic growth of the eddies. Previous observational studies have shown that the spatial structure of the horizontal components gives rise to an antisymmetric mean flow forcing that can explain the existence of a poleward tilt in the mean flow. In this study, we investigate what determines the structure and intensity of the eddy field and its interaction with the mean flow. In order to isolate the different mechanisms involved, we set a series of experiments where we control the barotropic and baroclinic properties of the system by changing the strength of the localized heating and the planetary rotation rate. We show that the poleward tilt increases as we increase the heating and rotation rate, and explain these results in terms of the relative contributions of the eddy flux tensor components. We discuss a simple interpretation of the results in terms of propagating Rossby waves and the corresponding length and time scales of the problem.