Simulating the seasonal cycle of the Northern Hemisphere storm tracks using idealized nonlinear storm track models

In this study, an idealized nonlinear model (details of the model are presented in the poster presentation by Chang and Guo) is used to investigate whether dry dynamical factors alone are sufficient for explaining the observed seasonal modulation of the Northern Hemisphere storm tracks during the cool season. By construction, the model does an excellent job simulating the seasonal evolution of the climatological stationary waves. Yet even under this realistic mean flow, the seasonal modulation in storm track amplitude predicted by the model is deficient over both ocean basins. The model exhibits a stronger sensitivity to the mean flow baroclinicity than observed, producing too large midwinter eddy amplitudes compared to Fall and Spring. This is the case not only over the Pacific, where the observed midwinter minimum is barely apparent in the model simulations, but also over the Atlantic, where the October/April eddy amplitudes are also too weak when the January amplitude is tuned to be about right.

The nonlinear model generally produces stronger eddy amplitude with stronger baroclinicity, even in the presence of concomitant stronger deformation due to the enhanced stationary wave. The same was found to be the case in a simpler quasi-geostrophic model, in which the eddy amplitude nearly always increases with baroclinicity, and deformation only limits the maximum eddy amplitude when the baroclinicity is unrealistically weak. Overall, these results suggest that it is unlikely that dry dynamical effects alone, such as deformation, can fully explain the observed Pacific midwinter minimum in eddy amplitude.

It is argued that one should take into account the seasonal evolution of the impacts of diabatic heating on baroclinic wave development in order to fully explain the seasonal cycle of the storm tracks. A set of highly idealized experiments that attempts to represent some of the impacts of moist heating is presented to suggest that deficiencies in the model simulated seasonal cycle of both storm tracks may be corrected when these effects, together with observed seasonal changes in mean flow structure, are taken into account.