Organization of Storm Track Anomalies by Recurring Low-Frequency Circulation Anomalies

Abstract

From previous studies it is known that anomalous momentum fluxes by bandpass eddies are important in maintaining long-lasting tropospheric flow anomalies. Evidence is presented that suggests that these anomalous fluxes do not occur at random but happen because the structure of storm track activity is modified by the presence of prominent low-frequency, large-scale circulation anomalies. This behavior is noted in an extended integration of a perpetual January simulation with a general circulation model (GCM). Because nonlinear feedbacks between low- and high-frequency variability make it difficult to establish causal relationships between these two ranges of variability, a model is constructed that approximates the storm track activity that would be expected to accompany a given low frequency without including the feedback of the high frequencies onto the low-frequency state. This model is based on the linearized primitive equations and uses a series of short integrations from random initial conditions to establish the statistics of the storm tracks. After first tuning this model to reproduce the storm tracks of the GCM climate, the model is verified by applying it to other climate states. Next it is used to determine the perturbations to the climatological storm track that result because of the presence of two prominent low-frequency anomalies that are observed to frequently occur in the GCM. The storm track anomalies that are caused by these low-frequency anomalies match the storm track anomalies that accompany the low-frequency anomalies in the GCM, thus establishing that in the GCM the low-frequency patterns cause the coincident storm track anomalies. Further experiments with the linear storm track model help to pinpoint which processes are important in organizing the storm track structure. It turns out that the barotropic component of the low-frequency disturbances has a large impact on the high-frequency disturbances. Group velocity calculations indicate that the barotropic component affects the distribution of storms by steering, rather than stretching and straining, the perturbations. Calculations also demonstrate that because the distribution of storms in the climatological state is nonuniform, some large-scale patterns may be able to organize storm track activity in such a way that the associated momentum fluxes positively feed back onto the large-scale anomalies while other patterns cannot induce such a positive feedback. The study concludes that since a two-way feedback has now been demonstrated between high- and low-frequency disturbances during episodes of prominent anomalies, the two timescales are inseparable and the description of recurring anomalies is incomplete without inclusion of the fast timescale fields. It is also suggested that the storm track model used in the study may serve as a useful means of parameterizing fluxes by bandpass perturbations in low-frequency models of the atmosphere.