Diabatic and Orographic Forcing of Northern Winter Stationary Waves and Storm Tracks

Abstract

Abstract In this study, a dry global circulation model is used to examine the contributions made by orographic and diabatic forcings in shaping the zonal asymmetries in the earth’s Northern Hemisphere (NH) winter climate. By design, the model mean flow is forced to bear a close resemblance to the observed zonal mean and stationary waves. The model also provides a decent simulation of the storm tracks. In particular, the maxima over the Pacific and Atlantic, and minima over Asia and North America, are fairly well simulated. The model also successfully simulates the observation that the Atlantic storm track is stronger than the Pacific storm track, despite stronger baroclinicity over the Pacific. Sensitivity experiments are performed by imposing and removing various parts of the total forcings. In terms of the NH winter stationary waves in the upper troposphere, results of this study are largely consistent with previous studies. Diabatic forcings explain most of the modeled stationary waves, with orographic forcings playing only a secondary role, and feedbacks due to eddy fluxes probably play only minor roles in most cases. Nevertheless, results of this study suggest that eddy fluxes may be important in modifying the response to orographic forcings in the absence of zonal asymmetries in diabatic heating. On the other hand, unlike the conclusion reached by previous studies, it is argued that the convergence of eddy momentum fluxes is important in forcing the oceanic lows in the lower troposphere, in agreement with one’s synoptic intuition. Regarding the NH winter storm-track distribution, results of this study suggest that NH extratropical heating is the most important forcing. Zonal asymmetries in NH extratropical heating act to force the Pacific storm track to shift equatorward and the Atlantic storm track to shift poleward, attain a southwest–northeast tilt, and intensify. It appears to be the main forcing responsible for explaining why the Atlantic storm track is stronger than the Pacific storm track. Tibet and the Rockies are also important, mainly in suppressing the storm tracks over the continents, forcing a clearer separation between the two storm tracks. In contrast, asymmetries in tropical heating appear to play only a minor role in forcing the model storm-track distribution.