FORMATION MECHANISM OF STORM TRACKS AND THEIR FEEDBACK EFFECTS TO THE TIME-MEAN CIRCULATION

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In the wintertime northern hemispheric circulations, storm tracks where the activity of synoptic-scale disturbances is enhanced in the extratropical troposphere are zonally localized over the Pacific and the Atlantic. However, the dynamical mechanism of the zonally localization of storm tracks has not been fully established. Their role on the formation of the time-mean tropospheric circulation is also controversial. In particular, the relevance of extratropical sea surface temperature (SST) distribution to the storm track formation, which is central to understand the dynamics of decadal climate variability remains unclear. In this study, we proceeded a series of long-term integration of an atmospheric general circulation model (AGCM) (CCSR/NIES AGCM with horizontal resolution of T42 and 20 vertical levels) to reveal the dynamics of storm track and its feedback onto the time-mean circulation. The numerical experiments were conducted under a perpetual January condition without topography by specifying idealized land-sea and SST distribution. The following results were obtained:1 In the absence of topographically forced Rossby waves, the zonal asymmetry of the low-level baroclinicity formed by the underlying SST distribution is indispensable to localize the storm, while that in the upper tropospheric circulation, such as subtropical jet and barotropic energy conversion, plays only a secondary role.2 The zonal asymmetry of the low-level baroclinicity is maintained by stationary planetary waves forced by the diabatic heating distribution associated with the storm track.3 Thus, the positive feedback mechanism between synoptic-scale eddies and planetary waves through the diabatic heating plays the most important role in localizing the storm track in the zonal direction.