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

在冬季北半球环流中，热带外对流层天气尺度扰动活动增强的风暴路径位于太平洋和大西洋的纬向局部。然而，风暴路径纬向局部化的动力机制还没有完全建立起来。它们在时间平均对流层环流形成中的作用也存在争议。特别是，温带海洋表面温度(SST)分布与风暴路径形成的相关性尚不清楚，而风暴路径的形成对于理解年代际气候变化的动力学至关重要。在这项研究中，我们进行了一系列大气环流模式(CCSR/NIES大气环流模式，水平分辨率为T42，垂直分辨率为20层)的长期积分，以揭示风暴路径的动力学及其对时间平均环流的反馈。数值试验是在没有地形的永久一月条件下，通过指定理想的海陆分布和海温分布进行的。结果表明：1在没有地形强迫Rossby波的情况下，底层海温分布所形成的低层斜压性的纬向不对称性对于风暴的局部化是必不可少的，而对流层高层环流中的低层斜压性的纬向不对称性(如副热带急流和正压能量转换)只起次要作用。2低层斜压性的纬向不对称性是由与风暴路径相关的非绝热加热分布所强迫的定常行星波维持的。3天气尺度涡旋与行星波之间通过非绝热加热的正反馈机制对风暴路径在纬向的局部化起着最重要的作用。

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.