Eddy-zonal flow interactions and the persistence of the zonal index
Eddy-zonal flow interactions and the persistence of the zonal index
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DOI：
10.1175/jas4006.1
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发表时间：
2007-09-01
影响因子：
3.1
通讯作者：
Vallis, Geoffrey K.
中科院分区：
文献类型：
--
作者：
Gerber, Edwin P.;Vallis, Geoffrey K.
用一个理想的大气环流模式研究了温带外大气季节内(10-100天)变化的时间尺度的控制因素。在这些时间尺度上的持续性存在于表现温带急流经向抖动的变率模式中。根据模式强迫的不对称程度，模式呈现出与纬向指数、环状模式和北大西洋涛动相似的性质。结果表明，急流曲折的时间尺度与明显的内模时间尺度不同，表明天气涡旋与大尺度气流的相互作用形成了一个独立的季节内时间尺度。提出了一种涡热和动量输运耦合的机制，延缓了急流的运动，减缓了急流的经向变化，从而延长了纬向指数和环状模式异常的持续时间。当模式强迫纬向均匀时，反馈很强，对模式参数非常敏感。然而，当引入纬向不对称时，急流变化的时间尺度下降，几乎所有对参数的敏感性都丧失了，这种不对称的形式是地形和接近陆地-海洋对比的热扰动。对纬向指数的纬向结构的诊断可以直观地了解指数和环状模式的物理性质，并提示为什么纬向不对称限制了涡旋-平均气流的相互作用。
An idealized atmospheric general circulation model is used to investigate the factors controlling the time scale of intraseasonal (10-100 day) variability of the extratropical atmosphere. Persistence on these time scales is found in patterns of variability that characterize meridional vacillations of the extratropical jet. Depending on the degree of asymmetry in the model forcing, patterns take on similar properties to the zonal index, annular modes, and North Atlantic Oscillation. It is found that the time scale of jet meandering is distinct from the obvious internal model time scales, suggesting that interaction between synoptic eddies and the large-scale flow establish a separate, intraseasonal time scale. A mechanism is presented by which eddy heat and momentum transport couple to retard motion of the jet, slowing its meridional variation and thereby extending the persistence of zonal index and annular mode anomalies. The feedback is strong and quite sensitive to model parameters when the model forcing is zonally uniform. However, the time scale of jet variation drops and nearly all sensitivity to parameters is lost when zonal asymmetries, in the form of topography and thermal perturbations that approximate land-sea contrast, are introduced. A diagnostic on the zonal structure of the zonal index provides intuition on the physical nature of the index and annular modes and hints at why zonal asymmetries limit the eddy-mean flow interactions.