Three-Dimensional Structure and Dynamics of African Easterly Waves. Part II: Dynamical Modes
Three-Dimensional Structure and Dynamics of African Easterly Waves. Part II: Dynamical Modes
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非洲东风波的三维结构和动力学。
DOI：
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发表时间：
2006
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通讯作者：
C. Thorncroft
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作者：
N. Hall;G. Kiladis;C. Thorncroft
采用原始方程模型研究了非洲东风急流的线性正态。夏季平均（6 - 9月；JJAS）流量的再分析数据提供了纬向均匀和波浪的基本状态。分析了西非在这些基本状态上生长的模式的结构和增长率。对于纬向均匀基本态，模态与以往许多研究中发现的非洲东风波相似，但偏压性强且表面增强。对于波浪基态，模态具有由AEJ决定的纵向结构。能量转换诊断证实，上游为表面强化斜压结构，下游为深部正压结构。这些模态看起来与作者在另一篇论文中发现的复合东风波结构非常相似。这种相似性延伸到垂直速度与流函数的相关系，类似于OLR复合材料，表明对对流有动力影响。无阻尼时，波浪基态的模态增长率为0.253天。通过合理数量的低水平阻尼，这种模式被中和。它的周期为5.5天，波长约为3500公里。月平均基本状态的进一步结果显示出轻微的变化，因为波包基本上跟随喷流核心的位移。1988年和1990年对东风波的具体活跃和被动年份进行的试验对模态的结果没有显著差异。这些结果，特别是系统的稳定性，得出的结论是，正斜压不稳定性不能单独解释预警的开始和间歇性，需要有限幅度的初始扰动。
A primitive equation model is used to study the linear normal modes of the African easterly jet (AEJ). Reanalysis data from the summertime mean (June–September; JJAS) flow is used to provide zonally uniform and wavy basic states. The structure and growth rates of modes that grow over West Africa on these basic states are analyzed. For zonally uniform basic states, the modes resemble African easterly waves (AEWs) as in many previous studies, but they are quite baroclinic and surface intensified. For wavy basic states the modes have a longitudinal structure determined by the AEJ. They have a surface-intensified baroclinic structure upstream and a deep barotropic structure downstream, as confirmed by energy conversion diagnostics. These modes look remarkably similar to the composite easterly wave structures found by the authors in a companion paper. The similarity extends to the phase relationship of vertical velocity with streamfunction, which resembles OLR composites, suggesting a dynamical influence on convection. Without damping, the mode for the wavy basic state has a growth rate of 0.253 day 1 . With a reasonable amount of low-level damping this mode is neutralized. It has a period of 5.5 days and a wavelength of about 3500 km. Further results with monthly mean basic states show slight variations, as the wave packet essentially follows displacements of the jet core. Experiments focused on specific active and passive years for easterly waves (1988 and 1990) do not yield significantly different results for the modes. These results, and in particular, the stability of the system, lead to the conclusion that barotropic–baroclinic instability alone cannot explain the initiation and intermittence of AEWs, and a finite-amplitude initial perturbation is required.