Destabilization of mixed Rossby gravity waves and the formation of equatorial zonal jets
Destabilization of mixed Rossby gravity waves and the formation of equatorial zonal jets
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DOI：
10.1017/s0022112008002656
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发表时间：
2008-08
影响因子：
3.7
通讯作者：
B. Hua;M. D'orgeville;M. Fruman;C. Ménesguen;R. Schopp;P. Klein;H. Sasaki
中科院分区：
文献类型：
--
作者：
B. Hua;M. D'orgeville;M. Fruman;C. Ménesguen;R. Schopp;P. Klein;H. Sasaki
使用连续分层原始方程的三维一致高分辨率模拟，对混合罗斯贝重力（MRG）波的稳定性进行了数值研究。对于足够短的纬向波长，向西传播的 MRG 波会因正压剪切不稳定性而严重不稳定，从而导致纬向急流的形成。纬向短波的大规模不稳定性会产生纬向急流，因为它主要由经向剪切运动组成，如最近的短正压罗斯贝波问题所示。在各种域几何形状中进行了模拟：周期性折返河道、在其西部部分受迫的短 MRG 波的盆地以及用区域局部 MRG 波初始化的非常长的河道。纬向射流的特性随几何形状的不同而变化。在周期性折返通道中，正压纬向急流主导赤道及其附近地区的总流动响应。在其他情况下，不稳定会导致具有完全不同特征的纬向急流，特别是在传播部分信号的向东群中。最引人注目的结果涉及赤道上纬向急流的形成，其符号在垂直方向上交替，与强迫或初始条件的规模相比，垂直规模较短。制定了简化扰动涡度方程的稳定性分析，以解释纬向喷流的空间尺度选择和增长率作为基本态 MRG 波特征的函数。对于两种类型的纬向喷流，模型预测它们的经向尺度与 MRG 波基本态的纬向尺度相当，而它们的增长率尺度为 μ ∝ Fr |k|，其中 Fr 是基本态经向速度分量的弗劳德数，k 是其无量纲纬向波数。斜压纬向喷流的垂直尺度对应于最快增长模式中基态的主要谐波 p 峰值，由 ppeak ≈ 0.55k2 给出。因此，基本态 MRG 波的纬向波长越短，纬向急流（正压和斜压）的经向尺度越窄，斜压急流的垂直尺度通过赤道变形半径与其子午尺度联系在一起，变形半径随着垂直波数的平方根而减小。空间尺度的预测与数值模拟在定性和定量上都一致，其中较短的垂直尺度斜压纬向急流有利于较短波长较长周期的MRG波基本态，垂直模式数随着MRG波周期的平方而增加。附录涉及带状长波和中波 MRG 波的情况，其中弱不稳定状态会导致涉及共振三重相互作用的适度调整，而不会导致射流形成。对于东向相位传播波，调整不会导致明显的角动量重新分布。
The stability of mixed Rossby gravity (MRG) waves has been investigated numerically using three-dimensionally consistent high-resolution simulations of the continuously stratified primitive equations. For short enough zonal wavelength, the westward phase propagating MRG wave is strongly destabilized by barotropic shear instability leading to the formation of zonal jets. The large-scale instability of the zonally short wave generates zonal jets because it consists primarily of sheared meridional motions, as shown recently for the short barotropic Rossby wave problem. Simulations were done in a variety of domain geometries: a periodic re-entrant channel, a basin with a short MRG wave forced in its western part and a very long channel initialized with a zonally localized MRG wave. The characteristics of the zonal jets vary with the geometry. In the periodic re-entrant channel, barotropic zonal jets dominate the total flow response at the equator and its immediate vicinity. In the other cases, the destabilization leads to zonal jets with quite different characteristics, especially in the eastward group propagating part of the signal. The most striking result concerns the formation of zonal jets at the equator, alternating in sign in the vertical, with vertical scale short compared to the scale of the forcing or initial conditions. A stability analysis of a simplified perturbation vorticity equation is formulated to explain the spatial scale selection and growth rate of the zonal jets as functions of the characteristics of the basic state MRG wave. For both types of zonal jets, the model predicts that their meridional scales are comparable to the zonal scale of the MRG wave basic state, while their growth rates scale as μ ∝ Fr |k|, where Fr is the Froude number of the meridional velocity component of the basic state and k its non-dimensional zonal wavenumber. The vertical scale of the baroclinic zonal jets corresponds to the dominant harmonic ppeak of the basic state in the fastest growing mode, given by ppeak≈0.55k2. Thus, the shorter the zonal wavelength of the basic state MRG wave, the narrower the meridional scale of the zonal jets, both barotropic and baroclinic, with the vertical scale of the baroclinic jets being tied to their meridional scale through the equatorial radius of deformation, which decreases as the square root of the vertical wavenumber. The predictions of the spatial scales are in both qualitative and quantitative agreement with the numerical simulations, where shorter vertical scale baroclinic zonal jets are favoured by shorter-wavelength longer-period MRG wave basic states, with the vertical mode number increasing as the square of the MRG wave period. An Appendix deals with the case of zonally long and intermediate wavelength MRG waves, where a weak instability regime causes a moderate adjustment involving resonant triad interactions without leading to jet formation. For eastward phase propagating waves, adjustment does not lead to significant angular momentum redistribution.