The MJO on the equatorial beta-plane: an eastward propagating Rossby wave induced by meridional moisture advection
The MJO on the equatorial beta-plane: an eastward propagating Rossby wave induced by meridional moisture advection
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DOI：
10.1175/jas-d-21-0071.1
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发表时间：
2021-07
影响因子：
3.1
通讯作者：
F. Ahmed
中科院分区：
文献类型：
--
作者：
F. Ahmed
在赤道β平面上的线性化波的解决方案进行检查的背景纬向湿度梯度的存在下。感兴趣的是一个缓慢的，向东传播的n = 1模式，在行星尺度上是不稳定的，只存在于一个小范围的纬向波数（106）。模式色散曲线表现为向西传播的赤道Rossby波解的东延。因此，这种模式被称为向东传播的赤道Rossby波（ERW）。纬向波数2 ERW的水平结构包括一个低层赤道辐合中心两侧的四极离赤道涡旋，并类似于所观察到的MJO的水平结构。的ERW的分析，领先的顺序色散关系表明，纬向水汽平流赋予向东传播，和一个总的湿稳定性一样的参数的小有助于缓慢的相速度。当低层东风湿润柱时，战争遗留爆炸物在行星尺度附近是不稳定的。这种增湿可能来自纬向水分平流或地面通量或其组合。当西风带湿润柱，ERW阻尼和向西传播的长Rossby波是不稳定的。当纬向湿度梯度太弱时，战争遗留爆炸物不存在。湿静态能量收支分析表明，战争遗留爆炸物尺度的选择部分是由于有限的时间尺度的对流调整和较小尺度的有效纬向风致增湿。相速度、优先尺度和水平结构的相似性表明，战争遗留爆炸物是MJO的β平面类似物。
Linearized wave solutions on the equatorial beta-plane are examined in the presence of a background meridional moisture gradient. Of interest is a slow, eastward propagating n = 1 mode that is unstable at planetary scales and only exists for a small range of zonal wavenumbers (≲ 6). The mode dispersion curve appears as an eastward extension of the westward propagating equatorial Rossby wave solution. This mode is therefore termed the eastward propagating equatorial Rossby wave (ERW). The zonal wavenumber 2 ERW horizontal structure consists of a low-level equatorial convergence center flanked by quadrupole off-equatorial gyres, and resembles the horizontal structure of the observed MJO. An analytic, leading order dispersion relationship for the ERW shows that meridional moisture advection imparts eastward propagation, and that the smallness of a gross moist stability like parameter contributes to the slow phase speed. The ERW is unstable near planetary scales when low-level easterlies moisten the column. This moistening could come from either zonal moisture advection or surface fluxes or a combination thereof. When westerlies instead moisten the column, the ERW is damped and the westward propagating long Rossby wave is unstable. The ERW does not exist when the meridional moisture gradient is too weak. A moist static energy budget analysis shows that the ERW scale selection is partly due to finite timescale convective adjustment and less effective zonal wind-induced moistening at smaller scales. Similarities in the phase speed, preferred scale and horizontal structure suggest that the ERW is a beta-plane analog of the MJO.