Planetary Scale Selection of the Madden–Julian Oscillation*
Planetary Scale Selection of the Madden–Julian Oscillation*
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DOI：
10.1175/2009jas2968.1
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发表时间：
2009-08
影响因子：
3.1
通讯作者：
Tim Li;Chunhua Zhou
中科院分区：
文献类型：
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作者：
Tim Li;Chunhua Zhou
用2.5层和2层模式进行了数值试验，考察了Madden-Julian振荡(MJO)的行星尺度选择机制。这里的策略是研究初始扰动的演变，该初始扰动在1到15的纬向波数处具有赤道Kelvin波的形式。在摩擦边界层存在的情况下，最不稳定的模式在线性加热下倾向于短波长；但在非线性加热下，纬向波数1增长最快。这与没有边界层的模式有很大的不同，在没有边界层的模式中，线性或非线性加热都不会导致长波选择。因此，数值模拟指出了非线性加热和摩擦边界层的共同作用在MJO行星尺度选择中的重要作用。非线性加热下这种尺度选择的原因是干Kelvin波和湿Kelvin-Rossby波之间的相速度不同。如果两个对流分支之间的距离小于一个临界距离(约16000公里)，则由一个对流分支触发的较快的干Kelvin波可能会追上并抑制另一个对流分支，后者以湿Kelvin-Rossby波耦合的相速度在其前面传播。干的开尔文波和湿的开尔文-罗斯比波耦合之间的干扰最终消散并“滤除”较短波长的扰动，导致选择长波。边界层通过摩擦水汽辐合使MJO不稳定，并对保持同相纬向风压结构起着重要作用。
Numerical experiments with a 2.5-layer and a 2-level model are conducted to examine the mechanism for the planetary scale selection of the Madden‐Julian oscillation (MJO). The strategy here is to examine the evolution of an initial perturbation that has a form of the equatorial Kelvin wave at zonal wavenumbers of 1 to 15. In the presence of a frictional boundary layer, the most unstable mode prefers a short wavelength under a linear heating; but with a nonlinear heating, the zonal wavenumber 1 grows fastest. This differs significantly from a model without the boundary layer, in which neither linear nor nonlinear heating leads to the long wave selection. Thus, the numerical simulations point out the crucial importance of the combined effect of the nonlinear heating and the frictional boundary layer in the MJO planetary scale selection. The cause of this scale selection under the nonlinear heating is attributed to the distinctive phase speeds between the dry Kelvin wave and the wet Kelvin‐Rossby wave couplet. The faster dry Kelvin wave triggered by a convective branch may catch up and suppress another convective branch, which travels ahead of it at the phase speed of the wet Kelvin‐Rossby wave couplet if the distance between the two neighboring convective branches is smaller than a critical distance (about 16 000 km). The interference between the dry Kelvin wave and the wet Kelvin‐Rossby wave couplet eventually dissipates and ‘‘filters out’’ shorter wavelength perturbations, leading to a longwave selection. The boundary layer plays an important role in destabilizing the MJO through frictional moisture convergences and in retaining the in-phase zonal wind‐pressure structure.