Slow Modes of the Equatorial Waveguide
Slow Modes of the Equatorial Waveguide
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DOI：
10.1175/jas-d-19-0281.1
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发表时间：
2020-02
影响因子：
3.1
通讯作者：
K. Emanuel
中科院分区：
文献类型：
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作者：
K. Emanuel
最近开发的线性模型向东传播的扰动有两个独立的不稳定模式：对流耦合开尔文波不稳定的风依赖的表面焓通量，和缓慢的，MJO样模式不稳定的云辐射相互作用和驱动向东的表面焓通量。后一种模式在弱温度梯度（WTG）近似下仍然存在，并且具有由表面通量湿润对流层柱所需的时间决定的时间尺度。在这里，我们扩展了该模型，包括高阶模式，并表明具有更复杂结构的行星尺度低频波也可以被云辐射相互作用放大。虽然这些波中的大多数在WTG近似下幸存下来，但它们的频率和增长率受到该近似的严重影响。而应用纬向地转的假设结果在一个更好的近似的全谱模式。对于小的云辐射和地面通量的反馈，开尔文波和赤道Rossby波是不稳定的，但当这些反馈是足够强的，除了在较高频率的开尔文和柳井波的情况下，频率不接近经典的赤道色散曲线。特别地，向东传播的n = 1模式具有类似于观察到的MJO结构的结构。
A recently developed linear model of eastward-propagating disturbances has two separate unstable modes: convectively coupled Kelvin waves destabilized by the wind dependence of the surface enthalpy flux, and slow, MJO-like modes destabilized by cloud–radiation interaction and driven eastward by surface enthalpy fluxes. This latter mode survives the weak temperature gradient (WTG) approximation and has a time scale dictated by the time it takes for surface fluxes to moisten tropospheric columns. Here we extend that model to include higher-order modes and show that planetary-scale low-frequency waves with more complex structures can also be amplified by cloud–radiation interactions. While most of these waves survive the WTG approximation, their frequencies and growth rates are seriously compromised by that approximation. Applying instead the assumption of zonal geostrophy results in a better approximation to the full spectrum of modes. For small cloud–radiation and surface flux feedbacks, Kelvin waves and equatorial Rossby waves are destabilized, but when these feedbacks are strong enough, the frequencies do not lie close to classical equatorial dispersion curves except in the case of higher-frequency Kelvin and Yanai waves. An eastward-propagating n = 1 mode, in particular, has a structure resembling the observed structure of the MJO.