Stratospheric Modulation of the MJO through Cirrus Cloud Feedbacks
Stratospheric Modulation of the MJO through Cirrus Cloud Feedbacks
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DOI：
10.1175/jas-d-22-0083.1
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发表时间：
2022-04
影响因子：
3.1
通讯作者：
Jonathan Lin;K. Emanuel
中科院分区：
文献类型：
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作者：
Jonathan Lin;K. Emanuel
最近的观测表明，北方冬季平流层准两年振荡(QBO)的位相对Madden Julian振荡(MJO)有显著的调制作用。MJO合成结果表明，对流层高空冰云分数与水汽异常一般配置在一起，并存在随云分数高度向东倾斜的现象。通过辐射传输计算表明，冰云比水汽异常具有更强的对流层辐射强迫，突出了在MJO的简单模式中考虑上对流层/平流层下部过程的重要性。对作者以前建立的对流层-平流层耦合线性模式进行了扩展，包括了平流层的平均风和卷云的预报方程，它们被动态地强迫并允许调制对流层的辐射冷却，类似于以前公式中对流层水汽的影响。在这些修改下，该模型仍然产生类似于MJO的缓慢的向东传播模式。结果表明，平流层纬向平均风的符号既控制着向上波的传播，又控制着对流层的垂直结构。在变化的平流层风和相互作用的卷云辐射下，类MJO模在平流层西风带下的增长速度弱于东风带，与观测到的MJO-QBO关系一致。这些结果直接归因于QBO东风下正压模式的增强。研究还表明，在平流层西风带下，卷云纬向平流层的差异性平流导致的增长速度弱于东风带。讨论了线性理论的含义和局限性。
Recent observations have indicated significant modulation of the Madden Julian Oscillation (MJO) by the phase of the stratospheric Quasi-Biennial Oscillation (QBO) during boreal winter. Composites of the MJO show that upper tropospheric ice cloud fraction and water vapor anomalies are generally collocated, and that an eastward tilt with height in cloud fraction exists. Through radiative transfer calculations, it is shown that ice clouds have a stronger tropospheric radiative forcing than do water vapor anomalies, highlighting the importance of incorporating upper tropospheric/lower stratospheric processes into simple models of the MJO. The coupled troposphere-stratosphere linear model previously developed by the authors is extended by including a mean wind in the stratosphere and a prognostic equation for cirrus clouds, which are forced dynamically and allowed to modulate tropospheric radiative cooling, similar to the effect of tropospheric water vapor in previous formulations. Under these modifications, the model still produces a slow, eastward propagating mode that resembles the MJO. The sign of zonal mean wind in the stratosphere is shown to control both the upward wave propagation and tropospheric vertical structure of the mode. Under varying stratospheric wind and interactive cirrus cloud radiation, the MJO-like mode has weaker growth rates under stratospheric westerlies than easterlies, consistent with the observed MJO-QBO relationship. These results are directly attributable to an enhanced barotropic mode under QBO easterlies. It is also shown that differential zonal advection of cirrus clouds leads to weaker growth rates under stratospheric westerlies than easterlies. Implications and limitations of the linear theory are discussed.