Convective organization and eastward propagating equatorial disturbances in a simple excitable system
Convective organization and eastward propagating equatorial disturbances in a simple excitable system
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简单可激发系统中的对流组织和向东传播的赤道扰动
DOI：
10.1002/qj.3792
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发表时间：
2020
影响因子：
8.9
通讯作者：
Vallis G
中科院分区：
文献类型：
--
作者：
Vallis G
我们描述和说明的机制，对流聚集和向东传播的赤道扰动，这是类似的在某些方面的马登-朱利安振荡（MJO），出现。我们构造了一个简单的显式系统，仅由浅水方程加上湿度变量组成;水分通过从湿表面蒸发进入，通过流动输送并通过冷凝去除，从而在高度场中产生异常。对于一个广泛的参数范围内的系统是兴奋的，即使线性稳定，与凝聚产生收敛和重力波，共同作用，触发更多的凝聚。在赤道β平面上，对流首先在赤道附近聚集，产生与松野-吉尔问题中遇到的模式相关的模式。然而，系统模式并不稳定，其东部边缘触发了更多的对流，导致一个持续的降水扰动向东发展。由于松野-吉尔型引起的来自东部的暖湿辐合空气比来自西部的辐合空气更不稳定，因此传播是向东的。该模式被限制在赤道几个变形半径内的区域，因为在这里对流可以产生组织自身成为自我维持模式所需的收敛;因此，较小的β参数值会引起更广泛的扰动。扰动的形成优先发生在地表较温暖的地方，并且在使扰动能够改革的条件出现之前必须经过足够的时间（通常是几十天），这是MJO的一个众所周知的特征。扰动的速度取决于蒸发的效率和冷凝释放的热量，通常为每秒几米，远低于开尔文波的速度。
We describe and illustrate a mechanism whereby convective aggregation and eastward propagating equatorial disturbances, which are similar in some respects to the Madden–Julian Oscillation (MJO), arise. We construct a simple explicit system consisting only of the shallow‐water equations plus a humidity variable; moisture enters via evaporation from a wet surface, is transported by the flow and removed by condensation, thus producing an anomaly in the height field. For a broad range of parameters the system is excitable even when linearly stable, with condensation producing convergence and gravity waves that, acting together, trigger more condensation. On the equatorial β‐plane the convection first aggregates near the Equator, generating patterns related to those encountered in the Matsuno–Gill problem. However, the pattern is unstable and more convection is triggered on its eastern edge, leading to a self‐sustaining precipitating disturbance that progresses eastward. The propagation is eastward because the warm, moist converging air from the east induced by the Matsuno–Gill pattern is more convectively unstable than the converging air from the west. The pattern is confined to a region within a few deformation radii of the Equator, as here the convection can create the convergence needed to organize itself into a self‐sustaining pattern; thus, smaller values of the beta parameter give rise to a wider disturbance. Formation of the disturbance preferentially occurs where the surface is warmer, and sufficient time (typically a few tens of days) must pass before conditions arise that enable the disturbance to reform, a well‐known characteristic of the MJO. The speed of the disturbance depends on the efficiency of evaporation and the heat released by condensation, and is typically a few metres per second, much less than the Kelvin wave speed.
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