The Role of Subtropical Rossby Waves in Amplifying the Divergent Circulation of the Madden–Julian Oscillation
The Role of Subtropical Rossby Waves in Amplifying the Divergent Circulation of the Madden–Julian Oscillation
复制标题
副热带罗斯贝波在放大马登朱利安振荡发散环流中的作用
DOI：
10.1175/jas-d-22-0259.1
复制
发表时间：
2023
影响因子：
3.1
通讯作者：
Kiladis, George N.
中科院分区：
文献类型：
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作者：
Barpanda, Pragallva;Tulich, Stefan N.;Dias, Juliana;Kiladis, George N.
长期以来，人们一直认为马登-朱利安振荡（MJO）的复合结构以副热带对流层上层明显的罗斯比环流为特征，其存在可以解释为副热带西风急流存在时对流加热异常的强迫响应。这里感兴趣的问题是，这些强迫环流是否有任何后续的影响，在热带地区的散度模式和开尔文模式的MJO的组成部分。一个非线性球浅水模式被用来研究如何引入不同的背景射流剖面影响模型的稳态响应施加MJO一样的固定热强迫。结果表明，强急流导致热带地区更强的Kelvin模响应，直到临界急流速度，沿着在强迫附近有更强的散度异常。为了理解这种行为，进行了额外的计算，其中在热带外施加了局部涡度强迫，而在热带没有任何热力强迫。如果喷流具有足够的振幅，则再次看到响应包括明显的赤道开尔文波。详细分析的涡度收支表明，纬向平均纬向风切变在放大赤道附近的开尔文模辐散风中起着关键作用，非线性的影响可以忽略不计。这些结果有助于解释为什么MJO往往是最强的，在北方冬季时，印度-太平洋急流通常是在其最强的。Significance StatementThe MJO是一个行星尺度的对流耦合赤道扰动，作为一个主要来源的大气可预测性的季节内时间尺度（30-90天）。由于其主导地位和自发重现，MJO具有显著的全球影响，影响热带飓风，风暴路径和中纬度地区的大气阻塞事件，甚至极地附近的天气系统。尽管亚季节到季节（S2 S）预测模型不断改进，但MJO预测技能仍未发挥其最大潜力。这一挑战的根源部分是由于我们缺乏对MJO如何与背景平均流相互作用的理解。在这项工作中，我们使用一个简单的一层大气模式与理想的加热和涡度源了解副热带急流的影响MJO振幅和水平结构。
The composite structure of the Madden–Julian oscillation (MJO) has long been known to feature pronounced Rossby gyres in the subtropical upper troposphere, whose existence can be interpreted as the forced response to convective heating anomalies in the presence of a subtropical westerly jet. The question of interest here is whether these forced gyre circulations have any subsequent effects on divergence patterns in the tropics and the Kelvin-mode component of the MJO. A nonlinear spherical shallow water model is used to investigate how the introduction of different background jet profiles affects the model’s steady-state response to an imposed MJO-like stationary thermal forcing. Results show that a stronger jet leads to a stronger Kelvin-mode response in the tropics up to a critical jet speed, along with stronger divergence anomalies in the vicinity of the forcing. To understand this behavior, additional calculations are performed in which a localized vorticity forcing is imposed in the extratropics, without any thermal forcing in the tropics. The response is once again seen to include pronounced equatorial Kelvin waves, provided the jet is of sufficient amplitude. A detailed analysis of the vorticity budget reveals that the zonal-mean zonal wind shear plays a key role in amplifying the Kelvin-mode divergent winds near the equator, with the effects of nonlinearities being of negligible importance. These results help to explain why the MJO tends to be strongest during boreal winter when the Indo-Pacific jet is typically at its strongest.Significance StatementThe MJO is a planetary-scale convectively coupled equatorial disturbance that serves as a primary source of atmospheric predictability on intraseasonal time scales (30–90 days). Due to its dominance and spontaneous recurrence, the MJO has a significant global impact, influencing hurricanes in the tropics, storm tracks, and atmosphere blocking events in the midlatitudes, and even weather systems near the poles. Despite steady improvements in subseasonal-to-seasonal (S2S) forecast models, the MJO prediction skill has still not reached its maximum potential. The root of this challenge is partly due to our lack of understanding of how the MJO interacts with the background mean flow. In this work, we use a simple one-layer atmospheric model with idealized heating and vorticity sources to understand the impact of the subtropical jet on the MJO amplitude and its horizontal structure.