Two Extratropical Pathways to Forcing Tropical Convective Disturbances
Two Extratropical Pathways to Forcing Tropical Convective Disturbances
复制标题
迫使热带对流扰动的两种温带途径
DOI：
10.1175/jcli-d-22-0171.1
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发表时间：
2022
影响因子：
4.9
通讯作者：
Dias, Juliana
中科院分区：
文献类型：
--
作者：
Cheng, Yuan-Ming;Tulich, Stefan;Kiladis, George N.;Dias, Juliana
通过对卫星OLR和ERA 5再分析资料的统计分析，证明了两条强热带对流路径的观测证据。强迫机制和由此产生的扰动强烈依赖于背景纬向风的结构。尽管Rossby波在东风中被禁止传播，但模拟研究表明，热带外强迫仍然可以通过热带和热带外之间的共振激发赤道波。在这里，这个“远程”强迫路径的背景下，对流耦合开尔文波在热带太平洋北方夏季首次进行了调查。热带外强迫表现为当热带外涡旋传播到副热带并遇到其临界线时产生的涡旋动量通量辐合。这种非线性强迫与开尔文波具有相似的波数和频率，并通过投射到纬向风中的纬向本征结构上来激发它们，作为共振的一种形式。这种共振也证明了动量预算分析，它揭示了非线性强迫项是必不可少的波的维护，而其余的线性项是必不可少的传播。相比之下，“本地”路径的extratrophilic强迫需要存在的西风管道在北方冬季，允许Rossby波传播到赤道东太平洋，而排除任何形式的共振与开尔文波由于多普勒频移效应。入侵的扰动主要通过准地转强迫激发热带“云羽”，同时保持其热带外的性质。这项研究表明，在热带环流强迫的热带外的多重角色，并阐明如何热带-热带外的相互作用和热带外的基本状态可以提供在S2 S时间scale.Significance StatementThis研究的可预测性的来源，旨在了解如何在中纬度地区的环流激发在热带地区的天气系统。结果表明，热带天气系统的激发机制和类型与大尺度平均风场结构密切相关。特别是，当大尺度风从东向西吹时，一种特殊类型的向东移动的热带天气系统，开尔文波，由于它与热带外遥远的向东移动的天气系统共振而被激发。相反，当平均风从西向东吹时，中纬度系统被观察到侵入低纬度地区，直接迫使热带对流，即云羽，同时保持其热带外性质。这些结果说明了中纬度地区如何在不同的气候风态下激发不同类型的热带天气系统。了解这些热带天气系统及其与中纬度地区的相互作用可能最终有助于改善对两周以上天气的预测。
Observational evidence of two extratropical pathways to forcing tropical convective disturbances is documented through a statistical analysis of satellite-derived OLR and ERA5 reanalysis. The forcing mechanism and the resulting disturbances are found to strongly depend on the structure of the background zonal wind. Although Rossby wave propagation is prohibited in easterlies, modeling studies have shown that extratropical forcing can still excite equatorial waves through resonance between the tropics and extratropics. Here this “remote” forcing pathway is investigated for the first time in the context of convectively coupled Kelvin waves over the tropical Pacific during northern summer. The extratropical forcing is manifested by eddy momentum flux convergence that arises when extratropical eddies propagate into the subtropics and encounter their critical line. This nonlinear forcing has similar wavenumbers and frequencies with Kelvin waves and excites them by projecting onto their meridional eigenstructure in zonal wind, as a form of resonance. This resonance is also evidenced by a momentum budget analysis, which reveals the nonlinear forcing term is essential for maintenance of the waves, while the remaining linear terms are essential for propagation. In contrast, the “local” pathway of extratropical forcing entails the presence of a westerly duct during northern winter that permits Rossby waves to propagate into the equatorial east Pacific, while precluding any sort of resonance with Kelvin waves due to Doppler shifting effects. The intruding disturbances primarily excite tropical “cloud plumes” through quasigeostrophic forcing, while maintaining their extratropical nature. This study demonstrates the multiple roles of the extratropics in forcing in tropical circulations and illuminates how tropical–extratropical interactions and extratropical basic states can provide be a source of predictability at the S2S time scale.Significance StatementThis study seeks to understand how circulations in the midlatitudes excite the weather systems in the tropics. Results show that the mechanisms, as well as the types of tropical weather systems excited, are strongly dependent on the mean large-scale wind structure. In particular, when the large-scale wind blows from east to west, a special type of eastward-moving tropical weather system, the Kelvin wave, is excited owing to its resonance with remote eastward-moving weather systems in the extratropics. On the contrary, when the average wind blows from west to east, midlatitude systems are observed to intrude into the lower latitudes and directly force tropical convection, the cloud plumes, while maintaining their extratropical nature. These results speak to how the midlatitudes can excite distinct types of tropical weather systems under different climatological wind regimes. Understanding these tropical weather systems and their interactions with the midlatitudes may ultimately help to improve predictions of weather beyond 2 weeks.