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非线性水汽-对流和海洋反馈在MJO事件形成中的作用
Alternative TitleRole of nonlinear moisture-convection and oceanic feedbacks in shaping MJO events
魏云涛
Subtype博士
Thesis Advisor穆穆 ; 任宏利
2019-05-14
Degree Grantor中国科学院大学
Place of Conferral中国科学院海洋研究所
Degree Name理学博士
Degree Discipline物理海洋学
Keyword热带大气季节内振荡(mjo) 条件非线性最优扰动(cnop) 水汽反馈 海-气相互作用 Mjo行星尺度选择 非线性 非线性对流加热 原发型mjo事件 海洋反馈 水汽-对流反馈
Abstract热带大气季节内振荡(MJO)作为热带大气最强的季节内变率(ISV)模态,对全球的天气和气候都产生了重要的调制作用。开展与MJO相关的机理和可预报性研究可为实现无缝隙天气气候预测提供可能。本文首次从非线性水汽-对流和海洋反馈的角度系统地研究了MJO事件的形成过程。研究结果主要揭示了以下具有重要科学意义和潜在应用价值的结论:非线性对流加热和海-气热力学相互作用在MJO如何由小尺度扰动发展为行星尺度的、缓慢东传的、环流-对流耦合的系统中扮演重要角色;非线性水汽-对流和海洋反馈对于原发型MJO事件的爆发起到了关键作用。本文首次将条件非线性最优扰动(CNOP)方法应用于原发型MJO事件的可预报性研究中,其结果不仅揭示了同化水汽场对预报原发型MJO事件的重要性,还强调了在次季节尺度(2周~1个月)的预报中考察非线性过程的作用是有必要的。 本文构建了适用于研究MJO行星尺度选择这一基本科学问题的海-气耦合动力水汽(ASDM)模型框架。非线性ASDM模型无论在纬向均匀还是类暖池的背景态下,都能较好地模拟观测中MJO的若干基本特征,包括:1)纬向行星尺度、赤道捕获、环流-对流耦合、混合Rossby-Kelvin波水平结构;2)暖池区的不稳定缓慢东传(4.2 m s-1)以及冷舌区的衰减和东传速度的增大(9.2 m s-1);3)东风以及正的海表温度(SST)异常超前MJO对流活动中心;4)环流-降水的纬向不对称性。MJO特有的行星尺度选择性只在采用了非线性对流降水参数化的ASDM模式中再现,而线性以及非耦合模式则不能模拟出该现象。这表明MJO行星尺度选择本质上是由非线性加热决定;海-气相互作用,特别是风-蒸发-夹卷反馈则会额外提供不稳定能源来支撑MJO行星尺度选择。另外,采用暖池背景态会加速MJO的行星尺度选择过程。MJO行星尺度选择的过程为:从初始小扰动开始,某个对流分支会随机地剧烈增长并领先其它对流分支。暖池背景态会加快这种随机选择过程。较强的对流分支激发出的干Kelvin波(东风异常)在5天之后会赶上其它缓慢东传的湿对流分支系统并部分抵消其西风异常。SST梯度和水汽辐合的减小会进一步减弱对流降水。最后,行星纬向一波的MJO结构被筛选出来。 本文选用对印-太暖池区气候变率,特别是ISV/MJO具有较好模拟能力的夏威夷大学耦合总环流模式并结合CNOP方法发展了一套适用于原发型MJO事件可预报性研究的非线性新方法框架。该方法框架从非MJO参考态入手旨在求解相应的非线性最优化问题来探究到底何种初始水汽扰动能够导致一次最强原发型MJO事件的爆发。研究表明,相比于随机扰动,具有特定空间结构的水汽CNOPs可以作为原发型MJO事件爆发的一种最优前期征兆,并且表现为沿赤道印度洋一致加湿模态,而在远离赤道方向上则表现为干异常。水汽CNOPs的真实性通过观测资料和长期模拟数据得以证实。通过水汽收支诊断揭示出水平水汽平流,特别是与高频经向风相关的非线性水汽平流是支配原发型MJO事件爆发的关键非线性物理过程。小波分析进一步表明与准3–4和准6–8天扰动相关联的高频赤道波动在调节非线性水汽平流中的重要性。而垂直柱过程,包括次网格尺度蒸发和夹卷等,对于大气边界层的增湿也起到了一定的作用。这些研究结果的价值体现在:它强调了同化水汽场以及低层对流层的非线性水汽-对流反馈在原发型MJO事件爆发中的重要性。海洋反馈对原发型MJO事件爆发的重要作用主要体现在两个方面:触发深对流云并维持其缓慢不稳定东传。海洋混合层热量收支诊断表明决定原发型MJO事件爆发前期SST增暖的主要因素包括:混合层底部夹卷效应以及海表热通量反馈。海-气相互作用在解释MJO正负位相振幅不对称性以及北半球冬季MJO在海洋性大陆南侧“绕流”现象方面具有重要作用。
Other AbstractAs the strongest intraseasonal variability (ISV) in the tropical atmosphere, the Madden-Julian Oscillation (MJO) plays an important role in modulating global weather and climate. Theoretical and predictability studies related to MJO can provide possibility for seamless weather and climate prediction. In this paper, the shaping of MJO events is systematically studied for the first time from the perspectives of nonlinear moisture-convection and oceanic feedbacks. The research results mainly reveal the following conclusions with important scientific significance and potential application value: nonlinearly convective heating and thermodynamically air-sea interaction play an important role in controlling how MJO develops from small-scale disturbances to a planetary-scale, slowly eastward-propagating, circulation-convection coupled system. Nonlinear moisture-convection and oceanic feedbacks play a key role in the onset of primary MJO events. The conditional nonlinear optimal perturbation (CNOP) method is firstly applied here to investigate the primary MJO initiation, and the results not only reveal the importance of assimilating moisture in the predictability study of primary MJO events, but also highlight the necessity of considering nonlinearity in subseasonal-scale (two weeks~one month) prediction. This paper has constructed air-sea coupled dynamical moisture (ASDM) model to study one of the most fundamental scientific questions related to the MJO, that is, the planetary scale selection. The nonlinear ASDM model can well simulate some basic characteristics of MJO in observation no matther in the zonally uniform or warm-pool-like basic state, including: 1) zonally planetary scale, equatorially trapped, circulation-convection coupled, mixed Rossby-Kelvin wave horizontal structure; 2) unstable and slowly eastward propagation (4.2 m s-1) in the warm pool area and the decaying while fast propagation (9.2 m s-1) in the cold tongue area; 3) easterly wind and positive sea surface temperature (SST) anomalies leading the MJO convetion activcity center; 4) zonal asymmetry in the circulation-precipitation. The planetary scale selectivity characteristic of MJO can only be reproduced in the ASDM model with nonlinear convective precipitation parameterization, while the linear and uncoupled models cannot simulate this phenomenon. This shows that the planetary scale selection of MJO is essentially determined by nonlinear heating, and the sea-air interaction, especially the wind-evaporation-entrainment feedback, will provide additional unstable energy to support the planetary scale selection of MJO. In addition, the adoption of warm pool background state will accelerate the planetary scale selection process of MJO. The process of planetary scale selection for MJO is as follows: starting from the initial small disturbance, one convection branch will randomly grow dramatically and lead the others. The warm pool background state will accelerate the process of random selection. The dry Kelvin wave (east wind anomaly) excited by the strong convection branch would catch up with other slowly eastward wet convection branch systems and partially offset its west wind anomaly after 5 days. The decrease of SST gradient and water vapor convergence will further weaken convective precipitation. Finally, the structure of the planetary-scale, zonal wavenumber-1 MJO is selected. In this paper, the coupled general circulation model of the University of Hawaii, which shows a good fidelity in simulating the Indo-Pacific climatic variabilities, especially the ISV/MJO, combined with the CNOP method has bee chosen to develop a new nonlinear methodology framework for the study of the predictability of primary MJO events. Starting from the non-MJO reference states, this methodological framework aims to solve the corresponding nonlinear optimization problem to explore what kind of initial moisture perturbations can lead to the onset of the strongest primary MJO event. Results have shown that, compared with random perturbations, CNOPs of moisture with specific spatial structure can be regarded as an optimal precursor in triggering the primary MJO initiation, and is manifested as uniform humidification mode along the equatorial Indian Ocean, while it is manifested as dry anomaly in the direction away from the equator. The authenticity of moisture CNOPs is confirmed by both observation data and long-term simulation data. Through the moisture budget analysis, it is revealed that horizontal moisture advection, especially the nonlinear moisture advection associated with high-frequency meridional wind, is the key nonlinear physical process dominating the onset of the primary MJO events. The wavelet analysis further shows the importance of high-frequency equatorial waves associated with quasi-3–4 and quasi-6–8 day perturbations in modulating the nonlinear moisture advection. Vertical column processes, including subgrid-scale evaporation and entrainment, also play a role in the humidification of boundary layer. These results are valuable in that they highlight the importance of assimilating humidity fields and nonlinear moisture-convection feedback in the lower troposphere in the onset of primary MJO events. Oceanic feedback plays an important role in preconditioning primary MJO events in two aspects: triggering the deep convective cloud and maintaining its slow and unstable eastward propagation. The oceanic mixed layer heat budget diagnosis indicated that the main factors determining the SST warming in the pre-onset stage of primary MJO event include the entrainment beneath the mixed layer and net surface heat flux feedback. The air-sea interaction plays an important role in explaining the asymmetry of positive and negative phase amplitudes of MJO and the "detour" of MJO in boreal winter via the southern Maritime Continent.
Subject Area海洋科学
MOST Discipline Catalogue理学::海洋科学
Pages321
Language中文
Document Type学位论文
Identifierhttp://ir.qdio.ac.cn/handle/337002/156812
Collection海洋环流与波动重点实验室
Recommended Citation
GB/T 7714
魏云涛. 非线性水汽-对流和海洋反馈在MJO事件形成中的作用[D]. 中国科学院海洋研究所. 中国科学院大学,2019.
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