On Northern Hemisphere Wave Patterns Associated with Winter Rainfall Events in China
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During extended winter (November–April), 43% of the intraseasonal rainfall variability in China is explained by three spatial patterns of temporally coherent rainfall. These patterns were identified with empirical orthogonal teleconnection (EOT) analysis of observed 1982–2007 pentad rainfall anomalies and connected to midlatitude disturbances. However, examination of individual strong EOT events shows that there is substantial inter-event variability in their dynamical evolution, which implies that precursor patterns found in regressions cannot serve as useful predictors. To understand the physical nature and origins of the extratropical precursors, the EOT technique is applied to six simulations of the Met Office Unified Model at horizontal resolutions of 200–40 km, with and without air–sea coupling. All simulations reproduce the observed precursor patterns in regressions, indicating robust underlying dynamical processes. Further investigation into the dynamics associated with observed patterns shows that Rossby wave dynamics can explain the large inter-event variability. The results suggest that the apparently slowly evolving or quasi-stationary waves in regression analysis are a statistical amalgamation of more rapidly propagating waves with a variety of origins and properties.
Key wordsrainfall in China spring flooding Rossby wave dynamics EOT analysis predictability teleconnections
本文通过对1982-2007年冬季(11月–4月)逐候降水异常开展经验正交遥相关(EOT)分解, 发现中国冬季降水季节内变化的三个主导型态, 可以解释其季节内变率的43%, 且与中纬度扰动有关. 然而, 对强EOT事件的分析发现, 不同EOT事件对应的波动动力演变过程存在较大差异, 说明回归分析得到的前期信号不能作为有用的预测因子. 为了理解热带外预测因子的物理本质, 该文进一步对比分析了英国气象局一体化模式(MetUM)不同分辨率的大气环流模式和耦合模式模拟结果. 所有模拟均能模拟出观测中基于回归方法得到的前期预报因子, 说明了相应动力机制的可靠性. 对观测中降水型动力过程的进一步诊断指出, Rossby波可以解释不同EOT事件间动力过程差异的产生原因. 该文研究表明, 回归分析得到的是明显的演变缓慢的波动或准静止波, 这是统计合并多种起源、多种特性的快速传播的波动的结果.
关键词中国降水 春季洪涝 罗斯贝动力学 EOT分析 可预报性 遥相关
Claudia C. STEPHAN was supported by the UK–China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China, as part of the Newton Fund. Nicholas P. KLINGAMAN was supported by an Independent Research Fellowship from the UK Natural Environment Research Council (NE/L010976/1). APHRODITE data are available from http://www.chikyu.ac.jp/precip/. The RWS function was computed using code from the python package windspharm v1.5.0, available at http://ajdawson.github.io/windspharm. We thank Matthias RÖTHLISBERGER for providing the Rossby wave initialization segment data and helpful discussions.
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