The Growth and Triggering Mechanisms of the PNA : A MJO-PNA Coherence
The Growth and Triggering Mechanisms of the PNA : A MJO-PNA Coherence
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DOI：
10.2151/jmsj.86.213
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发表时间：
2008-02
影响因子：
3.1
通讯作者：
M. Mori;M. Watanabe
中科院分区：
文献类型：
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作者：
M. Mori;M. Watanabe
在北方温带冬季，一种被称为太平洋/北美（PNA）遥相关型的低频变率的突出模式不仅在季节上而且在季节内时间尺度上普遍存在。在这项研究中，在1957-2002年期间，使用日常领域的季节内PNA的过程进行了调查。涡度收支分析的结果表明，PNA的正位相往往增长的线性过程，如正压能量转换的纬向非对称的气候流。对于PNA的负位相，非线性低频涡度平流与线性过程一样重要。PNA的合成生命周期表明，在峰值前9天，沿亚洲急流沿着观察到一个明显的波列，并最终发展到急流出口区附近的PNA。这个波列被发现是由主要与马登-朱利安振荡（MJO）的异常对流有关的辐散风激发的。概率密度函数的MJO计算分别以下的PNA的极性揭示了PNA和MJO之间的相位锁定。当与MJO相关的活跃（不活跃）对流到达孟加拉湾至西太平洋时，PNA的负（正）相出现频率最高。这种MJO触发解释了大约30%的总PNA事件，这表明，即使PNA可能是固有的outtratrophic大气，一个特定的热带强迫是重要的实现PNA为主模式。
A prominent mode of low-frequency variability in the northern extratropical winter known as the Pacific/North American (PNA) teleconnection pattern prevails not only on seasonal but also on intraseasonal timescales. In this study, processes governing the intraseasonal PNA are investigated using daily fields during 1957-2002. The results of the vorticity budget analysis illustrate that the positive phase of the PNA tends to grow by linear processes such as the barotropic energy conversion from the zonally asymmetric climatological flow. For the negative phase of the PNA, nonlinear low-frequency vorticity advection is as important as the linear processes. Composite life cycle of the PNA shows that at 9 days before the peak a pronounced wave train was observed along the Asian jet stream and it eventually developed to the PNA near the jet exit region. This wave train is found to be excited by divergent winds primarily associated with anomalous convection of the Madden-Julian Oscillation (MJO). Probability density functions of the MJO calculated separately following the polarity of the PNA reveal a phase locking between the PNA and the MJO. When the active (inactive) convection associated with the MJO reaches the Bay of Bengal to the western Pacific, occurrence frequency of the negative (positive) phase of the PNA is the highest. This MJO triggering explains roughly 30% of the total PNA events, suggesting that, even though the PNA may be inherent to the extratropical atmosphere, a specific tropical forcing is of importance to realize the PNA as dominant mode.