Interannual Variations in Synoptic-Scale Disturbances over the Western North Pacific
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The present study investigates the interannual variation of June–November synoptic disturbance activity over the western North Pacific (WNP) and its relationship with large-scale circulation for the period 1958–2014. Two leading modes of eddy kinetic energy for the disturbance variability over the WNP are obtained by EOF analysis, characterized by anomalous eddy kinetic energy over the subtropical WNP and around the Philippines, respectively. These modes explain a large portion of the interannual variance of synoptic disturbance activity over the WNP. Both are associated with lower-level cyclonic anomalies, but with different locations: over the subtropical WNP for the first mode and over the South China Sea for the second mode. Considering the impact of ENSO on synoptic disturbance activity over the WNP, we repeat the analyses after removing the effect of ENSO, which is simply defined as the components linearly regressed onto the Niño3.4 index, and find similar results, suggesting that the leading modes and their relationships with large-scale circulation exist without SST effects. Further analyses suggest that the meridional shear of zonal winds caused by cyclonic anomalies is crucial for maintaining the leading modes through barotropic conversion.
Keywordssynoptic disturbance activity interannual variability leading mode barotropic conversion
本文主要分析了1958–2014年6–11月西北太平洋(WNP)天气尺度扰动的年际变化及其与大尺度环流的联系. 该时期扰动动能的EOF结果表明, 第一模态表征副热带WNP扰动的年际变化, 第二模态则体现菲律宾附近扰动的年际变化, 它们可以解释WNP扰动的大部分方差. 这两个模态的形成与位于各自变率中心的气旋式异常环流关系密切. 考虑到ENSO对WNP天气尺度扰动的可能影响, 我们通过对Nino3. 4指数线性回归的方法去除ENSO信号后重复了上面的分析仍得到相似的结果. 这表明主模态与大尺度环流的联系在去掉海温影响后依然存在. 进一步的能量分析表明气旋式异常环流主要由纬向风经向切变通过正压能量转化来维持扰动活动.
关键词天气尺度扰动 年际变化 主要模态 正压转换
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We thank the two anonymous reviewers for their comments, which were very helpful for improving the presentation of this paper. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41320104007, 41475074 and 41475077).
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