ENSO循环与太平洋—北美型遥相关事件的关系

Relationship between Pacific-North American Teleconnection Events and ENSO Cycle

利用1950～2009年的Nifio3.4指数和冬季5个月逐日太平洋-北美型遥相关（PNA）指数，对ENSO冷、暖位相及中性年份冬季发生的PNA事件进行了统计分析，结果发现PNA事件个数有显著的差异。ENSO暖位相年份冬季的 PNA＋事件个数是 PNA-事件个数的4.13倍，而冷位相年份的 PNA-事件个数是 PNA＋事件个数的2.73倍。PNA 事件指数和 PNA 事件对应的300 hPa 流函数场合成分析结果显示 ENSO 暖（冷）位相年份的PNA＋（PNA-）事件的强度较小，生命史较长，PNA-（PNA＋）事件的强度较大生命史较短，位于北太平洋上空的南北向偶极子结构仅出现在 PNA＋事件过程的成熟阶段。同时，聚类分析也发现，ENSO 暖（冷）位相年份的PNA＋（PNA-）型的比例大于PNA-（PNA＋）型，PNA＋型波列结构中有太平洋区域的南北向偶极子结构，而PNA-波列结构中没有，与合成分析结果一致。通过计算北太平洋的300 hPa基本流发现，ENSO循环暖位相年份的副热带急流位置比ENSO循环冷位相年份的偏北，这解释了ENSO循环冷暖位相年份PNA＋事件和PNA-事件发生频率的差异。由于中性年份也存在PNA事件，ENSO循环对PNA事件的强度与发生频率有一定的调制作用。

The daily index of Pacific-North American teleconnection （PNA）, the Ni？o 3.4 index, and NCEP/NCAR reanalysis data were used. These data cover the months November-March for 1950-2009. We extracted the PNA events for the cold and warm ENSO phase years. We found that the number of PNA＋is 4.13 times the number of PNA-in the warm phase years of the ENSO cycle, while in cold phase years, the number of PNA-is 2.73 times the number of PNA＋. We composited the 300-hPa stream function field and the daily PNA index of the PNA＋and PNA-separately. The results show that the intensity of PNA＋（PNA-） is weaker than that of PNA-（PNA＋）, the life cycle of PNA＋（PNA-） is longer than that of PNA-（PNA＋） in the warm （cold） ENSO phase years, and a north-south dipole anomaly is present over the North Pacific during the mature period of all types of PNA＋events. Using K-means clustering algorithms, three weather regimes over the Pacific-North American region were obtained. In the warm （cold） ENSO phase years, the proportion of the PNA＋（PNA-） regime is greater than PNA-（PNA＋） regime, and a north-south dipole anomaly is present over the North Pacific for all PNA＋regimes. By calculating the basic flow over the North Pacific, we found that the subtropical stream jet of warm ENSO phase years is northerly, while the jet of cold ENSO phase years is southerly over the central part of the North Pacific, which explains the significant difference between the frequency of PNA events in the warm and cold ENSO phase years. As PNA events occur in neutral years, the ENSO cycle plays a role in the modulation of the frequency and intensity of PNA.