Relationship between Pacific-North American Teleconnection Events and ENSO Cycle

The daily index of Pacific-North American teleconnection (PNA), the Niño 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.