Study On The Variability Of PNA Events

In this study, daily PNA index, Nino3.4index and NCEP/NCAR (National Centerfor Environment Prediction/National Center for Atmospheric Research) reanalysisdata are used. The variation characteristics of frequency, distribution, structure ofPNA events are studied on. Some preliminary explanation of part of thecharacteristics is given in the study.It is found that there is a break point of PNA index between1976and1977winter,after that, PNA index increases and the increase trend become distinct from1980’s.We name the stage before the break point for P1(1950-1976) and after it forP2(1977-2009). We extract PNA events of P1and P2and we find that more PNA+events exist during P1, while more PNA-events exist during P2. The results ofcomposite analysis with PNA index suggest that during P1the strength of both PNA+events and PNA-events are larger than that during P2and during both P1and P2, inthe mature stage of PNA+events there is a north-south dipole structure over NorthPacific existing for at least2days, while for PNA-events the two activities display asnortheast-southwest. By calculating average of the300hPa zonal wind both of P1andP2, we found the North Pacific subtropical jet stream is northerly at the early stage ofPNA+events and southerly at the early stage of PNA-events. The subtropical steamjet over north Pacific is northerly during P1and southerly during P2.For further explain the variability of PNA events, we extract PNA events of thecold and warm ENSO phase years. It is found that the number of PNA+is4.125timesof the number of PNA-in the warm phase years of the ENSO cycle, while in the coldphase years, the number of PNA-is2.727times of the number of PNA+. Then wecomposite the300hPa stream function field and the daily PNA index of the PNA+and PNA-, separately. The results show that the intensity of PNA+(PNA-) is weakerthan that of PNA-(PNA+) in the warm (cold) ENSO phase years. Using K-meansclustering algorithms, three weather regimes over the Pacific-North American region are obtained. In the warm (cold) ENSO phase years, the proportion of PNA+(PNA-)regime is greater than PNA-(PNA+) regime. By calculating the basic flow over theNorth Pacific, it is found that the subtropical stream jet of warm ENSO phase years isnortherly, while the jet of cold ENSO phase years is southerly over the central ofNorth Pacific, which explains the significant difference between the frequency ofPNA events in the warm and cold ENSO phase years. As the PNA events really existin the neutral years, it indicates that the PNA is an internal process of the atmosphereand ENSO cycle play a role on the modulation of the frequency and intensity PNA.