Interannual and Decadal Variability of the North Pacific Blocking and Its Relationship to SST,Teleconnection and Storm Tracks

Climatological features of mid-latitude blocking occurring over the North Pacific Ocean during 52winters (December to February) of 1948/1949-1999/2000 are statistically analyzed based onNCEP/NCAR reanalysis data. Significant interannual variation with a period of about 3-7 years as wellas decadal variability is found by wavelet analysis and power spectrum analysis. A decreasing trend of the2-7 year bandscale-averaged variance occurs throughout the 52 years and an abrupt shift from a higherstate to a lower state during the 1970s is also found, which suggests an interdecadal variation of the NorthPacific blocking. The possible relationship between the variability of blocking and sea surface temperature(SST), storm tracks and teleconnection are shown using composite analysis. In strong blocking anomalywinter (SBW), the geopotential height anomaly at 500 hPa exhibits a typical PNA (Pacific-NorthAmerican)-like wave-train pattern in the North Pacific. The storm tracks, representing the activity of tran-sient eddies, extend northeastward to the western coast of North America along the mid latitudes of about40°-50°N, with the SST anomaly exhibiting a Pacific decadal oscillation (PDO) mode at mid-latitude and aLa Nina-like pattern along the equator. Contrasting features appear in weak blocking anomaly winter(WBW).