Changes in the variability of North Pacific Oscillation around 1975/1976 and its relationship with East Asian winter climate

[1] The variability of North Pacific Oscillation (NPO) and the associated atmospheric circulation in the boreal winter are studied with the HadSLP2, the National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR), and the ECMWF 40-year reanalysis (ERA-40) data. The results show that the variability of NPO is not stationary with a typical period around 3 years during 1957 - 1975 and shifting to around 5 - 6 years after that. Linear correlation and regression analysis indicates that in the winters before 1975 the geopotential height field related to the NPO is characterized by a barotropic north-south dipole around the Pacific sector, which is the traditional NPO mode. In this subperiod, the El Nino/Southern Oscillation (ENSO) has a dominant influence on the NPO. There is anomalous stationary wave propagation associated with the NPO from the subtropical-central Pacific to the northern Pacific. And the NPO has no significant relations to the East Asian climate. In the winters after 1975, however, the atmospheric circulation related to the NPO exhibits a circum-global wave train pattern over the extratropical regions in the Northern Hemisphere. Particularly, the northward wave activity propagation is enhanced over the extratropics of East Asia. The remote forcing from the tropical eastern Pacific sea surface temperature anomalies becomes small. Hence the NPO has a close relationship with the circulation over East Asia during this period.