The Pacific decadal oscillation (PDO) recently emerged in the literature as a robust signal in the Northern Hemisphere climate variability. Many studies reported that the relationships between PDO and East Asian monsoon (EAM) and climate variability in China are significant. Their possible mechanisms are, however, still unclear. Using the observational NCEP/NCAR reanalysis and Chinese station data during the period of 1969–2008, this study investigates the interdecadal relationship between Pacific decadal oscillation (PDO) and interhemispheric air mass imbalance or oscillation (IHO) between the Northern and Southern Hemispheres. The possible connection of PDO and IHO with both East Asian monsoon and climate variability in China are also assessed. It is found that the interdecadal components (11∼38 years) of PDO, IHO, and EAM contribute large variance to low frequency variations, and they are well‐matched with each other on (inter) decadal timescale. In particular, their negative phases mainly appeared in the 1970 s and late 1990 s, while positive phase in period from 1980 s to mid 1990 s. Decadal change of global mean air columnar temperature may be the key factor for the notable difference between PDO and IHO from mid 1970 s to mid 1990 s. The spatial distributions of PDO and IHO associated surface air temperature and surface pressure anomalies exhibit high similarity and large scale characteristics, indicative of their intimate linkage with air mass redistribution over the global domain especially over 30°S–50°N. The PDO associated columnar integral of velocity potential anomalies that maintain the air mass redistribution show a dipole pattern with air mass flux emanating mainly from the eastern hemisphere to the Pacific regions in positive PDO phase. This contributes to hemispherical and land‐sea mass exchange and redistribution, and also leads to the decadal displacement of both upward and downward branch of Walker circulation. In positive phase of PDO, an anomalous anticyclone is found in the Mongolian region in both boreal summer and winter seasons, inducing significant anomalous northerlies in the eastern China, and hence intensifying (weakening) the east Asian winter (summer) monsoon. Consequently, the interdecadal components of temperature and precipitation at most stations in east China are simultaneously correlated with the Pacific decadal oscillation index significantly.

The anomalous vertical structure of the austral summer (December∼February) zonal mean circulation [u] in association with Antarctic Oscillation (AAO) and ENSO has been investigated by means of 1979∼2004 NCEP/NCAR monthly reanalysis data. The Empirical Orthogonal Function (EOF) analysis has been performed for the zonal mean zonal wind anomalies in the present article. It is found that the EOF1 dominates the anomalous distribution of [u] in vertical with a positive-negative-positive sign pattern in the polar region, 70 • S∼50 • S and regions from 50 • S to the equator, respectively. The EOF1 exhibits the AAO signature and a close linkage with ENSO. The AAO is found to be statistically correlated with Nino3 index. To further clarify the effects of AAO and ENSO upon [u] variability in conjunction with the resulting structures, a one-variable regression scheme is utilized through Nino3 index to filter out the ENSO impact, and the remaining portion is decomposed with EOF analysis, obtaining AEOF1 for the [u] without the ENSO effect involved. It is displayed that AEOF1 represents the anomalous pattern of AAO-associated [u]. The residual of [u] after removing AAO-associated AEOF1 is analyzed by means of EOF, leading to a mode SEOF1, which is correlated with Nino3 index with a correlation coefficient of 0.9, thereby resulting in modes of [u] unrelated and related to ENSO. Besides the interdecadal and 3∼7-year interannual variations, the zonal mean zonal flow [u] shows weakening westerlies in the polar region, enhanced westerlies in the sub-polar zone, enfeebling subtropical westerlies and intensifying tropical easterlies as the long-term trends during the past 30 years.