The winter atmospheric circulation underwent two abrupt changes during recent two decades. One occurred in 1977 and the other in 1989. Significant changes in the atmospheric circulation over the Northern Hemisphere were found in geopotential height, sea level pressure and surface temperature. Both spatial distribution and temporal evolution of the two sudden changes are well captured by the leading EOF mode of winter 500mb height. The abrupt changes thus can be viewed as a part of decadal variability in the atmosphere. The first EOF mode of the 500mb height shows a significant correlation with the North Pacific sea surface temperature (SST).
To examine the relationship between this decadal variability and SST, a set of long-term simulations with an atmospheric general circulation model was performed, forced by the observed time-varying SST. An ensemble of twelve simulations (SST runs) is compared with that from a control simulation using a climatological annual cycle of SST. The circulation variations in the control run are an indication of internal variability of the model atmosphere. The first EOF mode of 500mb height in the control run has a decadal time scale with a spatial pattern similar to the observed. The two leading EOF modes of the 500mb height from the ensemble of the SST runs show significant variability over the Northern Hemisphere, associated with Pacific SST. The second mode also displays changes in circulation around 1977 and 1989. The SST-forced variance, however, is smaller than the internal variance. The results suggest the importance of internal atmospheric dynamics and the influence of Pacific SST in determining the atmospheric decadal variability.