Decadal changes in the atmospheric circulation and associated surface climate variations in the Northern Hemisphere winter

This study attempts to investigate decadal-scale climate changes in the mid- and high latitudes of the Northern Hemisphere in winter 1989 by using various observational data for atmospheric parameters, sea surface temperature (SST), and snow cover. Decadal-scale changes in the winter atmosphere after 1989 are characterized as follows: a dipole pattern of height anomalies between midlatitudes and polar regions with an equivalent barotropic structure, temperature changes with cooling in the polar region and warming in midlatitudes in the middle troposphere, and associated reduction of the subtropical jet stream. Statistical tests applied to the 500-hPa height field reveal that the changes in 1989 are a distinct discontinuity or shift on the decadal scale with hemispheric extent. The spatial structure is interpreted as a linear combination of three teleconnections: the North Atlantic oscillation, Pacific-North American, and Eurasian patterns, in addition to the zonally symmetric dipole. On the other hand, the sharpness of decadal changes in 1989 arises from synchronous phase shifts of interdecadal variations over the Pacific Ocean and quasi-decadal variations over the North Atlantic. Similar concurrence is also found in winter 1977. In agreement with previous studies, the interdecadal atmospheric variations over the North Pacific reveal a strong coupling with tropical/extratropical SST anomalies in the Pacific Ocean. A relationship between the quasi-decadal variability of the North Atlantic atmosphere and underlying SST anomalies is relatively tenuous. Surface air temperatures show a warming over all of Eurasia after winter 1989. It is shown by the analysis of snow cover in the Northern Hemisphere derived from NOAA satellites that a large decrease (increase) of the snow extent over the eastern part of the Eurasian continent occurred in autumn 1988 (1976) prior to winter 1989 (1977). It is suggested that the anomalies in the Eurasian snow cover during autumn play a role as an amplifier in the atmospheric shifts. These snow anomalies may be as important for the atmospheric changes as changes in SSTs.