| description abstract | The North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are compared from the standpoint of their association with Northern Hemisphere winter mean distributions of sea-level pressure (SLP) and 500 mb height. The NAO and SO are associated with significant SLP differences over much of the hemisphere except for Siberia and western North America. Significant SLP and 500 mb height differences occur in the NAO over the Atlantic Ocean and near Baja California, while in the SO they occur over the Pacific Ocean, India and the western Atlantic. Only over the latter region do large pressure and height variations consistently occur in the extremes of both oscillations; these are also associated with winter temperature variability over the southeastern United States. For example, during winter 1982?83, when the two oscillations simultaneously reached extremes, the NAO was associated with record December warmth east of the Mississippi River, but during January and February the SO dominated the height and air temperature distributions over the United States. The cospectrum of the NAO index and Darwin (Australia) pressure is largest at intermediate frequencies with periods of about 6 years, although the NAO itself has peak energy at 7.3 years. The NAO is characterized by a large trend toward lower index in the twentieth century through the 1960s; this is not associated with variations in the SO. In the 80 winters of data, simultaneous occurrences of particular modes of one oscillation with those of the other seem to occur by chance. | |
