The North Pacific Oscillation and Eigenvectors of Northern Hemisphere Atmospheric Circulation during Winter
INSTAAR Occasional Paper 31
1979, 177 pp.
A climatological study of the North Pacific Oscillation (NPO) between 1906 and 1978, and a related analysis of the eigenvectors of northern hemisphere sea level pressure (SLP) and 500 mb height departures for the winters 1946–47 through 1976–77, are performed. Two types of eigenvector analysis are carried out: the first is for mean winter pressure and height anomalies for the 31 winters and the second is for daily pressure and height departures during each individual winter. Eigenvector patterns based on daily data are compared between winters and also with the seasonal mean anomaly eigenvector patterns. Eigenvector patterns based upon unnormalized data appear to show real spatial variability in atmospheric anomaly fields whereas eigenvectors from normalized data are less useful. The associations between the time coefficients of the pressure and height eigenvector patterns, and their relationship to the climatological fields associated with the NPO and the Greenland-Europe winter temperature “seesaw,” or North Atlantic Oscillation (NAO), are elaborated.
The NPO results from longitudinal variations in the mean position of the Aleutian Low and is associated with regional variations in climatic parameters, particularly over North America. The NAO is due to variations in the intensity of the Icelandic Low and is associated with hemispheric variations in climatic parameters. Both oscillations, and their eigenvector pattern equivalents, are climatic features characterized by interannual variability. The NAO is associated with hemispheric trends in temperature, which vary with the relative frequency of the two modes of the oscillation. In contrast neither mode of the NPO has predominated over the other since 1906. The two oscillations represent separate patterns of teleconnection in pressure and winds.
Other results show that large, significant variations in air temperature and precipitation occur between modes of the NPO. Variations in the Bering Sea ice edge averaged about 275 km between occurrences of the two oscillation modes during recent winters, but there was little significant difference in sea surface temperatures in the North Pacific Ocean.
The patterns of January mean pressure variability during the modes of the NPO are shown to resemble the second eigenvector of SLP anomalies determined in an independent study employing January data. Both the eigenvector and pressure variability patterns change spatially when data for winters (DJF) are used, suggesting that the climatic fields associated with this oscillation vary during the course of the winter. This oscillation is compared with the climatic fields found for the NAO, which resembles the first eigenvector when either January or wintertime data are used. The NAO shows little change during winter months.
The time coefficients of the first eigenvector pattern of 500 mb height anomalies are highly correlated to the coefficients of both the first and second SLP eigenvectors. The thermal anomalies in the atmosphere suggested by the first 500 mb eigenvector pattern also resemble those which are associated with the NAO and NPO. The analysis of daily-data eigenvectors for individual winters shows between-year differences. Winters when daily-data eigenvector patterns resemble the mean winter anomaly field are usually those when one or both of the oscillations occur and particularly modes with persistent blocking. Most eigenvector patterns based on daily data resemble weather noise, but the percent of the total variance they explain increases when the data are filtered using five-day or eleven-day running means. The patterns change little when the data are smoothed.