A climatological study of the North Pacific Oscillation (NPO) during Januaries and winters (D-J-F) between 1906 and 1978 is presented. The NPO is defined in terms of opposition in sign of mean temperature anomaly between western Alaska-eastern Siberia (as represented by the stations at Dutch Harbor and St. Paul, Alaska) and western Canada (as represented by Edmonton). The two oscillation modes, Aleutians below normal temperature (Edmonton above normal) and Aleutians above normal temperature (Edmonton below normal), are associated with variations in the strength of the zonal westerlies over the Pacific and North America which in turn result from longitudinal variations in the mean position of the Aleutian low. A westward location of the mean low. over the Sea of Okhotsk, is associated with the development of intense mean high pressure cells over western North America while zonal flow is associated with a more eastward location of the mean low over the Gulf of Alaska. Significant spatial correlations exist between the strength of the zonal component of the geostrophic wind over the Pacific Ocean and the zonal component of the polar and subtropical easterlies over the Pacific sector.
The NPO modes are associated with large regional variations in air temperature and precipitation over North America, sea surface temperatures in the North Pacific, and Bering Sea ice. The NPO is found to resemble the second eigenvector of January and winter sea level pressure anomalies. In contrast to its more frequently occurring counterpart, the North Atlantic Oscillation, the NPO is not clearly associated with hemispheric mean temperature trends and its modes occur randomly with time. The two oscillations represent separate patterns of teleconnections of sea level pressure and zonal geostrophic winds over the Northern Hemisphere.