Large-Scale Changes in North Pacific and North American Weather Patterns in Recent Decades

Abstract

This paper compares long-term seasonal means of atmospheric and oceanic fields for the recent 1969?80 period, relative to the earlier 1947?66 base period. Difference maps of North Pacific sea surface temperature (SST), 700 mb height, and United States surface temperature and precipitation are examined. An appreciable cooling of the North Pacific SST field during the 1969?80 period was found to exhibit large coherent cold pools with the maximum average cooling over the period being ?1°C for the coldest locations. The center of maximum cooling varied with season; its position shifted eastward from winter to fall. The pattern of SST cooling corresponds well to the patterns of difference of 700 mb height over the North Pacific for the 1969?80 period, relative to 1947?66. The areas of cooling have generally occurred in regions of strengthened northerly wind components and/or low heights, a fact which can be rationalized by physical arguments. The winter and spring seasonal averages for 1969?80 are characterized by below normal heights in the region of the Gulf of Alaska and the Aleutians. Downstream, the field of 700 mb height over North America shows differences that are well related to those over the North Pacific, as shown by long-period teleconnections of seasonal averaged heights. The height differences over the continent are large and spatially coherent., for instance, the winter average shows a deep negative area affecting the eastern half of the United States. The patterns of height difference over the coterminous United States are easily related to those of surface temperature and precipitation. The temperature and precipitation differences also display large spatial scales, with average differences over the 1969?80 period large enough to designate them an extreme tercile class in many states. Thus, reasoning for very short period (e.g., seasonal) changes shows that climate variations may be applied to longer period (e.g., decadal) phenomena.