Abstract
Characteristics of the discontinuities in the National Meteorological Center (NMC) gridded winter 500-mb and 700-mb geopotential height fields were examined for the northeast Pacific, North America, and the northwest Atlantic. It was found that for the same 700-mb heights, the 500-mb heights during 1958–92 were mostly lower than those during 1948–57. The difference was especially significant for an area running across the study area between 25° and 45°N. In some regions, the areas with significant discontinuities extended to 60°N or even farther north. In these regions, the 500-mb heights seemed to be more sensitive to changes in observation time and analysis procedures than the 700-mb heights. It was also found that for the same surface temperatures across the United States, both 500- and 700-mb geopotential heights over the eastern United States were lower during 1958–92 than during the earlier period. Further investigation revealed several breaks during a 25-year period from 1952 to 1976. Although most of the discontinuities coincided with changes in analysis procedures and observation times, some may be related to real changes in the atmospheric conditions. The characteristics of the discontinuities in the early 1950s and mid-1970s indicate different relationships between geopotential height and surface temperature during warm and cold periods. In addition, strong El Niñlo–Southern Oscillation events in the 1970s and 1980s may have also influenced the geopotential height-surface temperature relationship. Results from this study suggest that caution should be exercised when the NMC geopotential height dataset is used in search for long-term variation patterns.
