DISCONTINUITIES IN THE NMC WINTER 500-MB AND 700-MB GEOPOTENTIAL HEIGHT DATA

Characteristics of the discontinuities in the National Meteorological Center (NMC) gridded winter 500-mb and 700-mb geopotential height fiel ds were examined for the northeast Pacific, North America, and the nor thwest Atlantic. It was found that for the same 700-mb heights, the 50 0-mb heights during 1958-92 were mostly lower than those during 1948-5 7. The difference was especially significant for an area running acros s the study area between 25 degrees and 45 degrees N. In some regions, the areas with significant discontinuities extended to 60 degrees N o r even farther north. In these regions, the 500-mb heights seemed to b e more sensitive to changes in observation time and analysis procedure s than the 700-mb heights. It was also found that for the same surface temperatures across the United States, both 500- and 700-mb geopotent ial heights over the eastern United States were lower during 1958-92 t han 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 obs ervation times, some may be related to real changes in the atmospheric conditions. The characteristics of the discontinuities in the early 1 950s and mid-1970s indicate different relationships between geopotenti al height and surface temperature during warm and cold periods. In add ition, strong Fl Nino-Southern Oscillation events in the 1970s and 198 0s may have also influenced the geopotential height-surface temperatur e 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.