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| NOAA's Paleoclimatology Program utilizes data from sources other than instrumental weather records (what are called proxy data which include, for example, tree rings, ice cores, and pollen analysis) to understand and model interannual to century-scale environmental variability. Proxy data have been used to reconstruct drought indices going back several hundred years to provide a long-term perspective on drought variability, particularly when it comes to multi-decadal droughts that have occurred prior to comprehensive instrumented records. Such an expanded view provides insight into what a "normal" climatic range is for a region. |
| The graph to the right shows the summer Palmer Drought Index for the last 325 years for the southeastern U.S. centered around South Carolina, southern North Carolina, and northeast Georgia. The time series in blue is the Palmer Hydrological Drought Index computed from June-August weather observations over the period 1900-2001. The time series in red is a reconstruction of the Palmer Index using tree ring data covering the period 1677-1979. The asterisk is the value for 2002 estimated from June-July data. (The methodology is discussed in an article by Cook et al., 1999; details are also available at the NOAA Paleoclimate web site.) The tree ring data successfully reproduce the droughts of the early 1910s, mid-1920s, 1930s, and 1950s, although due to its nature it is a conservative index of severity. The reconstructed index shows that droughts nearly as severe as the worst during the 20th century have occurred many times during the past three centuries. More importantly, prolonged severe droughts lasting many years have occurred, notably during 1743-1760. |
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The graphs to the left show spring precipitation for 1700-2002 (top graph) and 933-2002 (bottom graph) for the southeastern U.S. (the region consisting of the states North and South Carolina and Georgia). The time series in blue is the spring precipitation computed from 1895-2002 weather observations for April-June for North Carolina and March-June for South Carolina and Georgia. These months were chosen based on an analysis of tree ring data. The time series in red is a reconstruction of the spring precipitation using tree ring data covering the period 933-1985. (The methodology is discussed in an article by Stahle and Cleaveland, 1992; some details are also available at the NOAA Paleoclimate web site.) The tree ring data for the overlapping period 1895-1985 were most highly correlated with April-June precipitation for North Carolina and March-June precipitation for South Carolina and Georgia. The precipitation index reconstructed from the tree ring data successfully reproduces the overall wet-dry pattern over this period, although due to its nature it is a conservative index of precipitation and doesn't reach the annual extremes seen in the observed weather data. The tree-ring reconstructs indicate that spring rainfall extremes and decade-long regimes witnessed during the past century have been a prominent feature of southeastern U.S. climate over the past millennium. |
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References:
Cook, E.R., Meko, D.M., Stahle, D.W. and Cleaveland, M.K. 1999. "Drought reconstructions for the continental United States." Journal of Climate, 12:1145-1162. Stahle, D.W. and M.K. Cleaveland, 1992. "Reconstruction and analysis of rainfall over the southeastern U.S. for the past 1000 years." Bulletin of the American Meteorological Society, 73(12): 1947-1961. |
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