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| Short-term drought conditions in the Pacific Northwest improved substantially during March-May 2005. In south central Oregon (Division 7), winter-spring (November-May) precipitation was slightly above normal, bolstered by the third wettest May in 111 years of record. However, this area is still in long-term drought, with precipitation since June 2000 totaling 84% of the 60-month normal (based on 1950-2000). |
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Paleoclimatic Perspective
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Over the last three decades, November-May precipitation in Oregon's Division 7 has experienced large swings from dry (1975-77) to wet (1982-84) to dry (1990-92) to wet (1996-98) and back to dry (2000-03) (see graph to left). When the annual values are smoothed with a 5-year weighted average (blue line in graph below), these multi-year oscillations appear to have greater amplitude than those in the preceding 80 years of instrumental record. |
| The graph to the right also shows a millennial-length tree-ring record (1000-1996) that corresponds well to the variability in November-May precipitation (red line; 5-year weighted average of annual values). This record is the average of three tree-ring chronologies from central Oregon. The growth of the western juniper trees at these sites is influenced mainly by the moisture in the soil at the start of the growing season, delivered by storms the previous winter and spring. The correlation between annual values of the 3-chronology average and of November-May precipitation is 0.734, indicating a high degree of shared variance. |
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| The tree-ring record, as a proxy for precipitation, can put the variability of the last 30 years into a much longer perspective. While there is no exact analog for recent conditions, the tree-ring record shows periods in which there were similarly large oscillations, such as in the early 1200s, early 1400s, late 1500s, and mid-1700s. There are also long periods in which the amplitude of variability was generally lower, most notably 1800-1900. Also, there are a number of dry anomalies that match or exceed the extremes in the instrumental records, the most severe around 1071, 1352, and 1580. Another severe dry event in the mid-1600s has two closely separate peaks (1644, 1652). More recent significant dry events occurred around 1737, 1794, and 1844, the last one being less severe but more sustained. Knapp et al. (2004), examining the occurrence of severe, sustained droughts in the interior Pacific Northwest using a broader network of western juniper tree-ring chronologies, found widespread evidence for these last three dry events. Based on analyses of modern droughts, they ascribed the occurrence of these paleodroughts to persistent Pacific blocking highs which deflect winter and spring storms from the region. |
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Resources: Divisional climate data, including precipitation for Oregon Division 7 as shown above, can be obtained from NCDC. The three tree-ring chronologies used as a proxy record for Oregon Division 7 precipitation are from sites named Horse Ridge Recollection, Table Rock-Arrow Gap, and Frederick Butte Recollection. These data, contributed by Dave Meko and others from the University of Arizona's Laboratory of Tree-Ring Research, are available from the World Data Center for Paleoclimatology, International Tree-Ring Data Bank. The data for the three chronologies can be found by entering their names into the Tree-Ring Search Engine. Reference:
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NCDC /
Clim. Monitoring /
Climate-2005 /
May /
U.S. Regional Drought /
Search /
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