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| The Pacific Northwest, especially including the Cascades of Oregon and Washington, has experienced unusually dry weather this winter. Below-normal precipitation for this season and much of the last six years has resulted in drying soils, decreased streamflow, record low snowpack, and depleted reservoirs. During the last 25 years, Oregon has seen major swings in climate between prolonged wet and dry episodes. |
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Paleoclimatic Perspective
| The last four months have been unusually dry across parts of Oregon, including the Cascade Range of the northern half of the state (climate division 4). For this area, November 2004-February 2005 ranks as the fourth driest November-February in the 110-year instrumental climate record. The last 12 months (March 2004-February 2005) ranks as the fifth driest such period on record. Precipitation has been below normal for much of the last six years, resulting in a Palmer Hydrological Drought Index in the moderate to severe range. |
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| Paleoclimatic proxy records derived from tree rings can put the current winter drought in the Pacific Northwest into a multi-century perspective. The graph below shows the instrumental record of winter (November-February) precipitation, 1895-2005, for Oregon's Northern Cascades (climate division 4). The lighter blue line shows annual values, while the heavy blue line shows the record smoothed with a 5-weight filter. |
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The graph also shows a tree-ring record (annual values in light red, smoothed values in thick red line) that corresponds well to the Oregon Division 4 precipitation record. The tree ring record extends from 1750-1992. This record is the average of two tree-ring chronologies that are highly (negatively) correlated with the precipitation record. Together, they are a proxy record of past winter precipitation. The two chronologies are from mountain hemlock trees growing at high elevations on Mt. Hood and Mt. Jefferson in Oregon’s Cascade Range. |
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As explained by Peterson and Peterson (2001), tree growth at these high-elevation sites is most strongly controlled by length of the growing season. High winter precipitation creates deep, long-lasting snowpacks that shorten the growing season, reducing tree growth; conversely, low winter precipitation and shallow snowpacks are associated with longer growing seasons and enhanced tree growth. Thus, there is a negative relationship between winter precipitation and tree growth. Summer temperature also affects tree growth through its influence on growing season length. The correlation between the two-chronology average and winter precipitation is -0.495. The match between the two records is also visible in the graph. If the tree-ring record is considered to be a reasonable long-term proxy of precipitation, an assessment of periods of persistent low winter precipitation over the past 250 years can be made. Both the instrumental record and the tree-ring record show two significant multi-year periods of low winter precipitation in the past century: a decadal-scale event from about 1930 to 1945, and a shorter event from about 1988 to 1994. The tree-ring record indicates no similar departures below the long-term mean in the 1800s, but two such events in the late 1700s: one centered around 1776 and a particularly severe event centered around 1794. Resources: Divisional climate data, including precipitation for Oregon Division 4 as shown above, can be obtained from NCDC: The two tree ring chronologies used as a proxy record for Oregon Division 4 precipitation are from sites named Mt. Hood (High) and Mt. Jefferson (High). These data are available from the World Data Center for Paleoclimatology, International Tree-Ring Data Bank: The data for the two chronologies can be found by entering their names into the Tree-Ring Search Engine. Peterson, D. W. and Peterson, D. L. 2001. Mountain hemlock growth responds to climatic variability at annual and decadal time scales. Ecology 82(12):3330-3345.
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NCDC /
Clim. Monitoring /
Climate-2005 /
Feb /
U.S. Regional Drought /
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