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Paleoclimatic Perspective
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West North Central-Northwest Montana |
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Most of Montana has experienced persistent drought conditions since 1999. Precipitation in the northwest part of the state (Division 1) since September 2000 has totaled 84% of the 60-month normal (based on 1950-2000) (see graph to left). While only about one-quarter of the annual precipitation in northwest Montana falls during the summer growing season (June-August), this summer moisture -- or lack thereof -- has a disproportionate influence on both human activities and ecosystem processes. For example, extremely dry conditions in the summer of 2003 led to the ignition and spread of very large wildfires in northwest Montana. |
| The graph below left shows summer (June-August) precipitation, 1895-2005, for Montana Division 1. The most prominent feature of the record is the 17-year string of near to much drier than average summers (1924-1940) that includes the Dust Bowl drought of the 1930s. Summers tended to be wetter than average from 1975-1998, but the most recent drought (2000-2005) includes six near to much below average summers and two (2000, 2003) with less than 50% of normal precipitation. This year, June was quite wet, but drought returned in July and August with the precipitation for those months at 21% of normal. |
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To put the summer drought variability of the last century in northwest Montana into a much longer perspective, proxy data from tree rings can be used. The graph above right shows a 461-year tree-ring reconstruction (1540-2000) of mean summer deficit (MSD) for Kalispell, Montana (annual values in light red; 10-year running mean in dark red), developed by Pederson et al. (in press). Kalispell is located in a northern valley of Division 1. MSD is summer precipitation (mm) minus potential evapotranspiration (mm), and is calculated from June-August precipitation and mean temperature. The reconstruction is based on three long tree-ring chronologies (two Douglas-fir, one limber pine) from northwest Montana, and was calibrated on a record of MSD calculated from instrumental data for Kalispell Airport, also shown on the graph (annual values in light blue; 10-year running mean in dark blue). The correlation between the reconstruction and the instrumental record over their common period (1896-1996) is 0.683, indicating a high degree of shared variance. The Kalispell MSD record, as expected, captures the same major anomalies as the Division 1 summer precipitation record.
Comparison of the MSD reconstruction with the instrumental records of summer drought for northwest Montana leads to several observations. First, the most severe single-year summer droughts of the 20th century were probably matched or exceeded on only a few occasions in the prior four centuries. Second, the cumulative deficit during the extended period of dry summers in the 1920s and 1930s appears to be unprecedented in the 461-year reconstruction. This period was one of rapid retreat of glaciers in Glacier National Park (Pederson et al. in press). The current drought, though it includes two very dry summers, pales in comparison with that event and many other extended drought events seen in the tree ring record. |
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NCDC /
Clim. Monitoring /
Climate-2005 /
Aug /
U.S. Regional Drought /
Search /
Help
