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Water-Resources Investigations Report 02-4176

Interdecadal Changes in the Hydrometeorological Regime of the Pacific Northwest and in the Regional-to-Hemispheric Climate Regimes, and Their Linkages

By J.J. Vaccaro


Selected hydrometeorological (HM) data for the Pacific Northwest, and regional-to-hemispheric atmospheric-circulation data and sea-surface temperature (SST) data for the North Pacific, are examined for three successive interdecadal periods that are subsets of the instrumental record in order to estimate if their characteristics have changed. The HM data included monthly precipitation totals for 50 sites in western Washington and 29 climate divisions of the Pacific Northwest, and streamflow averages for 112 sites in Washington, Oregon, and Idaho. The atmospheric data included the Southern Oscillation Index (SOI), an index of the Pacific/North America (PNA) circulation pattern, measures of the westerly and northerly components of geostrophic flow, and a subset of the Northern Hemisphere 700-millibar geopotential height data; this subset of 162 grid points includes the area between 15 degrees and 75 degrees N, 110 degrees W and 130 degrees E. The SST data are for a 5-degree grid between 20 degrees N and 60 degrees N, 110 degrees W and 130 degrees E. The atmospheric and SST data were examined not only because the HM regime is linked to regional-to-hemispheric climate regimes, but also to estimate the extent of climate shifts displayed by these data.

Three subsets of the record were identified as pre-1947 (PRE), 1947-76 (BASE), and post-1976 (POST) water years, based on an analysis of the HM data and previous studies. For each subset, means were calculated for the water year (October-September), the runoff season (March-August), the winter season (October-February), and a baseflow season (August-September). Differences in means and in ratios of the means between the BASE period and the PRE and POST periods were examined for changes.

Winter-season mean precipitation during both the PRE and POST periods was smaller than the BASE period, indicating a spatially consistent and distinct change in the HM regime during winter during the PRE and POST periods. For the runoff season, mean precipitation at most sites, in comparison to the BASE period, was smaller during the PRE period and larger during POST period, indicating that different HM regimes occurred during the runoff season for the PRE and POST periods. Water-year mean precipitation was less for both the PRE and POST periods because of decreases in winter-season precipitation; however, the water-year values for the POST period were not as small as those of the PRE period because more precipitation was concentrated in the runoff season.

During both the PRE and POST periods, the mean water-year discharge was less than the BASE period for all but 15 of the 112 sites. Fourteen of the 15 sites were in a well-defined region (southern Idaho and southeastern Oregon), and 13 of the 14 had larger means only during the POST period. Winter-season streamflow was less for all but 11 sites during both PRE and POST periods; the largest decreases in the mean, more than 30 percent, were for an area in central Oregon. Except for the sites that had larger mean water-year discharge, runoff-season means also were less than those during the BASE period.

Changes in the SOI and PNA index from the BASE period were generally similar to and consistent with those of the majority of the hydrologic data; dissimilarities were in well-defined regions and are attributed to the evolutionary nature of the regime shifts. Negative values of the SOI for the POST period were more persistent than those that have occurred during both the PRE and BASE periods. The changes in the PNA index and the geostrophic flow components during the POST period are consistent with drier and warmer conditions in the Pacific Northwest. The 700-millibar data display trends and differences between the BASE and POST periods; differences in composite anomalies for selected winter months between these periods show a well-defined PNA pattern. For many areas of the North Pacific, the record of SSTs shows a significant long-term trend of both increasing and decreasing temperatures, reflecting well-defined patterns, over the period from 1947 to present, and there have been large changes in monthly values between the BASE and POST periods, most related to differences in the spatial patterns. Other long-term geophysical time-series also display distinct shifts, especially at the 1946-47 boundary.

The changes in atmospheric and SST data are clearly linked to and have influenced the changes in the HM regime. Together, these changes suggest that the HM regime in the post-1976 period was different than anything reflected in the record analyzed in this study--a regime with less water-year precipitation and streamflow in all but one region, and secondarily, more runoff-season precipitation over part of the Pacific Northwest.

Of the two major interdecadal-regime boundaries, the 1946-47 boundary appears to have been hemispheric in nature, and is documented in Arctic ice-concentration data and two sea-level pressure indices. On the other hand, the 1976-77 boundary (known as the 1977 climate shift) appears to be mainly contained in North Pacific information, suggesting the influence of the ocean-atmosphere system over the tropics and North Pacific in mediating the 1977 climate shift. Understanding the causes of these two major shifts will provide valuable information for the planning and management of water resources in the Pacific Northwest.


Methods of Analysis
References Cited

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Send questions or comments about this report to the author, John J. Vaccaro ( 253.428.3600 ext. 2620.

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