Climate in the Pacific Northwest

Among the most notable features about this region’s climate is that it is highly variable, both geographically and temporally. The mountainous topography of the Pacific Northwest carves the region into climatically distinct areas. The Cascade Mountains that lie along the western edge of the region effectively sever the maritime western coastal areas from the continental interior. The orographic effect of the Cascades is evident in the distinctive climate that characterizes these two regions (Figure 1).

Major topographic features in the Pacific Northwest.
Figure 1. Major topographic features in the Pacific Northwest.

Marine influences on the western side of the Cascades result in milder seasonal temperatures and greater precipitation compared to the region east of the mountains, where summers are warmer, winters cooler and the wet season (Oct – Mar) is drier (Figure 2). The Pacific storm track converges with the PNW during the wet season and delivers most of the region’s precipitation to the west side of the mountains, in the form of rain to the lowlands and snow to the cooler, higher elevations. In contrast the region to the east of the Cascades is notably drier and marked by warmer summers and cooler winters.

Spatial distribution of annual precipitation in the PNW.
Figure 2. Spatial distribution of annual precipitation in the PNW.

Image courtesy of Oregon State University

Climate Variability

The climatic variability arising from the region’s distinctive geography is a well-known phenomenon. Aside from these recognizable spatial contrasts, when the PNW climate departs from normal, the departures tend to remain consistent across the region. The correlations tend to be quite high (>0.8) when comparing any two parts of the region (i.e. southeastern Idaho and northwestern Washington).

Some fluctuations in regional climate are attributed to two modes of natural variability: the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These two key drivers of natural climate variability in our region affect the climate on shorter time scales, annual to decadal, than climate change. In their warm phases (i.e., El Niño conditions for ENSO), both ENSO and PDO increase the odds for a warmer-than-average PNW winter and spring and decrease the odds for a wetter-than-average winter. The opposite tendencies are true for cool phase ENSO (La Niña) and PDO: they increase the odds that PNW winters will be cooler and wetter than average. While tending to be warmer than average, very strong El Niño winters often have near-normal precipitation.

Climate Change

Global and national temperatures have increased throughout much of the 20th century. Many aspects of the earth’s physical and biological systems are changing in ways consistent with human-caused warming. Among these are rising sea levels, warming oceans, and changes in ocean chemistry. The Pacific Northwest is experiencing a suite of long-term changes that are aligned with those observed globally as a result of human-caused climate change. Regional trends include increasing temperatures, a longer frost-free season, decreased glacial area and spring snowpack, earlier peak streamflows in many rivers and rising sea level at most locations. Natural variability can result in short-term trends that are in contrast with those expected from climate change. However, studies investigating long-term trends in greater detail help clarify the role of variability within the scope the long-term trends associated with climate change.


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