Puget Sound's climate

The climate of Puget Sound is a product of the interaction between large-scale wind and weather patterns and the complex topography of the region. Seasonal changes in the movement of moisture-laden air that collides with the sudden barrier of the Olympic and Cascade mountains bring Puget Sound the record-breaking precipitation for which it is so famous. These circulation and topographic differences also lead to remarkable climate differences within Puget Sound itself, influencing the species and habitats that are found in the Sound.

The climate of Puget Sound is a product of the interaction between large-scale wind and weather patterns and the complex topography of the region. Seasonal changes in the movement of moisture-laden air that collides with the sudden barrier of the Olympic and Cascade mountains bring Puget Sound the record-breaking precipitation for which it is so famous. These circulation and topographic differences also lead to remarkable climate differences within Puget Sound itself, influencing the species and habitats that are found in the Sound.

Precipitation patterns

Beginning about mid-October, a semi-permanent low-pressure cell, commonly called the Aleutian Low, intensifies and migrates southeastward over the Aleutian Islands and Gulf of Alaska. Surface winds blow in a counterclockwise circulation around the Aleutian Low. Further south, winds blow in a clockwise circulation around a semi-permanent center of high pressure typically centered offshore of southern California. Together, these high- and low-pressure cells typically bring moist, mild onshore flow into the Pacific Northwest from October through early spring. As the moisture-laden air encounters the Olympic or Cascade mountains, it rises and cools, and the cooling causes water vapor to condense into liquid cloud and rain drops. Because of the seasonal shifts in large-scale wind patterns, the PNW’s wet season typically begins in October, peaks in midwinter, and ends in the spring; about 75% of the region’s annual precipitation falls in the period October-March. During late spring, the Aleutian Low retreats to the northwest and becomes less intense, while the high-pressure cell to the south expands northward and intensifies. The result is a strong reduction, from late spring through summer, of landfalling storms for the Pacific Northwest.

Mountains and rainfall

The west (windward) slopes of the Cascade and Olympic mountains receive enormous quantities of rain and snow, exceeding 200 inches (5 meters) of water equivalent per year at some locations on the Olympic Peninsula. At Paradise Ranger Station on Mount Rainier, the average spring sees an end-of-season snow depth of 4.1 meters (~170 inches). The Cascades are often among the snowiest places on Earth; in 1956 the snow at Paradise piled to a depth of nearly 9.1 meters (30 feet) during a year in which that location’s annual snowfall was nearly 28.5 meters (94 feet); the Mount Baker Ski Area, located in the North Cascades near the US/Canada border, set a new world record for the highest annual snowfall ever recorded (October-September) in 1998-99 with a total of 29 meters (96 feet) (Bell et al. 2000).

Although the west side of the Cascades is generally a very wet region, it contains several areas that receive significantly less precipitation than the west-side average. Washington’s Puget Lowlands, the northeast extreme of the Olympic Peninsula, and the San Juan Island archipelago are relatively dry areas that lie in “rain shadows.” Rain shadows in these areas are caused by the Olympic Mountains located to the west and southwest that shield them from the direct impact of storms that follow the wet season’s prevailing storm track.

The Cascade Mountains also bear strongly on seasonal variations in the region’s climate. West of the Cascades, the low-lying valleys have a maritime climate with typically abundant winter rains, infrequent snow, dry summers, and mild temperatures year-round (usually above freezing in winter, so that snow seldom remains for more than a few days. East of the Cascade crest, the region’s climate is much more continental, with rainfall and cloudiness less common and sunshine and dry conditions more common year-round. On a finer scale, gaps and low-elevation passes in specific locations provide connections between the east-west climate differences, affecting habitat-forming processes and the spatial distribution of the biota of the west slopes of the Cascade mountain range.

Climate variation

The year-to-year and decade-to-decade changes in the strength and location of the Aleutian Low have been a prominent feature of Pacific climate variation in the last century. Climate records indicate that one of the most prominent features of Pacific climate variation is expressed through annual and decadal changes in the strength and location of the Aleutian Low pressure pattern. This is of special importance for Puget Sound because an intense Aleutian Low brings relatively warm and dry winters to the region, while a weak Aleutian Low favors a cooler and wetter winter. Variability in the Aleutian Low comes from a variety of sources, but two important influences on the Aleutian Low are the large-scale variations in sea-surface temperatures known as the El Nino-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO).

El Nino-Southern Oscillation (ENSO)

ENSO is a natural part of Earth’s climate that spontaneously arises from interactions between tropical trade winds and ocean surface temperatures and currents near the equator in the Pacific. Warm extremes of the ENSO cycle, commonly called El Nino, favor an especially intense Aleutian Low that is associated with the displacement of the storm track in the eastern North Pacific. As the displacement moves the track southerly towards California, the conditions favor a warm and mild Puget Sound winter. Cold extremes of the ENSO cycle, commonly called La Nina, favor the opposite. Individual ENSO events (either El Nino or La Nina) typically occur over the course of a single year, and over the past century one year in four (on average) has been an El Nino extreme, and one year in four has been a La Nina extreme.

Pacific Decadal Oscillation (PDO)

The Pacific Decadal Oscillation plays out over a longer time-scale, with a typical lifetime of 20 to 30 years for the extremes. Warm eras of the PDO prevailed during 1925-46 and 1977-98, while cold eras prevailed during 1900-24 and 1947-76. Warm eras of the PDO are associated with a prevalence of intense Aleutian Low winters, while cold eras are associated with weak Aleutian Low winters. Because the PDO and ENSO have similar impacts on the character of the Aleutian Low, both La Nina and cool PDO periods are typically accompanied by cooler than average sea surface temperatures, shallow thermocline (temperature layer), and high productivity. Warm periods (typical of warm PDO and Nino periods) are characterized by warmer sea surface temperatures, deeper thermocline and lower productivity. Biological changes throughout the ecosystem in the northeast Pacific Ocean are associated with different climate regimes – including changes in zooplankton, benthic algae, meso-crustaceans, rocky-reef fishes, and apex predators. The effects of regime shifts vary geographically – Pacific salmon from Alaska increased in abundance in response to the 1976 regime shift while populations from the Pacific Northwest declined.

Global climate change

Potential changes in global climate patterns are likely to have important consequences to Puget Sound ecosystem processes. Changes in precipitation, temperature, and the frequency of intense Aleutian Low systems will alter freshwater input, nutrient cycling, and stratification with ramifications throughout the food web. The Climate Impacts Group at the University of Washington has published two important reports (Snover et al. 2005; Mote et al. 2005) that document a number of potential impacts that climate change may have on the Puget Sound ecosystem.

About the Author: 
The development of the Sound Science document has been a collaborative process among scientists from a variety of disciplines and institutions throughout Puget Sound. The content reflects the wealth of knowledge in existing plans, research projects and personal expertise. The open dialogue and vigorous discussion about the interactions between components of the ecosystem, key threats to the system and critical science needs is almost as significant as the findings themselves. This document is the product of over 30 authors and almost 100 reviewers from federal, tribal, state, local, non-governmental, and academic institutions across the Puget Sound region. In total, hundreds of natural and social scientists have contributed either as co-authors, through extensive reviews, or by participating in workshops to debate and improve the information. We believe that the resulting content of the document thus reflects the collective views of the broad community of natural and social scientists familiar with Puget Sound. Key contributing agencies: King County Department of Natural Resources and Parks, National Oceanic and Atmospheric Administration (NOAA), Northwest Indian Fisheries Commission, People for Puget Sound, Puget Sound Action Team, The Nature Conservancy, US Environmental Protection Agency, US Geological Survey, University of Washington, Washington State Department of Ecology, Washington Department of Fish and Wildlife, Washington State Department of Health, Washington State Department of Natural Resources.