About Pacific Northwest Climate

Climate Variability

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Impacts of Natural Climate Variability on Pacific Northwest Climate

Research conducted by the Climate Impacts Group (CIG) finds that variations in Pacific Northwest (PNW) climate are strongly shaped by two large-scale patterns of climate variability: the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). ENSO and PDO are not the sole drivers of PNW climate, however. Even with perfect predictions of ENSO and PDO, about 70% of the region’s winter climate variability remains unexplained. Other climate patterns, combined with “noise” and the chaotic nature of climate system, also contribute.

ENSO Impacts on PNW climate. An examination of monthly averaged temperature and precipitation values for El Niño versus La Niña years (1931-1999) in the PNW finds that El Niño winters tend to be warmer and drier than average. La Niña winters tend to be cooler and wetter than average. More specifically:

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[gragh] Composite 1931-1999 monthly temperature differences associated with warm (El Niņo) versus cool (La Niņa) phases of ENSO for the PNW[graph] Composite 1931-1999 monthly precipitation differences associated with warm (El Niņo) versus cool (La Niņa) phases of ENSO for the PNW

Figure 1 Composite 1931-1999 monthly (a) temperature and (b) precipitation differences associated with warm (El Niño) versus cool (La Niña) phases of ENSO for the PNW (data source: NOAA's Climate Diagnostics Center).

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Box-and-whisker plots showing the influence of ENSO on October-March (a) temperature (1899-2000).Box-and-whisker plots showing the influence of ENSO on October-March (a) precipitation (1899-2000).

Figure 2 Box-and-whisker plots showing the influence of ENSO on October-March (a) temperature and (b) precipitation (1899-2000). For each plot, years are categorized as cool (La Niña), neutral (ENSO neutral), or warm (El Niño). For each climate category, the distribution of the variable is indicated as follows: range of values (whiskers); mean value for the phase category (solid horizontal line); regional mean for all categories combined (dashed horizontal line); 75th and 25th percentiles (top and bottom of box). Area-averaged Climate Division data are used for temperature and precipitation.

PDO Impacts on PNW climate. Analysis of past (1931-1999) PDO events shows that warm phase PDO winters tend to be warmer and drier than average while cool phase PDO winters tend to be cooler and wetter than average. The largest differences between warm and cool PDO years (1931-1999) occur in the fall, winter and spring seasons. More specifically:

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[graph] Composite 1931-1999 monthly temperature differences associated with warm phase versus cool phase PDO for the PNW assuming that the PDO pattern is truly regime-like, with continuous epochs switching phase in 1925, 1947, and 1977.[graph] Composite 1931-1999 monthly precipitation differences associated with warm phase versus cool phase PDO for the PNW assuming that the PDO pattern is truly regime-like, with continuous epochs switching phase in 1925, 1947, and 1977.

Figure 3 Composite 1931-1999 monthly (a) temperature and (b) precipitation differences associated with warm phase versus cool phase PDO for the PNW assuming that the PDO pattern is truly regime-like, with continuous epochs switching phase in 1925, 1947, and 1977 (data source: NOAA's Climate Diagnostics Center).

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Box-and-whisker plots showing the influence of PDO on October-March  temperature  (1899-2000).Box-and-whisker plots showing the influence of PDO on October-March precipitation (1899-2000).

Figure 4 Box-and-whisker plots showing the influence of PDO on October-March (a) temperature and (b) precipitation (right) (1899–2000). For each plot, years are categorized as cool phase, neutral, or warm phase. For each climate category, the distribution of the variable is indicated as follows: range of values (whiskers); mean value for the phase category (solid horizontal line); regional mean for all categories combined (dashed horizontal line); 75th and 25th percentiles (top and bottom of box). Area-averaged Climate Division data are used for temperature and precipitation.

Combined ENSO and PDO Impacts. The potential for temperature and precipitation extremes increases when ENSO and PDO are in the same phases and thereby reinforce each other (Figure 5). This additive effect is also seen in the region’s streamflow and snowpack (Figure 6).

What happens when ENSO and PDO are in opposite phases? There is no evidence at this time to suggest that either PDO or ENSO dominates with respect to temperature and precipitation when the two climate patterns are in opposite phases (i.e., an El Niño during a cool phase PDO or a La Niña during a warm phase PDO). The opposite effects on temperature and precipitation can cancel each other out, but not in all cases and not always in the same direction. Similar effects are seen on regional streamflow.

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Impacts of ENSO and PDO on average PNW temperature (1900-1999)Impacts of ENSO and PDO on average PNW precipitation (1900-1999)

Figure 5 Impacts of ENSO and PDO on average PNW (a) temperature and (b) precipitation (1900-1999). Figure 5 illustrates the additive effects of ENSO and PDO when in-phase (shown here as deviations from average), calculated using temperature and precipitation data for 1900-1999. Increases in average temperature and decreases in average precipitation are more significant when warm ENSO (El Niño) and PDO phases occur simultaneously than when either occurs alone. Conversely, decreases in average temperature and increases in average precipitation are more significant when cool ENSO (La Niña) and PDO phases occur simultaneously. Effects are most pronounced during October through March. The dotted line represents ½ standard deviation from the mean.

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[graph] Impacts of ENSO and PDO on snowpack at specific locations in the PNW[graph] Impacts of ENSO and PDO on streamflow at specific locations in the PNW

Figure 6 Impacts of ENSO and PDO on (a) snowpack and (b) streamflow at specific locations in the PNW. Average total winter snow depth is shown for the winter season (January 15 to April 15) at Snoqualmie Pass, Washington. Streamflow is for April-September average flow on the Columbia River at The Dalles after removing the effects of the dams on streamflow. Averages are computed during the warm phases (white) and cool phases (black) of ENSO and PDO separately and combined. Note the cumulative effect when ENSO and PDO are in-phase (i.e., El Niño and warm phase PDO, La Niña and cool phase PDO). Percent changes from normal winter snow depth and summer streamflow associated with ENSO and PDO show that El Niño and/or warm PDO winters tend to have lower than average snowpack and streamflow. The reverse is true for La Niña and/or cool PDO.

For More Information

Climate Impacts on PNW Resources

The CIG has demonstrated numerous linkages between changes in ENSO and PDO and variability in PNW precipitation, temperature, snowpack, streamflow, flooding, droughts, forest productivity, forest fire risk, salmon abundance, and quality of coastal and near-shore habitat. For more information on these impacts, see Climate Impacts in Brief.

Climate Forecasting and Planning

The exciting message from recent successes in climate prediction is that forecasts associated with major climate events – like the El Niño of 1997-98 and the La Niña event of 1998-2000 – can skillfully predict shifts in the odds for realizing distinct climate conditions (e.g., wet vs. dry, or cool vs. warm) in select regions of the world. The potential utility of these forecasts in the PNW is enhanced by the PNW's location in one of the more predictable parts of North America with respect to seasonal to interannual climate variations. Furthermore, the PNW has the added bonus of having an amplified response in the region’s water cycle to the most predictable swings in Pacific/North America winter climate.

Understanding the connections between ENSO, PDO, and PNW resources provides valuable opportunities for improved climate-based resource forecasting and management. Climate forecasts give resource managers opportunities to consider how projected climate conditions may affect resource management decisions and outcomes. PNW water supply managers may, for example, choose to compensate for an El Niño forecast by allowing reservoirs to fill higher than normal in anticipation of warmer, drier conditions (see Seattle Public Utilities case study). Columbia River hydropower managers could use seasonal streamflow forecasts to increase hydropower revenue on the order of 4%, or $150 million, per year on average without compromising other operational objectives (Hamlet et al. 2002). By taking these forecasts into account and adjusting operational practices to reflect potential conditions, resource managers can reduce their climate-related risk, positioning themselves to better meet resource management objectives.

The CIG is actively researching and developing forecast methodologies and tools based on global climate forecasts for use within the PNW resource management community. For more information on climate and resource forecasts and forecasting techniques, see Forecasts and Planning Tools. For more information on planning for climate variability, see Planning for Climate Variability and Change.