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Seasonal to Interannual Forecasts

Climate Outlook

March 2005 archive copy

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The Climate Impacts Group (CIG) translates global-scale climate forecasts and conditions into regional-scale climate forecasts for Pacific Northwest (PNW) resource managers and the general public. The El Niño/Southern Oscillation (ENSO) is the most important factor for seasonal forecasting, changing the odds for different types of winter and spring weather (e.g. warmer/drier, cooler/wetter) in the PNW. The climate outlook also provides the basis for natural resource forecasts, including the CIG's annual streamflow forecasts.

What's Next for the Pacific Northwest?

Winter-Spring 2005
Updated March 23, 2005
The climate outlook is reviewed monthly and updated as needed.

Current indicators for Pacific climate:

Over the past few weeks sea surface temperatures (SSTs) in the eastern tropical Pacific have been trending to near average values (and even below average values in the far eastern equatorial Pacific) indicating that the weak El Niño event of 2004-05 has ended (see definition). In contrast, tropical SSTs in the western half of the tropical Pacific remain about 1°C warmer than average. The warmth of the western tropical Pacific has been relatively stable since late 2001. Because the greatest warming has remained in the western half of the tropical Pacific many climate scientists have questioned whether the recent warm period deserves the standard "El Niño" label, or whether it should be characterized as something else. Either way, beginning in January 2005 the region east of the date line has recently cooled, and the majority of current forecasts call for either near average or slightly above average SSTs in the eastern tropical Pacific for the next few seasons. It is interesting to note that a pattern of dry weather in Australia/Indonesia and heavy rainfall further east developed in February, and along with this came very strong western wind anomalies and an extreme negative swing in the Southern Oscillation Index (SOI for February was -4.1 standard deviations!). This strong month-long disturbance in the western Pacific has since faded away, but it did force strong downwelling along the equator that is now slowly moving eastward along the equator (as a packet of equatorial Kelvin wave signals). It is not now clear how this one-month event will influence tropical climate in the next few months and seasons, but it is likely to favor a warming trend in eastern equatorial Pacific SSTs for the next month or two. For more information on the current ENSO state and forecast, see the forecast summaries provided by the NOAA Climate Prediction Center and the International Research Institute for Climate Prediction .

Seasonal to interannual forecasts for the state of the Pacific Decadal Oscillation (PDO) index (based on a pattern of North Pacific SSTs) are an emerging science. A major source of uncertainty in developing PDO forecasts is our lack of understanding of what causes the observed multi-year persistence in the PDO index and, more importantly, what triggers PDO regime shifts. However, a strong tendency for year-to-year persistence of the PDO index along with a well-established statistical relationship with the state of ENSO provides a means for making skillful 1-year projections of the PDO index.

Using that simple statistical method with the observed PDO index values from July 2003-June 2004, combined with a prediction that SSTs in the NINO3.4 region of the tropical Pacific would be in the range of +0.4°C to +1.2°C, yields a prediction for a July 2004-June 2005 PDO index value ranging from ~ +0.5 to +1. Observed PDO index values for July 2004 to January 2005 are: 0.44, 0.85, 0.75, -0.11, -0.63, -0.17, 0.44 and 0.81 (averaging out to ~0.30). Note that the average value for the Nino34 index from July 2003-February 2004 is ~0.70, about in the middle of the range predicted last fall.

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What will it mean for the PNW in coming months?

The latest seasonal forecasts from NOAA's Climate Prediction Center call for a slight tilt in the odds favoring a warm spring and summer for the entire Pacific Coast of North America.

At this time it seems unlikely that the PDO pattern has played a major role in influencing North Pacific climate in recent months, but instead recent changes in the PDO index reflect a strong SST response to recent atmospheric circulation patterns. While circulation patterns for the fall and early winter did not resemble those most often observed in "classic" El Niño fall/winter periods, the jet stream pattern that prevailed in February did. This "El Niño-like" circulation pattern included a persistent blocking ridge over the PNW region that extended into the Gulf of Alaska, and most of the storm activity during February was directed at central and southern California and the interior southwest US.

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Even though the Pacific Northwest has experienced warmer and drier than average fall and winter climate, the large-scale circulation patterns causing these conditions only resembled those typically associated with an El Niño event during our exceptionally dry month of February. It was also during February that the most significant shift in tropical rainfall has been observed during the past 6 months, and there too the patterns of anomalously wet and dry weather resembled those often seen in past El Nino episodes. In the past few weeks the strong shift in tropical rainfall patterns has faded away, as have the weak warm SST anomalies in the eastern equatorial Pacific. NOAA's Climate Prediction Center forecast calls for an increased likelihood that western Oregon and Washington will experience above average temperatures for the next few months, and the remainder of the region has climatological odds for both precipitation and temperature this spring and summer.

It is important to note that these climate forecasts indicate relatively subtle shifts in the odds for warmer/cooler temperatures and more/less precipitation in the PNW rather than a deterministic (or exact) climate prediction for the next 2 seasons. Simply stated, expectations for continued very weak El Niño to ENSO neutral conditions in the tropics and trends for warmer spring and summer west coast temperatures yield a climate outlook for the PNW that has higher than average odds for a warm spring and summer in western Oregon and Washington.

Pacific Northwest Resource Outlooks

Climate Prediction Resources

The links below provide access to the latest information on the current state of global and regional climate, as well as links to global and regional climate predictions.

The Current State of the Tropical Pacific

Predictions of Tropical Pacific Conditions

The Current State of the Globe

Current and Predicted U.S. Conditions

Pacific Northwest Conditions

State Climatologist Offices

Special Areas