2.2 Pacific Decadal Oscillation Index and El Nino/La Nina

Last Updated: 
June 1, 2015


This indicator tracks the Pacific Decadal Oscillation (PDO) and El Niño/La Niña, both patterns of Pacific Ocean and climate variability.

The PDO and El Niño occur in slightly different regions of the Pacific – the PDO is a northern Pacific phenomenon; El Niño is centered in the tropics.

Building Clouds
Building Clouds

They also behave differently. PDO phases last decades. Because each PDO phase is long, and the air-sea interactions require many years to adjust, the effects of PDO phase changes, or ‘regime shifts”, are predictable for up to 10 years. During warm (positive) phases of the PDO, warmer than average ocean water sits very near the western coast of North America. During cool (negative) phases, cooler waters are present there.

PDO positive (Warm) phase and negative (Cool) phase
PDO positive (Warm) phase and negative (Cool) phase. Image from the NASA Jet Propulsion Laboratory webpage. Courtesy of Stephen Hare and Nathan Manua. University of Washington. Surface temperature units are in degrees Celsius. Arrows are wind stress patterns.

El Niño events last as little as six to 18 months and usually peak near Christmas (El Niño means the little boy in Spanish, referring to the Christ child). During an El Niño event, warm waters concentrate at the surface of the Pacific Ocean in a large band west of Peru. During La Niña phases, these waters are cooler.

El-Niño/La Niña
El-Niño/La Niña. Images from the NASA Jet Propulsion Laboratory webpage. Courtesy of Stephen Hare and Nathan Mantua. University of Washington. Surface temparture units are in degrees Celsius. Arrows are wind stress patterns.

PDO values are obtained from the Joint Institute for the Study of Atmosphere and Ocean (University of Washington and NOAA) webpage1. El Niño/La Niña events are tracked using the Multivariate Niño Southern Oscillation (ENSO) Index. ENSO values are obtained from NOAA’s Earth System Research Laboratory, Physical Sciences Division webpage2. Analysis of the effects of El Niño/La Niña events on weather in the NWT is obtained from Environment Canada’s webpage3.

NWT focus

The map below shows typical regions of Canada impacted by the different phases of ENSO, PDO and NAO during the cold season. The Pacific North American is strongly influenced by ENSO with positive PNA tending to occur during winters associated with El Niño episodes, and negative values during La Niña events. 

Warmer than normal winters occur during El Niño events, positive PDO and positive PNA in forested NWT, and during negative NAO in Nunavut. 

Large-scale climate oscillations influencing Canada 1900-2008
Figure from Bonsal, B. and A. Shabbar. 2011. Large-scale climate oscillations influencing Canada, 1900-2008, Canadian Biodiversity: Ecosystem Status and Trends 2010. Technical Thematic Report No. 44.

This indicator tells us about variations in "normal" climate conditions. As climate affects many aspects of northern ecosystems, both indices are important to understanding changes in northern ecosystems occurring over decades (PDO) and from year to year (El Niño/La Niña). Why changes in PDO phases and El Niño occur is not clearly understood, but simply knowing when they change helps us better understand the climate in northwest North America, including the climate in mainland NWT.

The effects of the PDO and El Niño are more evident in the south and western (forested) part of the NWT. For example, increased summer lightning storms over forested parts of the NWT between 1976-1999 have been linked to a positive (warm) PDO phase5, 6, 7. El Niño events have been shown to result in warmer winter weather and slightly higher than normal snow fall in the southern NWT3, 4, and increased spring water discharges in rivers5. Both El Niño events and positive (warm) PDO phases also result in drier summers in the southern NWT4, which in turn have been correlated to slower growth rates in spruce trees (Picea glauca and P. mariana)8.

Current view - status and trend

Pacific Decadal Oscillation


Mean annual (January through December) values of the Pacific Decadal Oscillation index, 1900-current
Mean annual (January through December) values of the Pacific Decadal Oscillation index, 1900-current. Source courtesy of jisao.washinton.edu/pdo.

The PDO fluctuates in two general cycles: shifting every 15-to-25 years, and shifting every 50-to-70 years. The PDO shifted regime twice in the past century. It shifted towards a negative phase in 1999 for the first time since the 1970s, then back to positive in mid-2000s. As of 2008, it has moved to a negative phase.

El Niño – La Niña

During the past 70 years, there have been nine strong El Niño events (Oceanic Niño Index (ONI) ≥1.5)  (in 1957-58, 1965-66, 1972-73, 1982-83, 1986-88, 1991-92, 1997-98, 2002-03,and 2009-2010 ). La Niña conditions have been in place since summer 2010. The current ONI index can be tracked monthly on US National Oceanic and Atmospheric Administration (NOAA) webpage2. NOAA has advised that an El Niño period may have started in March 2015.

Looking forward

The expected effects of El Niño – La Niña events on NWT’s weather can be found on Environment Canada webpage3. The effects of phase shifts in PDO and El Niño events on NWT ecosystems are not well understood. Only a few studies look at these long-term changes with the NWT in mind4. Some studies have looked at weather patterns in relation to long-term fluctuations in NWT ecosystems such forest fire regimes6, droughts8, but more research is needed on links to flooding and effects on wildlife populations. Long-term datasets for each of these patterns and others do exist for the NWT, and more studies can be expected in the future. This will greatly help our efforts to plan for and adapt to a changing climate.

"... there is currently no consensus on how increases in greenhouse gas concentrations have impacted the occurrence of these large-scale climate oscillations. Furthermore, the effects of projected future climate change on the major teleconnection patterns affecting Canada remain uncertain since there is a lack of agreement among the various climate models concerning the future frequency and structure of large-scale atmospheric and oceanic modes. With respect to ENSO for example, the ability of current Glocal Climate Models (GCMs) to simulate observed El Niño and La Niña events differ consderably from one model to the next, however, these events are much better simulated using an ensemble of models. At present, the majority of GCMs do not indicate any discernible changes in the projected ENSO amplitude or frequency in the 21st century in summary, further advancements in GCMs are needed in order to detect future changes to large-scale teleconnections and their resultant impacts on Canadian climate." Quote from B. Bonsal and A. Shabbar. 2011. Large-scale climate oscillations influeing Canada, 1900-2008. Canadian Biodiversity: Ecosystem Status and Trends 2010, Technical Thematic Report No. 44.

"In addition to robust multi-decadel warming, global means surface temperature exhibits substantial decadal and interannual variabililty... Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998-2012: 0.05 [-0.05 to 0.15] oC per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951-2012; 0.12[0.08 to 0.14] oC per decade)..." Quote from the Fifth Assessment Report of the International Panel on Climate Change 2013 - The Physical Science Basis - Summary for Policymakers9.

For more information

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Contact us

Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


Ref. 1. NASA JPL. Current Pacific Decadal Oscillation (PDO). Science - El Niño/ La Niña & PDO. Jet Propulsion Laboratory - California Institute of Technology.

Ref. 2. NOAA National Oceanic and Atmospheric Administration. Current Multivariate ENSO Index (MEI). NAOO. US Defence of Commerce - Eath System Research Laboratory. Physical Sciences Division. Available at www.elnino.coaa.gov

Ref. 3. Environment Canada. 2008. El Niño - Canadian Effects - Mackenzie District. Environment Canada. 

Ref. 4. Bonsal, B. and A. Shabbar. 2011. Large-scale climate oscillations influencing Canada, 1900-2008, Canadian Biodiversity: Ecosystem Status and Trends 2010. Tehcnical Thematic Report No. 4. Canadian Councils of Resource Ministers. Ottawa, ON.

Ref. 5. Déry, S.J. and E.F. Wood. 2005. Decreasing River discharge in northern Canada. Geophysical Research Letters, 32: CitelD L10401.

Ref. 6. Fauria, M.M. and E.A. Johnson. 2006. Large-scale climatic patterns control large lightning fire occurrence in Canada and Alaska forest regions. Journal of Geophysical Research 111: 1-17.

Ref. 7. Hogg, E.H., J.P. Brandt, and B. Kochtubajda. 2005. Factors affecting interannual variation in growth of western Canada aspen forests during 1951-2000. Canadian journal of forest research 35: 610-622.

Ref. 8. Sauchyn, D. and J. Barichivich. 2007. American Geophysical Union (AGU). Acapulco, Mexico.

Ref. 9. International Panel on Climate Change. 2013. AR5 - The Physical Science Basis.