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Natural Fluctuations

Large-scale annual and decadal fluctuations in weather are caused by changes in patterns of ocean circulation and atmospheric pressures.

In the NWT, indices for two of these phenomena, the Arctic Oscillation (AO), and the Pacific Decadal Oscillation (PDO) - El Niño, are particularly important to track to understand large natural fluctuations and changes occurring in NWT’s weather from year to year and decade to decade. These large fluctuations in weather are natural and began long before human-caused climate change. Natural fluctuations in weather have direct impacts on drivers of ecosystem change such as drought, forest fire, flooding, permafrost melt, forest pest outbreaks, timing of vegetation greening. Natural fluctuations need to be taken into account if we want to track the effects of human-caused climate change on NWT’s ecosystems.

A pattern in variation in atmospheric pressure over the North Pacific, called the Pacific/ North American teleconnection pattern (PNA), is also an important source of variability in weather north of the tropics. This pattern is strongly influenced by El Niño events. El Niño events are associated with positive (warm) phases in PNA. The PNA is known to be correlated with some weather patterns in the NWT, and may be tracked using an additional indicator in the future.

2.1 Arctic Oscillation Index

The Arctic Oscillation (AO) is a pattern of variability in the atmospheric pressures of the Arctic and North Atlantic oceans, resulting is large changes in weather from year to year, and decade to decade. The North Atlantic Oscillation and AO are different ways of describing the same phenomenon.

The Arctic climate is highly variable. The AO index gives us information on the phase of the variation. It tells us about "normal" weather conditions that can greatly vary over decades.

The AO has the largest effect during winter (January, February and March), so the index is usually represented as patterns in winter climate in the Arctic(1).

When the Arctic Oscillation index is in “positive phase” (left globe), high atmospheric pressure persists south of the North Pole, and lower pressures sits on the North Pole. In the positive phase, very cold winter air does not extend as far south into the middle of North America as it would during the negative phase. The AO positive phase is often called the “Warm” phase in North America.

Positive (left) and Negative (right) AO Phases © Figures courtesy of J. Wallace, University of Washington) from the National Snow and Ice Data Centre

When the AO index is in "negative phase", relatively high atmospheric pressure sits over the Beaufort Sea (called the Beaufort High) and the North Pole, and low pressures stay further south, about 45 degrees N. Cold winter air extends far to the south in North America. The AO negative phase is often called the “Cold” phase in North America. Weather patterns in the negative phase are in general "opposite" to those of the positive phase. The AO phases also have effects on Western Europe and Africa as shown on the diagrams.

AO values are obtained from NOAA/ National Weather Service National Centers for Environmental Prediction, Climate Prediction Center(15).

NWT Focus

As weather and climate affect many aspects of northern ecosystems, understanding the AO is essential to understanding changes in northern ecosystems.

A positive AO index is related to a decadal (for about 10 years) weaker clockwise circulation in the Beaufort Sea (weaker Beaufort Gyre)(9,10), which results in change in currents across the Arctic Ocean and a decrease in old thicker sea ice at the pole(9). A positive AO is also linked to warmer winter temperatures on average north of 60(9).

The effects of AO on weather patterns in the NWT are clearer in the north (Beaufort Sea) and northeast (tundra) part of the territory(4,7). The effects of the Pacific Decadal Oscillation appear to have a stronger effect on weather in the south and western (forested) part of the territory(4,6). There is evidence(18) that the AO, which has been associated with climatic changes in the Arctic and North Atlantic, may be a good predictor of shifts in the Pacific Decadal Oscillation ( see index 3.2).

Current view - status and trend

The standardized seasonal mean AO index during cold season (blue line) is constructed by averaging the daily AO index for January, February and March for each year. The black line denotes the standardized five-year running mean of the index. . Source courtesy of: NOAA/ National Weather Service National Centers for Environmental Prediction , Climate Prediction Center

Over most of the past century, the AO alternated rapidly between its positive and negative phases. However, in the 1970s, and then again from late 1980s to late 1990s, the index remained “stuck” in a strong positive (warm) phase, with a record high in 1990. This extended positive phase is being studied extensively(2,11), . The current pattern (since about 2005) is more consistent with the rapid flip-fops patterns observed before this exceptionally long positive phase(9).

Looking forward

The variability in the AO is a natural phenomenon that can reduce or amplify the effects on Arctic climate caused by increased greenhouse gas emmissions(2). In decades when the natural effects of AO are similar to the predicted effects of human-caused climate change, it is difficult to distinguish between the two(8). The current pattern of fast changes in positive and negative phases in AO offers scientists a renewed opportunity to study the effects of human-caused climate change in the Arctic(11).

Looking around

  • “Many Pacific Arctic changes are continuing, despite the observation that climate indices such as the Arctic Oscillation were negative or neutral for six of the last nine years. The Pacific Arctic may be having a larger role in shaping the persistence of Arctic change than has been previously recognized.”

Quote From the NOAA’s Arctic Theme Page, Observations in the Pacific Arctic:(14)

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2.2 Pacific Decadal Oscillation Index and El Niño/ La Niña

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.

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. © Image from the NASA Jet Propulsion Laboratory web page. Courtesy of Stepen Hare and Nathan Mantua, University of Washington. Surface Temperature units are degrees Celsius. Arrows are wind stress patterns.

El Niño events last as little as 6 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 Nino/La Nina, © Image from the NASA Jet Propulsion Laboratory web page. Courtesy of Stepen Hare and Nathan Mantua, University of Washington. Surface Temperature units are 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) web pages(13). 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 webpage(16). Analysis of the effects of El Niño / La Niña events on weather in the is obtained from Environment Canada’s webpage(5).

NWT Focus

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 are 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 phase(4,7,17,6). El Niño events have been shown to result in warmer winter weather and slightly higher than normal snow fall in the southern NWT(5), and increased spring water discharges in rivers(4). Both El Niño events and positive (warm) PDO phases also result in drier summers in the southern NWT, which in turn have been correlated to slower growth rates in spruce trees (Picea glauca and P. mariana)(3).

Current view - status and trend

Pacific Decadal Oscillation

Mean annual (January through December) values of the Pacific Decadal Oscillation index, 1900-current. Source courtesy of: http://jisao.washington.edu/pdo/PDO.latest, Graph from http://jisao.washington.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(12).

El Niño – La Niña

El Niño – La Niña events are tracked using the ENSO index. Positive values represent El Niño events, negative values represent La Niña events. 1950-current. Source courtesy of: http://www.cdc.noaa.gov/people/klaus.wolter/MEI/

During the past 60 years, there were seven strong El Niño events (positive ENSO index) (in 1957-59, 1965-67, 1972-74, 1982-84, 1986-88, 1991-93, 1997-99, and 2002-04). La Niña conditions have been in place since about August 2007, and is predicted to continue into 2009(16). The current ENSO index can be tracked monthly on NOAA web pages(16).

Looking Forward

The expected effects of El Niño – La Niña events on NWT’s weather can be found on Environment Canada webpage(5). The effects of phase shift in PDO and El Niño events on NWT’s ecosystems are not well understood. Only a few studies look at these long-term changes with the NWT in mind. Some studies have looked at weather patterns in relation to long-term fluctuations in NWT’s ecosystems, such fire regimes(6), droughts(3), but more is needed on 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.

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Reference List

  1. 2008, The Arctic Oscillation, National Snow and Ice Data Center, Arctic Climatology and Meteorology, Education Center, University of Colorado, Boulder http://nsidc.org/arcticmet/patterns/arctic_oscillation.html
  2. Cohen, J. and M.Barlow, 2005, The NAO, the AO, and global warming: how closely related?, J.Climate, 18,(4498- 4513
  3. Dave Sauchyn and Jonathan Barichivich, 2007, Long-term Drought in the North-Western Interior of North America and Linkages to ENSO, PDO and AMO., http://www.agu.org/meetings/sm07/sm07-sessions/sm07_U41B.html
  4. Déry, Stephen J and Wood, E. F, 2005, Decreasing river discharge in northern Canada, Geophysical Research Letters, 32,(10):CiteID L10401-
  5. Environment Canada, 2008, El Nino - Canadian Effects - Mackenzie District, Environment Canada, Environment Canada http://www.msc-smc.ec.gc.ca/education/n/region/
  6. Fauria, M. M. and Johnson, E. A., 2006, Large-scale climatic patterns control large lighting fire occurrence in Canada and Alaska forest regions, Journal of Geophysical Research, 111,(G4008):1- 17
  7. Hogg, E. H., Brandt, J. P., and Kochtubajda, B., 2005, Factors affecting interannual variation in growth of western Canadian aspen forests during 1951-2000, Canadian journal of forest research, 35,(610- 622
  8. International Panel on Climate Change., 2007, Climate Change 2007 - The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change . Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller, World Meteorological Organization and the United Nations Environment Programme, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
  9. J.Richter-Menge, S.Nghiem, D.Perovich, and I.Rigor, 2008, Sea Ice Cover, Arctic Report Card 2007, http://www.arctic.noaa.gov/reportcard/seaice.html http://www.arctic.noaa.gov/reportcard/seaice.html
  10. Lukovich JV and Barber DG, 2007, On the spatiotemporal behavior of sea ice concentration anomalies in the Northern Hemisphere., Journal of Geophysical Research, 112,(d13):D13117-
  11. McGuire, A. David, Chapin, F. S., Walsh, John E., and Wirth, Christian, 2006, Integrated Regional Changes in Arctic Climate Feedbacks: Implications for the Global Climate System*, Annual Review of Environment and Resources, 31,(1):61- 91
  12. NASA - JPL, 2008, New Release - Larger Pacific Climate Event Helps Current La Nina Linger -April 21, 2008, NASA - Jet Propulsion Laboratory - California Institute of Technology, NASA http://www.jpl.nasa.gov/news/news.cfm?release=2008-066
  13. NASA JPL, 2008, Pacific Decadal Oscillation (PDO), Science - El Nino/LaNina & PDO, NASA Jet Propulsion Laboratory - California Institute of Technology http://sealevel.jpl.nasa.gov/science/pdo.html
  14. National Oceanic and Atmospheric Administration (US), 2008, ACTIVITIES OF NOAA THAT SUPPORT THE OBJECTIVES OF THE INTERNATIONAL POLAR YEAR (IPY) MARCH 2007-MARCH 2009, Arctic Theme Page (Web), OBSERVATION 2. Causes and Impacts of Recent Changes in the Pacific Arctic,National Oceanic and Atmospheric Administration (US) http://www.arctic.noaa.gov/ipy-noaa.html
  15. NOAA, 2008, Arctic Oscillation, US National Weather Centre, Climate Prediction Service, NOAA http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/current
  16. NOAA National Oceanic and Atmospheric Administration, 2008, Multivariate ENSO Index (MEI), NOAA, U.S. Department of Commerce - | Earth System Research Laboratory | Physical Sciences Division
  17. Quan-fa Zhang and Wen-jun Chen, 28-3-2007, Fire cycle of the Canada's boreal region and its potential response to global change, Journal of Forestry Research, 18,(1):55- 61
  18. Sun Jianqi and Wang Huijun, 2006, Relationship between Arctic Oscillation and Pacific Decadal Oscillation on decadal timescale, Chinese Science Bulletin, 51,(1):75- 79
 
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