Natural Climate Fluctuations

Large-scale annual and decadal fluctuations in climate and 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. 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.

Natural Fluctuations indicators

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 natural phases of this 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 Arctic1.
When the Arctic Oscillation index is in a “positive phase” (left globe), high atmospheric pressure persists south of the North Pole, and lower pressures sit over the North Pole. In a positive phase, very cold winter air does not extend as far south into the middle of North America as it would during a 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 a "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 Center16.

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 (about 10 years) weaker clockwise circulation in the Beaufort Sea (weaker Beaufort Gyre)10, 11, which results in changes in currents across the Arctic Ocean and a decrease in old thicker sea ice at the pole10. A positive AO is also linked to warmer winter temperatures on average in northwestern North America 10, but is linked to winters colder than average in Nunavut ^{2, 10}.
The effects of AO on weather patterns in the NWT are clearer in the north (Beaufort Sea) and northeast (tundra) part of the territory5, 8. The Pacific Decadal Oscillation appears to have a stronger effect on weather in the south and western (forested) part of the territory5, 7. There is evidence19 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.

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 extensively3, 12. The current pattern (since about 2005) is more consistent with the rapid flip-fop patterns observed before this exceptionally long positive phase10.

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 emissions3. 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 two9. The current pattern of fast changes between positive and negative phases in AO offers scientists a renewed opportunity to study the effects of human-caused climate change in the Arctic12.

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 Arctic¹⁵

Find More

For more information on AO and its related phenomenon the North Atlantic Oscillation go to http://en.wikipedia.org/wiki/North_Atlantic_oscillation

Other Focal Points

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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 pages14. 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 webpage17. Analysis of the effects of El Niño / La Niña events on weather in the NWT is obtained from Environment Canada’s webpage6.