Skip booklet index and go to page content

Technical Thematic Report No. 4. - Large-scale climate oscillations influencing Canada

Main Oscillations Influencing Canada

El Niño/Southern Oscillation

The El Niño/Southern Oscillation (ENSO) represents large-scale ocean-atmosphere oscillations in the tropical Pacific that influence climatic conditions around the globe including Canada. It consists of two phases, namely warm El Niño events and cool La Niña events which tend to occur on average, every two to seven years. ENSOvariability is often measured by the difference in surface pressure anomalies between Tahiti and Darwin (known as the Southern Oscillation Index or SOI) and sea-surface temperatures (SSTs) in the equatorial Pacific. Figure 1 shows time series of seasonal SOI values from 1900 to 2008 with negative (positive) values representative of El Niño (La Niña) conditions. The graph reveals considerable variability over time. Strong events occurred through the first 25 years of the 20th century with few events of note through the middle portion of the 1900s. There is, however, a discernible shift in the mid 1970s toward more prolonged and intense El Niño episodes (that is, negative SOI values). This shift was influenced by a decadal-scale change in atmosphere-ocean conditions over the Pacific and western North America around 1976 (e.g. Trenberth and Hurrell, 1994) that is also evident in the Pacific Decadal Oscillation (see page 3). The most notable El Niño events in recent times occurred during the winters of 1982-1983 and 1997-1998.

Figure 1. Southern Oscillation Index for the period 1900 to 2008.

Long Description for Figure 1

This bar chart shows the Southern Oscillation index (SOI) values from 1900 to 2008. Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e. difference from the long-term mean) by the long-term standard deviations. Negative SOI values are representative of El Niño conditions, and positive SOI values are representative of La Niña conditions. An overlain line on the graph shows the five year running mean. The chart shows considerable variability over time. Strong events occurred from 1900 to 1925, with few events of note through the mid-1900s. There is a shift in the mid-1970s toward more prolonged and intense negative SOI values, or El Niño episodes.

Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e. difference from the long-term mean) by the long-term standard deviation. Five-year running means for each series are also provided (purple line).
Source: data for 1901 to 1950 are from Stand Tahiti - Stand Darwin) Sea Level Press Standardized Data and data for 1951 to 2008 are from Stand Tahiti - Stand Darwin) Sea Level Press Anomaly. Accessed 09/11/09.

Top of Page

Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is a measure of SSTs in the North Pacific that has a very strong correlation with the intensity of the Aleutian Low (Mantua et al., 1997). Positive PDO are characterized by colder than normal (i.e., long-term average) SSTs in the east-central North Pacific and warmer SSTs along the west coast of North America and vice versa. Although Pacific SSTs associated with the PDO and ENSO are somewhat spatially similar (with extra-tropical anomalies being emphasized in the PDO relative to ENSO), they have very different behaviour in time with PDO cycles persisting for 20 to 30 years. This is clearly evident in Figure 2 which shows the decadal nature of the PDO with shifts occurring around 1925, 1947, and 1976. As noted previously, the change toward positive PDO in the late 1970s also coincides with a shift toward more frequent El Niño events (see Figure 1).

Figure 2. Pacific Decadal Oscillation Index for the period 1900 to 2008.

Long Description for Figure 2

This bar chart shows the Pacific Decadal Oscillation (PDO) Index values from 1900 to 2008. Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e. difference from the long-term mean) by the long-term standard deviations. Positive PDO are characterized by colder than normal (i.e., long-term average) sea surface temperatures (SSTs) in the east-central North Pacific and warmer SSTs along the west coast of North America and vice versa. An overlain line on the graph shows the five year running mean. The chart shows PDO cycles persisting for 20 to 30 years with shifts occurring around 1925, 1947, and 1976. There is change toward positive PDO values in the late 1970s, which coincides with more frequent El Niño episodes.

Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e., difference from the long-term mean) by the long-term standard deviation. Five-year running means for each series are also provided (purple line).
Source: PDO IndexPDO.latest. Accessed 09/11/09
.

Top of Page

Pacific North American Pattern

The Pacific North American (PNA) pattern represents an upper-atmospheric wave structure featuring a sequence of high and low-pressure anomalies stretching from the sub-tropical west Pacific to the east coast of North America. The positive phase of the PNA features anomalous high pressure near Hawaii and over western Canada, and below average pressures in the central and eastern North Pacific (i.e., a deeper than normal Aleutian Low) and over the southeastern United States. Although the PNA is a natural internal mode of climate variability, it is also strongly influenced by ENSO with positive PNA tending to occur during winters associated with El Niño episodes, and negative values during La Niña events. Based on 1950 to 2008 data, approximately 51% of the PNA occurrences coincide with ENSO. Time series of seasonal PNAvalues from 1950 to 2008 (Figure 3) show considerable variability although, as with ENSO and PDO, there is a tendency for more positive PNA since the mid 1970s, particularly during winter.

Figure 3. Pacific North American Pattern Index for the period 1950 to 2008.

Long Description for Figure 3

This bar chart shows Pacific North American (PNA) Index values from 1950 to 2008. Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e., difference from the long-term mean) by the long-term standard deviation. Positive PNA values feature anomalous high pressure near Hawaii and over western Canada, and below average pressures in the central and eastern North Pacific (i.e., a deeper than normal Aleutian Low) and over the southeastern United States. An overlain line on the graph shows the five year running mean. The chart shows considerable variability with a tendency for more positive PNA since the mid 1970s, particularly during winter.

Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e., difference from the long-term mean) by the long-term standard deviation. Five-year running means for each series are also provided (purple line).
Source: NOAA - Earth Science Research Labratory  Accessed 09/11/09

Top of Page

North Atlantic Oscillation/Arctic Oscillation

The North Atlantic Oscillation (NAO) represents the dominant mode of atmospheric variability over the North Atlantic and is most pronounced during the cold season. The NAO index (Figure 4) is a measure of the difference in the strength of the Icelandic Low and the Azores High and has been shown to affect climate over Europe and eastern Canada. Positive NAO are associated with a deeper than normal Icelandic Low and a stronger than normal Azores High, while negative values represent opposite conditions. During the last century, the NAO has demonstrated considerable inter-annual and inter-decadal variations with little evidence of a long-term trend. The Arctic Oscillation (AO) represents atmospheric circulation variability over the extra-tropical Northern Hemisphere where sea-level pressures over the polar regions varies in opposition with that over middle latitudes (about 45°N) (Thompson and Wallace, 1998). It has also been referred to as the Northern Hemisphere Annular Mode. Note that the NAO and the AO are shown to be highly correlated in time, suggesting the NAO is merely a regional manifestation of the hemispheric-scale (AO) (Thompson and Wallace, 1998). Furthermore, the effects of both oscillations on temperature and precipitation in Canada are nearly the same and as a result, only relationships with the NAO are presented in this review.

Figure 4. North Atlantic Oscillation Index for the period 1900 to 2008.

Long Description for Figure 4

This bar chart shows North Atlantic Oscillation (NAO) Index values from 1900 to 2008. Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e. difference from the long-term mean) by the long-term standard deviation. Positive NAO are associated with a deeper than normal Icelandic Low and a stronger than normal Azores High, while negative values are associated with the opposite conditions. An overlain line on the graph shows the five year running mean. The chart shows considerable variability and little evidence of a long-term trend.

Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e. difference from the long-term mean) by the long-term standard deviation. Five-year running means for each series are also provided (purple line).
Source: Climatic Research Unit.  Accessed 09/11/09

Top of Page

Atlantic Multi-Decadal Oscillation

The Atlantic Multi-Decadal Oscillation (AMO) is a mode of natural variability representing multi-decadal changes in North Atlantic SSTs with cool and warm periods lasting for 20-40 years over an approximate range of +/- 0.4°C. It is defined as the first rotated empirical orthogonal function of global SSTs from which intra-seasonal ENSO and local trends have been removed (Mestas-Nuñez and Enfield, 1999). To the authors’ knowledge, no dynamical relationship has been established between the atmospheric NAO and the oceanic AMO. Whereas the AMO has a uniform polarity in SSTsacross the entire North Atlantic and operates on a multi-decadal scale, the NAOrepresents atmospheric variability with a dipole pattern in the North Atlantic and varies mainly on a quasi-decadal temporal scale (Arguez et al., 2009). The AMO series displays cool phases during 1905 to 1925 and 1970 to 1990, and warm phases from 1930 to 1960 and beginning again in the mid-1990s through the present (Figure 5).

Figure 5. Atlantic Multi-Decadal Oscillation Index for the period 1900 to 2008.

Long Description for Figure 5

This map of Canada displays where winter temperature is impacted by phases of El Niño/Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Pacific North American (PNA) pattern, and the North Atlantic Oscillation (NAO). Generally, ENSO is shown to impact the temperature of the western coast to central Canada, from the northern border of the Boreal Cordillera Ecozone+, through to the eastern edge of the James Bay. The PDO and PNA patterns impact a similar region as the ENSO. The PDO range extends from the western coast of Canada to around the western edge of the Hudson Bay, roughly following the northern boundary of the ENSO, while the PNA range extends to the northwest tip of Canada, roughly following the Arctic Ecozone+ boundary, ending before the Hudson Bay. The NAO range covers the northeastern regions of the country, influencing temperatures from Baffin Island, and southeast to the northern parts of the Atlantic Maritime Ecozone+.

Values are presented as standardized anomalies that are calculated by dividing each anomaly (i.e., difference from the long-term mean) by the long-term standard deviation. Five-year running means for each series are also provided.
Source: AMO unsmoothed from the Kaplan SST V2   Accessed 09/11/09

Top of Page