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

Impacts on Canadian Climate

Cold season temperature

The strongest links between large-scale teleconnections and Canadian climate occur with cold-season temperature. The greatest inter-annual variation in winter temperature is, to a large extent, controlled by ENSO. In particular, the anomalous heating in the eastern tropical Pacific associated with El Niño, triggers upper-atmospheric waves which give rise to a positive PNA-like pattern over North America (Wallace and Gutzler, 1981). The initiation of this pattern leads to warmer than normal temperatures that spread eastward from the west coast to central Canada during late autumn to early spring following the onset of El Niño episodes (Shabbar and Khandekar, 1996) (see Figure 6). In addition, the frequency and duration of winter warm spells are significantly enhanced and cold spells significantly decreased during El Niño winters (Shabbar and Bonsal, 2004). Temperature responses to La Niña events are for the most part, opposite with colder than normal winters (including a higher frequency of cold spells) over western and central Canada. However, La Niña effects are often more concentrated in the west relative to those associated with El Niño (Hoerling et al., 1997; Hsieh et al., 2009). It should also be noted that individual ENSOevent temperature responses vary considerably, both temporally and spatially. As alluded to previously, the PNA also occurs in the absence of large-amplitude ENSO events. On average, the positive phase of the PNA pattern is associated with above-average temperatures over western Canada in a region similar to that for El Niño, but extending further into northern Canada and confined to the westernmost four provinces (Figure 6). Negative PNA winters are associated with colder temperatures in this region (see Climate Prediction Center Internet Team, 2005b).

On longer time scales, cold-season temperature variations over Canada have been associated with North Pacific atmosphere-ocean variability as measured by the PDO. Bonsal et al. (2001) determined that positive PDO periods were significantly related to warmer than normal winter temperatures over western and central regions of the country on both inter-decadal and inter-annual scales (and vice versa) (Figure 6). They also found that El Niño (La Niña) related temperature responses were stronger and more consistent during positive (negative) phases of the PDO (for more details see Figure 4 and Figure 5in Bonsal et al., 2001).

Figure 6. Typical regions of Canada impacted by the different phases of ENSO, the PDO, the PNA pattern, and the NAO during the cold season: temperature.

Long Description for Figure 6

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+.

Warmer (colder) than normal temperatures occur in the designated regions during El Niño (La Niña) events, positive (negative)PDO, positive (negative) PNA, and negative (positive) NAO.

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Teleconnection relationships over eastern Canada are generally not as strong when compared to the west. The main climatic driver in this region is the NAO which during the cold season, impacts temperatures over northeastern regions of the country (Figure 6). This includes colder than normal winters in association with positive NAO and vice versa (Bonsal et al., 2001). The colder temperatures are related to an intensified Icelandic Low that results in increased frequencies of northerly flow into northeastern Canada. Due to the strong association between the NAO and the AO, anomalously cold temperatures are also observed over in this region of Canada during positive phases of the AO.

It should be noted that the aforementioned associations between these large-scale oscillations and cold-season temperatures over various regions of Canada have also resulted in significant relationships between these teleconnection patterns and several ecosystem-related variables over the country. These include for example, the duration of lake and river ice (Bonsal et al., 2006) the timing of snowmelt and spring peak streamflow (Stewart et al., 2005; Burn, 2008), the onset of spring (Bonsal and Prowse, 2003; Schwartz et al., 2006), and even the occurrence and mortality of the mountain pine beetle in western Canada (Stahl et al., 2006a).

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Cold season precipitation

Although not as strong as temperature, significant relationships between large-scale teleconnections and Canadian cold-season precipitation have also been identified (Figure 7). In particular, winters following the onset of El Niño are generally associated with a distinct pattern of below normal precipitation stretching from southern British Columbia, through the Prairies, and into the Great Lakes region. La Niña events lead to an opposite response (Shabbar et al., 1997). Links between the PDO and winter precipitation are also evident, but these effects are confined to western Canada (southern Yukon, British Columbia and Alberta, and southern Saskatchewan and Manitoba) where positive PDO are associated with below normal precipitation and vice versa (see Figure 7) (Bonsal et al., 2001).

Figure 7. Typical regions of Canada impacted by the different phases of ENSO, the PDO, the PNA pattern, and the NAO during the cold season: precipitation.

Long Description for Figure 7

This map of Canada displays where winter precipitation 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). The ENSO range covers southern Canada, from British Columbia, through the prairies, and into the Great Lakes region. The PDO range is confined to western and central Canada, from the northern boarder of the Boreal Cordillera Ecozone+ stretching southeast to include the Lake Winnipeg region. The PNA pattern is generally confined to Alberta. The NAOrange covers the southeastern region of Baffin Island and Labrador.

Below (above) average precipitation is associated with El Niño (La Niña) events, positive (negative) PDO, positive (negative) PNA, and positive (negative) NAO.

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As with temperature, El Niño (La Niña) precipitation responses in western Canada tend to be amplified during positive (negative) PDO phases (e.g. Kiffney et al., 2002; Stahl et al., 2006b). The PNA winter precipitation response tends to be confined to most of Alberta (Figure 7) (see Climate Prediction Center Internet Team, 2005a) where positive PNA is associated with below average values and vice versa. Some investigations have also found that positive PNAperiods were significantly correlated with reduced snow cover in western Canada during all seasons (Brown and Goodison, 1996). The effect of NAO on Canadian precipitation is modest and generally restricted to northeastern regions where positive values are associated with lower than normal winter precipitation (Figure 7). The positive NAO allows for more frequent outbreaks of cold, dry Arctic air thus resulting in less precipitable moisture.

Note that these teleconnection – precipitation relationships have also been reflected in streamflow characteristics over various regions of the country. In particular, over western Canada, there is a higher frequency of low streamflow events in association with the drier conditions during El Niño events and positive phases of the PDO and the PNA pattern (and vice versa). In northeastern regions of the country, reduced stream flows occur during positive phases of NAO and the AO (see Bonsal and Shabbar, 2008 and references therein for a detailed summary of relationships between large-scale oscillations and stream flow in Canada).

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Summer climate

Relationships between teleconnections and summer climate over Canada are not as strong and/or consistent as compared to the cold season. Nonetheless, there have been some studies that have identified various associations with drought-like conditions during the summer period. Bonsal and Lawford (1999) indicated that during the majority of El Niño events, persistence of a North Pacific SST pattern consisting of anomalously cold water in the east-central North Pacific and anomalously warm water along the west coast of North America led to extended summer dry spells on the Canadian Prairies. In addition, Shabbar and Skinner (2004) found that El Niño events led to a summer moisture deficit in western Canada, while La Niña events produced an abundance of moisture, mainly in extreme western Canada. They also determined that the positive phase of the winter AMO tends to be associated with summer dry conditions over the central and northern regions of the Canadian Prairies, the lower Great Lakes and St. Lawrence Valley, and portions of the west coast of Canada (see Figure 8). Other ecosystem-related studies have revealed that El Niño events and the positive phase of PDO lead to drier conditions and higher forest fire severity in western, northwestern, and parts of northeastern Canada. Conversely, La Niña events and negative PDO produce an excess in summer moisture and low fire severity over western regions of the country (Skinner et al., 2006). In addition, an analysis of tornado frequencies and ENSO suggest that the La Niña events tend to suppress tornadic activity in western Canada, while El Niño episodes tend to enhance it (Etkin et al., 2001).

Figure 8. Typical regions of Canada impacted by ENSOand the AMOduring the summer season.

Long Description for Figure 8

This map of Canada displays where summer precipitation is impacted by El Niño/Southern Oscillation (ENSO) and the Atlantic Multi-Decadal Oscillation (AMO). The ENSO range primarily covers southwestern and central Canada, from southern British Columbia the northern Lake Winnipeg region. The AMO range covers three regions: a small region in northern BC and the Yukon; a central region starting at the western boundary of the Boreal Plains Ecozone+, extending east to the Hudson Bay; and a third region, covering lower Great Lakes and St. Lawrence valley.

Drier (wetter) than normal conditions occur in the designated regions during El Niño (La Niña) events and positive (negative) phases of the AMO.

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Trends in Large-Scale Oscillations and Canadian Climate

It has been documented that the different phases in large-scale teleconnections have acted to amplify (or in some cases, dampen) observed climate trends over various regions of North America including Canada. As a result, a number of the observed 20th century climate changes can be attributed at least in part, to changes in these various teleconnection patterns (Solomon et al., 2007). Hurrell (1996), for example, found that the NAO, ENSO, and variations in North Pacific circulation collectively explained a significant portion of Northern Hemisphere winter temperature variability during the 20th century. Specifically for North America, the mid 1970s climate shift to positive PDO and more frequent El Niño events appear to have led to contrasting changes across the continent, as the west has warmed more than the east (Trenberth et al., 2007). This shift has therefore also been associated with the trend toward warmer winter and spring temperatures over western Canada. However, 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 ENSOfor example, the ability of current Global Climate Models (GCMs) to simulate observed El Niño and La Niña events differ considerably 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 (Meehl et al., 2007). 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.

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