The Impact of ENSO on the Canadian Climate
Amir Shabbar and Barrie BonsalAtmospheric Environment Service
Downsview, Ontario, Canada
amir.shabbar@ec.gc.ca
Introduction
The effects of El Niño-Southern Oscillation (ENSO) events are usually strongest during the Northern Hemisphere winter. Recent investigations into the impact of ENSO on Canadian climate also demonstrate that the tropical influence is strongest during the cold season. In this study, Canadian temperature and precipitation and Northern Hemisphere mid-tropospheric data are statistically analyzed in the context of El Niño, La Niña and non-ENSO years, (Shabbar and Khandekar, 1996; Shabbar et al., 1997). The analysis incorporates 25 moderate to strong El Niño and 17 moderate to strong La Niña events that have occurred this century. Differences in the Pacific North American (PNA) oscillation and in particular, the height anomaly over western Canada, provide the strongest ENSO signal over North America and also explain the significant temperature and precipitation responses over Canada. However, results indicate that the effects of La Niña are not exactly opposite to those associated with El Niño. Given that the nature of ENSO events introduces a time lead in the climate over Canada, the established relationships in this study have been used, through the canonical correlation analysis (CCA) technique, to produce long-lead forecasts of Canadian temperature and precipitation. Recent ENSO events have shown that these forecasts provide usable skill.
Temperature impact
Our analysis shows that statistically significant positive surface temperature anomalies spread eastward from the west coast of Canada to the Labrador coast during the late fall to early spring (November through May) following the onset of El Niño episodes. The accompanying temperatures in the lower troposphere show a transition from the PNA pattern to the Tropical/Northern Hemisphere (TNH) pattern over the North American sector during the same period. Conversely, significant negative surface temperature anomalies spread southeastward from the Yukon and extend into the upper Great Lakes region during the winter season following the onset of La Niña episodes. The lower tropospheric temperatures show a negatively-phased PNA-like pattern in early winter which weakens considerably by May of the following year. Thus, while western Canadian surface temperatures are influenced during both phases of ENSO, those over eastern Canada are affected only during El Niño. The impact of ENSO on the Canadian surface temperatures is the strongest during the winter season and nearly disappears by spring (April and May). In both El Niño and La Niña years, the largest temperature anomalies are centered over two separate regions; one over the Yukon and the other just west of Hudson Bay. Over western Canada, the mean temperature distribution of the El Niño (La Niña) years is shifted toward warmer (colder) values relative to the distribution of non-ENSO years.
Precipitation impact
A detailed investigation of the spatial and temporal behavior in precipitation responses over Canada shows significant responses over southern Canada, during the first winter following the onset of the ENSO events. Composite and correlation analyses indicate that precipitation over a large region of southern Canada extending from British Columbia, through the Prairies, and into the Great Lakes region is significantly influenced by the ENSO phenomenon. The results show a distinct pattern of negative (positive) precipitation anomalies in this region during the first winter following the onset of El Niño (La Niña) events. Statistical and field significance of the responses are established by non-parametric and Monte Carlo procedures. The significant precipitation anomalies can be explained by the associated mid-tropospheric flow pattern, which following the onset of El Niño (La Niña) events resembles the positive (negative) phase of the PNA pattern. While the temperature anomalies for both El Niño and La Niña show a west-to-east progression starting in the autumn of the onset year and persisting through the following spring, no progression of precipitation anomalies is evident in our analysis. Furthermore, the areas encompassed by significant temperature anomalies differ from those of precipitation. Over the southern Canadian region, an investigation of the intraseasonal variability shows no apparent difference in the intensity of precipitation. In addition to the ENSO signal, winter precipitation over Canada also appears to be influenced by decadal-scale oscillations in the Pacific Ocean. There is also considerable interannual variability in both temperature and precipitation responses associated with individual ENSO events.
Recent ENSO impacts over Canada
Each El Niño/La Niña has distinct characteristics. Examples of a few recent major impacts are listed below.
- El Niño: With the exception of the northeast Arctic, winter temperatures were 2 to 8°C above normal, and precipitation was generally one-half of the normal amount in association with the 1997-98 El Niño. The warm, dry weather contributed to severe brush fires in Alberta in December. A temporary northeastward displacement of the unusually strong El Niño-related sub-tropical jet stream is also believed to have played a key role in the disastrous Ice Storm over eastern Ontario and southwestern Quebec in January. During the 1982-83 and 1997-98 warm events, the mild winter weather significantly reduced snow removal budgets for municipalities. Heating costs were 5 to 15 per cent below normal for homeowners and businesses (an average savings of $200 Canadian per household).
- La Niña: During the 1988-89 event, a build up of bitterly cold air in the high Arctic was responsible for the highest pressure reading ever recorded in North America; 107.5 kilopascals at Northway, Alaska. All-time high pressure readings were also set in many stations on the Canadian Prairies. During early February 1989, Vancouver experienced its longest cold spell, as the overnight temperatures dropped below -10°C. Frigid air pushed as far south as southern Texas and even blanketed southern California with snow. During the 1995-96 La Niña, most of Canada suffered from a particularly long winter. Even in southern Canada, most locations had continuous snow cover from October to March. Winter temperatures were as much 11°C below normal in Winnipeg; the second coldest on record. In addition, overnight temperatures remained below a frigid -30°C for 19 days in a row. Higher than normal snowfall in the Red River valley contributed to an extensive spring flooding in southern Manitoba and the Dakotas in the U.S.
References:
La Niña from a Canadian Perspective - http://www1.tor.ec.gc.ca/lanina/
Shabbar, A. and M. Khandekar, 1996: The impact of El Niño-Southern Oscillation on the temperature field over Canada. Atmos.-Ocean, 34, 401-416.
Shabbar, A., B. Bonsal and M. Khandekar, 1997: Canadian precipitation patterns associated with the southern oscillation. J. Climate, 10, 3016-3027.
| Temperature Distribution over Western Canada | Typical Winter Precipitation response following the onset of La Niña. Values shown are standardized anomaly |
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