During the winter of an El Niño event, the air temperature tends to be warm over most of Canada, with the greatest warming centred around Manitoba-western Ontario, where a temperature anomaly of up to +3 degrees Celcius (averaged over the last nine El Niño events) can be found (Hoerling et al., 1997; Shabbar and Khandekar, 1996). Southern Canada also tends to be drier during an El Niño winter (Shabbar et al., 1997). Southern British Columbia tends to receive less snow (Hsieh and Tang, 2001).
In the case of the cold La Niña event-- the opposite of the warm El Niño event-- the coastal waters off British Columbia tend to be cool. In a La Niña winter, the Canadian air temperature (especially west of Quebec) tends to be below normal (Hoerling et al., 1997; Shabbar and Khandekar, 1996), while the precipitation in southern Canada tends to be above normal (Shabbar et al., 1997). Southern British Columbia tends to receive more snow (Hsieh and Tang, 1999). Poor Canadian prairie wheat harvests often follow La Niña events (Garnett and Khandekar, 1992; Hsieh et al., 1999).
Note that La Niña does not give anomaly patterns which are exact opposites of El Niño, as can be seen from the nonlinear projection of the El Niño index to the extratropical Northern Hemisphere winter climate anomalies (Hsieh et al. 2006).
More information from Environment Canada.
Garnett, E.R. and Khandekar, M.L. 1992. The impact of large-scale atmospheric circulations and anomalies on Indian monsoon droughts and floods and on world grain yields-- a statistical analysis. Agricultural and Forest Meteorology, 61: 113-128.
Hoerling, M.P., Kumar, A. and Zhong, M. 1997. El Niño, La Niña and the nonlinearity of their teleconnections. Journal of Climate, 10: 1769-1786.
Hsieh, W.W. and B Tang, 2001. Interannual variability of accumulated snow in the Columbia basin, British Columbia. Water Resources Res. 37: 1753-1759.
Hsieh, W.W., B. Tang and E.R. Garnett, 1999. Teleconnections between Pacific sea surface temperatures and Canadian prairie wheat yield. Agricul. Forest Meterol. 96: 209-217.
Hsieh, W.W., A. Wu and A. Shabbar, 2006. Nonlinear atmospheric teleconnections. Geophys. Res. Lett. 33, L07714, doi:10.1029/2005GL025471. (preprint in PDF) (Videos).
Shabbar, A. and Khandekar, M. 1996. The impact of El Niño-Southern Oscillation on the temperature field over Canada. Atmosphere-Ocean 34:401-416.
Shabbar, A., Bonsal, B. and Khandekar, M. 1997. Canadian precipitation patterns associated with the Southern Oscillation. Journal of Climate, 10: 3016-3027.