Extreme cold weather alerts in Toronto, Ontario, Canada and the impact of a changing climate

• Examined record of extreme cold weather alerts (ECWAs), 2004–12, for Toronto, Ontario.
• −10 °C threshold was found to be a good proxy for both extreme cold and wind chill events.
• Projected a decrease in ECWAs for Toronto for the 2020s, 2050s and 2080s.

The extreme cold weather alerts (ECWAs) were examined for Toronto, Canada for the winters of 2004–05 to 2011–12. ECWAs are triggered by extreme cold temperature, wind chill and intense winter precipitation. Just over 40% of the ECWAs occurred when the temperature fell below a −15 °C threshold. All but two of the alerts had a wind chill below −15 °C. The use of a −10 °C threshold captured well the frequency of wind chill events with half the −10 °C or lower events meeting the wind chill threshold. The modified −10 °C threshold and the −15 °C threshold were subsequently used to first assess how well climate models reproduced contemporary climate conditions, as well as three projection periods (2020s, 2050s, 2080s) to assess the impacts of a changing climate. The climate models reproduced current conditions well. In all projection cases the frequency of occurrence of events below the two thresholds decreases throughout the projection period but do not completely disappear. Interannual variability of projected events indicates a range of frequencies with some years similar to the contemporary climate. This suggests that thermal and wind driven ECWAs will continue for Toronto under climate change scenarios, although with gradual decreasing frequency.