The Occurrence of Near-0? Surface Air Temperatures in the Current and Pseudo-Global Warming Future Over Southern Canada

Temperatures near 0? represent a critical threshold for many environmental processes and socio-economic activities. This study examines surface air temperatures (T) near 0? (-2? = T = 2?) across much of southern Canada over a 13 year period (October 2000-September 2013). It utilized hourly data from 39 weather stations and from 4-km resolution Weather Research and Forecasting model simulations that were both a retrospective simulation as well as a pseudo-global warming simulation applicable near the end of the 21st century. Average annual occurrences of near-0? conditions increase by a relatively small amount of 5.1% from 985 hr in the current climate to 1,035 hr within the future one. Near-0? occurrences with precipitation vary from <5% to approximately 50% of these values. Near-0? occurrences are sometimes higher than values of neighboring temperatures. These near-0? peaks in temperature distributions can occur in both the current and future climate, in only one, or in neither. Only 4.3% of southern Canada is not associated with a near-0? peak and 65.8% is associated with a near-0? peak in both climates. It is inferred that latent heat exchanges from the melting and freezing of, for example, precipitation and the snowpack contribute significantly to some of these findings.