Trends and Variability in Spring and Autumn 0 °C-Isotherm Dates over Canada

In cold-regions climates, numerous environmental processes and socio-economic activities are significantly impacted by the timing of the seasonal advance and retreat of the 0 °C isotherm. This investigation examines 20th century trends and variability in spring and autumn 0 °C-isotherm dates over Canada. Results reveal considerable variability across the country. Significant trends toward earlier springs are observed over most of western Canada including a dramatic shift to earlier dates during the last 20–30 years. Central regions areassociated with smaller, generally insignificant earlier spring trends. Conversely, extreme eastern areas experience later springs. During autumn, isotherm dates show little change over the majority of the country. The observed spatial and temporal characteristics in the 0 °C-isotherm trends are reflected in past variations in several hydro-cryospheric variables over many regions of Canada including the timing of snowmelt and the dates of freshwater ice break-up. Investigation into the potential causes of observed variability in 0 °C-isotherm dates reveals that large-scale oscillations representing atmospheric/oceanic variations in the north Pacific and north Atlantic significantly relate to isotherm dates over western and eastern Canada, respectively, during both spring and autumn. Although significant, a relatively small amount of overall variance in isotherm dates is explained by the oscillations indicating the influence of other factors. Results from this study improve our knowledge of past trends and variability in 0°C-isotherm dates and resultant impacts on hydro-cryospheric processesover Canada. They also provide insight into potential future climatologic impacts given the possibility of climate change.