Abstract

Indices of extreme of daily observed temperature during the 1971-2000 period using several meteorological stations in the Greater Toronto Area (GTA) are examined. The indices that are based on daily minimum temperature exhibit statistically significant decreasing trends at all stations in the GTA. The trend analysis of the extremes based on daily maximum temperature shows increasing trends at urban and suburban stations. The temporal and spatial distributions of cold extreme indices are more coherent throughout the region compared to the warm extreme indices. The trend analysis of seasonal extreme temperatures for warm days, warm nights, cold days, and cold nights show statistically significant trends leading to more days with extreme high temperature in winter and spring, but no indication of any significant trend of extreme high temperature in summer. The percentage of cold nights shows decreasing trends in the winter, spring, and summer, while the percentage of warm nights shows increasing trends for the same seasons. The significant decreasing trend in diurnal temperature range, mainly in the urban and suburban areas, is, therefore, due to the nighttime warming of these stations, which could be a positive feedback effect from urban warming. The regional analysis of the seasonal extremes reveals that there is a shift towards warmer climate in the GTA region with an increase in extreme high temperature and a reduction in extreme low temperatures during the study period, which are more apparent in winter and spring compared to summer and autumn. There are also fewer nights with extreme low nighttime temperatures, which are observed in summer, but more nights with extreme high nighttime temperatures are found in all seasons. The impact of climate warming in addition to the urban warming could lead to an increase in the intensity and frequency of heat-related extreme events in the GTA.