Summer Severe-Rainfall Frequency Trend and Variability over Ontario, Canada

Abstract

During the last two decades (1979?2002), there has been an ever-increasing frequency of summer severe-rainfall events over Ontario, Canada. This observed upward trend is robust as demonstrated through the Mann?Kendall test with consideration of removing a lag-1 autoregressive process. It is shown through composite analyses using the NCEP reanalysis data that in the presence of warming conditions the summer severe-rainfall events occur more frequently over Ontario, especially under atmospheric conditions with stronger low-level cyclonic circulations and more precipitable water. Further analyses indicate that over north and central Ontario the summer severe-rainfall frequency is linked with a positive trend of precipitable water whereas over central and south Ontario there is a strong interannual response of summer severe-rainfall frequency to the changes in precipitable water through the variations of air temperature.