VARIABILITY AND TRENDS OF TOTAL PRECIPITATION AND SNOWFALL OVER THE UNITED-STATES AND CANADA

The biases and large-scale inhomogeneities in the time series of measu red precipitation and snowfall over the United States and Canada are d iscussed and analyzed. The spatial statistical characteristics of mont hly and annual snowfall and total precipitation are investigated and p arameterized. After adjustments and selection of the ''best'' network, reliable ''first guess'' estimates of North American snowfall and pre cipitation are obtained. Century-long time series of unbiased annual p recipitation over the regions to the south of 55 degrees N and 40-year time series of unbiased area-averaged annual precipitation and snowfa ll for all of North America are developed. The analysis of their trend s shows the following. 1) During the last 100 years, annual precipitat ion has increased in southern Canada (south of 55 degrees N) by 13% an d in the contiguous United States by 4%; however, the main domain of t his century-scale precipitation increase is eastern Canada and adjacen t to it northern regions of the United States. 2) Up to a 20% increase has occurred in annual snowfall and rainfall during the last four dec ades in Canada north of 55 degrees N. The relationships between centur y-long precipitation time series over North America with Northern Hemi sphere surface air temperature and the South Oscillation index (SOI) a re investigated. It is shown that ENSO (negative anomaly of SOI) is us ually accompanied by an increase of precipitation whenever it affects the United States (especially in the southwestern region of the countr y).