Canadian Prairie growing season precipitation variability and associated atmospheric circulation

Canadian Prairie growing season (May to August) precipitation variability and associated atmospheric circulation patterns are examined for the period 1905 to 1996. The majority of precipitation and atmospheric variables generally displayed substantial interannual, as well as interdecadal, variability. The analysis identifies 5 decadal scale periods when the temporal distribution of precipitation (as measured by changes in the weighted centre of 30 d moving window precipitation totals) significantly shifted to earlier or later in the growing season. Monthly precipitation variability accounted for the observed shifts in temporal distribution including significantly low July-August rainfall from the mid 1910s to the mid 1940s, and a significant increase in May precipitation in the early 1970s. In addition, since 1980, July has replaced June as the month of maximum precipitation. Mid-tropospheric and sea-level pressure atmospheric indices are developed to examine the synoptic scale flow characteristics associated with precipitation variability. Indices at the 500 hPa and, to a lesser extent, surface levels explain a substantial portion of variance in monthly precipitation, and are also generally related to shifts in the temporal distribution. Since 1974, May 500 hPa circulation has been characterized by a lower frequency of ridging (troughing) over western (eastern) Canada resulting in significantly higher May precipitation over the Prairies. The interdecadal variability in July and August precipitation was influenced by decadal variations in surface circulation involving the continental low over the southcentral United States, the high pressure over Arctic Canada, and the low pressure over eastern Canada. In particular, a substantial lowering of surface pressure over the continental United States from approximately the mid 1970s to the present appears to have contributed to the recent increase in later growing season precipitation over the Prairies.