THE RELATIONSHIP BETWEEN CANADIAN PRAIRIE DRY AND WET MONTHS AND CIRCULATION ANOMALIES IN THE MIDTROPOSPHERE

We identify Dry and Wet months for the Canadian Prairies during the 40‐year period 1946–1985 and investigate their relationship to the Northern Hemisphere circulation at the 50‐ and 100‐kPa levels. The target area is first subdivided into five zones on the basis of characteristic differences in precipitation climatology, and for each zone the dates of occurrence of dry and wet months are determined. These events are then stratified into five “seasons”. Much of the analysis focuses on the two growing seasons: Spring, defined as April and May, and Early Summer, June and July. Composite anomaly fields for the Northern Hemisphere are constructed for the 50‐kPa level by “season” and zone, and for the Dry and Wet groupings. For each zone, the two 50‐kPa anomaly fields Dry and Wet are clearly distinguishable, not only over North America but in many instances, upstream and downstream of the continent. Composite anomalies are shown to be zone‐sensitive. The 100‐ and 50‐kPa anomalous wind fields associated with the Dry and Wet regimes, respectively, are found to be consistent with dynamic and thermodynamic processes that control the production of precipitation. Anomaly field structures over the oceans and North America are related to Northern Hemisphere tropospheric circulation modes, including the PNA, NPO and NAO. High‐latitude blocking over the North Atlantic and North Pacific is often associated with Wet months, particularly in Spring, whereas “in situ” blocking over western Canada or simply amplified ridging extending northward from the western United States is almost invariably associated with the Dry months. © 1990 Taylor & Francis Group, LLC.