105 Chapter 5 Weather and Climate Patterns in Canada’s Prairie Grasslands Sean M. McGinn Agriculture and Agri-Food Canada Lethbridge, Alberta, Canada T1J 4B1 Abstract. The underlying uniqueness of Canada’s prairie grasslands is its aridity. The dominant air mass of this region is continental in nature and is associated with a relatively low amount of precipitation and high potential evaporation during the summer. Drought in this region is frequent, lasting one to two months on average, and presents a significant barrier to ecological capacity and agriculture. Severe droughts are associated with a stalled anticyclonic system that directs advected moist air around the region. Canada’s prairie grasslands receive on average of between 300 and 550 mm of precipitation, most of which is received as rain in spring and summer. Over the last century, changes to the climate have resulted in a general warming trend that is projected to increase further as a result of an enhanced greenhouse gas effect. Native grassland plant diversity and insect populations may shift as a result of expected climate changes. Résumé. Les prairies canadiennes doivent leur caractère unique à leur aridité. La masse d’air dominante dans cette région est continentale, et se caractérise par un degré relativement faible de précipitations et une évaporativité élevée en été. Les sécheresses sont fréquentes dans cette région; elles durent en moyenne de un à deux mois et réduisent sensiblement la capacité limite du paysage et le rendement agricole. Les sécheresses graves sont causées par un système anticyclonique stable qui empêche les courants d’air humide de pénétrer dans la région. Les prairies canadiennes reçoivent en moyenne de 300 à 550 mm de précipitations, la plupart sous forme de pluie au printemps et en été. Au cours du dernier siècle, le changement climatique a provoqué un réchauffement général qui devrait s’intensifier sous l’effet de l’augmentation des concentrations de gaz à effet de serre. La diversité des espèces végétales indigènes et les populations d’insectes pourraient évoluer sous l’effet des changements climatiques attendus. Introduction The prairie grasslands of Canada possess unique characteristics because of their topography, soil, fire frequency, climate, grazing, and imposed pressure from human activity (Ripley 1992). Although the current distribution of grasslands is attributed to both natural and anthropical forces, climate remains the dominant factor (Barnes 1959; Padbury et al. 2002). It is not surprising, therefore, that production of prairie flora is strongly related to weather elements such as precipitation (Smoliak 1986; Thorpe et al. 2008) and is well adapted to fluctuations in weather (Coupland 1958). For Canada’s prairie grasslands, an extension of the Northern Great Plains, the climate component is characterized as predominantly continental. Although the makeup of prairie grasslands has changed because of the introduction of agriculture (i.e., cropped grasslands), grasslands remain a distinctive landscape. Prairie grasslands extend eastward from the Canadian Rockies to the Assiniboine–Red River Valley in Manitoba and are bordered to the north by the boreal forest (see Chapter 3). Here, the seasonal extremes fluctuate between long, cold winters and short, warm summers. The McGinn, S. M. 2010. Weather and Climate Patterns in Canada’s Prairie Grasslands. In Arthropods of Canadian Grasslands (Volume 1): Ecology and Interactions in Grassland Habitats. Edited by J. D. Shorthouse and K. D. Floate. Biological Survey of Canada. pp. 105-119. © 2010 Biological Survey of Canada. ISBN 978-0-9689321-4-8 doi:10.3752/9780968932148.ch5 106 S. M. McGinn continental polar air mass that typically resides over this region produces relatively low amounts of precipitation. Periodic interruptions of moist maritime polar air in winter and summer by colder and drier arctic air masses generate the variety of weather experienced across the prairie grasslands. The seasonal extremes in air temperature, and the low amount of precipitation, are common elements of grassland ecosystems throughout the world. The extreme nature of climate on the prairies was first highlighted in relation to agricultural capacity during the 1856 expedition of Captain John Palliser (Spry 1959). Having spent a short time in the region, Palliser reported that southern Alberta and Saskatchewan, and a small area in southwestern Manitoba, would not support viable agriculture. The normal climate of this arid region, which became known as Palliser’s Triangle, was later proven to be capable of supporting agriculture if specialized practices were used (Grace 1987a). However, even with the advancements in agriculture today, periods of extreme weather, especially that of drought, continue to shape the agricultural and ecological capacities of the region. The climate of the prairie region prior to direct weather observations has been reconstructed by using many types of proxy data (Nkemdirim 1991). Such reconstruction of the climatic record has given insight into the potential for future climate. For example, drought intensity as experienced in the 1930s may have been more frequent in the past (Laird et al. 1996) and suggests that we are currently experiencing a climate without these extreme fluctuations. Precipitation across the Prairie Grasslands Precipitation amounts and timing are the principal climate features of the Canadian prairie grasslands that effectively determine the vitality of this ecosystem. In the absence of regional changes in topography and coastlines, the main force controlling regional precipitation patterns is the circulation of the atmosphere (Borchert 1950). In particular, the movement of cyclonic (low-pressure) systems across this region is a critical process that delivers advected moisture. The Canadian prairies receive an annual average of 454 mm of precipitation (McGinn and Shepherd 2003), which is much less than the Canada-wide average of 535 mm (Phillips 1990). However, the distribution of precipitation across these grasslands is highly variable, ranging from an annual low of 300–350 to a high of 500– 550 mm. In general, the driest region coincides with Palliser’s Triangle (Fig. 1). The distribution of precipitation is seasonal. Approximately 70–80% of the annual precipitation across the prairies is received as rain (Wheaton 1998). The rain received in June and July accounts for 20–35% of the yearly total precipitation and 42–54% of the growing- season precipitation (Chakravarti 1972). For the southern grasslands (between 49 and 52–54° N latitude), the peak month for rainfall is June, coinciding with the early summer position of the jet stream (a narrow band of strong winds in the high troposphere) and more frequent tracking of cyclonic systems. At higher latitudes, the peak month for precipitation is July, again coinciding with the predominant position of the jet stream at that time of the year. Within the Canadian prairies, the province of Saskatchewan receives the least annual precipitation (395 mm), followed by Alberta (482 mm) and Manitoba (486 mm) (Shepherd and McGinn 2003). In winter, the southern corridor of Alberta and central Saskatchewan receives 40 mm or less precipitation, whereas the southern border of Alberta and Saskatchewan (around Cypress Hills) receives more precipitation (Fig. 2). More than 60 mm of precipitation is received in northern Alberta and along the foothills, as well as in Manitoba. During the summer, areas of southeastern Alberta and southwestern Saskatchewan receive precipitation of between 120 and 145 mm. The maximum average precipitation in the summer occurs in central-northern Alberta (200–300 mm) and in eastern Manitoba (220 mm). Weather and Climate Patterns in Canada’s Prairie Grasslands 107 Fig. 1. Average annual precipitation (mm) across the prairie provinces (from Wheaton 1998). Prairie Drought Three basic weather patterns dominate the Canadian prairie grasslands in the summer: a mid-troposphere (approximately 10 km in height) ridge that occurs 72% of the time, a mid-troposphere trough with a 14% occurrence, and a zonal flow occurring 14% of the time (Dey 1982). A prolonged decrease in precipitation on these grasslands coincides with the mid-troposphere ridge pattern that remains stationary. An anticyclonic system (high- pressure area) associated with the ridge drives the jet stream northward, allowing moist maritime polar air to flow into the prairie region. This advection of atmospheric moisture into the prairie region accounts for about 65–76% of the rainfall received in the summer (June, July, and August; Raddatz 2000). The causes of drought continue to be evaluated. Bonsal and Wheaton (2005) reported that, unlike the typical 1961 and 1988 droughts of the Canadian prairies, where the formation of ridges and troughs had a key role, the 2001–2002 drought on the Canadian prairies had no relationships to large-scale circulation patterns. Instead, the drought was caused by a unique circulation pattern related to a northward extension of a persistent drought of the continental United States. Although the meteorological causes of drought are well recognized after they occur, they are often difficult to predict. In addition, the impact of drought is not always straightforward. A clear definition is needed when describing drought because the degree of impact is related to the intensity and duration of a drought event. Drought on the prairies is not characterized solely by a prolonged period of reduced precipitation (meteorological