3.1 Autumn

Projected mid-tropospheric (500 hPa) circulation changes resemble a westward-shifted negative PNA pattern (Fig. 2a). An anomalous trough is projected to occur above British Columbia/Yukon and extend into the Pacific off the western coast of the US, while an anomalous high is projected to occur over the southwestern vicinity of the Aleutian Islands. These circulation anomalies, as well as those to be discussed below for other seasons, are deep structures that extend to the top of the troposphere. Similar circulation patterns are typically found during negative PNA conditions, with an anomalous low centered above the southern Prairies and a high above the Aleutians. Corresponding changes in anomalous mean sea-level pressure (MSLP) include a projected high centered just east of the upper high and a trough that extends from the Arctic Ocean into central northern Canada. Circulation changes at higher levels effectively reduce the waviness of the upper flow and jet stream by weakening both the climatological Aleutian low and the upper ridge over western Canada.

Figure 2CMIP5 RCP8.5 projected changes in the 39-model ensemble mean between the periods (2081–2099) and (1981–2000) for autumn (SON). For each 3-month period, the four panels show differences in (a) 500 hPa height (m), (b) sea-level pressure (hPa), (c) near-surface (2 m) air temperature (^∘C), and (d) precipitation rate (mm d⁻¹).

The time series of the SON PNA index (Fig. 3a) suggests that the afore-mentioned circulation change is projected to commence around the mid-century. Although inter-model spread as measured by the interquartile range is large, a statistically significant (at 5 % level) decreasing trend of the ensemble median is projected to occur during the second half of the century. The mean near-neutral PNA condition that characterizes the first half of the century is replaced by mean negative (−0.26) conditions and 9 out of the 10 strongest negative PNA autumns are found after 2060; this latter result provides support to the assumption that changes in the mean circulation pattern are partly manifested in changes in the frequency and/or intensity of the extreme similar anomaly patterns.

Figure 3The (a) autumn (SON) and (b) winter (DJF) PNA index over the 21st century computed by applying CMIP5 ensemble model information to the formula given in Wallace and Gutzler (1981). The lines indicate the median, 25th percentile, and 75th percentile. Also shown are piecewise linear regression lines separated at the “break” year indicated by the dashed line. Assessments using the Mann–Kendall method (Kendall, 1955) show that the after (before) break trend for SON (DJF) is significant at 5 % (10 %), while the other trends are not significant. The before and after break trends are significantly different at 5 % (10 %) for SON (DJF).

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Projected regional climate responses to the circulation changes are consistent with those found during negative PNA, but are shifted in association with the projected circulation features. In particular, the cold air advection into northwestern Canada by the anomalous anti-cyclonic flow associated with the surface North Pacific high found in typical negative PNA conditions is much reduced due to the westward-shifted location of the high. Instead, the surface high enhances low-level flows towards the Pacific coast of Canada, which when combined with the upper low over British Columbia would substantially enhance precipitation over the coastal regions (Fig. 2d). The enhanced cross-barrier flow and associated precipitation induce subsidence and adiabatic warming over the Prairies (Szeto, 2007; Szeto et al., 2007; Szeto, 2008). The warming over the south effectively reduces the S–N gradient of net anthropogenic warming (Fig. 2c) and partly offsets its detrimental effects on the low-level background baroclinicity and synoptic storms that affect southwestern Canada.

Quasi-geostrophic theory (e.g., Holton, 1979) predicts that cyclone activities would be enhanced in the downstream vicinity of the upper anomalous trough, i.e., over southwestern Canada. Despite the potential increases in autumn cyclones and atmospheric moisture in the warming environment, Prairie precipitation is projected to increase by only ∼10 %. This is likely related to the significant depletion of Pacific moisture over the coastal mountains and enhanced lee-side subsidence that are associated with the enhanced cross-barrier flow discussed earlier. As a result, although the precipitation increase is statistically significant (i.e., the increase is larger than the natural variability of historical modelled precipitation for the region), it is substantially lower than the relative increases projected for the winter and spring. These results indicate that the complex topography of the region, as well as changes in the synoptic-scale forcing, could play an important role in affecting the autumn precipitation over the Prairies under this warming scenario.

3.2 Winter

The winter anomaly pattern is projected to be characterized by a pronounced upper low centered in the eastern Pacific and enhanced ridging over central northern Canada (Fig. 4a). In contrast to projected SON changes, the circulation change resembles an eastward-shifted positive PNA pattern. An increasing trend, albeit merely significant at the 10 % level, in the ensemble-median DJF PNA index is found only during the first half of the century (Fig. 3b). An abrupt “jump” is projected to occur at the end of the increasing trend during the late 2050s where the piecewise linear regression lines over the two periods are separated by a statistically significant gap that is larger than the variability of the index. The significance of the mid-century change is reflected in the marked increase in the mean PNA index from 0.10 in the first half of the century to 0.37 in the second half as well as in the fact that all of the 10 projected strongest PNA winters occur after 2060. This latter result also provides support to the assumption we made about the relationship between changes in the mean and extreme patterns.

Figure 4As in Fig. 2 but for winter (DJF).

These results might be related to those of Zhou et al. (2014), who showed that the eastward shift of tropical convective anomalies under climate warming would cause the ENSO-forced winter PNA pattern to move eastward and intensify. These projected changes effectively enhance the waviness of the upper flow and jet over the North Pacific by strengthening and broadening the North Pacific upper trough towards the western coast of the US while broadening the climatological upper ridge and making the flow more zonal over northwestern Canada. Some changes at the lower levels (Fig. 4b) are similar to those found in positive PNA conditions, with an anomalous trough extending from the Aleutians into areas off the western coast of North America. But a strong anomalous surface ridge that is typically located over the western US under positive PNA conditions is projected to be centered over southwestern Canada.

Similar to positive PNA conditions, the low-level high–low couplet allows the warm Pacific air to be advected into the Yukon and the Mackenzie basin. On the other hand, the reduced onshore flow could decrease the precipitation along the British Columbia coast and the associated weakening of adiabatic warming and lee cyclogenesis over the southern Prairies. These combined effects enhance the S–N anthropogenic warming gradients (Fig. 4c), weaken the background surface frontal zone, and contribute to the development of the anomalous surface ridging in the region. For example, the mean DJF N–S near-surface temperature gradient over the southern Prairies is projected to decline by ∼25 % towards the end of the century. In addition, the weakened mean upper northwesterly flow over northwestern Canada reduces the number of North Pacific systems that enter Alaska to migrate down to the southern Mackenzie basin. Collectively, these changes in circulation and dynamic features are expected to reduce the frequency and intensity of weak cyclones that typically affect the region in winter.

Despite these considerations, winter precipitation at the end of the century is projected to increase by approximately 19 % over the Prairies and by larger amounts at higher latitudes. This 19 % increase is significantly higher than that predicted for SON. This result can be explained by considering the anomalous upper- and low-level troughs projected to occur off the US western coast which allow more moisture-laden Pacific systems to develop and affect the western US, as reflected in the enhanced troughing and precipitation projected over this region (Fig. 4b and d). With the weakened mean upper northwesterly flow over western Canada, some of these moisture-laden southern systems could be able to track into the Prairies and produce more frequent extreme winter precipitation events.

Preliminary analysis of projected daily Prairie precipitation provides strong support for the above inference. It is a preliminary analysis because we only have access to daily data for 21 out of the 39 models for this analysis. The results show that, although the ensemble mean frequency of precipitation days (daily precipitation P > 0.5 mm d⁻¹) only increases marginally from 743 d during 2006–2020 to 778 d during the last 15 years of this century, the corresponding frequency of extreme precipitation days (daily P > Pc, the 99th percentile of daily P during DJF 2006–2020) increases from 10 to 29. In addition, although the mean frequency of extended (longer than 1 d) precipitation events hardly changes between the two periods (157 versus 158), the frequency of extended extreme precipitation events (multi-day events with daily P > Pc) increases 5-fold from 0.6 to 3.3 between these two periods.

3.3 Spring

The most prominent circulation anomaly feature is the quasi-stationary upper low centered over the northwestern US (Fig. 5a). This large-scale setting favors the development of cyclones that bring warm-season precipitation to southwestern Canadian regions (Szeto et al., 2011, 2015, 2016). The intensity of the low can be quantified by the H index as detailed earlier and the development of this feature is shown in Fig. 6. The low is a robust feature projected by most models (Fig. 6a). The mean magnitude of the low (i.e., negative H index) is projected to intensify mainly during the mid-century (Fig. 6a). In fact, the 20-year mean H (not shown) decreases by 15 m (from −10 to −25 m) during 2045–2065, suggesting that some radical changes in the MAM large-scale circulations are projected to occur mid-century. It is noteworthy that this mid-century decrease in 20-year mean H is even larger than the standard deviation of H over the century (13.9 m). In addition, 8 of the 10 lowest H index springs, i.e., springs that are likely to be associated with extreme wet conditions over the Prairies, occur after 2040. At the lower levels, the S–N warming gradient is relatively weak (Fig. 5c) and thus has little effect on the mean frontal zone across the southern Canadian regions. As a result, cyclone activity that affects the region is expected to increase, as reflected in the anomalous N–S surface trough that extends from Hudson Bay into the eastern Prairies and central US (Fig. 5b).

Figure 5As in Fig. 2 but for spring (MAM).

Figure 6H index for (a) the spring (MAM) anomalous low and (b) summer (JJA) anomalous high centered over the northwestern USA during the 21st century projected from CMIP5 ensemble model information. The lines indicate the median, 25th percentile, and 75th percentile. Also shown are piecewise linear regression lines separated at the “break” year indicated by the dashed line. Assessments using the Mann–Kendall method (Kendall, 1955) show that only the after break trend for JJA is significant at the 5 % level. The before and after break trends are significantly different at 5 % for JJA only.

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Consequently, spring precipitation is projected to increase significantly over southern Canada in general (Fig. 5c). For example, Prairie MAM precipitation increases by 26 %, the largest among the seasons. This Prairie spring precipitation is projected to increase starting from the 1990s and continue until around 2060 (not shown) following the stabilization of the anomalous upper low. It is noteworthy that the predicted intensification of the upper low and associated increasing trends of mean and extreme precipitation over the eastern Prairies during the turn of the century are also evident in observations (Szeto et al., 2015).

Results from the analysis of daily Prairie precipitation provide further insight into the regional precipitation response to the circulation change. In particular, the frequency of extreme precipitation days (daily P > Pc, where Pc is the 99th percentile of daily P during MAM 2006–2020) doubles from 10.5 d during the early century period to 20.4 d towards the end of the century (2086–2100). In addition, although the mean frequency of extended precipitation events hardly changes between the two periods (153 versus 156), the frequency of extended extreme precipitation events increases 3-fold from 1.2 to 3.7 between these two periods. Although both the DJF and MAM results suggest substantial future increases in extreme precipitation events, it is noteworthy that, although the relative seasonal precipitation increase for MAM is higher, the increase in extreme precipitation event frequency is somewhat higher for DJF. The apparent discrepancy is likely related to the difference in the model ensembles (21 versus 39 members for daily and monthly analysis, respectively) that were used in the assessments.

The long-term mean large-scale upper low pressure anomaly would allow more upper low systems to enter the continent through the northwestern US. Analyses of historical extreme Prairie precipitation events (not shown) suggest that the location of heavy precipitation is sensitive to the location of the upper low due to the topography that characterizes the region. In particular, strong upslope rainstorms over southern Alberta, similar to the one that caused the 2013 Calgary flood (Pomeroy et al., 2015; Liu et al., 2016; Kochtubajda et al., 2016; Li et al., 2017), could result from upper lows that are located over the northwestern US, whereas flood-producing extreme rain events over the eastern Prairies (see for example Brimelow et al., 2014; Szeto et al., 2015) could result from upper lows that were centered only slightly to the east. Furthermore, some systems that track slowly across the region could bring extreme precipitation to both the eastern and western regions (e.g., Szeto et al., 2011). When combined with the increased winter precipitation and earlier snowmelt and freshet in a warmed climate, the expected increase in extreme spring precipitation could substantially increase the risk of extreme Prairie spring floods over both the western and eastern Prairies.

3.4 Summer

In contrast to the projected spring conditions, the most prominent circulation anomaly feature is the quasi-stationary upper high centered over the northwestern US and southern British Columbia (Fig. 7a). This blocks cyclones that bring warm-season precipitation to southwestern Canadian regions. The decreased summer cyclone activity is also reflected in the anomalous N–S surface ridge over the western continent (Fig. 7b). Schubert et al. (2016) had previously pointed out that SST values over the Pacific also affect precipitation deficits over the continent, and Li et al. (2018) indicated that expected higher SST values in the central Pacific by the end of the century will affect the Madden–Julian Oscillation in a manner conducive to reduced summer precipitation over the Canadian Prairies. It is not clear though whether the anomalous upper high is related to such pattern changes over the central Pacific.

Figure 7As in Fig. 2 but for summer (JJA).

The development of the anomalous upper high is evident in the time series of the JJA H index (Fig. 6b). Unlike the spring upper low that is projected to be located at the same general location, the intensification of the high is expected to accelerate after the mid-2040s when a significant (at 5 %) increasing trend commences. The mean index increases by 19.1 after the “break” which is larger than the standard deviation of the index during the century (17.2). It is also notable that all of the top 10 highest H index summers, i.e., summers with extreme warmth and dryness likely occurring in the Prairies, occur after 2050.

Under the influence of the upper high, downward solar radiation is projected to increase up to ∼5 W m⁻² over the southern Prairies by 2100. JJA near-surface air temperature is expected to increase by ∼2 ^∘C over the next 30 years (by 2050) and by an additional 4 ^∘C over the following 50 years (by 2100) along with even stronger warming over its southern vicinity. As a result, a “hot spot” with maximum summer warming that extends into southwestern Canada is projected to be induced under the upper high (Fig. 7c). The projected large-scale changes induce a significant decrease in precipitation over the Great Plains (Fig. 7d). In accordance with the temporal development of the upper high, the Prairie JJA precipitation is projected to remain rather constant until approximately 2070 and then decrease by 5 % towards the end of the century. Although this is not a significant decrease, summer is the only season with projected reduction in precipitation over the study region. In contrast, evapotranspiration (not shown) is projected to increase slowly until 2060 (∼6 % of historical values) and then decrease very slowly again towards the end of the century.

The reduction in summer precipitation, along with the enhanced evapotranspiration induced by the strong surface warming, could increase the potential for summer drought and wildfires over western Canada. Increases in surface sensible and latent heat fluxes that would accompany the projected strong warming could also enhance convective activity. On the other hand, the projected upper high is expected to suppress convection when fully developed. As a result, it is not clear how summer convection might change during the mid-to-late century; it may be enhanced or suppressed.

4.1 Cold season and near 0 ^∘C conditions

The location of the 0 ^∘C isotherm is a critical aspect of this region's climate. It is closely linked with the melting of snow at the surface which in turn affects albedo, land–atmospheric energy exchange, and precipitation runoff (Jennings et al., 2018). Precipitation near this temperature furthermore varies greatly in occurrence and type and can be linked with major hazards (e.g., freezing precipitation). Changes in these features are examined here.

4.1.1 0 ^∘C isotherm movement and pattern

In association with overall warming, the near 0 ^∘C region will move northward. To quantify this, monthly average locations of the 0 ^∘C isotherm were calculated from different model datasets. The NCEP/NCAR re-analysis was used to compute locations for 1976–2005. The future time periods were computed by adding the CMIP5 39-model ensemble median of the RCP8.5 10-year mean air temperature delta (future period – 1976–2005 historical) projected for three different periods (2046–2055, 2066–2075, 2086–2095) to the NCEP/NCAR climatology. Locations of the isotherm for each ensemble member were estimated by interpolating air temperatures onto a 1^∘ by 1^∘ latitude–longitude grid.

Results of these calculations are shown for 2 months (March and November) that illustrate some of the greatest movements (Fig. 8). In central regions of the country, the movement of the 0 ^∘C isotherm is of order 50–100 km per decade (especially in November), although it is much less in some areas of high terrain in the western Cordillera. The high terrain means that the near 0 ^∘C region would move vertically but little horizontally.

Figure 8Locations of the 0 ^∘C isotherm during (a) March and (b) November over four different time periods. The NCEP/NCAR re-analysis was used to compute locations for 1976–2005. The future time periods were computed by adding the CMIP5 39-model ensemble median of the RCP8.5 10-year mean air temperature delta (future period – 1976–2005 historical) projected for three different periods to the NCEP/NCAR climatology. The grey area shows the region bounded by the 0 ^∘C isotherms in the 25th and 75th ensemble percentiles of the 2086–2095 mean air temperatures.

Note that there is considerable variation between models in the actual locations of this isotherm. Some of the narrowest spreads occur in the interior of the country, far from oceans and mountains. In the western Cordillera, the spread is large in part due to different regions having high terrain which strongly influences the locations of this isotherm. Oceanic regions also exhibit large spreads, with the East Coast and Hudson Bay being impacted by variable sea-ice cover.

This large spread in projected western Cordillera patterns is accentuated when considering whether rain or snow will fall. For example, in the spring of 2015 in the Kananaskis area of the Alberta foothills, a mixture of rain and snow has been observed at temperatures as high as 9 ^∘C in some events, whereas it only occurred below 2–3 ^∘C in other events (Thériault et al., 2018).

Critical factors behind such varying observations include the near-surface vertical profiles of temperature and atmospheric moisture as well as the density of the falling solid precipitation. For example, low (high) values of particle density lead to rapid (slow) melting and a low (high) upper temperature threshold. This dependence of surface precipitation phase on particle density has been quantified using a one-dimensional column cloud model coupled to a microphysical scheme (Sankaré and Thériault, 2016). Snow pellets (high density of 400 g m⁻³) would fall approximately 200 m more before melting than would snowflakes (low density of 100 g m⁻³) under the same vertical temperature and moisture profile (Duplessis et al., 2016). This can lead to solid, instead of liquid, precipitation at the surface. Such factors make the determination of the rain–snow transition a challenging issue even in the present climate, let alone the future one.

4.1.2 Freezing rain

The movement of near 0 ^∘C conditions must be linked with changes in freezing rain occurrence. This type of precipitation, not even considering its accumulation, is difficult to simulate and project into the future. Atmospheric factors driving its formation over the CCRN region typically include extra-tropical cyclones with warm fronts, although chinook-associated patterns are also important (Kochtubajda et al., 2017a); surface factors include the degree of sea ice over Hudson Bay that, when present, acts to maintain the necessary cold near-surface temperatures. The generally low occurrence of freezing precipitation, in comparison with other regions, is partially attributable to particle sublimation or evaporation below cloud (Kochtubajda et al., 2017a).

To assess future changes in the occurrence of freezing rain, the fifth generation of the Canadian Regional Climate (CRCM5) model with a 0.44^∘ grid mesh was used (Fig. 9). CRCM5 was driven by the GCM from the Max Planck Institute for Meteorology Earth System Model (MPI-ESM-MR) for the 1981–2000 and 2081–2100 periods using the RCP8.5 scenario (Moss et al., 2010). Freezing rain was diagnosed using the technique developed by Bourgouin (2000); this approach is used operationally at ECCC. Results shown in Fig. 9a indicate that the model driven by MPI-ESM-MR reproduced the general pattern of the mean annual number of hours of freezing rain over the domain and basically shows the same features as a hindcast simulation driven by reanalysis data (not shown), but somewhat weaker. The main differences are a strong negative bias west of Hudson Bay as well as in an area stretching from northern Alberta to southern Manitoba. This latter area is likely due to the cold bias for that region as suggested by Šeparović et al. (2013); it is produced by the cold sea surface temperature bias in the North Pacific in the driving data.

Figure 9The mean annual number of hours of freezing rain (a) observed and simulated by the CRCM5 driven by MPI-ESM-MR for the 1981–2000 period, and (b) the change in the number of hours based on the difference between 2081–2100 and 1981–2000 assuming the RCP8.5 scenario. Observational locations are indicated by circles in (a) and stars in (b).

Projections suggest little change in the southern portion of the region but increases in excess of 20 h yr⁻¹ in the Northwest Territories. Such increases are comparable to current annual values. This pronounced increase in the north is due, at least in part, to the northward movement of near 0 ^∘C temperatures (Sect. 4.2.1).

4.2 Spring and early summer flooding

Flooding often occurs across this region in the spring and early summer. One such area is the eastern Prairies and it has experienced devastating events recently. Future aspects of these floods are examined here from an atmospheric perspective.

Large- to synoptic-scale atmospheric forcing is critical to the likelihood of spring and early summer flooding over the eastern Prairies. In particular, persistent atmospheric patterns often bring extended periods of precipitation extremes, either wet or dry, depending on location across the region relative to the circulation pattern (Brimelow et al., 2014, 2015; Szeto et al., 2015). Such persistent patterns were linked with spring or early summer rainfall that contributed to flooding on the Assiniboine River in 2011 and 2014; this precipitation also coincided with snowmelt.

A few studies have examined how such persistent patterns are expected to change across different regions of the Prairies. Szeto et al. (2015) found that persistent patterns conducive to eastern Prairie spring and early summer enhanced precipitation and flooding may become more pronounced. Using 500 hPa output from several RCM/GCM combinations, Bonsal et al. (2017) and Bonsal and Cuell (2017) identified future (2041–2070) changes to the frequency of key summer (JJA) circulation patterns associated with extreme dry and wet conditions over the southwestern Canadian Prairies and the Athabasca River basin (ARB), respectively. Most of the models simulated general features of observed circulation patterns, and these also occur in the future but with some changes to their average frequency. However, there was considerable inter-model variability.

Flooding events in Canada are often associated with numerous factors (including extreme precipitation) that occur in combination. For example, one critical aspect of the 2014 Assiniboine flood was a cool spring followed by rapid snowmelt combined with above-normal precipitation (Szeto et al., 2015). MAM temperatures were approximately 2.5 ^∘C below the 1995–2014 normal and AMJ precipitation was approximately equal to the 90th percentile of AMJ precipitation that occurred during 1950–2005.

CMIP5 information was examined to determine whether the likelihood of this combination would change in the future. Comparisons of historical and projected future surface temperatures over the eastern Prairies (not shown) suggest that spring melt would commence in February and be completed in March towards the end of the century. As such, the frequency of wet MAM and cool FM during 2081–2100 is compared to historical (1986–2005) wet AMJ combined with cool MA, using the anomalous conditions for the 2014 flood as criteria for each model. The focus area is the eastern Prairies (47.5–55^∘ N latitude by 255–262.5^∘ E longitude).

Results indicate that the ensemble mean frequency of wet MAM (i.e., MAM with a P > 90th percentile of MAM precipitation during 1950–2005) is projected to be 4.6 seasons per decade, which is substantially higher than the corresponding mean frequency of 1.2 seasons per decade for wet AMJ during 1986–2005. In addition, the mean frequency of combined wet MAM and cool FM (i.e., FM temperature 2.5 ^∘C cooler than the 2081–2100 mean) was projected to be 0.8 seasons per decade, which is 8 times larger than the frequency of 0.1 seasons per decade estimated for the co-occurrence of wet AMJ and cool MA during 1986–2005. The results suggest that the projected large-scale atmospheric conditions that are favorable for the development of wet MAM could substantially increase the risk for eastern Prairie floods that are associated with cool and wet springs.

4.3 Summer severe conditions

Summer across the region can be associated with severe conditions ranging from widespread drought to severe thunderstorms. Their combination can furthermore be key factors linked with forest fires. Expected changes in these phenomena are examined here.

4.3.1 Drought

The projected large-scale summer changes (Fig. 7) are expected to impact future drought conditions. Given that past droughts over western Canada have been associated with a persistent mid-tropospheric (500 hPa) large-amplitude ridge centered over the area (e.g., Bonsal et al., 1999), it is anticipated that the quasi-stationary anomalous upper high centered over the northwestern US and southern British Columbia (Fig. 7a) will result in more drought-like conditions in this region. Some evidence for these changes was found by Bonsal et al. (2017) and Bonsal and Cuell (2017), who examined future (2041–2070) changes to summer (JJA) Standardized Precipitation Evapotranspiration Index (SPEI, Vicente-Serrano et al., 2010) values over two southern Canadian Prairie watersheds and the ARB, respectively. For the southern basins, results indicated an uncertain future ranging from a substantial increase in drought, with a higher degree of inter-annual variability, to relatively no change from current conditions. Farther north in the ARB, projections revealed an average change toward more drought-like summer conditions, but there was a substantial range among the climate models. Over a larger study area that included all western Canadian river basins, Dibike et al. (2018) incorporated six CMIP5 GCMs to assess future SPEI changes on annual and summer scales for the periods 2041–2070 and 2071–2100 (relative to 1971–2000) using the RCP4.5 and RCP8.5 emission scenarios. They found that southern watersheds showed a gradual increase in annual water deficit throughout the 21st century, whereas the opposite was true for northern basins. In contrast, for summer, all river basins with the exception of the extreme northern ones were expected to experience decreasing water availability.

A comprehensive Canada-wide drought study assessed changes in the SPEI using outputs from 29 CMIP5 models (Tam et al., 2018). In agreement with Fig. 7, results showed strong relative summer drying during the 21st century over much of western Canada, including interior southern British Columbia, as well as western–central portions of the country from the Prairies to the Arctic. In addition, the frequency of extended relatively dry periods (e.g., consecutive years that are characterized by strongly negative summer SPEI) is projected to increase markedly during the second half of this century. Compared to other locations in Canada, the southern Prairies exhibit the largest likelihood of extended severe drought during the latter part of this century under the RCP8.5 scenario (Fig. 10a). The intensification, following approximately 2050, is consistent with the accelerated intensification of the upper ridge during the second half of the century (Figs. 6b and 7a).

Figure 10CMIP5 29-model ensemble medians of projected (a) summer (JJA) and (b) annual SPEI for RCP8.5 from 1900 to 2100 over the southern Prairies (defined in Sect. 2). Negative (red) values indicate surface water deficit relative to 1950–2005 conditions. Grey lines denote the 25th and 75th percentiles.

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On annual scales, a dry-south–wet-north pattern characterizes projected drought changes over the CCRN domain (see Fig. 10b for drying over the south) (Tam et al., 2018). This pattern is largely accounted for by the combined results of projected dry conditions during the summer and autumn over southern regions, and the projected wet conditions during winter and spring over the northern and coastal regions of Canada. The projected surface water deficit during summer and autumn would thus play a dominant role in affecting the future annual water budget over the southern domain.

An important issue concerning drought is its future intra-seasonal character. Many droughts tend to be hot with almost no precipitation. In contrast, others are not associated with especially elevated temperatures and can even have cool periods (Stewart et al., 2012), can have rain showers (Evans et al., 2011), and/or can experience occasional large precipitation events (Szeto at al., 2011) that may or may not increase in a warmer world (as discussed in Sect. 4.2.2). Such differences in character can substantially affect their impact, but the CMIP5 information does not have sufficient resolution to resolve this issue.

4.3.2 Convection and hail

Analysis of future convection-related precipitation requires higher spatial resolution than available from global climate models. Suitable datasets are available with dynamically downscaled RCMs, such as NARCCAP (Mearns et al., 2012), previously mentioned in Sect. 2. However, future scenarios are only available to mid-century when, as discussed in Sect. 3.4, dry conditions are not expected to be so dominant over the southern Prairies (Fig. 10).

NARCCAP historic (1971 to 2000) and mid-century future (2041 to 2070) model output was used to assess future changes in convective precipitation and hail over the Canadian Prairies, southern Northwest Territories, and US northern plains. Convective precipitation is defined as occurring when the model convective scheme is triggered to release latent energy and convective instability through simulated vertical motion. Brimelow et al. (2017) suggested that the three most consistent NARCCAP model pairings to assess convective precipitation and hail for the regions of interest herein included MM5-HadCM3, MM5-CCSM, and HRM3-HadCM3, based on their ability to reproduce the precipitation climatology. No other NARCCAP studies focused on hail or warm season convection-only precipitation, although Mearns et al. (2013) and Mailhot et al. (2011) looked at ensemble summer total precipitation and annual maximum precipitation, respectively, while other studies focused on other seasons (e.g., Gutowski et al., 2010; Kawazoe and Gutowski, 2013).

Changes in future summer (JJA) convective precipitation are shown in Fig. 11 for three NARCCAP model pairs under the SRES A2 scenario. All three model pairs show increases over much of the Prairies but with varying amounts (near zero to 50 mm). MM5-CCSM and HRM3-HadCM3 are consistent with CMIP5 RCP8.5 results for the same future period (not shown) and the spatial patterns in other studies (that is, increases in Canada but decreases in the central/southern US Plains) (e.g., Mearns et al., 2013; Mailhot et al., 2011). These results are also consistent with general increases in CAPE and surface dew points in a warming climate over much of the Prairies (e.g., Brimelow et al., 2017).

Figure 11Changes in future warm season (JJA) convective precipitation (mm) for three NARCCAP model pairs of (a) MM5-HadCM3, (b) MM5-CCSM, and (c) HRM3-HadCM3. Positive values imply greater future precipitation (i.e., 2041–2070 minus 1971–2000).

The future occurrence of hail is also important. A recent study by Brimelow et al. (2017) highlighted future changes (2041–2070 minus 1971–2000) in hail character over North America based on simulations from a one-dimensional cloud–hail model (HAILCAST; Brimelow et al., 2002) forced with the same three NARCCAP model pairs discussed above. Over the CCRN domain, results show that the number of hail days generally declines in Manitoba and Saskatchewan in summer (JJA), while increases occur in the western half of Alberta and extreme southwestern Northwest Territories (see Brimelow et al., 2017, Fig. 1). Over much of Alberta and the southwestern Northwest Territories, there are general increases in accumulated kinetic energy (AKE) and maximum hail size, even in some regions where the number of hail days does not change in the future. That is, when it does hail, it will potentially be larger and more destructive (larger AKE). Parts of Saskatchewan and central to northern Manitoba may also see increases in AKE and hail size even though the number of hail days decreases. This is thought to be primarily due to more moisture and energy available to summer storms when they do occur (Brimelow et al., 2017).