Barrie Bonsal

Canadian Precipitation Patterns Associated with the Southern Oscillation

Abstract Precipitation responses over Canada associated with the two extreme phases of the Southern Oscillation (SO), namely El Nino and La Nina, are identified. Using the best available precipitation data from 1911 to 1994, both the spatial and temporal behavior of the responses are analyzed from the El Nino/La Nina onset to several seasons afterward. Composite and correlation analyses indicate that precipitation over a large region of southern Canada extending from British Columbia, through the prairies, and into the Great Lakes region is significantly influenced by the SO phenomenon. The results show a distinct pattern of negative (positive) precipitation anomalies in this region during the first winter following the onset of El Nino (La Nina) events. During this same period, significant positive precipitation anomalies occur over the northern prairies and southeastern Northwest Territories in association with El Nino events. Statistical significance of the responses is tested by the Student’s t-test a...

An Assessment of changes in winter cold and warm spells over Canada

The recent Third Assessment Report (TAR) of the Intergovernmental Panel onClimate Change (IPCC) indicated that observed 20th century changes in severalclimatic extremes are qualitatively consistent with those expected due to increasedgreenhouse gases. However, a lack of adequate data and analyses make conclusiveevidence of changing extremes somewhat difficult, particularly, in a global sense.In Canada, extreme temperature events, especially those during winter, can havemany adverse environmental and economic impacts. In light of the aforementionedIPCC report, the main focus of this analysis is to examine observed trends andvariability in the frequency, duration, and intensity of winter (Jan–Feb–Mar) cold and warm spells over Canada during the second half of the 20th century.Cold spell trends display substantial spatial variability across the country. From1950–1998, western Canada has experienced decreases in the frequency, duration, and intensity of cold spells, while in the east, distinct increases in the frequency and duration have occurred. These increases are likely associated with morefrequent occurrences of the positive phase of the North Atlantic Oscillation (NAO)during the last several decades. With regard to winter warm spells, significantincreases in both the frequency and duration of these episodes were observedacross most of Canada. One exception was found in the extreme northeasternregions, where warm spells are becoming shorter and less frequent. The resultsof this study are discussed within the context of climate warming expectations.

Effects of Changes in Arctic Lake and River Ice

Climatic changes to freshwater ice in the Arctic are projected to produce a variety of effects on hydrologic, ecological, and socio-economic systems. Key hydrologic impacts include changes to low flows, lake evaporation regimes and water levels, and river-ice break-up severity and timing. The latter are of particular concern because of their effect on river geomorphology, vegetation, sediment and nutrient fluxes, and sustainment of riparian aquatic habitats. Changes in ice phenology will affect a wide range of related biological aspects of seasonality. Some changes are likely to be gradual, but others could be more abrupt as systems cross critical ecological thresholds. Transportation and hydroelectric production are two of the socio-economic sectors most vulnerable to change in freshwater-ice regimes. Ice roads will require expensive on-land replacements while hydroelectric operations will both benefit and be challenged. The ability to undertake some traditional harvesting methods will also be affected.

Trends and Variability in Spring and Autumn 0 °C-Isotherm Dates over Canada

In cold-regions climates, numerous environmental processes and socio-economic activities are significantly impacted by the timing of the seasonal advance and retreat of the 0 °C isotherm. This investigation examines 20th century trends and variability in spring and autumn 0 °C-isotherm dates over Canada. Results reveal considerable variability across the country. Significant trends toward earlier springs are observed over most of western Canada including a dramatic shift to earlier dates during the last 20–30 years. Central regions areassociated with smaller, generally insignificant earlier spring trends. Conversely, extreme eastern areas experience later springs. During autumn, isotherm dates show little change over the majority of the country. The observed spatial and temporal characteristics in the 0 °C-isotherm trends are reflected in past variations in several hydro-cryospheric variables over many regions of Canada including the timing of snowmelt and the dates of freshwater ice break-up. Investigation into the potential causes of observed variability in 0 °C-isotherm dates reveals that large-scale oscillations representing atmospheric/oceanic variations in the north Pacific and north Atlantic significantly relate to isotherm dates over western and eastern Canada, respectively, during both spring and autumn. Although significant, a relatively small amount of overall variance in isotherm dates is explained by the oscillations indicating the influence of other factors. Results from this study improve our knowledge of past trends and variability in 0°C-isotherm dates and resultant impacts on hydro-cryospheric processesover Canada. They also provide insight into potential future climatologic impacts given the possibility of climate change.

Past and Future Changes in Arctic Lake and River Ice

Paleolimnological evidence from some Arctic lakes suggests that longer ice-free seasons have been experienced since the beginning of the nineteenth century. It has been inferred from some additional records that many Arctic lakes may have crossed an important ecological threshold as a result of recent warming. In the instrumental record, long-term trends exhibit increasingly later freeze-ups and earlier break-ups, closely corresponding to increasing air temperature trends, but with greater sensitivity at the more temperate latitudes. Broad spatial patterns in these trends are also related to major atmospheric circulation patterns. Future projections of lake ice indicate increasingly later freeze-ups and earlier break-ups, decreasing ice thickness, and changes in cover composition, particularly white-ice. For rivers, projected future decreases in south to north air-temperature gradients suggest that the severity of ice-jam flooding may be reduced but this could be mitigated by changes in the magnitude of spring snowmelt.

An assessment of Canadian prairie drought: past, present, and future

Within Canada, the Canadian Prairies are particularly drought-prone mainly due to their location in the lee of the western cordillera and distance from large moisture sources. Although previous studies examined the occurrence of Canadian Prairie droughts during instrumental, pre-instrumental and to a lesser extent, future periods, none have specifically focused on all time three scales. Using two different drought indicators, namely the Palmer Drought Severity Index (PDSI) and Standardized Precipitation Index (SPI), this investigation assesses the variability of summer drought duration and intensity over a core region of the Prairies during (a) the pre-instrumental record extending back several centuries (inferred from tree rings), (b) the instrumental record (1901–2005), and (c) the twenty-first century using statistically downscaled climate variables from several Atmosphere–Ocean Global climate models with multiple emission scenarios. Results reveal that observed twentieth century droughts were relatively mild when compared to pre-settlement on the Prairies, but these periods are likely to return (and even worsen) in the future due to the anticipated warming during the course of the twenty-first century. However, future drought projections are distinctly different between the two indices. All PDSI-related model runs show greater drought frequency and severity mainly due to increasing temperatures. Conversely, the precipitation-based SPI indicates no significant changes to future summer drought frequency although there tends to be a higher persistence of multi-year droughts in central and southern portions of Canadian Prairies. These findings therefore stress the importance of considering anticipated warming trends when assessing future regional-scale drought, especially given the uncertainties and lack of consistency in future precipitation signals among climate models. This study can be considered an initial step toward quantifying and understanding Canadian Prairie drought occurrence and severity over several centuries as determined from paleo, instrumental, and climate model data sources.

Impacts of Large-Scale Circulation Variability on Low Streamflows over Canada: A Review

Low streamflows constitute an important component of hydro-climatic extremes. This is particularly true for Canada where reduced flows can affect several economic and environmental activities ranging from less hydro-electric production and increased freshwater transportation costs, to reduced water quality and ecological habitat destruction. This paper reviews past research regarding the impacts of large-scale circulation patterns on streamflow variability (including low flows) over Canada. Results from the various studies reveal that streamflow responses are generally consistent with those observed for large-scale climate. For western Canada, this includes a higher frequency of low-flow events in association with the warmer/drier conditions during El Niño events and positive phases of the Pacific Decadal Oscillation (PDO) and the Pacific North American (PNA) pattern. Relationships in northern/northeastern regions of the country are less robust but in general, reduced streamflows occur during positive phases of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO). However, it is clearly evident that the spatial and temporal aspects of these relationships are greatly influenced by hydro-climatic complexities associated with individual watersheds, especially in the cordilleran areas of Canada. Future research requires more in-depth analyses into the spatial and temporal aspects of relationships between circulation variability and low-flow occurrences over critical watersheds within Canada including the combination of climatic patterns and associations with regional, synoptic-scale circulation. The incorporation of fully coupled climate and hydrologic models to assess the impacts of projected climate change on future low-flow occurrences is also needed. This research would result in a better understanding and enhanced prediction of low streamflow events that is critical for the current and future efficient management of water resources throughout the country.

Climatic Conditions in Northern Canada: Past and Future

Abstract This article reviews the historical, instrumental, and future changes in climate for the northern latitudes of Canada. Discussion of historical climate over the last 10 000 years focuses on major climatic shifts including the Medieval Warm Period and the Little Ice Age, and how these changes compare with those most recently experienced during the period of instrumental records. In reference to the latter, details are noted about observed trends in temperature and precipitation that have been recorded over the last half century, which exhibit strong west to east and north to south spatial contrasts. A comprehensive review of future changes is also provided based on outputs from seven atmosphere–ocean global climate models and six emission scenarios. Discussion focuses on annual, seasonal, and related spatial changes for three 30-year periods centered on the 2020s, 2050s, and 2080s. In summary, substantial changes to temperature and precipitation are projected for the Canadian North during the twenty-first century. Although there is considerable variability within the various projections, all scenarios show higher temperature and, for the most part, increasing precipitation over the entire region.

Arctic Freshwater Ice and Its Climatic Role

Freshwater ice dominates the Arctic terrestrial environment and significantly impacts bio-physical and socio-economic systems. Unlike other major cryospheric components that either blanket large expanses (e.g., snow, permafrost, sea ice) or are concentrated in specific locations, lake and river ice are interwoven into the terrestrial landscape through major flow and storage networks. For instance, the headwaters of large ice-covered rivers extend well beyond the Arctic while many northern lakes owe their genesis to broader cryospheric changes. The effects of freshwater ice on climate mostly occur at the local/regional scale, with the degree of influence dependent on the magnitude, timing, location, and duration of ice cover, and the size of the water body. Freshwater-ice formation, growth, decay, and break-up are influenced by climatic variables that control surface heat fluxes, but these differ markedly between lakes and rivers. Despite the importance of freshwater ice, there has been a recent reduction in observational recordings.

Associations between low frequency variability modes and winter temperature extremes in Canada

Abstract Relationships between patterns of low frequency climate variability including the El Niño‐Southern Oscillation (ENSO), the Arctic Oscillation (AO) and the Quasi‐Biennial Oscillation (QBO), and the occurrence of Canadian winter temperature extremes are examined for the 1950–98 period. Composite analyses reveal that ENSO plays a dominant role in the frequency and duration of both cold and warm spells. In particular, the warm phase of ENSO is associated with a significant increase (decrease) in the occurrence of warm (cold) spells and the number of extreme warm (cold) days across most of Canada. The findings are generally the opposite for the cold ENSO phase. The singular value decomposition (SVD) technique further shows that ENSO‐like interdecadal sea surface temperature variability plays a significant role in the variability of winter cold and warm spells over Canada. Differences between the high and low index phases of the AO show a significantly higher frequency and duration of cold spells over eastern Canada during positive AO winters, while the frequency of winter warm spells increases over the Canadian Prairies. These results are also confirmed by the SVD analysis using northern hemisphere 1000‐hPa circulation. The QBO, as determined by the phase of the stratospheric winds, has an effect on the spatial pattern of Canadian extreme temperatures that is similar to that of the AO. The most significant result involves increases in the frequency of warm spells and the number of extreme warm days over the southern Prairies during the westerly phase of the QBO. The diagnostic results from this study improve the understanding of the imapact of the low frequency variability modes on temperature extremes over Canada. They may also assist in the attribution of past trends and variability in extremes, and potentially, provide insight into future changes to extreme temperature events.