Recent trends in Canadian lake ice covercirrus.unbc.ca/454/week12/Duguay2006.pdf · Recent trends in Canadian lake ice cover ... Lenormand et al., 2002). The CID is a national historical

HYDROLOGICAL PROCESSESHydrol. Process. 20, 781–801 (2006)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.6131

Recent trends in Canadian lake ice cover

Claude R. Duguay,1* Terry D. Prowse,2 Barrie R. Bonsal,3 Ross D. Brown,4

Martin P. Lacroix2 and Patrick Menard5

1 Geophysical Institute, University of Alaska Fairbanks, PO Box 757320, Fairbanks, AK 99775-7320, USA2 National Water Research Institute, Water & Climate Impacts Research Centre, University of Victoria, PO BOX 1700 STN CSC, Victoria,

British Columbia V8W 2Y2, Canada3 National Water Research Institute, Saskatoon, Saskatchewan S7N 3H5, Canada

4 Service meteorologique du Canada—Region du Quebec, Place Bonaventure, 800, rue de la Gauchetiere Ouest, bureau 7810, Montreal,Quebec H5A 1L9, Canada

5 145 Dew Street, King City, Ontario L7B 1L1, Canada

Abstract:

Recent studies have shown that ice duration in lakes and rivers over the Northern Hemisphere has decreased overthe 19th and 20th centuries in response to global warming. However, lake ice trends have not been well documentedin Canada. Because of its size, considerable variability may exist in both freeze-up and break-up dates across thecountry. In this paper, results of the analysis of recent trends (1951–2000) in freeze-up and break-up dates acrossCanada are presented. Trends toward earlier break-up dates are observed for most lakes during the time periods ofanalysis which encompass the 1990s. Freeze-up dates, on the other hand, show few significant trends and a lowdegree of temporal coherence when compared with break-up dates. These results are compared with trends in autumnand spring 0 °C isotherm dates over the time period 1966–95. Similar spatial and temporal patterns are observed,with generally significant trends toward earlier springs/break-up dates over most of western Canada and little changein isotherm and freeze-up dates over the majority of the country in autumn. Strong correlations (r > 0Ð5) between0 °C isotherm dates and freeze-up/break-up dates at many locations across the country reveal the high synchrony ofthese variables. These results are also consistent with more recent observations of other cryospheric and atmosphericvariables that indicate, in particular, a general trend toward earlier springs in the latter part of the 20th century. Theresults of this study provide further evidence of the robustness of lake ice as a proxy indicator of climate variabilityand change. Copyright 2006 John Wiley & Sons, Ltd.

KEY WORDS lake ice; freeze-up and break-up dates; 0 °C isotherm dates; climate change; Canada

INTRODUCTION

Lake ice is an important component of the Canadian terrestrial cryosphere. Surface–atmosphere interactionsare altered by the presence of ice on a lake and the formation of lake ice has important ecological andeconomic implications (Brown and O’Neill, 2002). The dates of ice freeze-up and break-up have beenshown to be good indicators of climate variability and global climate change (e.g. Palecki and Barry,1986; Robertson et al., 1992; Barry and Maslanik, 1993; Assel and Robertson, 1995; Livingstone, 1997;Magnuson et al., 2000; Assel et al., 2003). For example, using long historical ice records from a limitedset of lakes (and rivers), Magnuson et al. (2000) have shown that freeze-up and break-up dates provideconsistent evidence of later freezing and earlier break-up around the Northern Hemisphere from 1846 to1995. Over that period, changes in freeze-up dates averaged 5Ð8 days per 100 years later, and changes inbreak-up dates averaged 6Ð5 days per 100 years earlier; these translate to increasing air temperatures of about1Ð2 °C per 100 years. These trends are in general agreement with various studies suggesting that each 1 °C

* Correspondence to: Claude R. Duguay, Geophysical Institute, University of Alaska, Fairbanks, Fairbanks, AK 99775-7320, USA.E-mail: [email protected]

Received 11 June 2005Copyright 2006 John Wiley & Sons, Ltd. Accepted 23 September 2005

782 C. R. DUGUAY ET AL.

change in autumn or spring air temperature causes a 4 to 6 day change in the mean freeze-up or break-update for lakes in the middle latitudes of the Northern Hemisphere, all other things being equal (Walsh, 1995).However, recently, the linear temperature response of lake ice break-up has been questioned: Weyhenmeyeret al. (2004) showed that the relationship between the timing of break-up and air temperature from fourdecades of lake ice break-up observations from 196 Swedish lakes was best described by an arc cosinefunction.

Over the last 10 years or so, many studies have used long time-series of historical observations of lakeice break-up dates for clarifying the relationship between break-up and air temperature, and establishing thesignificance of break-up as an indicator of climate variability and change (Anderson et al., 1996; Livingstone,1999; Magnuson et al., 2000; Hodgkins et al., 2002). Some of the most recent studies describe trends andvariability in ice cover in relation to changes in air temperature and large-scale (atmospheric circulation)oscillations (e.g. North Atlantic oscillation, Arctic oscillation, and the North Pacific index) in Europe,Scandinavia and the USA (Benson et al., 2000; Yoo and D’Odorico, 2002; Todd and Mackay, 2003; Blenckneret al., 2004; George et al., 2004).

In Canada, few investigations have examined patterns and trends in lake ice cover time series, andmost of the results have appeared in the grey literature (Da Silva, 1984, 1985; Anderson, 1987; Skinner,1992). Schindler et al. (1990) examined climatic and hydrologic records from the 1960s to the 1980s forthe Experimental Lakes Area (ELA) of northwestern Ontario (boreal region) and found that the ice-freeseason duration had increased by about 20 days over the 20-year period due mainly to earlier break-up dates in spring. Autumn freeze-up dates were not observed to change significantly. The earlier springbreak-up of the lakes in the ELA was attributed to the increased April–May air temperatures and reducedsnow cover and warmer temperatures in March causing earlier snowmelt and increased solar radiationabsorption by the lake in early spring. Skinner (1992) used freeze-up and break-up records from 30 lakesacross various regions of Canada (most regions with data over the period 1956–57 to 1988–89, otherregions with shorter records 1971–72 to 1988–89) and showed that trends toward earlier and warmerspring seasons in western and central Canada were reflected in break-up dates for most lakes. As in theSchindler et al. (1990) study, no specific trends were seen in either air temperatures or regional freeze-up dates during the autumn season in any part of the country. The trends toward shorter ice seasons inmost parts of Canada reflected the earlier spring break-up in response to warmer temperatures during thespring season. More recently, Futter (2003) presented results from the analysis of 46 break-up and 15 ice-free season phenology time series from lakes of southern Ontario (1853–2001) obtained largely throughvolunteer monitoring efforts. He observed monotonic trends toward earlier break-up dates and longer ice-free seasons across this region of Canada over the full length of the time series and in the last 30 yearsof data. The trends are believed to be indicative of the warming in spring temperature across the studyarea.

The primary objective of this study was to analyse spatial and temporal trends in lake ice freeze-up andbreak-up dates across Canada for the period 1951–2000. This study extends the scope of previous lake iceinvestigations by providing a Canada-wide picture of the response of lake ice covers to climate in Canadaat different time intervals during the last five decades. More specifically, this paper addresses the followingquestions:

1. What has been the magnitude of changes in freeze-up and break-up dates in different regions of Canadafor the 30-year climatological periods (1951–80, 1961–90, and 1971–2000)?

2. How do the lake ice trends compare with trends in other cryospheric and atmospheric variables observedby other workers (e.g. Serreze et al., 2000) over a comparable time period (1966–95)?

3. How do the trends and variability in freeze-up and break-up dates compare with those of the autumn andspring 0 °C isotherm dates?

Copyright 2006 John Wiley & Sons, Ltd. Hydrol. Process. 20, 781–801 (2006)

TRENDS IN CANADIAN LAKE ICE COVER 783

DATA AND METHODOLOGY

Data sources

Lake ice observations. The lake ice observations were extracted from the Canadian Ice Database (CID;Lenormand et al., 2002). The CID is a national historical ice database that contains records relating to the stateof Canadian ice covers (lake ice, river ice, landfast sea ice) since 1822. The CID contains in situ observationsfrom 757 sites across Canada that were originally kept on digital or paper records at the MeteorologicalService of Canada Headquarters (MSC-HQ) and the Canadian Ice Service (CIS). As shown in Figure 1, thenetwork of lake ice observation sites reached a peak between the early 1950s and the mid 1980s, but has sincethen experienced a dramatic decline. A complete description of the ice database and its historical evolutionare given in Lenormand et al. (2002).

Data provided by the CIS relate to (near-)weekly ice thickness and on-ice snow depth measurements madeduring an ice season, and the records from the MSC-HQ contain detailed information pertaining to severalparameters describing the freeze-up and break-up processes. These include, to name a few, the first ice date,the date of complete freeze over (CFO), the maximum ice thickness, the date of beginning of ice melt, andthe date when the water body becomes clear of ice (WCI). The terms freeze-up and break-up dates as usedthroughout this paper correspond to CFO dates and WCI dates respectively. Freeze-up and break-up dates, theterms most often used in the literature, are also referred to at times as ice-in and ice-out dates (e.g. Hodgkinset al., 2002). In this paper, the term ‘lake site’ is favoured over ‘lake’, since shore-based ice observationsmay not represent ice conditions over an entire lake.

Since the main objective of this study is to analyse spatial patterns in freeze-up and break-up dates acrossCanada, a decision was taken to examine trends over 30-year time periods for which there were relativelygood spatial distributions of lakes across the country with at least 20 years of observations during thoseperiods. The use of a longer 50-year period would have been preferred, but there were only 13 lake sites inthe CIS with data covering a 50-year period. Of these 13 sites, a number had periods of missing data and theperiods of observations did not overlap sufficiently to compare time series over the same time period.

Three 30-year periods (1951–80, 1961–90, and 1971–2000) corresponding to the conventional periods usedin the calculation of climate normals and anomalies in Canada were selected for trend analysis. An additional

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Figure 1. Evolution of lake ice observation sites from the CID



period, 1966–95, was included so that trends could also be compared with trends in other cryospheric andatmospheric variables from recent studies, as well as trends in autumn and spring 0 °C isotherm dates calculatedover the same time period. Lake sites with at least 20 years of records over the four periods of analysis areshown in Figure 2. Noteworthy is the larger number of sites for the periods 1961–90 and 1966–95 comparedwith the previous (1951–80) and later (1971–2000) periods. The establishment of the DEW line (short forDistant Early Warning Line) in 1957 contributed to the growth of the lake ice network in the north. TheDEW line was an integrated chain of 63 radar and communications systems stretching 3000 miles from thenorthwest coast of Alaska to the eastern shore of Baffin Island, Canada, roughly along the 69th parallel (TheDEW Line Sites in Canada, Alaska & Greenland, 2005). The operations ceased for many locations in the late1980s to early 1990s, which is reflected in the smaller number of lake sites in northern Canada with at least20 years of observations during the period 1971–2000.

Air temperature data. Air temperature data employed in this study consisted of daily mean values for187 high-quality, relatively evenly distributed MSC stations across Canada for the period 1966–95. The 0 °Cautumn and spring isotherm dates were determined at each meteorological station using the approach describedin Bonsal and Prowse (2003). Briefly, the autumn (spring) 0 °C isotherm dates are defined as the date whenmean daily temperature falls below (rises above) 0 °C. Because of the large degree of variability inherent indaily temperatures, mean daily temperature values are filtered using a 31-day running mean and autumn andspring isotherms are defined as the date when the running mean daily temperature crosses 0 °C. The MSCstation network does not adequately represent mountainous regions of Canada, but this is not consideredproblematic for this study as few of the lakes were located in mountain regions.

The strength and the significance of relations between freeze-up/break-up dates and autumn/spring isothermdates were determined using the Pearson product-moment correlation r. Freeze-up and break-up dates at lakesites were correlated with 0 °C isotherm dates calculated from their closest meteorological station. A high(low) correlation means that there is a high (low) degree of synchrony between the 0 °C isotherm dates andice freeze-up/break-up dates. In total, 66 correlation coefficients were derived relating freeze-up dates and0 °C autumn isotherm dates and 74 relating break-up dates to 0 °C spring isotherm dates. Maps displayingisotherm trends across Canada were also produced through spatial interpolation (ordinary kriging methodavailable in ArcGIS 9Ð0 software package) using spring and autumn 0 °C isotherm dates derived from the 187meteorological stations.

Trend analysis

The presence of first-order trends in freeze-up/break-up and 0 °C isotherm time series was tested usingthe non-parametric Mann–Kendall test and the magnitude (slope) of the trends was estimated with Sen’smethod (Sen, 1968). The Mann–Kendall test is widely used in environmental science because it can copewith missing values and values below a detection limit. Recently, the test has been utilized to detect lineartrends in long-term time series of river ice and lake ice observations (e.g. Smith, 2000; Hodgkins et al., 2002;Futter, 2003).

With the Mann–Kendall test, the presence of a statistically significant trend is evaluated using thestandardized Z-statistic, which has a normal distribution. A positive (negative) value of Z indicates an upward(downward) trend. To test for either a downward or an upward monotone trend (a two-tailed test) at ˛ levelof significance, the null hypothesis (H0: no trend) is rejected if the absolute value of Z is greater thanZ1�˛/2, where Z1�˛/2 is obtained from the standard normal cumulative distribution table. The estimation ofthe slope of the trends was determined using Sen’s non-parametric method (Sen, 1968). The Mann–Kendalltest and Sen’s slope estimates were both performed using the Excel template implementation (MAKESENS:Mann–Kendall test for trend and Sen’s slope estimates) developed by Salmi et al. (2002).



(a) 1951-1980

(b) 1961-1990

Climatic RegionPacific

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Figure 2. Distribution of lake ice observation sites for periods (a) 1951–80, (b) 1961–90, (c) 1966–95, and (d) 1971–2000. The thick linesrepresent the limits of the climatic regions of Canada (Hare and Thomas, 1974)



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Figure 2. (Continued )



RESULTS AND DISCUSSION

Trends for periods 1951–80, 1961–90, and 1971–2000

Freeze-up dates. The spatial distribution of freeze-up trends across Canada for periods 1951–80, 1961–90,and 1971–2000 is shown in Figure 3. With a few exceptions, clear spatial patterns are difficult to discernowing to the large regional variability. One exception can be found in the map of 1961–90, a period duringwhich there appears to be group responses of lakes (spatial coherence) toward later freeze-up dates alongthe DEW line (69th parallel) and in the Quebec–southeastern Ontario region, and earlier freeze-up dates inthe Cape-Breton–Newfoundland region. During the period 1951–80, there is a greater concentration of sitesshowing later freeze-up dates in western Canada (including sections of British Columbia and the NorthwestTerritories) and earlier dates in the central and eastern parts of the country (Great Lakes–St Lawrence andAtlantic regions). For the period 1971–2000, lakes in the Great Lakes–St Lawrence region show trends towardlater freeze-up dates (three out of four that are significant at the 10% level), and earlier dates for a group oflakes in the Boreal region of northern Ontario and Manitoba, and those of the Atlantic region. It is worthnoting that trends toward earlier freeze-up dates (colder climate) persist for the latter region throughout thethree 30-year periods. Overall, the spatial coherence in freeze-up trends appears to be more local or regionalin nature than for break-up trends, which will be discussed in the next section. Also, of all the trends, feware statistically significant at the 10% level over the three periods.

The large spatial variability in freeze-up trends contrasts with Canadian river freeze-up results reported byZhang et al. (2001) that showed widespread trends toward earlier freeze-up over the period 1967–96. Thisdifference may be related to the different spatial distribution of observing stations in the two studies. Trends insnow cover onset date over the Northern Hemisphere for the period 1972–98 from the NOAA dataset (Hallet al., 2004) confirm the results presented here, i.e. of strong regional contrasts in fall-season cryosphericresponse (e.g. earlier snow cover onset over the Maritimes, southern Manitoba and the Mackenzie Valley, andlater snow cover onset dates over British Columbia, northern Quebec and the high Arctic).

Lake sites that have experienced some of the strongest freeze-up trends are shown in Figure 4. Freeze-up anomalies are computed as departures from the 30-year mean. Some lakes have experienced verysignificant trends, especially toward later freeze-up dates for their respective periods. For example, LakeUtopia (1971–2000), which has a complete 30-year record, shows the most significant trend (significant atthe 0Ð1% level), with its freeze-up date occurring 37 days later (1Ð23 days per year). Two of the lakes inthe Atlantic region, Grand Lake and Deadman’s Pond, both show significant trends toward earlier freeze-up dates (about 2 weeks earlier over the 30-year periods). In addition to the trends, some lake sites showlarger interannual variability than others, as well as some indications of regime shifts. Without discarding therole of local factors on the interannual variability of freeze-up dates, some of the fluctuations and shifts arelikely associated with the influence of large-scale atmospheric/oceanic circulation variability (oscillations).This subject merits further investigation, but is beyond the scope of this paper.

Break-up dates. In contrast to freeze-up dates, break-up dates show a greater spatial coherence (Figure 5).There is a dipole-like distribution of break-up dates across the country for the period 1951–80, with mostof the lake sites west of Hudson Bay experiencing earlier springs, which is particularly strong along theManitoba–Saskatchewan border and in the Northwest Territories. Later break-up dates, on the other hand,are more prominent in eastern Canada, but with few sites displaying statistically significant trends at the10% level. The period 1961–90 shows a strong, generalized trend toward earlier break-up dates over all ofCanada: 71 (88%) of the 81 lake sites exhibit this trend (21 sites significant at the 10% level). Noteworthyis the fact that none of the sites experiencing later break-up dates shows statistically significant trends at the10% level. Trends during the period 1971–2000 reveal the persistence of the warmer spring conditions formuch of Canada: 31 (72%) of the 43 lake sites show a trend toward earlier break-up dates. During that period,a ‘boomerang shape’ distribution of lakes displaying earlier break-up dates (negative trends) is noticeable inFigure 5, beginning in Labrador and ending in northern British Columbia. A similar pattern is observed in



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Figure 3. Trends in freeze-up dates across Canada for the periods (a) 1951–80, (b) 1961–90, and (c) 1971–2000. Triangles pointing upindicate later freeze-up dates (warming trend) and those pointing down indicate earlier freeze-up dates (cooling trend). Lake sites withsignificant trends at the 10% level are denoted by filled triangles. The upper and lower limits of the number of days of change determined

for each period are given on the left-hand side of the freeze-up legend



(c) 1971-2000






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trends of last date of spring snow cover over the period 1972–98 (Hall et al., 2004). A few lake sites innorthern Manitoba and Saskatchewan, as well as the Northwest Territories, though, show trends toward laterbreak-up dates (colder conditions). Some significant late break-up trends appear in the Great Slave Lake area,which also shows up as an area with a trend toward later spring snow cover over the period 1972–98 (Hallet al., 2004).

Break-up anomalies for the lakes showing some of the strongest positive (later dates, colder climate) andnegative (earlier dates, warmer climate) trends in the country over the three 30-year periods are illustratedin Figure 6. From this set of graphics it can be seen that break-up dates became earlier (or later) by about10–15 days, with earlier dates being the norm, as shown in the maps of Figure 5. With the exception of theColpoys Bay site (Georgian Bay, Lake Huron), break-up dates appear to show smaller departures from theirmeans than freeze-up dates. From the sample of lake sites of Figure 6, break-up anomalies rarely exceed20 days, whereas for freeze-up dates (see Figure 4) the anomalies more frequently surpass 20 days. Theexamination of a larger number of sites would be necessary, however, before making any strong generalizationon this particular point.

Trends for the period 1966–95

Albeit with a few exceptions, trends in freeze-up and break-up dates for the period 1966–95 (Figure 7)mirror those of the period 1961–90 (Figures 3b and 5b), with break-up dates becoming increasingly earlier formost of the country and no strong regional signals in freeze-up dates, except perhaps for the Great Lakes–StLawrence region (later freeze-up) and the Atlantic region (earlier freeze-up). Also shown in Figure 7, as abackdrop, are the trends in 0 °C isotherm dates presented in the form of classes. As explained in an earliersection, these were obtained through interpolation of autumn and spring 0 °C isotherm dates derived from 187meteorological stations across Canada. Their trends in relation to freeze-up and break-up trends will be thefocus of a discussion in a later section.



(a) Frame Lake - Boreal region Grand Lake – Atlantic region

(b) Lake Athabasca – Boreal region Deadman's Pond – Atlantic region

(c) Lake Utopia – Great Lakes/St. Lawrence region Island Lake – Boreal region

Figure 4. Freeze-up anomalies for lake sites showing some of the strongest positive (left: later dates, warmer) and negative (right: earlierdates, colder) trends for the periods (a) 1951–80, (b) 1961–90, and (c) 1971–2000. The number of years n for each record, the number ofdays per year of change (in parentheses) and the significance level of the trends (C: 10%; Ł: 5%; ŁŁ: 1%; ŁŁŁ: 0Ð1%) are also indicated on

the graphics

Comparison with trends in other cryospheric and atmospheric variables. In recent years, several papershave been published on the trends in atmospheric and cryospheric variables (e.g. Brown and Goodison, 1996;Brown and Braaten, 1998; Serreze et al., 2000; Zhang et al., 2001; Huntington et al., 2003; Overland et al.,2004), as well other parameters such as the greenness of vegetation (e.g. Myneni et al., 1997; Slayback et al.,



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Figure 5. Trends in break-up dates across Canada for the periods (a) 1951–80, (b) 1961–90, and (c) 1971–2000. Triangles pointing upindicate later break-up dates (cooling trend) and those pointing down indicate earlier break-up dates (warming trend). Lake sites withsignificant trends at the 10% level are denoted by filled triangles. The upper and lower limits of the number of days of change determined

for the periods are given on the left-hand side of the break-up legend



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2003) and plant phenology (e.g. Beaubien and Freeland, 2000; Cayan et al., 2001; Menzel, 2003) in responseto 20th century climate warming in the Northern Hemisphere. Trends in freeze-up/break-up dates for theperiod 1966–95 were examined, as they can be compared with trends in other cryospheric and atmosphericvariables previously documented over the same time period. The major trends identified in these referencesare summarized in Table I. Of all the variables listed in the table, air temperature is the one that has thelargest influence on freeze-up and break-up dates. As stated by (Magnuson et al., 1997):

Ice formation and break-up are dependent on many climatic forcing variables, such as air temperature,solar radiation, wind, and snow depth. Yet, air temperature alone often provides a reasonable predictionof ice phenologies.

The air temperature trends (winter and spring) are generally consistent with the break-up trends from thisstudy, in that most of Canada, with the exception of northeastern Canada and Newfoundland, has experienceda warming trend over the period 1966–95. As discussed earlier, freeze-up trends are highly variable acrossthe country, so that it is difficult to establish a clear relation with the air temperature trends summarized inTable I.

Relation to 0 °C isotherm dates. It has been shown in several lake ice studies that freeze-up and break-up dates correlate most strongly with air temperatures in the 1 to 3 months before the event (e.g. Paleckiand Barry, 1986; Robertson et al., 1992; Livingstone, 1997, 1999). Magnuson et al. (2000) reported that, inmore northern areas of the Northern Hemisphere (such as Lake Kallavesi, Finland), freeze-up dates reflect theclimate prevailing around October to November, whereas in more southern areas, such as Grand Traverse Bay(connected to Lake Michigan), the dates reflect the climate from January to February. Break-up dates, on theother hand, reflect February to March climates in more southern areas, such as Lake Mendota in Wisconsin



(a) Colpoys Bay (Lake Huron) − Great Lakes/St. Lawrence region

(b) Gull Lake − Great Lakes/St. Lawrence region

(c) Back Bay (Great Slave Lake) − Boreal region

Diefenbaker Lake − Prairies region

Lake Utopia − Great Lakes/St. Lawrence region

Brochet Bay (Reinder Lake) − Boreal region

Figure 6. Break-up anomalies for lake sites showing some of the strongest positive (left: later dates, colder) and negative (right: earlierdates, warmer) trends for the periods (a) 1951–80, (b) 1961–90, and (c) 1971–2000. The number of years n for each record, the numberof days per year of change (in parentheses) and the significance level of the trends (C: 10%; Ł: 5%; ŁŁ: 1%; ŁŁŁ: 0Ð1%) are also indicated

on the graphics



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Figure 7. Trends in (a) freeze-up and (b) break-up dates across Canada for period 1966–95. Triangles pointing up indicate laterfreeze-up/break-up dates and those pointing down indicate earlier freeze-up/break-up dates. Lake sites with significant trends at the 10%level are denoted by filled triangles. Also shown in the background are the trends in autumn and spring 0 °C isotherm dates estimated across

Canada (number of days change over the period 1966–95)



Table I. Summary of trends in other cryospheric and atmospheric variables in Canada

Variable Documented trendb

Air temperature Autumn. Colder over part of central Canada, Hudson Bay, and into western Nunavut,Northwest Territories and Yukon. Close to no change over the rest of the country

Winter. Warmer west of Hudson Bay; particularly strong over western provinces,Northwest Territories, and Yukon. Colder from Hudson Bay eastward. Close to nochange over Ontario

Spring. Warmer over most of the country; particularly strong over western Canada. Colderon southern portion of Baffin Island. Close to no change over Hudson Bay, northernQuebec, and Atlantic provinces

Summer. Warmer over part of western Canada and the western Arctic in particular. Closeto no change over the rest of the country

Precipitation Positive trends in annual precipitation, as well as snowfall (up to 20% increase), during thepast 40 years over Canada north of 55°N

Snow-covered area Statistically significant decreases in spring snow cover (1946–95) over most of westernCanada and the high Arctic

Snow depth Decrease in January–March snow depths for most of Canada (1946–95), with the largestdecreases in March. Decreases are most prominent in Mackenzie basin, Prairies, andlower St Lawrence Valley. Increases are noted only on the east coast. Snow coverduration declined over most of western Canada and in the Artic in summer. Sharptransition to lower depths in the mid 1970s

River ice freeze-up/break-up Statistically significant trends toward earlier freeze-up, particularly in eastern Canada, andearlier break-up, especially in British Columbia (1967–96)

River ice duration Increased ice cover duration over the Maritimes; variable response in other regions ofCanada

Glaciers Canadian mountain glaciers characterized by generally negative mass balance and retreatduring the second half of 20th century. Glacier mass balance over the western Cordilleraclosely linked to North Pacific climate variability, e.g. Pacific decadal oscillation

Arctic sea ice extent Small negative trend since 1979, with more pronounced reduction since the late 1980sSea ice duration Statistically significant increase in length of ice-free season in southwestern Hudson Bay

region (1971–2003). Much of the increase is attributed to earlier break-up (3 days perdecade)

Permafrost Temperature increases for western Canada, but not consistentDecrease in temperatures for northern Quebec from mid-1980s, tentatively attributed to

lower air temperatures in this region, to about mid-1990s

a Period 1966–95 unless specified otherwise.b Sources: Brown and Goodison (1996); Brown and Braaten (1998); Serreze et al. (2000); Zhang et al. (2001); Brown et al. (2004); Goughet al. (2004); Overland et al. (2004).

(Magnuson et al., 2000). How well freeze-up and break-up dates correlate with a simpler air temperatureindex, such as the 0 °C isotherm date, had not yet been investigated.

The arrival of the 0 °C isotherm date in autumn initiates, for example, snow accumulation and animalhibernation. In spring it brings about, for example, snowmelt, early plant growth, and flood hazards. In arecent paper, Bonsal and Prowse (2003) summarized some relations between trends in autumn and spring0 °C isotherm dates and trends in hydro-cryospheric variables (e.g. snowmelt, river and lake ice) in generalterms. Here, the strength of the relation between autumn/spring 0 °C isotherm dates and freeze-up/break-updates is quantified. Trends in both sets of dates, as well as their degree of synchrony, are described.

Figure 7 allows one to get a sense of the spatial distribution of trends in both freeze-up/break-up dates and0 °C isotherm dates, and their possible relation from a qualitative standpoint. In Figure 7a, autumn isothermtrends are represented in three classes. The first class shows areas of the country with a trend toward later0 °C isotherm dates (warming trend) and the two other classes a trend toward an earlier arrival of the 0 °Cisotherm. Few lake sites exhibit any statistically significant trends in freeze-up dates. However, for those that



do, they tend to fall in regions of the country where trends in autumn isotherms generally follow the samedirection (i.e. positive or negative trends). The Great Lakes–St Lawrence and Atlantic regions, as well asnorthern Ellesmere Island (Upper Dumbell Lake–Eureka weather station), are good examples of such trends.As shown in Figure 7b, the break-up dates follow much the same trends as the spring 0 °C isotherm dates,with a strong signal toward earlier break-up and 0 °C isotherm arrival dates. Only one small region of thecountry shows a trend toward later 0 °C isotherm arrival dates (eastern Newfoundland), and the lake site(Deadman’s Pond) closest to this region exhibits a trend toward later break-up dates.

The maps of Figure 8 provide a clear picture of the spatial and temporal coherence between freeze-up/break-up dates and 0 °C autumn/spring isotherm dates: 52% (34/66) of the paired sites (lake and closestmeteorological station) have r > 0Ð5 in the relation freeze-up/autumn isotherm dates. For the spring break-upperiod, r > 0Ð5 for 78% of the paired sites (58/74). So, although 0 °C isotherm dates mark the beginning ofthe freeze-up and break-up periods, they show a high degree of synchrony at many locations throughout thecountry, and generally more so during the spring break-up period.

However, it should be noted that, although the 0 °C isotherm dates and freeze-up/break-up dates are oftenin tempo, there is a lag (delay) between the onset of melting and freezing conditions and the completefreeze-over and disappearance of ice from lakes. This point is clearly illustrated in Figures 9 and 10. Fromthe sample of sites shown in these figures (one per climatic region), it appears that the lag (i.e. the numberof days of difference) is shorter during the freeze-up period (10–15 days on average) than the break-upperiod (25–30 days on average). There are no complete explanations that can be provided at this point,other than the likelihood that the lake sites selected to show the synchrony between freeze-up dates andautumn 0 °C isotherm dates are generally shallow. Shallow lakes have thermal turnover rates in the order ofa week (they store less heat); hence, they form ice more quickly than larger lakes. However, no lake depthmeasurements are available for these lakes to support this affirmation. The greater lag during the spring period(see Deadman’s Pond in Figures 9 and 10 for comparison) can be attributed to some degree to the amountof time needed to melt the snow on the ice covers and to melt the slab of ice formed on the lakes duringthe winter period. Overall, the correlations obtained between the 0 °C isotherm dates and freeze-up/break-updates are as strong as the correlations involving the use of air temperatures from the 1 to 3 months precedingthe events (break-up/freeze-up).

SUMMARY AND CONCLUSION

The primary intent of this paper was to analyse spatial and temporal trends in lake ice freeze-up and break-updates across Canada for various time periods between 1951 and 2000. Three questions were being addressed:

1. What has been the magnitude of changes in freeze-up and break-up dates in different regions of Canadafor the 30-year climatological periods (1951–80, 1961–90, and 1971–2000)?

2. How do the lake ice trends compare with trends in other cryospheric and atmospheric variables observedby other workers over a comparable time period (1966–95)?

3. How do the trends and variability in freeze-up and break-up dates compare with those of the autumn andspring 0 °C isotherm dates?

The main findings, which provide answers to the above questions, can be summarized as follows:

1. Trends in freeze-up dates. Trends toward later and earlier freeze-up dates were found at various locationsacross Canada. However, many of these trends were not significant at the 10% level and their spatialcoherence was generally weak. The latter may be due to the effect of lake morphometry (depth and area)and local meteorological conditions such as wind, which can play a significant role during the freeze-up



(a)

(b)

Correlation

< 0.2

0.2 - 0.4

0.4 - 0.6

0.6 - 0.8

0.8 - 1.0

Correlation

< 0.2

0.2 - 0.4

0.4 - 0.6

0.6 - 0.8

0.8 - 1.0


Cordillera

Prairies

Boreal

Arctic


Atlantic

2

2

3

3

1

1

4

4

7

7

6

6

5


Cordillera

Prairies

Boreal

Arctic


Atlantic

2

2

3

3

1

1

4

4

7

7

6

6

5

5

5

Figure 8. Correlation coefficients between (a) freeze-up and autumn 0 °C isotherm dates and (b) break-up and spring 0 °C isotherm datesacross Canada for the period 1966–95



(a) Watson Lake, Cordillera region (d) Unnamed Lake 684009748 − Arctic region

(b) Ekapo Lake − Prairies region (e) Base Lake − Great Lakes/St. Lawrence region

(c) Attawapiskat Lake − Boreal region (f) Deadman’s Pond − Atlantic region

Figure 9. Comparisons between freeze-up and autumn 0 °C isotherm dates for: (a) Watson Lake, Yukon; (b) Ekapo Lake, Saskatchewan;(c) Attawapiskat Lake, Ontario; (d) Unnamed Lake 684 009 748, Nunavut; (e) Bass Lake, Ontario; (f) Deadman’s Pond, Newfoundland.Each lake is representative of a different climatic region. Correlation coefficients r are based on annual time series for the period 1966–95



(a) Watson Lake, Cordillera region (d) Unnamed Lake 684009748 – Arctic region

(b) Ekapo Lake – Prairies region (e) Bass Lake – Great Lakes/St. Lawrence region

(c)Attawapiskat Lake – Boreal region (f) Deadman’s Pond – Atlantic region

Figure 10. Comparisons between break-up and spring 0 °C isotherm dates: (a) Watson Lake, Yukon; (b) Ekapo Lake, Saskatchewan;(c) Attawapiskat Lake, Ontario; (d) Unnamed Lake 684 009 748, Nunavut; (e) Bass Lake, Ontario; (f) Deadman’s Pond, Newfoundland.Each lake is representative of a different climatic region. Correlation coefficients r are based on annual time series for the period 1966–95

period. Where trends toward later freeze-up were observed they corresponded to areas with increasing fallsnow cover from the NOAA dataset.

2. Trends in break-up dates. Trends toward earlier and later break-up dates were found at various locationsacross Canada, though earlier break-up dates dominated over the periods of analysis. Several trends were



significant at the 10% level and their spatial coherence was strong. Western Canada showed the mostconsistent trends toward earlier break-up dates over all 30-year periods. The spatial patterns were consistentwith changes in spring snow cover duration derived from the NOAA dataset.

3. Comparison with trends in other cryospheric and atmospheric variables. The trends observed in lake icecover are consistent with other cryospheric variables (e.g. snow cover, river ice) and provide further evidenceof the enhanced spring warming that began over North America during the second half of the 20th century.Warming has played a role, but changes in the main modes of atmospheric circulation over North Americahave also played a role (Walsh et al., 2005).

4. Relation between freeze-up/break-up dates and 0 °C isotherm dates. A strong relation was found between0 °C isotherm dates. Although autumn and spring 0 °C isotherm dates mark the beginning of the freezingand thaw periods respectively, there was a high degree of synchrony between freeze-up and break-up datesand the 0 °C isotherm dates. This was well reflected in both the trends and interannual variability depictedin the 1966–95 time series. Ice freeze-up and break-up dates lagged behind the 0 °C isotherm dates by afew days to about 1 month.

Finally, the results of this study leave the door open for future, regionally focused investigations onthe response of lake ice to climate over longer time periods and in relation to large-scale atmosphericoscillations/teleconnections. The recent availability of lake ice databases such as CID (Lenormand et al.,2002) and IceWatch on the Canadian Cryospheric Information Network (CCIN) will facilitate these types ofinvestigations.

ACKNOWLEDGEMENTS

This research was made possible with funding obtained from the Natural Sciences and Engineering ResearchCouncil of Canada, Environment Canada and a start-up grant from the Geophysical Institute, University ofAlaska Fairbanks, to C. Duguay. The CID was developed at Laval University with financial support from theMSC to C. Duguay. The CID is available through the CCIN at http://www.ccin.ca. We are grateful for thehelpful comments of Andrew Klein and two anonymous reviewers.

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