autumn synoptic conditions and rainfall in the subarctic canadian shield of the northwest...

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 25: 1493–1506 (2005)

Published online 1 August 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1185

AUTUMN SYNOPTIC CONDITIONS AND RAINFALL IN THE SUBARCTICCANADIAN SHIELD OF THE NORTHWEST TERRITORIES, CANADA

CHRISTOPHER SPENCE* and JARA RAUSCHEnvironment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada

Received 4 June 2004Revised 17 December 2004Accepted 8 February 2005

ABSTRACT

Autumn precipitation is important to the annual water budget and spring streamflow magnitude in the subarctic CanadianShield portion of the Mackenzie Basin in northwestern Canada. The objective of this study is to improve understandingof the synoptic mechanisms generating this autumn precipitation by investigating a wet (1998) and a dry (1999) autumnover a portion of this region north of Great Slave Lake. Seven synoptic map patterns were identified using a hybridmanual–revised Kirchhofer correlation methodology. Cyclones were found to be the most important map pattern providingautumn rainfall. These cyclones tend to occur 2 to 3 days following the presence of a low in the Gulf of Alaska near theAleutian Islands. The results of this study corroborate past research that shows Pacific atmospheric moisture transportedinland provides the source for much of the cyclonic rainfall in the Mackenzie Basin. Aleutian lows, which encourage thistransportation, were as common in 1998 as in 1999, but the mechanisms necessary to transfer moisture to the North GreatSlave region were more entrenched in 1998 than in 1999. Winds along storm tracks through the region were weaker andmore dispersed in 1999. In addition, conditions for cyclogenesis in the region were less favourable in 1999. Copyright 2005 Royal Meteorological Society.

KEY WORDS: synoptic climatology; rainfall; subarctic; Northwest Territories; Canadian Shield

1. INTRODUCTION

Most of the few synoptic climatology studies of Canada’s north have concentrated on the High ArcticArchipelago (Bradley and England, 1979; Walsh and Chapman, 1990). Of those concerned with the mainland,research has focused on the winter months, because this is when cyclogenesis has the strongest synopticsignal (Lackmann and Gyakum, 1996) and the most adverse effects on day-to-day activities, such as aviation(Scholefield, 1976). The only warm-season synoptic climatologies developed for locations on the mainland ofCanada’s northern territories were developed by Petrone and Rouse (2000) for Inuvik, Northwest Territoriesand Churchill, Manitoba. They subsequently applied the Churchill climatology to an investigation of theimpact of El Nino events on surface conditions at that location (Petrone et al., 2000).

One of the key results of Petrone and Rouse (2000) was that low-pressure systems are the most efficientsynoptic systems at transporting moisture into the mainland of northern Canada. The moisture influx associatedwith low-pressure systems and cyclogenesis has been identified as very important to the atmospheric waterbudget of the Mackenzie Basin (Walsh et al., 1994). Spence and Rouse (2002) illustrated the hydrologicalimportance of the timing and magnitude of wet conditions in the late summer and autumn in the CanadianShield physiographic region of the Mackenzie Basin. Wet conditions in the fall of 1998 increased the storageof water on the landscape prior to freeze up. When compared with a lesser runoff response in 2000 followinga dry fall in 1999, this increase in storage in the fall of 1998 was shown to increase significantly the proportion

* Correspondence to: Christopher Spence, Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada;e-mail: [email protected]

Copyright 2005 Royal Meteorological Society

1494 C. SPENCE AND J. RAUSCH

of snowmelt that ran off the landscape in the spring of 1999. The work of Spence and Rouse (2002) shows thatweather conditions in autumn (i.e. late August and September) have a profound effect on the surface waterand energy budgets 9 months and 11 months into the future respectively. No previous synoptic climatologystudy has focused on this particular region, and especially the autumn period in the region. The objectiveof this study is to improve understanding of the synoptic mechanisms generating this autumn precipitationby investigating a wet (1998) and a dry (1999) autumn over a portion of the Canadian Shield region northof Great Slave Lake. This will be accomplished by developing a synoptic climatology for the region anddiscussing the results in the context of previous atmospheric moisture studies of the Mackenzie Basin.

2. STUDY AREA

This study area’s focus is the Canadian Shield region north of Great Slave Lake. It is bounded bythe city of Yellowknife on the north shore of Great Slave Lake (114°23′W, 62°45′N) and the Ekatidiamond mine in the Coppermine River headwaters (110°36′W, 64°42′N) with Lower Carp Lake (114 °W,63°34′N) at its centre (Figure 1). This area occupies the eastern edge of the Mackenzie River basin.The environment and landscape typify the Coppermine River Upland ecoregion of the Taiga Shieldecozone (Ecological Stratification Working Group, 1996). Glacial action has scoured the area repeatedlyduring the Quaternary, exposing the Precambrian bedrock. Drainage courses often follow the bedrockstructure, showing a trellis drainage pattern. Hummocky rock surfaces formed by glacial erosion resultin a large number of lakes in the region; so much so that surface water accounts for almost 30% of thearea.

The 1971–2000 climate of the region, as characterized at the city of Yellowknife, can be describedas subarctic continental with short, cool summers (July daily average temperature of 17 °C) and long,cold winters (January daily average temperature of −27 °C). July is the warmest month of the year,and fall conditions begin no later than late August, with an average day of first fall frost of 20September. Mean annual unadjusted precipitation is roughly 280 mm, with just under half falling as

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Figure 1. Map of northwestern Canada illustrating the GEM analysis grid (black dots), and locations of Great Slave Lake (GSL), thecity of Yellowknife (black box), Lower Carp Lake (X), and Ekati (white circle). For comparison, boundaries of the grids of Petrone

and Rouse (2000) (thick black lines) and the locations of Inuvik (I) and Churchill (C) are included

Copyright 2005 Royal Meteorological Society Int. J. Climatol. 25: 1493–1506 (2005)

SYNOPTIC CONDITIONS AND RAINFALL, NORTHWEST TERRITORIES, CANADA 1495

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Figure 2. Monthly air temperature (top) and precipitation (bottom) at the city of Yellowknife. The black boxes represent 1961–90averages, and the bars represent ±1 standard deviation

snow (http://www.climate.weatheroffice.ec.gc.ca/climate normals/index e.html). Snowfall has been observedin every month of the year, and a contiguous snow cover usually develops on the landscape by mid October.Figure 2 illustrates the variability of monthly precipitation and the intrinsic variability of the convective pre-cipitation that can dominate June, July and August (Szeto, 2002). The two months of interest in this study,August and September, experienced mean precipitation of 40 ± 31 mm and 31 ± 15 mm respectively between1942 and 2002.

3. DATA AND METHODS

A synoptic climatological approach provides a means to identify relationships between synoptic conditionsand the surface environment. The methodology seeks to explain key interactions between the atmosphereand the surface of the Earth through classification of atmospheric circulations and the assessment of anyrelationships between these classes and a region’s weather elements or surface environment (Yarnal, 1993).There are two types of synoptic climatological study. The first is circulation to environment studies, whensynoptic patterns are identified independent of surface conditions. The surface data are then assessed relativeto the pattern of synoptic types. The second approach is to fit the classes of atmospheric patterns usingvariation in selected surface variables. This study applies the former.



12Z surface pressure fields were obtained from the Canadian Meteorological Centre (CMC) for the twosummers (1 June to 30 September) prior to each spring melt observed by Spence and Rouse (2002). Theseanalysis data were the result of data assimilation procedures among available observations and data generatedfrom Environment Canada’s weather forecast model, the Global Environmental Multi-scale (GEM) model(Cote et al., 1998a, b). This model produces a non-uniform horizontal 24 km grid. A coarser 240 km versionwas applied, to reduce computational time to operable levels. A 35 × 10 point grid was created over an areabounded by 56 °N, 100 °W and 70 °N, 140 °W (Figure 1). A new climatology was deemed necessary for thisarea of interest because it lies on the extreme outer edges of both regions investigated by Petrone and Rouse(2000) (Figure 1).

Two years of synoptic data were considered adequate, because the two encapsulated the range of autumnprecipitation conditions experienced by the region. The use of both summer and autumn synoptic data wasto ensure that all the relevant synoptic types could be identified. Synoptic data used by Petrone and Rouse(2000) for their climatologies ranged from 21 April to 7 September. A dataset more inclusive of Septemberconditions was considered necessary for this study in case there were any distinctly autumn synoptic patterns.

The principal synoptic types (or key days) were identified using a revised Kirchhofer technique describedby Blair (1998). The original Kirchhofer (1974) technique calculates grid scores using normalized pressurevalues to avoid similar pressure patterns with different pressure values from being classified as the samecirculation type following:

Zi = Xi − X

σ(1)

where Zi is the normalized value at point i, Xi is the pressure value at i, and X and σ are respectively themean and standard deviation of all 350 values in the field. Each field is compared with all the other fieldswith a sum of squares approach, using

S =N∑

i=1

(Zai − Zbi)2 (2)

where N is the number of values in the field, Zai is the normalized value for point i on day a, Zbi thenormalized value for the same point on day b, and S is the Kirchhofer score between those 2 days. Themethod of Blair (1998) also calculates S scores for each corresponding column and row by using the standarddeviation of each column and row rather than that of the entire field. Each pair of days was consideredsimilar if the S score for the field was less than 61 (one-quarter of the number of fields) and the row andcolumn scores were less than 17.5 and 5 respectively. The fields with the largest number of scores beloweach threshold were defined as the key days.

Surface weather maps at 0Z, 6Z, 12Z and 18Z were obtained for the same period from the Arctic WeatherCentre of Environment Canada. The predominant synoptic features on each key day were identified using the12Z map. Maps 12 h before and after 12Z were also consulted to ensure that these features were representativeof the entire day (Yarnal, 1993). These key-day maps were classified into map patterns based upon theirpredominant synoptic features. All the remaining surface weather maps were manually analysed relative tothe centre at Lower Carp Lake and these key-day features. Each daily map was assigned to one of the mappatterns, if possible. If not, it was given an unclassified designation. This hybrid approach differs markedlyfrom that used by Petrone and Rouse (2000) and Frakes and Yarnal (1997), but provides an objective selectionof the predominant synoptic types. The manual classification of individual synoptic maps to predefined mappatterns is among the most subjective exercises in manual classification exercises (Yarnal, 1993). This isreflected in some of the results.

Daily precipitation data were acquired for the only observing sites in the region. These were MeteorologicalService of Canada stations at Yellowknife and Ekati (http://www.climate.weatheroffice.ec.gc.ca/climateData/canada e.html) and a tower at Lower Carp Lake associated with the study outlined by Spence and Rouse(2002). Values from each station were averaged to derive a regional precipitation time series for the study



period. Additional meteorological data useful for characterizing the surface conditions of each map patternwere available only from Lower Carp Lake. These data include air temperature, relative humidity and netradiation. A thorough discussion of the collection of this data is outlined in Spence and Rouse (2002).

In addition to these surface meteorological terms, the raininess index Rxy of Bradley and England (1979)was also calculated for each day to determine the efficiency of each map pattern in generating rainfall:

Rxy(%) = PxyNy

NxyPy

× 100 (3)

where Pxy is regional precipitation from map pattern x in period y, Nxy is the frequency of map pattern x inperiod y, Ny is the total number of days in period y and Py is the regional precipitation in period y.

4. RESULTS

4.1. Map patterns

The revised Kirchhofer correlation method selected seven key days. Seven distinct map patterns were foundin the surface weather maps (Figure 3). Type A was a cyclone centred over Lower Carp Lake. An anticycloneover the same location typified Type B. The predominant feature in Type C was a trough from Victoria Islandto northern Alberta. The outstanding characteristic of Type D, back of high, was a high in southern Nunavutand a low in the Beaufort Sea. Type E was the reverse of Type D, with a high to the northwest and a low tothe southeast. The most predominant feature on the Type F weather map was an Aleutian or Gulf of Alaskalow. Type G is a high-pressure ridge close to the location of the trough in Type C.

Two of the indices of how well a synoptic classification technique works are the percentage of classifieddays and the S scores associated with each map pattern. Unclassified days constituted 12 of the 122 evaluated(9.8%), the majority of which were in 1998 (Table I). This is comparable to the method applied by Petroneand Rouse (2000), but less than that of Bradley and England (1979). Low values of S reflect less internalvariability of an individual map pattern. This study’s average S scores are 30% higher (22.8 versus 17.7)than those of Petrone and Rouse (2000). These two comparisons suggest that the manual classification wasliberal. This was reflected in some of the results.

There were notable variations in the precipitation and surface conditions within map patterns C and E. In1998, Type C was wet, and vapour pressure deficits and net radiation were low, implying wet and cloudyconditions. 1999 was distinctly different, with higher net radiation and low rainfall efficiency (Table II).

Map pattern E, although not overly wet, was always a good generator of precipitation. Variations in surfaceconditions, however, suggest that the source of this precipitation may have varied between the 2 years. The

Table I. List of key days, their associated map patterns and average Kirchhofer scores. The frequencies f

of each pattern are summarized for each autumn the classification was applied to

Key day Map pattern S f (%)

Both autumns 1998 1999

11 August 1998 A 28 29.5 39.3 19.74 July 1998 B 15 4.9 3.3 6.629 September 1999 C 23 9.8 8.2 11.519 September 1999 D 28 13.9 6.6 21.313 June 1998 E 24 5.7 3.3 8.230 September 1998 F 28 18.0 19.7 16.45 July 1999 G 14 8.2 3.3 13.1

Unclassified ? n/a 9.8 16.4 3.3



A-low B-high

C-trough D–back of high

E–back of low F –Aleutian low

G–ridge

Figure 3. Surface analysis weather maps on the seven key days used to derive the map patterns. The black square denotes the locationof Lower Carp Lake



Table II. Conditions of total precipitation P , rainfall efficiency, average daily vapour pressure deficit VPD, and averagedaily net radiation Q∗ during each autumn for each map pattern

Map 1998 1999pattern

P (mm) Rxy (%) VPD (kPa) Q∗ (W/m2) P (mm) Rxy (%) VPD (kPa) Q∗ (W/m2)

A 68.3 161 0.30 22.8 46.7 235 0.18 10.5B 1.6 41.0 0.61 48.9 0.7 8.7 0.60 64.7C 11.8 131 0.13 13.1 4.5 40.0 0.27 37.8D 0.8 11.0 0.39 45.2 11.3 56.0 0.40 43.7E 6.3 188 0.10 10.6 9.6 128 0.52 75.3F 7.9 37.0 0.22 28.2 8.5 49.0 0.24 31.7G 2.2 64 0.87 76.6 17.4 130 0.47 51.4

? 6.6 50.0 0.43 66.8 1.2 29.0 0.45 34.9

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Figure 4. Surface conditions occurring within map pattern. From top to bottom, total precipitation, average vapour pressure deficit,average net radiation and raininess index

low vapour pressure deficits and net radiation in 1998 (Table II) suggest overall cloudy conditions and a wetair mass from outside the region. 1999 was dry and sunny (Table II), so the rainfall then may have beenconvective in origin. This is a distinct possibility given the large number of lakes in the region. If this werethe case, then it would be exceedingly difficult to characterize this particular source of precipitation usingthe methods described earlier. There could have been a change in source region of the air associated withthe Type E map pattern between the 2 years (Petrone et al., 2000). Another, more likely, explanation is thatthe present method was not always convincingly able to identify synoptic patterns that could represent eitherprecipitation or surface conditions.



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Figure 5. Frequency (%) of synoptic types in August and September by year

Consistent surface conditions associated with the other map patterns allow for more confidence to be placedin them. Type A was the wettest map pattern when evaluated for both autumns (Figure 4), with 63% and 47%of all precipitation falling on Type A days in 1998 and 1999 respectively. Type A was a disproportionatelygood generator of rainfall, with Rxy values exceeding 100% during both falls (Table II). Consistently lowvapour pressure deficits and net radiation on Type A days imply that these days were moist and cloudy(Figure 5, Table II). Type B was a strong high-pressure system. The strong southerly geostrophic flow overthe area associated with a Type D synoptic pattern, in conjunction with expected westerly flow aloft, implies aveering wind profile, warm advection and synoptic-scale lift (Figure 3). Both B and D map patterns producedconsistently sunny and dry days (Table II).

Type A was the most frequent map pattern (Table I). Type A was twice as frequent in 1998 as in 1999(Figure 5). The decrease in Type A days in 1999 resulted in more frequent instances with Type D, in particular.Type B was the rarest map pattern in both years. The Aleutian low map pattern (Type F) occurred, on average,18% of the time. This value did not vary much between 1998 and 1999.

4.2. Aleutian lows and North Great Slave cyclones

Aleutian lows (Type F) had a profound influence on the map pattern sequence in the study region. Oncemoved into the area, Aleutian lows persisted for 2 days 38% of the time (Figure 6). This 2-day period wasoften followed by a cyclone (Type A) over the North Great Slave region 42%, 56% and 44% of the time2 days, 3 days and 4 days respectively after the original appearance of the Aleutian low (Figure 4). Thepattern in Figure 4 shows that a North Great Slave cyclone (Type A) was the most common synoptic featurefollowing an Aleutian low (Type F).

In order to illustrate the transition to Type A from Type F, a case study is presented. There were 17.5 mmof rainfall between 2 and 5 September 1998 in the region. Although this was not a relatively large amount, theevent is typical of the conditions experienced in August and September 1998. There were two predominantatmospheric features over western Canada on 31 August (Figure 7). The smaller of the two was an upper coldlow that was progressing west from the Northwest Territories into Nunavut. This system had just provided8 mm of rainfall to the study region. The second feature was a very large and deep Aleutian low in theGulf of Alaska. This system brought moisture east as a warm front approached the British Columbian coast.By 2 September, strong winds evident at 500 hPa carried this moisture across the Western Cordillera intonortheastern British Columbia. Cold air associated with a ridge of high pressure near the Mackenzie Deltaand the Beaufort Sea coast of Alaska contrasted warmer air in the Canadian Prairies. This helped generatea subsequent cyclone by 3 September, which was well established just northeast of Great Slave Lake on 4



Figure 6. Pie charts showing percentages of occurrence of a particular map pattern after an initial Type F day

September. This is when the bulk of the event rain fell in the region. Two similar sequences to the 31 Augustevent followed, contributing 5 mm and 16 mm of rainfall from 7 to 9 September and 12 to 18 September,respectively.

5. DISCUSSION

This type of sequence has been documented previously using observed data, teleconnection indices andclimate model output. Using the MC2 model, Misra et al. (2000) and Lackmann et al. (1998) show that,during cyclonic activity over the Mackenzie Basin similar to this study’s Type A map pattern, the nethorizontal flux convergence predominated the water vapour budget. This suggests that the source of moisturewas from outside the basin. In an evaluation of European Centre for Medium-Range Weather Forecasting(ECMWF) data, Smirnov and Moore (1999) discussed that typical September events that transport moistureinto the Mackenzie Basin were associated with a low near the Bering Strait. Both Smirnov and Moore (1999)and Misra et al. (2000) demonstrated that an original system over the Pacific tended to remain for a coupleof days while a pulse of moisture propagated inland. The results from these previous studies imply that themoisture associated with North Great Slave cyclones (Type A) often comes from Aleutian lows (Type F).

These and other studies also support the temporal sequence suggested by the synoptic climatology results.Both regional climate model (Mackay et al., 1998) and gridded observation data (Lackmann and Gyakum,1996; Stewart et al., 2000) have shown the development of an inland low with a band of precipitation 6 hafter the presence of a low in the Gulf of Alaska. The band of precipitation modelled by Mackay et al. (1998)was associated with a lee cyclone that developed after about 24 h and continued over the next 36 h. Theband of precipitation tended to advance over the study region between 24 and 48 h after it developed. Theresults shown in Figure 6 and the case study of the 31 August 1998 event (Figure 7) corroborate this temporalpattern.

The difference in rainfall between 1998 and 1999 was not due to ‘dry’ cyclones, such as those documentedby Lackmann and Gyakum (1996), where the North American ridge is farther east than Saskatchewan and


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the Pacific trough is northwest of its ‘wet’ location. The Type A cyclones in the fall of 1999 were not dry.In fact, they were wetter, at least in terms of Rxy , than their 1998 counterparts (Table II). Results show thatthe fall of 1998 was wetter than the fall of 1999 because Type A synoptic conditions were more common.

This discussion could imply that Aleutian lows produce greater cyclonic activity and ample rainfall in theregion north of Great Slave Lake. Notably, however, the frequency of Aleutian lows was similar, whereasthere was a 50% decrease in North Great Slave cyclones between the autumns of 1998 to 1999. The presenceof Aleutian lows does not necessarily mean it will rain north of Great Slave Lake. The differences between1998 and 1999 highlight the substantial variability in moisture convergence over the Mackenzie Basin inautumn (Walsh et al., 1994). The pathway by which moisture approaches the eastern Mackenzie Basin is atlevels of the atmosphere near 850 hPa (Walsh et al., 1994), but may be as high as 700 hPa (Liu et al., 2002).Evaluation of National Centers for Environmental Prediction–National Center for Atmospheric Researchreanalysis data (Kalnay et al., 1996) demonstrates that in August and September of 1998 the 850 hPa zonalwinds out of the Pacific consistently passed over the Queen Charlotte Islands and the Western Cordilleratowards Lake Athabasca, as during the 31 August 1998 event (Figure 8). This pattern was not nearly aspronounced in 1999 (Figure 8). A similar pattern was observed at 250 hPa (Figure 9), a level commonlyassociated with the jet stream and mid-latitude storm tracks. These data imply that the ‘atmospheric river’transporting moisture from the North Pacific inland referred to by Smirnov and Moore (1999) was in placein 1999, but not necessarily as entrenched as in 1998.

In addition to a consistent supply of moisture from the North Pacific, there needed to be conditionsfavourable to cyclogenesis in order for the rainfall observed at Lower Carp Lake to materialize. The air

Aug to Sep: 1998

Aug to Sep: 1999

0 3 5 8

NOAA−CIRES/Climate Diagnostics Center

NOAA−CIRES/ClimateDiagnostics Center

Figure 8. Mean 850 hPa level zonal wind speeds (m/s) during August and September 1998 (top) and 1999 (bottom)



Aug to Sep: 1999

Aug to Sep: 1998

2 8 16 22

NOAA_CIRES/ClimateDiagnostics Center


Figure 9. Mean 250 hPa level zonal wind speeds (m/s) during August and September 1998 (top) and 1999 (bottom)

temperature differences noted during the 31 August 1998 event were very consistent during that autumn.This created a consistent positive 500 hPa geopotential height anomaly on the Canadian Prairies and aneven stronger negative geopotential height anomaly towards the Yukon and Alaska (Figure 10). The largebaroclinicity in between in the Great Slave region improved conditions for cyclogenesis (Bjornsson et al.,1995). The 31 August storm track through British Columbia to Great Slave Lake followed this predominantarea for cyclogenesis, along the zero contour of the geopotential height anomalies (Figure 10). A comparablezone separating similar warm and cold air in 1999 was located far from the eastern Mackenzie Basin nearthe British Columbia coastline (Figure 10). The geopotential height and wind results from 1999 imply thatthe atmospheric moisture link from the Pacific to the Mackenzie Basin was not as strong as in 1998 becauseit was interrupted by conditions on the coast of British Columbia. The 1999 autumn precipitation along thenorthern British Columbia coast that was 12% higher than normal, and 34% higher than in 1998, providesfurther evidence that this was the case.

6. CONCLUSIONS

A synoptic climatology was constructed for the North Great Slave region using 1998 and 1999 growing-seasonsurface pressure data and weather maps for the purpose of identifying the atmospheric circulations importantin producing autumn rainfall in this region. Seven classifications were identified. Cyclones were found tobe the most important pattern supplying rainfall to the region. Their frequency is dependent upon severalfactors. First, a low-pressure system must be present in the North Pacific that is large enough to provide



Aug to Sep: 1998


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Figure 10. Anomaly 500 hPa geopotential height (m) during August and September 1998 (top) and 1999 (bottom)

moisture inland towards the North Great Slave. Second, there must be a means to deliver this moisture to thearea. In 1998, the mid-latitude storm track was generally just south of the region. Third, some impetus forcyclogenesis is needed. Again, in 1998 conditions were favourable, with significant temperature anomalies inthe troposphere. These were not the conditions in 1999. Strong Aleutian lows were just as frequent in 1999as in 1998, but the mechanisms to transfer moisture from the North Pacific to the eastern Mackenzie Basinwere not in place. Much of the moisture was likely intercepted by the Coastal Mountains of western BritishColumbia.

The results of the synoptic climatology are similar to those from dynamic meteorological studies ofatmospheric circulations above the Mackenzie Basin. The result that North Pacific atmospheric moisturefeeds precipitation in the Mackenzie Basin is not novel. Several previous studies have found the same. Thisresearch is the first, however, to investigate the synoptic influence over precipitation in this particular partof the basin, and focus on a part of the water year that has only recently been identified as particularlyhydrologically important, i.e. the autumn.

ACKNOWLEDGEMENTS

We wish to thank Luigi Romolo, who kindly provided the revised Kirchhofer correlation software writtenby Danny Blair, of the University of Winnipeg. His patience with us as we learned the system was trulyappreciated. Edward Hudson and Steve Knott of the Arctic Storm Prediction Centre, and Ron Goodson andBob Kochtubajda of Environment Canada’s Hydrometeorology and Arctic Laboratory provided exceptionaladvice on earlier drafts of the manuscript. Dave Fox of Environment Canada helped program severaldata manipulation tools. Field assistance was provided by Jennifer Dougherty, Shawne Kokelj, Claire



Oswald, Mark Dahl, Iain Stewart, Bob Reid, and Andrea Czarnecki. Images in Figures 8, 9 and 10 wereprovided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, USA, from their Website athttp://www.cdc.noaa.gov. We wish to thank two anonymous reviewers for their constructive criticism.

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autumn synoptic conditions and rainfall in the subarctic canadian shield of the northwest...

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