Decay Patterns of Fast Sea Ice

in Canada and Alaska

MICHAEL A. BILELLO

ABSTRACT

Weekly measurements of the thickness of fast sea ice made over a period of 10-15 years in Canada and Alaska were analyzed. That portion of the data relating to maximum ice thickness and decay (the decrease in ice thickness) is presented and examined. For example, at Sachs Harbour, N.W.T., maximum ice thicknesses of 160-230 cm have been observed during April or May of each year, and the pattern of subsequent ice decay was similar each year. Average ice thickness curves for seven coastal locations in Canada and Alaska revealed individual patterns of ice decay and major contrasts in the amount of ice and the time of its occurrence because of latitudinal differences.

This study investigates the effects of air temperature and solar radiation on ice decay. Complete and reliable air temperature data for each station made it possi­ble to analyze the relationship between accumulated thawing degree-days (ATDD)

and sea ice ablation. Concurrent values of ATDD (base of 0°C) during 10-day increments of ice decay were compiled for all the years with useful ice data. Linear grid plots of the average decrease in ice thickness versus ATDD for six of the seven stations studied revealed a near straight-line relationship and similar slopes. The results indicate that the equation y = I„, — X, where y = thickness of decaying ice (cm), A' = ATDD (>0°C) and /„, = maximum ice (cm), is a good first approximation for estimating the rate and amount of decrease in sea ice.

Since solar radiation (SR) data were also available for Resolute, N.W.T., the relationship between ice decrease and daily accumulated SR was investigated; the results were comparable to those derived when ATDD was used as the depend­ent variable. Other factors affecting ice ablation and breakup, such as snow-ice formation, snow cover depth, and wind, are also discussed in the study.

313

314 MICHAEL A. B I L E L L O

I N T R O D U C T I O N

Most of the literature on the decay and breakup of ice bodies relates to studies of rivers and lakes for navigational purposes (Zubov, 1945; Burbidge and Lauder, 1957). Few investigations have been made of the rate at which sea ice decreases in thickness. The freshwater ice studies are based on observations of deteriorating ice conditions including some measurements of the decrease in ice thickness, from both the top and the bottom. Theoretical equations similar to those available for sea ice growth have not been applied extensively to studies of sea ice decay. The many physical and mechanical variables which affect sea ice decay, and also the lack of data and observational methods, have prevented proper investigation of the problem.

In a cooperative program between a number of Canadian and U.S. govern­ment agencies, a network of observing stations was established throughout North America in 1956. Detailed information on this program, including the names of the participating agencies, the ice measuring equipment, the names and locations of the stations, and tabulations for 14 consecutive winters, is available in a series of Canadian (Department of the Environment, Canada, 1963-74), and U.S. (Bilello and Bates, 1961-75) reports. This network provides reports of weekly ice thickness measurements and surface ice conditions along the seacoasts of North America. Although no emphasis was placed on continuing ice thickness measurements during the ablation period, sufficient data have been collected to justify an analysis of maximum ice thickness and decay for a number of coastal locations in Canada and Alaska.

MAXIMUM ICE T H I C K N E S S AND DECAY CURVES

Data on maximum ice thickness and decay were assembled for the network of stations previously described. Although 24 sea ice locations in Canada and 6 in Alaska are contained in the network, only 7 sites (Cartwright, Coral Harbour, Eureka, Mould Bay, Resolute, and Sachs Harbour, Canada, and Kotzebue, Alaska) are studied here. They were selected on the basis of sufficient data, contrasting ice conditions, and geographical distribution (Fig. 1). A north-to-south variation is provided by the stations at Eureka, Coral Harbour and Cart-wright, and an east-to-west distribution by Resolute, Sachs Harbour, and Kot­zebue. Included in the tabulated data extracted from the records are maximum ice thickness at the end of each winter, the subsequent decrease in thickness during the period of ice decay, the depth of snow on the ice, surface conditions during breakup, the dates on which vehicular traffic across the ice ceased, and the dates on which the body of water became clear of ice. Some of the information in the last two categories was obtained from a Canadian Meteorological Service report on ice freezeup and breakup dates (Allen and Cudbird, 1971). A sample tabula­tion for 5 of 16 years of record at Sachs Harbour is shown in Table 1. (Similar tables for 30 salt or brackish water bodies and 36 lake and river sites in Canada

Decay Patterns of Fast Sea Ice in Canada and Alaska 315

Figure 1. Station map.

and Alaska are available from the author.) The additional information included in these tables, such as weather conditions, can often be used to explain anomalies in ice deterioration and breakup.

A plot of ice thicknesses at Sachs Harbour from 1959 through 1974 is shown in Figure 2. Although large variations in maximum ice thickness (160-230 cm) are observed during April or May of each year, the pattern of subsequent ice decay is similar throughout the years of record. The fast sea ice at Sachs Harbour begins to decay in late May or early June and ablates rapidly through June and early July; the harbor is generally clear of ice by about 20 July.

Latitude is one obvious condition that determines how thick the fast sea ice grows and when the body of water becomes clear of ice. This factor can best be shown by comparing Eureka (80°N), Coral Harbour (64°N), and Cartwright (54°N). Instead of annual curves of the decrease in ice thickness, as in Figure 2, upper and lower boundaries of the observed thicknesses (i.e., envelope curves) are drawn (Fig. 3). Sea ice accretion at Eureka is greater than twice that at Cartwright, and maximum ice thickness occurs from four to six weeks later at Eureka. Ice deterioration and ablation are observed during June and July at Eureka, in contrast to April and May at Cartwright. The envelope curve for Coral

316 MICHAEL A. BILELLO

TABLE 1 MAXIMUM ICE THICKNESS AND SUBSEQUENT DECAY, SACHS HARBOUR, N.W.T., CANADA

Date Ice Thickness (cm) and Conditions Snow Depth (cm)

1961 May 19

26 27

June 2 5 9

16 22 23

July 6

1962 May 25

25 June 1

1 8

10 12

July 2

1963 May 24 June 7

14 14 21

July 1 12

229. 226.

Open water visible on horizon. 224.

Ice conditions becoming unsafe for vehicles. 185. 152.

Ice breaking up due to strong SE winds. Some ice moving in harbor. Harbor clear of ice.

183. Lead 8 km south of station, width variable.

185. Snow drifts 23 cm in height.

170. Ice conditions becoming unsafe for vehicles. Ice cover starting to deteriorate. Harbor clear of ice.

207. 199. 152.

Surface 50% puddled. Ice unsafe, numerous deep puddles and cracks Ice starting to break up. Harbor clear of ice.

30.

10.

Harbour (Fig. 3) reflects changes in ice conditions that lie between those of the other two sites.

A comparison of maximum ice and subsequent decrease in thickness for the seven locations was made by plotting the average curve for all years of record at each station (Fig. 4). A minimum of three years and maximums of from nine to fifteen years of record were included in the averages. Although each station displays individual decay patterns, the ice decrease curves for four of five sta­tions in the Northwest Territories of Canada (Eureka, Mould Bay, Sachs Har­bour, and Coral Harbour) are similar. The ice reaches its maximum thickness at these stations in late May, the ice sheet deteriorates and some ablation occurs in early June, melting proceeds rapidly in late June and early July, and, on the

Decay Patterns of Fast Sea Ice in Canada and Alaska 317

TABLE 1 (continued)

Date

1965 May 28 June 18

25 25

July 8 18

1966 May 13

20 25 27 27

June 3 11 19 25

July 15

Ice Thickness (cm) and Conditions

188. 175. 163.

Shore lead about 2 m wide and 400 m long. Ice cover deteriorating rapidly. Harbor clear of ice.

196. 191.

Ice conditions becoming unsafe for vehicles. 182.

Few cracks observed in ice sheet. 178. 163. 100. 97.

Harbor clear of ice.

Snow Depth (cm)

8. 0 0

8. 0

3.

0 3. 0 0

average, the ice clears out between 10 July and 10 August. The exception to this trend is Resolute, N.W.T., where the sea ice sheet deteriorates through most of June before the onset of rapid melting.

No east-to-west variation in the sea ice decay patterns in northern Canada can be discerned in Figure 4. However, differences in maximum ice thickness are revealed, even at latitudes between 72°N (Sachs Harbour) and 80°N (Eureka). A marked variance is also noted between two stations at similar latitudes, Kotzebue (67°N) and Coral Harbour (64°N). The difference in ice conditions at these sites, again, is probably associated with local environmental conditions. Like Cartwright, Kotzebue has higher winter air temperatures (Hogue, 1956) and possibly lower sea water temperatures than Coral Harbour. Figure 4 can therefore serve as a good forecasting tool for predicting fast sea ice conditions during the season of maximum growth and decay. The diagram provides a good indication of how thick the shore ice becomes at widely separated locations in the North American Arctic, how fast the ice can be expected to decay and erode, and when the bodies of water generally become clear of ice.

DECREASING SEA ICE T H I C K N E S S AND T H A W I N G AIR T E M P E R A T U R E S

Many meteorological and oceanological factors, such as air and water tem­perature, solar radiation, cloudiness, snow cover, albedo, tides, and currents, participate in the ice deterioration and melt process. The following problems are

318 MICHAEL A. B I L E L L O

740

230

220

2Î0

200

- i — | — i | i | — i | r " i — i — r

Sachs Harbour, MWT

_ ^ i L

61 63 59 65

_l L J L

Figure 2. Ice decay curves for Sachs Harbour, N.W.T., Canada.

encountered in any attempt to develop models designed to simulate the event: (1) Data to determine the relative importance of many parameters are insufficient or unavailable. (2) A complete model would have to include the effects of both the thermal regime and mechanical disruptions, as, for example, ice breakup due to wind. (3) In regard to thermal modeling, it is important to first determine whether the principal erosion is taking place at the top or at the bottom of the ice sheet.

Sea ice temperature profiles made on Slidre Fiord, Eureka (Fig. 5), show that when the ice begins to deteriorate (late May or early June) the temperatures within the 200-cm-thick ice are nearly isothermal (Bilello, 1965). Since the

Decay Patterns of Fast Sea Ice in Canada and Alaska 319

temperature at the water/ice interface remains at or near freezing in Arctic re­gions, it seems logical to assume that most of the melting occurs at the top of the ice sheet. Warm ocean currents washing against the bottom of an ice sheet in late spring may also cause erosion. However, this phenomenon may be less important

LAND FAST SEA ICE

•M"

200'

150 •

100 '

50-

Q

EUREKA/

V-CORAL HARBOUR /

L^ C A R T W R I G H T ^ X .

^ \

March | April j May

\ — \

m. st K \ N

\ June

v \

A IN i m July

Figure 3. Envelope of ice decay curves for three locations in Canada.

Figure 4. Average ice decay curves for Cartwright (C), Coral Harbour (CH), Kotzebue (K), Eureka (E), Mould Bay (MB), Resolute (R), and Sachs Harbour (SH).

320 MICHAEL A. B I L E L L O

WATER TEMPERATURE

SEA-ICE TEMPERATURE CURVES, 1950-51

Figure 5. Sea-ice temperature curves 1950-51, Slidre Fiord, Eureka, Canada, Air tem­peratures taken from a thermoscreen at standard height at Eureka weather station.

in fast sea ice sheets than in pack ice. Such critical information needs to be obtained before useful heat balance equations can be developed. During the ice growth stage, the heat flux is unidirectional (vertical), which allows for the development of a model similar to that presented by Untersteiner (1964) for the Central Arctic.

As a first step in the development of ice decay models, this study empirically investigates the contributing effects of two parameters: air temperature and solar radiation. Reliable records of these meteorological parameters are available; they also are elements that generally can be forecast with some confidence and are key factors in the decay process. Other environmental conditions, such as snow depth and wind effect, will be noted when their influence becomes apparent.

In a previous paper (Bilello, 1961) the observed decrease in ice thickness and concurrent accumulated thawing degree-days (ATDD) were investigated for four sea ice locations in the Canadian Archipelago. (Accumulated thawing degree-days (base 0°C) are obtained by summing the daily difference between positive mean air temperatures and 0°C.) A least-squares computation on 29 sets of obser­vations yielded a correlation coefficient of 0.93 in the equation h — 0.55 20, where h = decrease in thickness (cm) and £# = accumulated degree-days above — 1.8°C. These results were compared with a similar Soviet study by D. B. Karelin as quoted by Armstrong (1955); apparently the Canadian Arctic requires more accumulated degree-days than the Russian Arctic to produce the same amount of ice decay. This study investigates the relationship further and in­cludes more stations and a wider geographic region of the North American Arctic.

After discussions with other CRREL colleagues (W. F. Weeks and G. D.

Decay Patterns of Past Sea Ice in Canada and Alaska 321

Ashton, personal communication, 1977) it was decided that ATDD with a base of 0°C would be better than ATDD with a base of — 1.8CC. There are two principal reasons for taking this approach: (1) the salt (or brine) content in the sea ice is reduced drastically during the deterioration and melt stage; and (2) fresh water undoubtedly exists at the top of the ice sheet and perhaps occasionally beneath a fast sea ice sheet that is melting. Any fresh melt water reaching the bottom of the sea ice sheet which comes in contact with sea water that is just below freezing would tend to accrete new ice. Of course, for calculations on the growth of sea ice, accumulating freezing degree-days below — 1.8CC would probably be more nearly correct.

Because equipment to measure temperature continuously is only rarely in­stalled on the sea ice sheet, average daily air, temperatures were used from weather stations adjacent to the bodies of water under study. These meas­urements are taken by trained observers and undergo reliable quality control processes before being published (Dept. of Environment, Canada, 1959-1974; U.S. Dept. of Commerce, 1962-1974).

Assuming that most melting of the fast sea ice sheet in Arctic regions is due to the heat flux occurring at the snow/air or ice/air interface, the relationship was investigated between 10-day increments of ATDD and the corresponding amounts of ice decrease. Intervals of 10 days allow for the effects of a time lag between the variables and are necessary to provide some smoothing of the data. The ATDDS for day 10, 20, and 30 (or 31) of each month during the ice ablation period were used. Shorter or longer intervals were used when ice observations occurred between these dates or when weekly ice measurements were not available.

The results of this analysis for Resolute (Fig. 6) show that except for two of the ten years of record analyzed (1967 and 1971) the relationship appears good. For most years, the relationship between rate of ice decay and ATDD can be defined by two lines of different slope. From the start of ice decay until about 90-100 cm of ice have melted at Resolute the average ablation rate is approximately 9 cm per 10 ATDD. After about 90-100 cm of fast sea ice has melted, the average ablation rate becomes approximately 17 cm per 10 ATDD.

The ice ablation rates at Resolute differ markedly during 1967 and 1971 (Fig. 6). Investigation of the weather conditions during the ice growth and decay period in these two seasons shows that air temperatures did not depart appreci­ably from the normal, but that during 1970-71 the snow cover was from two to four times as deep as it was in 1966-67 at the beginning of winter, and almost twice as deep (48 versus 28 cm) at the beginning of ice deterioration in May. The extreme variation in the rate of ice ablation for these two seasons, therefore, can be attributed to the difference in the amount of snow on the ice, and this factor should be considered in any expanded model of sea ice decay.

Using the results shown in Figure 6, an average curve for the ten years of record was drawn (Fig. 7). Similar curves compiled for the other six stations studied are also shown in Figure 7. Although there are some irregularities, the decrease in fast sea ice and ATDD on the linear grid paper shows a near straight-

322 MICHAEL A. B I L E L L O

line relationship for most of the stations. The rate of decrease was more closely evaluated by computing the average values obtained during each year for every station (Table 2). Since ice thickness measurements during the ablation period at some stations are incomplete (e.g., at Mould Bay) the number of years that can be analyzed for some stations is reduced. Except for some cases (Eureka in 1967, Kotzebue in 1968, Cartwright in 1972) the slope of the line defining rate of ice decay with respect to ATDD is rather uniform (Table 2). Further investigation of these anomalies would be useful in determining the relative influence of other meteorological or oceanological parameters in the decay process.

In 49 of the 55 years studied, the rate of ice ablation ranged from about 6 to 12 cm per 10 ATDD. An average value, then, of 9.8 cm or, for purposes of sim­plification, 10 cm of ice decrease for every 10 ATDD seems reasonably accepta­ble. Therefore, the equation y = /„, — X, where y = thickness of decaying ice (cm),X = ATDD (base 0°C), and/,,, = maximum ice thickness (cm), would serve

50 100 150 200

Accumulated Thawing Degree Days (Base 0°C)

Figure 6. Relationship between rate of ice decay and accumulated thawing degree days, 10 years of record, Resolute, Canada.

Decay Patterns of Fast Sea Ice in Canada and Alaska 323

»00

150

0 0

50

1

.E

- S h K \

\K

sC ^

— \

1

1 ' 1

1 , 1

1 1

1 1

1

. \ R

CH

1

^ M B

1

1

\ ^ E

i

-

-

-

-

100 150 2 0 0 Accumulated Thawing Degree Days (8ase0°C)

250

Figure 7. Relationship between average rate of ice decay and accumulated thawing degree-days (see Fig. 4 for station names).

as a good first approximation for estimating the rate and amount of decrease in sea ice as a function of air temperature.

DECREASING SEA ICE THICKNESS AND O T H E R C L I M A T I C PARAMETERS

As noted earlier, other meteorological elements contribute to ice deterioration and melt, but the relative importance of these factors is difficult to evaluate. For example, the important role of solar radiation in the process would necessitate continuous measurement of (1) long-wave incoming radiation, (2) short-wave back and reflected radiation, (3) the penetrating effects of radiation through snow, snow-ice, and sea ice, and (4) the resultant effects of surface erosion including ponds of melt water. Very little information of this kind exists for sea ice sheets that are rotting rapidly and are also subject to movement and breakup.

As an alternative, incoming daily solar radiation (ISR) data available for one station (Resolute) were related to the decrease in ice thickness. Since meas­urements of ISR are not affected by the surface on which it falls, the values recorded at the weather station would be about the same as those over the sea ice. The same computations used to obtain the curves in Figure 6 were used here; that

324 MICHAEL A. BILELLO

TABLE 2 AVERAGE RATE OF DECREASE IN FAST SEA-ICE THICKNESS AS RELATED TO

ACCUMULATED THAWING DEGREE-DAYS (ATDD) (cm of ice/10 ATDD; base 0°C)

Year

1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1974

Cartwright

9.2 12.4

11.8 11.5 8.0

10.4

19.4 11.7

Coral Harbour

9.4 8.3 9.5

11.4 6.3 8.8

13.9 8.5 8.7

8.7

8.8

Eureka

8.1

6.7 6.2

10.9 12.3 15.8 9.2

10.3

Kotzebue

11.6 9.7

5.0

15.2 9.9 6.7

Mould Bay

9.8

6.8

8.2

8.2

Resolute*

6.0 9.5 7.9 7.8

10.4 12.3 8.6

10.7 6.9 5.9

Sachs Harbour

11.2 11.6 11.7

8.5 9.2

13.5 11.6 9.8

* Values for Resolute pertain to the first 100 cm of ice decrease only.

is, concurrent values of accumulated ISR and ice decrease over 10-day intervals for the same years of record were used for Resolute. This was done in order to compare the resultant curves with those in which ATDD was used as the dependent variable.

The outcome is shown in Figure 8. The relationship between accumulated ISR and ice decrease is similar to that shown in Figure 6, except that the change in slope of the two nearly straight lines in Figure 8 is more pronounced and the inflection point occurs after about 70 cm of ice has melted. Notice that the same two years (1967 and 1971) discussed in Figure 6 appear as anomalies in Figure 8.

According to this comparison, attempts to use ISR data instead of ATDD appear fruitless, unless it can be shown that in specific years ISR values during the melt period were abnormally high or low because of clear or cloudy conditions. Of course, if forecasting reliable ISR values is far easier than forecasting air tempera­tures, then use of the relationship shown in Figure 8 may be advisable. It is possible that combining the effect of both air temperature and ISR would provide better results than considering them separately. Evaluation of a pilot study on this type of multivariable relationship for data from two stations in Canada provided a slight improvement in the results for Goose Bay, Labrador, and a negligible effect for Resolute, N.W.T.

Wind action at any particular location can be a critical component in the breakup and ice clearance process. For example, in Sachs Harbour the fast sea

Decay Patterns of Fast Sea Ice in Canada and Alaska 325

ice sheet appears to be affected by winds. Note that in Figure 2 a dashed line defines the decrease in ice thickness in 1959. This estimate line was necessary because on 14 July the observer noted that strong north and northeast winds had cleared the harbor of all ice except for a small patch near the shore. Another relatively thick ice sheet (152 cm) observed in Sachs Harbour on 16 June 1961 moved out of the harbor abnormally rapidly (see Fig. 2). Investigation of the weather revealed that the daily air temperature for June 1961 was 2.3°C above normal and, according to the ice observer (Table 1), strong southeast winds on 22 June 1961 helped to break up the ice sheet even though this particular wind direction is on-shore or perhaps parallel to the shoreline.

CONCLUSION

This study attempts to evaluate the importance of certain meteorological parameters that are known to contribute to the decrease in landfast sea ice thickness. Naturally, the changing combination and intensity of these (and other) environmental factors would alter the pattern of ice decay from year to year.

250] 1 1 1 1 1 1 1 1 r

50 100 150 200 250

Accumulated Solar Radiation (!Oslangleys)

Figure 8. Relationship between rate of ice decay and accumulated solar radiation, Reso­lute, Canada. One langley (the unit of radiation) equals 1 gram calorie per square centi­meter.

326 MICHAEL A. BILELLO

Also, disruptive factors and unknown quantities such as wave action, flooding, and ocean currents can produce erratic patterns at specific sites. However, the results shown here provide useful information on the relative significance of some key parameters and can form a basis for predicting the rates and amounts of ice decay on fast sea ice sheets.

ACKNOWLEDGMENTS

This work has been supported by Corps of Engineers, USACRREL In-House Laboratory Independent Research (ILIR) funds.

Appreciation is extended to the following CRREL personnel: Mary Lynn Brown for her assistance in extracting, calculating, and plotting the data; Dr. Anthony Gow for his technical review of the study; Ruth Barrup for preparing the final manuscript; and Matthew Pacillo, Thomas Vaughan, and Robert Demars for preparing the figures.

REFERENCES

Allen, W. T. R., and B. S. V. Cudbird. 1971. Freeze-up and break-up dates of water bodies in Canada. Canadian Meteorological Service Report CLI-l-71, Toronto, Canada.

Armstrong, T. 1955. Soviet work on sea-ice forecasting. Polar Record, 7(49), 302-11. Bilello, M. A. 1961. Formation, growth and decay of sea ice in the Canadian Arctic

Archipelago. Arctic, 14(1), 2-24. Bilello, M. A. 1965. Sea-ice temperature curves for Slidre Fiord, Canada. CRREL Inter­

nal Report 332, Hanover, N.H. Bilello, M. A., and R. E. Bates. 1961-1975. Ice thickness observations, North American

Arctic and Subarctic, 1958-59 through 1971-72. CRREL Special Report 43, pts. 1-7, Hanover, N.H.

Burbidge, F. G., and I. R. Lauder. 1957. A preliminary investigation into break-up and freeze-up conditions in Canada. Meteorological Division, Dept. of Transport, Canada, Circular 2939, Technical 252.

Department of the Environment, Canada. 1959-1974. Monthly Record, Meteorological Observations in Canada, Atmospheric Environment, Toronto, Canada.

Department of the Environment, Canada. 1963-1974. Ice thickness data for selected Canadian stations, freeze-up to break-up. Ice Circulars, Toronto, Canada.

Hogue, D. W. 1956. Temperatures of northern North America. Research Study Report RER-9 (rev. Oct. 1957), U.S. Army Quartermaster Research and Engineering Center, Natick, Mass.

U.S. Department of Commerce. 1962-1974. Climatological Data, Alaska. National Oceanic and Atmospheric Administration, Environmental Data Service, Asheville, N.C.

Untersteiner, N. 1964. Calculations of temperature regime and heat budget of sea-ice in the central Arctic. Journal of Geophysical Research, 69, 4755-66.

Zubov, N. N. 1945. L'dy arktiki [Arctic ice], 360 pp., Glavsev-morput (Northern Sea Route Administration), Moscow.