Sea Ice Climatic Atlas for the Northern Canadian Waters 1981-2011: chapter 2

The Ice Regime

In northern Canadian waters, ice is normally present in many areas throughout much of the year. In some sectors, much of the ice does not melt completely each year. Thus, for example, in the Arctic Ocean the differences between a typical chart showing the ice cover in summer and one in winter are the ice concentration and presence of openings in the pack and around the coastline. In the Canadian Arctic Archipelago, the period when air temperatures reach above freezing is very brief, so freeze-up can begin as early as August.

Ice in the sea is at the interface between the ocean and the atmosphere, and the ice may impact or attach to the land. The ice  can have an enormous impact on human activities - either enabling them, or hindering or preventing them. Ice can also reduces the direct atmosphere-ocean interactions - so the ice makes a difference to the atmosphere as well as the other way around. The understanding of ice and its interactions with the atmosphere and the ocean is a fascinating and complex study.

Factors affecting ice in the sea

Atmospheric Factors

Sea ice forms largely as a result of removal of thermal energy from the sea and is lost principally by addition of thermal energy from solar radiation. Variations in these energy transfer processes are largely controlled by atmospheric events.

The most significant heat removal process is the evaporation of water into the atmosphere,. Roughly, the rate of heat energy removal by the atmosphere is proportional to the difference in the temperatures of the water and the air over it, and also to the rate at which the water vapour can be removed from the interface, basically related to wind and atmospheric stability. In practice, the air temperature and knowledge of its changes can be used to estimate a date for beginning of ice formation. If there is a  fairly complete ice cover, further thickening of the ice continues from radiation losses from the upper ice surface. Snow can accumulate on top of the ice and can provide insulation, reducing the loss of heat and so variations in the snow cover can have a significant effect on the growth in the thickness of the ice. In the absence of a snow cover, air temperature alone can be used to give a reasonable estimate of the thickening of the ice throughout the winter.

The ice is open to the action of winds and water currents as long as it isn’t “stuck” to the land, or “shore-fast.” Complex calculations can be done to estimate the dynamic interactions of the forces of air and water, as well as internal forces within the ice itself. For all practical purposes, free-floating ice will respond very quickly to any change in the water motion around it. On the other hand, the response by ice to the force of the wind takes time because of the great density difference between air and ice. The component of ice motion due to the wind is similar to the wind-driven current in situ - in fact open drift ice and the roughness of the ice surface can contribute to the development of a wind-driven current.

After ice forms along a coastline, cold seaward-moving winds often drift the ice farther away from the coast. The ice will either melt or continue to thicken, depending on whether heat energy is available in the water.

For most of the area depicted in this atlas, the ice coverage grows in the fall and early winter, reaching a limit where the thermal energy available in the oceanic water column does not permit further expansion. The ice conditions then remain much the same for several months, although there would be changes in the details.

In the spring, the main heat transfer process operating is radiation. The increasing height of the sun in the sky allows solar radiation to add heat energy to the water just as the intensity of cold air incursions and evaporative heat loss diminishes. Melting of the snow begins, and increasing incursions of warmer air allow a net positive balance in thermal energy at the surface. Puddles from the melting snow develop on the surface of the ice. The puddles are much more effective at capturing incoming short-wave radiation than ice and snow, hastening the melt process. Similarly, where there is open water, such as a polynya or a shore or flaw lead, there is also greatly enhanced absorption of incoming radiation. This warmed water moves with the tides and currents and the heat energy also transfers to the bottom surface of adjacent ice. Thus, polynyas act as centres around which the break-up process spreads. Once the ice has warmed up to the melting point, it too can begin to melt. The temperature of the ice and the water beneath it essentially remains at the melting point until the ice is gone. Also, as the ice warms up, it begins to shrink, and internal stresses develop within the ice. This process is amplified wherever there are discontinuities in the ice, and cracks and openings are created which can be acted on by waves, currents, winds and tides to initiate further break-up of the ice sheet.

Oceanographic factors

As noted above under atmospheric factors, ice can begin to form once sufficient thermal energy is removed from the water. How much cooling is necessary before ice can form depends on the characteristics of the water column. As long as the water being cooled at the surface is denser than the water below it, there will be upward mixing of warmer water, and ice does not form, barring exceptional circumstances.

Similarly, the ice melts if the wind pushes it into warmer waters. The ice cools the surface water then convective overturning in the water column brings warmer water back in contact with the ice, and melt continues. If ice incursions into the warmer water continue, and the water is shallow enough, the whole water column becomes cooled and a new edge will become established.

Currents in the ocean are very important in understanding where ice may form, and then how it moves. Because of the small (about 10%) difference in density between ice and water, ice will respond very quickly to a change in the current. Water movements near shore are strongly affected by tidal motions and surface water runoff variations, as well as local winds.

The principal driving force for the circulation of water in the area covered by this atlas is the North Atlantic current system. Density and wind driven, the Gulf Stream and its extension, the North Atlantic Drift, moves vast quantities of water between Iceland and Scandinavia into the Arctic Basin. After circulating in the Arctic Ocean, most of this excess water exits the Arctic Ocean via the East Greenland current, aptly named, which also moves heavy Arctic pack ice southward between Greenland and Iceland. Much of this ice melts but some of it continues westward past Cape Farewell and then northward again in the north-flowing West Greenland Current before melting completely. Some of this current turns westward in Davis Strait and some of it continues northward, into Baffin Bay, making a large counterclockwise gyre moving at about 10 to 20 km per day. In northwestern Baffin Bay, this gyre is joined by almost all of the remaining volume of outflow from the Arctic Basin, which has filtered through amongst the islands of the Canadian Arctic Archipelago or through Nares Strait. The augmented southward flowing portion of the Baffin Bay gyre reaches Davis Strait, making as much as 20 to 30 km per day and accepts some West Greenland waters as described above before becoming the Labrador Current. The main Labrador Current has two branches, the current from Baffin Island which is the most fresh and close to shore at about 10 km per day, and the outer portion from West Greenland. At about 100 km from the coast its rate is about 20 to 30 km per day.

From northern Baffin Bay to southern Labrador Sea, the long term average ice motion may be generally described as following the shoreline at about 10 to 15 km/day. Variations in wind speed may increase this motion or stop it entirely for short periods. If an average speed of 15 km/day is maintained, multiyear ice off Devon Island at the beginning of October would arrive near the mouth of Hamilton Inlet about mid-February. This agrees with dates of aerial ice reconnaissance reporting older ice in the area.

Offshore to the northwest of the Canadian Arctic Archipelago, there is a slow, broad southward-setting current, which gradually turns westward across the northern portions of the Beaufort Sea. However within and adjacent to the Archipelago, it could be said that each major island or island group has a clockwise current around it. Because of the net transport southward through the Archipelago, and for dynamic reasons, the southward and eastward- portions of these currents are both broader and stronger than the other portions.

In the shallow waters of Hudson Bay, there is a counterclockwise gyre, driven partly by winds, partly by runoff, which flows out along the south side of Hudson Strait and joins the inner section of the Labrador Current.

Along most coastlines, the ice can become attached to the land (shore fast) and can become extensive. However, the seaward extent of fast ice will be limited if tidal action is strong, and fast ice is generally within the shallower areas. Unless a body of water is very wide, or water motions strong, ice can form a continuous cover from shore to shore, such as amongst the islands of the Canadian Arctic Archipelago. In the broader or more dynamically active channels, the location of the edge of the shore fast ice can differ markedly from month to month and vary from year to year. In most of the channels the shore to shore fast ice breaks in the summer but the northern section of Nansen Sound remains fast for most years.

Land Geography

The topography of the land has an impact on the ice since it affects the behaviour of surface winds, and in some cases even causes winds. During the colder season, over higher terrain or glaciers, very strong drainage winds can develop, affecting near shore ice. For certain atmospheric stability conditions, funnelling can cause severe wind events, and in some cases even break up a consolidated ice area.


The continental shelf is the most significant single feature of the ocean bottom that affects Canadian ice regimes. Off eastern Canada the shelf extends out to about 300 km off the coast abeam the Strait of Belle Isle and gradually narrows northward to 130 km wide at approximately 56 N, and then expands to about 200 km off Cape Chidley and Cape Dyer. A submerged ridge extends from the coast of Baffin Island to Greenland at about latitude 66 N. Seaward of this line, the deep waters provide a reservoir of heat energy which can readily reach the surface and melt any ice incursions. Such a large heat energy reservoir is not available in any of the other waters depicted in this atlas.

However, waters are fairly shallow in eastern Foxe Basin and in much of the western waterways. The continental shelf in the southern Beaufort Sea is 100 km wide, except near Barter Island and Herschel Island where the shelf break is less than 50 km from shore. Very shallow waters extend as much as 20 km offshore and sea ice is often grounded.

In the Canadian Arctic Archipelago, depths are generally in excess of 100 m. However, the waters around King William Island are well known for being shallow.

General Description of the Ice Regime


The following is a general description of the ice breakup season for a typical year. During the winter, frigid air masses develop over continental areas, then weather systems move the cold air over the adjacent seas. In spring, as the sun's elevation in the sky increases, and the land warms up, the cold winter blasts diminish rapidly in intensity. In southern portions of the area of this atlas, ice formation stops, but on average, winds continue to drift the existing ice towards warmer waters where convection in the water column can always bring a supply of warmer water to melt the ice. So the first signs of break-up appear in southern Labrador waters and in James Bay near the end of April. Break-up gradually spread northward during May and June. At the same time in areas of consolidated ice, puddling of the melted snow cover begins while the thin ice in polynyas disappears. In June, decay has begun throughout the atlas area. Because of absorption of solar heat by polynyas, particularly the North Water, and also the northwestern portions of Hudson Bay and Foxe Basin, decay and break-up also spread southward and eastward from these areas in June and July. At the end of the typical melt season, usually early September, high concentrations of ice are present in Nares Strait, Norwegian Bay, Queens Channel, Viscount Melville Sound, M'Clintock Channel and Victoria Strait. The Arctic Ocean pack lies 50 to 100 km off the coast in the Beaufort. Also ice usually remains in Committee Bay and southern Gulf of Boothia.

However, it is worth emphasising that in many years, not all the ice will melt in other areas, notably Foxe Basin and northwestern Davis Strait. Only James Bay, the southern two thirds of Hudson Bay, and the Labrador Sea always clear completely of sea ice.


In August, summer comes swiftly to an end in the areas north of Parry Channel. Around the lingering floes from previous winters, new ice is able to form almost as soon as air temperatures drop below the freezing point. This new ice thickens rapidly so by early October, first-year ice from the new ice season is mixed with first-year ice remaining from the previous winter. On the 1st of October, first-year ice remaining from the previous winter is reclassified as second-year ice. It will be nearly salt free, and much harder than the recently formed ice. In December, the first-year ice normally becomes a consolidated sheet with embedded multi year and second year ice. This old ice is often predominant in the Canadian Arctic Archipelago except around Baffin Island. The rest of the Atlas area becomes encumbered with ice moving with weather systems and currents, except for offshore portions of the Labrador Sea.

Fast ice becomes well established along the Baffin Island, Greenland and Labrador coasts. The width of this fast ice may reach 50 km at times in some areas. Offshore, the pack remains mobile throughout the winter and floes ranging from small to vast in size are repeatedly frozen together and broken apart.

Arctic sea ice carried by the east Greenland current rounds Cape Farewell (southern tip of Greenland) in January, reaches its maximum extent near 63 N in May, but disappears from waters west of Cape Farewell in August. This sea ice is normally located within 100 km of the Greenland coast.


Wide variations in ice conditions can occur from one year to the next for the same date, and in some areas, from week to week. Furthermore, the entire nature of the ice cover may differ from year to year. For example, Amundsen Gulf ice remains light and mobile in some years: in others it consolidates, sometimes with embedded old ice. A warm summer in the High Arctic results in greater old ice break-up in the Sverdrup Basin, giving heavier ice the following spring and summer in Parry Channel. Parry Channel consistently develops a consolidated ice cover in western Barrow Strait, but the eastern edge may lie at Bylot Island or at Somerset Island, or most anywhere in between, and break up and re-form more than once during the winter season. Similar variations occur in the timing of consolidation in Nares Strait, but the extent of consolidation there is remarkably consistent. The width of the pack off Labrador and in Davis Strait is sensitive to extended periods of on-shore or off-shore winds.

The Total Accumulated Coverage (TAC) is defined as the geographical area multiplied by the ice concentration and is an index used to compare ice severity. For this atlas we compare the Total Accumulated Coverage (or ice conditions) for each year of the atlas for the period June 25 to October 15. 2010 was the year with the minimal ice coverage for the Northern Canadian waters as a whole while 1983 was the year with the maximal ice coverage. The graphic depicting the variations of the Total Accumulated Coverage over the 30 year period is included in this atlas.  

Ice Thickness

During the course of a single winter in northern portions of the Canadian Arctic Archipelago, undisturbed bare ice can grow to a maximum of about 240 cm. In the central and western Arctic, maximum thickness is about 200 cm. Farther south, in James Bay and along the Labrador coast, the thickness of locally developed ice can reach about 120 cm.

Multi-year ice found in the Archipelago reaches a thickness of 300 to 450 cm. However Ice Shelf fragments can be as thick as 2000 cm. The ice shelves consist of fresh-water and sea-water ice, formed over many years along the northwestern shore of Ellesmere Island. Some pieces of the shelf there have broken off in recent years, and these very distinctive ice features are occasionally found far from their point of origin. They are much like tabular icebergs, except not formed from snow.

Old Ice

The presence of old floes within an area of predominantly first-year ice has a direct impact on the penetrability of an ice area even for the most powerful ships. For this reason, charts examining the concentration and frequency of occurrence of old floes are included in this atlas.

In September, there may be some old ice present from earlier years, some first-year ice from the previous winter, which has failed to melt, and also recently formed ice, which is at the first-year stage of development by the end of the month. Although second and multi-year ice are difficult to identify separately at any time, it is useful to separate these three ice types which have different hardness. For this reason any first-year ice which survives to October 1st  is promoted to second-year ice on that date. Thus, there is an increase in the amount of old ice in the October charts due to this promotion.

In May, the median concentration of old ice indicates an elongated area of 1 to 3 tenths of old ice in south-western Baffin Bay extending southward to western Davis Strait. This pattern doesn’t change much through June to mid-July. This old ice melts after mid-July and the area is for the most part free of old ice in August through the fall.

At first, one might think that increasing old ice amounts during the melt season is not correct, but what occurs here is a melting of the thinner forms of ice, allowing the old floes to accumulate in an area rather than being dispersed through the pack.

Two series of charts show the percent frequency of presence of old ice: in the 4 to 10 tenths range; and in the 1 to 10 tenths range. Note that traces are not considered in these charts. There is always a small chance of 1 tenth or more of old ice in western Davis Strait and almost all of Baffin Bay at all times of year

In Foxe Basin, the median amount of old ice never rises above zero except in the Igloolik-Fury and Hecla Strait area, but it is evident from our climate data that old floes do infest many sectors of the Basin. The increase in the frequency of old ice in October (but not its amount) identifies areas where clearing did not occur by the end of September.

Both the amounts and frequency of occurrence of old ice are notable in southern Gulf of Boothia and Committee Bay, as well as in M'Clintock Channel, Larsen Sound and Victoria Strait.

The median old ice concentration in western Barrow Strait lies in the 1 to 3 tenths range, but bumps up to the 4 to 6 tenths range with the October 1st ice promotion. The charts depict the heavier concentrations generally present along the south side of Parry Channel in the summer months.

In Sverdrup Basin, old ice is usually predominant. However in warm summers, break-up can leave large areas where first year ice will predominate in the following year. In Norwegian Bay, old ice concentrations and frequencies are lower in eastern sections, as low as the 1 to 3 tenths range. In Eureka sound, small amounts of old ice usually persist through the melt season.

In the Beaufort Sea the Arctic Pack of the Arctic Ocean is a dominating feature. As might be expected, both the amount and frequency of occurrence of old ice increases with the distance from the coast. Except within the shallows of the Mackenzie Delta there is always a small percentage frequency of old ice. In fact, as the first-year pack near the coast melts out in the summer, incursions of old ice increase the percentages near the coast.

Regional Details

Some of the regions below do not have clearly defined  boundaries. For purposes of this document, the Labrador coast includes the waters between the Strait of Belle Isle and the approaches to Frobisher Bay. To its north, Davis Strait has a northern boundary to the north of Home Bay, and Baffin Bay lies to the north of Davis Strait. Other areas referred to are fairly clear geographical entities.

Labrador Coast

As spring temperatures rise, melting normally begins in southern Labrador waters around the end of April, reaching the Resolution Island area about mid-June. The pack slowly narrows and loosens, and the southern ice edge retreats from the Strait of Belle Isle to north of the approaches to Hamilton Inlet in June and the approaches to Hudson Strait and Frobisher Bay in July, although patches of ice may linger through August.

A small percentage of old ice is usually present within the Labrador pack. After all of the level first-year ice has melted at the end of the ice season, there is nothing but ridge remnants and old ice, and the latter may in fact be predominant. The offshore ice drifting in from Davis Strait may be over 150 cm thick. Many storms affect the area, and ice ridges up to 5 metres high can easily develop under pressure caused by winds and currents. As a rule of thumb, ice keels are in the order of three times the vertical extent of associated ice ridges. Westerly winds are frequent so a flaw lead develops, while along the outer edge the ice organises into strips and patches. In periods of persistent east to northeast winds, the ice compacts near the coast and ice deformation processes can be very intense. Because of incoming swells and wave action the ice breaks up into small floes near the ice edge.

In December, first-year ice begins to appear off northern Labrador and new ice off southern Labrador. For the rest of the winter the pack is mostly first-year ice and an equilibrium edge establishes some 150 km off the Labrador Coast. Ice condition comparisons between years can be largely related to the mean wind-flow experienced during the winter and spring months. Whenever low pressure weather systems persistently track across the Newfoundland area, easterly winds along the Labrador coast can compress all the ice into a 100 km wide belt against the coast. However when the low pressure systems track north of the area, westerly winds spread the ice up to 500 km seaward.

Freeze-up on the Labrador coast has started as early as the second half of October and as late as the second week of December. The Labrador coast has completely cleared of sea ice as early as the end of June but can persist until August.

Hudson Bay

Ice melt starts in May, as an open water area develops along the northwestern shore, and a narrow coastal lead develops around the rest of the Bay. In June and July, open water leads expand around the shoreline so that at the end of July, only large patches remain in southern portions of the Bay. In August the last vestiges disappear. Intrusions of ice from Foxe Basin may occur in the northeastern part of the bay in some years.

In late October, the ice begins to form along the northwestern shores of the Bay. Some years there may also be a simultaneous development in the cold waters near Foxe Channel. In November, the ice thickens as prevailing winds move it east and southeast. In December the Bay becomes covered with thickening first-year ice. During the winter, a 10 to 15 km wide fringe of shore-fast ice develops along most of the coastline and in many years a distinctive consolidated ice area develops between the Belcher islands and the Quebec coast. Meanwhile, the pack responds to winds and the slow counterclockwise current gyre in the bay.

In Hudson Bay, freeze-up has commenced as early as the first week of October and as late as the first week of November, while complete melting has occurred as early as mid- July and as late as the first week of September, except for incursions from Foxe Basin.

James Bay

Ice melt begins in late April. By mid-July much of the Bay is open water. Complete clearing normally occurs in early August but the northwest portion may receive occasional intrusions of ice from Hudson Bay until late August. Freeze-up is usually quick beginning after mid November. However, freeze-up has begun as early as the first week of November and as late as early December. Complete clearing has occurred as early as late June and as late as late August.

James Bay ice is noted for its discoloration, caused by freezing of shallow muddy water, or by run-off concentrating sediments on the surface of the ice. A sizable open water area often develops south of Akimiski Island. Old ice does not reach James Bay.

Foxe Basin

Ice normally forms in northern and western portions near mid-October, thickening rapidly and spreading southward and seaward to cover the Basin and Foxe Channel early in November. The ice becomes predominantly first-year ice by December.

Melting starts by June. The polynya near Hall Beach slowly enlarges. Open water leads expand around the shoreline in July. In the central Basin, the ice very gradually decreases in amount but more rapid disintegration occurs in August. Patches of ice persist during September.

In Foxe Basin shallow water combines with large tidal ranges and strong winds to keep a large amount of bottom sediments in suspension. Thus the ice is very rough, much of it in small floes and muddy in appearance. In northern and southwestern sectors there are large areas of shore-fast ice. In some years, all the ice will melt throughout Foxe Basin and Foxe Channel, while in other years with a cold summer, significant concentrations of ice will remain as freeze-up begins again. Thus second year ice may affect Foxe Basin and adjacent waters through the following winter and spring.

In Foxe Basin, freeze-up has started as early as late September and as late as the third week of October. Complete clearing does not occur every year but has occurred as early as the first week of September.

Hudson Strait and Ungava Bay

Freeze-up usually begins near the shore in western Hudson strait in November, then ice formation progresses to cover the entire area by early December, and by mid December the first-year stage predominates. Except for quite extensive shore-fast ice among the islands from Big Island to Cape Dorset, the ice is in constant motion because of strong currents and frequent gale force winds. Ridging, rafting and hummocking are continually taking place, and ice congestion often affects Ungava Bay and the south side of Hudson Strait. Conversely, a shore or flaw lead is frequently present on the north side of the Strait. At times small concentrations of second year ice drift into the area from Foxe Basin. Multi Year ice also enters eastern portions from Davis Strait.

Open water leads develop in May, slowly expand in June. Clearing becomes extensive during July but Ungava Bay often remains encumbered with heavy deformed ice, with some embedded old ice in July. Complete clearing has taken place as early as mid-July and as late as the end of August. However it is worth noting that incursions of second year ice from Foxe Channel occur in some years.

In Hudson Strait, freeze-up has started as early as mid October and as late as the first week of December, while complete clearing has occurred as early as late July and as late as early September. Freeze-up in Ungava Bay has begun as early as late October and has been delayed until the second week of December.

Baffin Bay and Davis Strait

In late May and June, any thin ice in the North Water polynya in northern Baffin Bay disintegrates, and then clearing extends southward across the approach to Lancaster Sound. The pack deteriorates more quickly around the eastern shores than it does in the centre of the bay. Thus at the beginning of August ice remains near the coast from Cape Dyer to Clyde River and in central parts of the Bay northward to near latitude 74º N. The pack is finally reduced to offshore patches between Cape Dyer and Home Bay late in August. Clearing occurs on the average by late August.

The north-flowing current along the Greenland coast is relatively warm, and the south-flowing current along the east Baffin Island is relatively cold. Thus ice formation along the west side of the Bay begins earlier than on the Greenland side. In September, new ice begins to form in the northwestern reaches of Baffin Bay. By the end of the month a fringe forms all along the Baffin Island coast. Ice formation accelerates through October and November, such that first-year ice becomes predominant north of Cape Dyer near mid-November. On average, the southern extent of sea ice achieves equilibrium near a line from the Greenland Coast near latitude 68 N generally southwestward to a point some 200 km off Resolution Island.

First-year ice predominates in Baffin Bay and Davis Strait throughout the winter. Because an area of low pressure is often centered in Baffin Bay, winds may develop a flaw lead along the Baffin Island coast. A percentage of multi-year ice originating mainly from Smith Sound and sometimes Lancaster Sound infests the western side. This ice is mostly in the range of 240 to 320 cm thick. Ridging, rafting and hummocking are significant, and icebergs abound.

An open water route across northern Baffin Bay has occurred as early as the third week of June and has been as late as the last week of August. Frobisher Bay has cleared of sea ice as early as late June and as late as early October. Baffin Bay and Davis Strait have cleared of all sea ice as early as mid-August, in other years some ice has remained until freeze-up began. In the latter situation the floes remaining are usually well dispersed throughout the area by autumn storms. Freeze-up in northwestern Baffin Bay has developed as early as the last week of August and been delayed until the middle of October. In Frobisher Bay, new ice formation has begun as early as mid October, and as late as the second week of November.

Arctic Archipelago

As temperatures move above freezing in the high Arctic, polynyas and open areas start to expand slowly. Then during June, the mobile ice in Lancaster Sound clears from the west followed by break-up of its consolidated ice. On the consolidated ice in the Archipelago, puddling begins, becoming extensive in early July. Fracturing in much of the Archipelago usually occurs in July, but often waits until August in Barrow Strait, Norwegian Bay, Viscount Melville Sound, Peel Sound, Larsen Sound and M'Clintock Channel.

In Dolphin and Union Strait, Coronation Gulf and Dease Strait clearing usually comes before the end of July. Complete clearing in Admiralty Inlet and in Pond Inlet usually occurs in early August and in Queen Maud Gulf and south and east of King William Island during the second week of August. Wellington Channel and Jones Sound normally clear by late August, but incursions from the north may occur. Peel Sound, Prince Regent Inlet and the Gulf of Boothia will often clear in early September. However, the southern end of the Gulf of Boothia as well as Committee Bay usually remains encumbered with old ice throughout the summer. In Sverdrup Basin, the area of fracturing is quite variable, and ice is usually present as freeze-up begins in the fall.

In Parry Channel, and in central portions of the Archipelago, new ice begins to form in September, and thickens rapidly to first-year ice in October, and then most of the area consolidates. However, in Lancaster Sound, freeze-up events may be delayed by a month because of strong winds and tidal activity. New ice begins to form around King William Island during the first week of October, consolidating in early November. Freeze-up spreads to Coronation Gulf, and consolidation is usually complete in mid November.

In central portions of Viscount Melville Sound, and M'Clintock Channel the ice may remain in restricted motion during December. Small tidal openings are common in Penny Strait and Bellot Strait while a significant polynya exists in Hell Gate all winter. In eastern Parry Channel, the rate of consolidation varies considerably from year to year. Some years, consolidation reaches almost to the eastern entrance to Lancaster Sound, while in other years, consolidation only reaches Barrow Strait, but the median consolidation edge is at Prince Leopold Island.

East and south of a line from King William Island to Bathurst Island to southern Ellesmere Island, first-year ice predominates, with a small percentage of multi-year ice floes here and there. Committee Bay is an exception, where much of the ice is of the multi-year variety. West and north of this line, the predominant ice type is multi-year and the concentration of first-year ice depends upon the extent of break-up during the previous melting season.

The area of Larsen Sound and surrounding waters, and also the Committee Bay area noted above acts as a trap for old ice that periodically invades from more northern areas, because there is no effective exit for the ice. Incoming heat energy during the summer is sufficient to reduce the thickness of the old floes by more than the normal winter growth of this ice. The cycle may take several years before melt of an old floe is complete, but a "new" supply of old ice invades the area every few years.

Ice conditions can vary greatly from one year to the next. During colder winters, Lancaster Sound and Prince Regent Inlet can develop a consolidated ice cover and Lancaster Sound may still have loose ice as freeze-up begins. During easy years, Lancaster Sound can become bergy water by the end of May and remain open until new ice forms in October. During colder summers, many of the channels remain consolidated, or retain close pack ice leading to early freeze-up. On the other hand, during a warmer summer, most channels break up, with extensive clearing ensuing. This may allow old ice broken from the ice cover in the Queen Elizabeth Islands to drift south in the fall into Parry Channel, contributing to difficult ice conditions there the following year.

Beaufort Sea

Old or multi-year ice up to 450 cm thick - the Arctic Pack - continuously circulates with currents and winds in the Arctic Ocean, and it is present year round. Its degree of penetration into the Beaufort Sea at any given time is dependent on the wind regime of the year. On average, the boundary of the Arctic Pack lies from near Cape Prince Alfred southwestward to some 200 km north of Herschel Island and then westward some 200 km off the Alaska North Coast. Between the Arctic Pack and the coastal shore-fast ice, mobile first year ice is predominant through the winter.

The edge of consolidation in Amundsen Gulf can be quite different from year to year, but commonly it lies near Cape Baring or Cape Lambton, or less frequently at Cape Kellett. In spring, northwest winds die off, and east and southeast winds become predominant, so that a polynya develops there. In June, melt begins in the Mackenzie Delta and an open water area also develops quickly. Typically, Amundsen Gulf fractures in late June and the ice drifts out and decays. The fast ice along the Tuktoyaktuk Peninsula fractures in late June or early July, and by the end July an open water route usually develops from Mackenzie Bay to Cape Bathurst. Amundsen Gulf usually clears before August.

West of the Mackenzie Delta to Point Barrow, a narrow shore or flaw lead develops in July. Open drift ice conditions do not develop along the coast until the first week of August and an open water route not until the first week of September.

Freeze-up in the Beaufort depends to a very great extent upon the location of the southern limit of the Arctic Pack. New ice formation starts among the multi-year floes in late September and spreads southward while it also spreads seaward from the coast. By late October much of the ice is at the first-year stage right out to the Arctic Pack. Shore-fast ice is extensive and grows seaward to the vicinity of the 20 metre water depth. Onshore winds during the winter months hold the mobile pack ice tight to shore-fast ice.

During a cold summer, the shore-fast ice along the Tuktoyaktuk peninsula may not completely break until mid July. These cold summers occur when northwesterly winds keep the Arctic Pack close to shore. Open water along the Alaskan coast can develop as early as the third week of July.

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