Polynya refresher: open water in spring is due to winds & currents, not ice melt

Arctic sea ice begins to open up in spring at predictable locations due to currents and prevailing winds and this was as true in the 1970s as it is today. Polynyas and widening shore leads that most often get mistaken for early sea ice melt are those in the Beaufort and Chukchi Seas and in Hudson Bay.

Beaufort Sea male polar bear USGS_2005 Amstrup photo

But contrary to concerns expressed about possible negative implications of these early patches of open water, these areas have always been critical congregation areas for Arctic seals and are therefore important feeding areas for polar bears.

Seal habitat frozen open lead_Beaufort 2008_Miller
Seals hauled out beside a lightly frozen over lead in Beaufort Sea ice, 2008. USFWS.

Polynyas and shore leads_Smith and Rigby 1981

From one of my posts three years ago on the topic (20 May 2015):

“The map of sea ice extent in Canada at 20 May 2015 is an almost-perfect example of the placement of recurring patches of open water polynyas that were present in the Canadian Arctic at this time of year in 1975-1979. Notes from field work on shore leads in Hudson Bay ice at May, 1948 offer further insight into the current pattern of sea ice cover on the bay.”

Polynyas and shore leads vs sea ice at 20 May 2015_PolarBearScience

Here is what Smith and Rigby (1981:24) had to say about the development of open water in eastern Beaufort in the spring (from this 2015 post), see image below:

“Some open water can be found in virtually all months somewhere in western Amundsen Gulf in the area of Cape Bathurst, Cape Parry, and Cape Kellet (Banks Island). Open water can appear as early as sometime in December, although it is not until April that a characteristic form to the polynya appears.”

The eastern Beaufort polynya is known as Cape Bathurst and the one further west, in the Chukchi Sea, as the West Barrow polynya. Smith and Rigby (1981:24) pointed out, based on records from the early 1970s, this western polynya usually became prominent in May:

“In contrast to the region discussed above, sizeable areas of open water rarely occur between Mackenzie Bay [mouth of the Mackenzie River] and Point Barrow for any length of lime during the winter. The lead along this part of the coast tends to remain closed. In the area west of Point Barrow, however, open water and new ice is evident in most months, but again, it does not become continuous and extensive until about mid May.”

Bathurst and W Beaufort polynyas_1975 vs 2015_PolarBearScience

And here is what marine mammal biologists Ian Stirling and colleagues had to say about polar bears and the Cape Bathurst polynya in spring (Stirling et al. 1981:49):

“Polar bears prey mainly upon ringed seals and, to a lesser degree, on bearded seals. Polar bears appear to be more abundant in polynya areas and along shoreleads, probably because the densities of seals are greater and they are more assessable. For example, between March and June in the Beaufort Sea from 1971 through 1975, 87% of the sightings of polar bears were made adjacent to floe edges or in unstable areas of 9/10 or 10/10 ice cover with intermittent patches of young ice.” [my bold]

Later, they discussed why these areas of open water can be so important in the Southern Beaufort area (Stirling et al. 1981:54):

“One useful approach is to ask what would happen if the polynya was not there? Obviously this is impossible to evaluate on an experimental basis, but by examining the consequences or natural seasonal variation, some useful insights can be gained. For example, the influence of rapidly changing ice conditions on the availability of open water, and consequently on populations of seals and polar bears, has been observed in the western Arctic. Apparently in response to severe ice conditions in the Beaufort Sea during winter 1973-74, and to a lesser degree in winter 1974-75, numbers of ringed and bearded seals dropped by about 50% and productivity by about 90%. Concomitantly, numbers and productivity of polar bears declined markedly because of the reduction in the abundance of their prey species. …If the shoreleads of the western Arctic or Hudson Bay ceased opening during winter and spring, the effect on marine mammals would be devastating.” [my bold]

Dunbar (1981:32) had this to say about Hudson Bay’s persistent flaw leads:

“The largest flaw leads normally found in Canada are in Hudson Bay. The Hudson Bay lead, seaward of the fast ice, is so wide as to have generated the belief that the whole of the bay, except for the fast ice region along the shore, stayed unfrozen all winter…In the Hudson Bay instance, the myth of an open bay all winter was dispelled by Hare and Montgomery (1949), who showed that the pattern of of air temperatures over the whole region made an open Hudson Bay in winter very improbable. By overflying the area, they demonstrated that in fact the central bay is covered with ice in winter, although there normally exists a large flaw lead seaward of the fast ice on both sides of the bay and extending into northern James Bay. This flaw lead varies in width according to the direction of the wind from “about a mile and a half to 30 or 40 nautical miles” (Hare and Montgomery 1949).” [my bold]

Hare and Montgomery (1949:160, 163) described the formation of a wide shore lead that formed in eastern Hudson Bay in early May 1948:

“The shore lead, which seems to have caused so much confusion in estimating the ice cover of Hudson Bay, may at times be entirely absent. Along the east coast from Great Whale River to Port Harrison the “Ice” reconnaissance of 8 March 1949 found no suggestion of open water. There were traces of old refrozen leads but none of them as large or as continuous as the one found along this same coast by the “Cariberg” reconnaissance of 6 May 1948. At that time the lane of open water off Port Harrison [now called Inukjuak, on the east coast] was 25 to 30 miles wide and seemed to stretch north and south along the coast as far as could be seen. It should be noted that this wide shore lead resulted after several days of NE winds which had effectively driven the ice offshore. [my bold]

In recent years the Hudson Bay shore lead has tended to widen on the western side of the bay.  See maps below from April 26, 2018 and May 3, 2018 where an early widening of the shore lead in late April was largely closed by May 3 (note also the lack of a polynya opening in the southern Beaufort, as occurred in 2015 and 2016):

Sea ice Canada 2018 April 26

Sea ice Canada 2018 May 3

References

Dunbar, M.J. 1981. Physical causes and biological significance of polynyas and other open water in sea ice. In: Polynyas in the Canadian Arctic, Stirling, I. and Cleator, H. (eds), pg. 29-43. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

Hare, F.K. and Montgomery, M.R. 1949. Ice, Open Water, and Winter Climate in the Eastern Arctic of North America: Part II. Arctic 2(3):149-164. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/3985 Pdf here.

[see also: Hare, F.K. and Montgomery, M.R. 1949. Ice, Open Water, and Winter Climate in the Eastern Arctic of North America: Part I. Arctic 2(2):79-89. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/3976 ]

Stirling, I. and Cleator, H. (eds). 1981. Polynyas in the Canadian Arctic. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

Smith, M. and Rigby, B. 1981. Distribution of polynyas in the Canadian Arctic. In: Polynyas in the Canadian Arctic, Stirling, I. and Cleator, H. (eds), pg. 7-28. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

Stirling, I, Cleator, H. and Smith, T.G. 1981. Marine mammals. In: Polynyas in the Canadian Arctic, Stirling, I. and Cleator, H. (eds), pg. 45-58. Canadian Wildlife Service, Occasional Paper No. 45. Ottawa.

Pdf of pertinent excerpts of above papers here.

via polarbearscience

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May 4, 2018 at 12:46AM

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