Polar bear habitat update: Hudson Bay sea ice breakup on track

Hudson Bay sea ice coverage is only slightly below average for this time of year (week of 14 May, below) but well above levels for 2006, when Western Hudson Bay breakup was relatively early).

Sea ice Canada 2015 May 14_CIS

Figure 1. Sea ice coverage over Hudson Bay for the week of 14 May, 1971-2015. Canadian Ice Service. Click to enlarge.

Figure 1. Sea ice coverage over Hudson Bay for the week of 14 May, 1971-2015. Canadian Ice Service. Click to enlarge.

Even at its highest extent in April, Hudson Bay is only 97-98% ice covered (due to persistent shoreleads and polynyas), which means ice levels are currently only 10% or so below maximum. In other words, there is still lots of polar bear hunting habitat over the bay.

That’s a bit lower than ice coverage was for the last two years at this date (2013-2014), which had average or above average ice cover. However, there is currently a bit more ice on Hudson Bay than there was in 2011 and 2012 – and much more than there was in 2010 and 2006.

Note that in mid-late May 2013 (Fig. 2), sea ice was breaking up in southwest Hudson Bay, around Churchill, as well as along the east coast.

The difference from 2015 is almost certainly due to currents and prevailing winds (known to drive the formation of spring polynyas, as discussed in my last post regarding open water in the Beaufort Sea), not temperature (Danielson 1990).

Figure 2. Sea ice coverage over Canada at 23 May 2013, showing patches of open water and/or low ice concentrations along the southwest coast of Hudson Bay, near Churchill (a bit south of where an opening occurs this year), and along the west coast.

Figure 2. Sea ice coverage over Canada at 23 May 2013, showing patches of open water and/or low ice concentrations along the southwest coast of Hudson Bay, near Churchill (a bit south of where an opening occurs this year), and along the east coast. Click to enlarge.

Breakup of sea ice over Hudson Bay usually proceeds from the east and north, with the last remaining ice converging on the southwest shore (Fig. 3, discussed previously here). The pattern and ultimate date of breakup is highly variable year to year (Fig. 5).

Figure 3. The peculiar pattern of ice melt on Hudson Bay is illustrated by this figure from Stirling et al. (2004). Ice along the southwest coast of Hudson Bay (darkest gray) is the last to melt each season. As a consequence, the southwest quadrant is where most Western Hudson Bay bears come ashore. Note that the “time of breakup” on this map uses the old, out-of-date method (50% ice coverage).

Figure 3. The peculiar pattern of ice melt on Hudson Bay is illustrated by this figure from Stirling et al. (2004). Ice along the southwest coast of Hudson Bay (darkest gray) is the last to melt each season. Note that the “time of breakup” on this map uses the old, out-of-date method (50% ice coverage). Click to enlarge. See previous discussion here.

Sea ice Canada 2015 May 18_CIS

Figure 4. Sea ice coverage over Canada at 18 May 2015. Not much has changed since last week (at 14 May, opening image above). Click to enlarge.

Figure 5. Hudson Bay breakup patterns for 17 May (Region 10, day 137) compared: 2015, 2011, 2009, 2010, 2007, 2006. NSIDC MAISE products archive. Note that 2009 had one of the latest breakup dates on record (30 July), while 2006 was relatively early (mid-June). Comparing the current year to only one previous year is uninformative because breakup patterns and dates are so variable year to year. Click to enlarge.

Figure 5. Hudson Bay breakup patterns for 17 May (Region 10, day 137) compared: 2015, 2011, 2009, 2010, 2007, 2006. NSIDC MAISE products archive. Note that 2009 had one of the latest breakup dates on record (30 July), while 2006 was relatively early (mid-June). Breakup patterns and dates are variable year to year. Click to enlarge.

References
Danielson Jr., E.W. 1990. Hudson Bay ice conditions. Arctic 24(2):90-107. pdf here.

Stirling, I., Lunn, N.J., Iacozza, J., Elliott, C., and Obbard, M. 2004. Polar bear distribution and abundance on the southwestern Hudson Bay coast during open water season, in relation to population trends and annual ice patterns. Arctic 57:15-26. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/479/509

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