I'm going to try and regularly report on new scientific papers that discuss some aspect or other of Arctic sea ice. Creating a special segment on the blog that categorizes all important research papers (a bit like the AGW Observer blog) is still on my to-do list.
Following yesterday's blog post that referred to Chris R's blog where he devotes several posts to multiyear ice, I now call attention to a new paper by Maslanik, Stroeve, Fowler and Emery called Distribution and trends in Arctic sea ice age through spring 2011, which was published in Geophysical Research Letters on July 14th.
From the abstract:
Analysis of a satellite-derived record of sea ice age for 1980 through March 2011 shows continued net decrease in multiyear ice coverage in the Arctic Ocean, with particularly extensive loss of the oldest ice types. The fraction of total ice extent made up of multiyear sea ice in March decreased from about 75% in the mid 1980s to 45% in 2011, while the proportion of the oldest ice declined from 50% of the multiyear ice pack to 10%. These losses in the oldest ice now extend into the central Arctic Ocean and adjacent to the Canadian Archipelago; areas where the ice cover was relatively stable prior to 2007 and where long-term survival of sea ice through summer is considered to be most likely. Following record-minimum multiyear ice coverage in summer 2008, the total multiyear ice extent has increased to amounts consistent with the negative trend from 2001–2006, with an increasing proportion of older ice types. This implies some ability for the ice pack to recover from extreme conditions. This recovery has been weakest in the Beaufort Sea and Canada Basin though, with multiyear ice coverage decreasing by 83% from 2002 to 2009 in the Canada Basin, and with more multiyear ice extent now lost in the Pacific sector than elsewhere in the Arctic Ocean.
Ice age is estimated by treating each grid cell that contains ice as a discrete, independent Lagrangian parcel, and then transporting the parcels at weekly time steps. In cases where particles of different ages fall within a single grid cell, the cell’s age is assigned that of the oldest particle.
Because the grid cell is assigned the age of the oldest ice present, a cell with a total concentration as low as 15% at the end of the melt period is coded as multiyear ice even though the majority of ice present after freeze‐up is first‐year ice.
This graph nicely shows how multi-year ice has recovered from the 2007/2008 massacre:
They go on to explain how much multiyear ice extent has declined in the last 30 years in specific regions. These graphs show the extents of multiyear ice and 5+ ice for the third week of March and at the September minimum for (a) Canada Basin, (b) Beaufort Sea, (c) East Siberian Sea, (d) Nansen Basin, and (e) north of Canadian Archipelago (regions 1, 2, 4, 6, and 7 in the map at the top of this post):
Maslanik et al. explain that the recent recovery of multiyear ice is the result mainly of unusual SLP patterns:
Strong and sustained negative AO periods during winter [Stroeve et al., 2011] not seen since the late 1960s were present in 2010, with an annual meanAOindex of −1.04. This was the lowest annual mean from 1950 through 2010, over two standard deviations from the mean of −0.14, and one of only three years with a negative AO in 11 of the 12 months. The year 2010 was also unusual as the only year over the same period with a negative North Atlantic Oscillation (NAO) in all months, with an annual mean NAO index of −1.30; the lowest observed over the 61‐year period and more than three standard deviations less than the long‐term mean of −0.10. As a result, while mean ice transport was cyclonic in 2009 (see http://www.arctic.noaa.gov/reportcard), transport was more anticyclonic in 2010.
Perhaps that the extreme compaction in 2007 also had to do something with it, by making much of the remaining ice thicker than usual. A small negative feedback, I guess.
In the final part of the paper the authors show how most of summertime reduction of multiyear ice has occured on the Pacific side of the Arctic, ie within the Beaufort Sea and Canada Basin (regions 1 and 2; red), versus loss elsewhere in the Arctic Ocean (regions 3 through 8; blue) for (a) multiyear ice, and (b) 5+ ice:
Multiyear ice extent in the area North of the Canadian Archipelago (region 7 on the map at the top of the post) was relatively stable until 2006. Above normal transport observed during the winter of 2009/2010 to the Beaufort Sea and increased export through the Nares Strait in recent years contributed to this. But little of this transported ice has made it through the summer in recent years:
The mean survival rate of multiyear ice extent in the Beaufort Sea as a whole declined from 93% over 1981–2005 to 73% during 2006–2010, with most of the surviving ice located in the eastern portion of the region (e.g., Figure 1). Regions 1 and 2 now account for the majority of summertime reductions in multiyear ice extent, exceeding the losses elsewhere in the Arctic Ocean.
While convergence and deformation or transport to adjacent regions between mid May and the September minimum contribute to some of the loss in extent, the short time period and the directions of transport in the regions suggest that nearly all of the loss is due to melt. This changing significance of the Pacific sector is particularly apparent for the oldest ice types. Most of the reduction in coverage of the 5+ ice over summer now occurs within the Arctic Basin itself, with relatively little loss through the Fram Strait.
So to conclude: multiyear ice has recovered somewhat from the 2007/2008 cryocide due to unusual SLP patterns, which could point to a capacity for the Arctic sea ice to bounce back after extreme ice loss. At the same time we see that most of the multiyear ice is now being lost on the Pacific side of the Arctic. This could have something to with an increase of warm water transport through Bering Strait, with open water areas absorbing more solar heat, with a negative Arctic Dipole, or a combination of all three.
The paper ends with the relevant question:
A key question then is whether the increases in the extent and age of the multiyear ice seen over the last three years can be sustained ‐ continuing the recovery observed since the 2007/2008 record minima and thickening enough to become less vulnerable to another extreme melt season [e.g., Lindsay et al., 2009].
We might find out this melting season, as it has been pretty extreme so far.
I'd like to thank Dr. Julienne Stroeve for allowing me to cite from this concise, but very instructive paper. I actually understood all of it.