Coincidentally, as others including Neven have noted, Beaufort is very interesting right now, this winter's export of multi year ice makes me suspect it will stall, but an early start in Beaufort is promising for a more exciting season than the last two years. Here's hoping...
The BCE region is shown in the modification of a graphic from Cryopshere Today shown below. The BCE region is filled in red.
April PIOMAS thickness calculated on a per grid box basis for the BCE region shows a continuing decline in April thickness. Behaviour from 2004 onwards my evolve into a slower decline rate of April thickness, but I will get onto that.
Gice is the sub grid thickness distribution used in the PIOMAS model, described more fully here. It uses 12 thickness bands to allow the model to cope with a smaller scale than the size of a grid box, which is typically several hundred square km in area. The following graphic shows thickness distributions for December from 1979 to 2014. Individual lines are not important, the colour scale is from dark blue for earlier years to pale blue for the latest years and shows qualitatively what has been happening to PIOMAS thickness distribution in the BCE region.
Growth of new ice after September shows as the peak around 1.46m thick, this has declined in volume somewhat and has spread out, but the major source of loss of December volume is from the thickest ice. Moving through the winter to April, the month of peak volume for the whole PIOMAS model domain, it can be seen that while ice below 3.3m thick has remained fairly level (actually increasing slightly), ice above 3.3m thick has been the source of the overall volume decline. 3.3m is the demarcation between the 2.61m and 4.23m thickness bands in the above graphic.
The following graphic has the vertical axes deliberately offset to bring volume and extent in line. It shows April PIOMAS gice volume from ice above and below 3.3m thickness plotted together with September extent, all for the BCE region.
Extent in September is seen to track the decline in April ice volume over 3.3m thick, as I have said before for other regions, this is because a decline in thick ice increases open water formation efficiency. So leaving aside the last two years, which were a result of weather so tell us nothing of the underlying process in question, for the 2007 to 2012 period the BCE region has seen extremely low extent. The general decline of extent seems to be related to the decline in the thicker ice, and a causal relationship between the two indices is to be expected due to increased open water formation efficiency. For a given seasonal thinning thinner ice is more likely to melt out to give open ocean.
Turning back to thickness. Using the data behind the first graphic of this post I have a series of April thickness for the BCE Region. I have taken the thickness for the regional seas of the BCE region, Beaufort Sea, Chukchi Sea and East Siberian Sea, and have plotted them as thickness differences from the overall thickness for the whole BCE Region.
The effect seen in the above graphic is one of decreasing variance as time proceeds, as the ice approaches 2m thick in April the plots of the three regions converge. That 2m thick is the typical sort of thickness expected for growth of ice from open water is not a coincidence. This was found by Dromicosuchus in the comments of my original Slow Transition post.
I have taken the data behind the above graph and have recalculated the data for each sea as absolute values (i.e. all positive, no sign +/- used) and have calculated the average for the three seas. This is presented as the following graph, which basically shows the decline in variation of thickness within the BCE region.
Obviously this decline cannot carry on to zero, since that would imply a pack of uniform thickness throughout the entire BCE region. So what does this mean?
The clue is in the thickness distribution plot posted earlier. But for this part I have re-drawn for April gice and annotated.
As argued in my previous post, The Mono-modal Spike, the relative lack of change at the thin ice end of the distribution is marked compared to the decline in thicker ice. This is because of the thermodynamic thickening of ice over winter, a process described previously. That post was done using grid box effective thickness, and the same process is seen in the sub grid thickness distribution, gice. Note that the wall is drawn as a fuzzy line, it is fixed only by winter temperatures, and as the winter warms that will dictate thinner ice. However the winter is not warming fast enough for claims of a virtually ice free September this decade to be at all plausible.
What years like 2007 to 2012 mean is that most of the ice melts out. This prevents the BCE region being a transit for the exported multi year ice from the Central Arctic, and turns it into a killing ground for that ice. As 2013 and 2014 show, this does not happen every year, it just needs to happen most years. And even 2013 and 2014 did not bring extent up to the levels before 2003, because thicker ice volume was so much less. 2003 may be a significant year because of the dominance of the Arctic Dipole from around that time, i.e. here and the references therein. So for as long as the BCE region remains in the state seen since 2007 it can be expected to revert to its new default state; virtually seasonal.
I should justify the description of virtually sea ice free for 2007 to 2012. For the BCE region, the 2007 to 2012 period has an average September extent 14% of that for the 1980s. For comparison with the whole Northern Hemisphere extent, 14% of the 1980s average would be 0.979M km^2, which fits the commonly accepted definition of below 1M km^2 for which the Arctic be considered virtually ice free in September. Therefore BCE can be considered virtually ice free in September for the period 2007 to 2012.
So the BCE region is already at the stage where, after 2007, most years have been what can be described as a virtually seasonal pack. The future is likely to see that condition entrench and continue. However that does not mean they are out of the equation of the transition of the entire Arctic Ocean to a virtually seasonal ice pack (<1M km^2 extent for September). This is because as long as there is ice there to melt during the summer that ice must melt out to enable the ice edge to make significant inroads into the Central Arctic. The melt season is fixed by the solar cycle, melt has from March until September, and unless we see a crash in April thickness, there will be similar volumes of ice to act as a barrier to large ice edge encroachment into the Central Arctic region.
The issue is that 'the wall' described above suggests that ice growth over winter will continue to keep winter volume in the BCE region up, countering the past losses from thicker ice. In such a situation the melt season will still have a lot of ice to melt and the post 2007 behaviour of summer extent would become a new normal, not a prelude to a collapse of the Arctic Ocean's ice cover.
The obvious question then becomes: What is going on in the Central Arctic and what are the prospects for a collapse of summer ice cover in the Central Arctic? That will be the subject of my next post.