However I will start this post by stating the sum total of my findings with regards that more detailed data: I see an Arctic Ocean with two regimes, only the Central Arctic shows a gain in volume, the other regions show little difference from recent years, including April 2012. I also find little evidence of such an export of multi year ice into Beaufort, Chukchi, or the East Siberian Sea that would support my view that those regions are not likely to melt out this year. I may be about to change that view.
Extent for 2012 was ranked 21st lowest, while the top ten lowest April extent years are predominantly unremarkable years.
Looking at the situation on a regional basis I switch to anomalies. Anomalies are the difference from a long term average, here as usual I use the NCEP/NCAR reanalysis baseline of 1981 to 2010.
The red lines are the ranges between the highest and lowest extents for each region between 1979 and 2015 on 30 April. Okhotsk, Bering, and Barents are all near lowest recorded extent. The seas within the Arctic Basin, (Beaufort to Laptev), also Kara, Central and the Canadian Arctic Archipelago, are all near zero anomaly because extent is at maximum with full ice coverage over 15% concentration at this time of year.
A selection of seas playing a role in the current decline in extent are shown below.
Bering contributes to the record low April extent and has begun the drop to ice free conditions in line with previous years.
Meanwhile in the Atlantic, the Barents Sea is low through April and is on a trajectory towards dropping to near zero extent in the months to come.
Baffin Bay has had high extent all winter, this is associated with below average temperatures through April.
The Gulf of St Lawrence has had high extent through the winter, but is now dropping to zero.
Looking at the year so far in terms of sector extent, the Pacific sector (Okhotsk & Bering) has started to drop significantly. This has happened as temperatures over the region have crossed the 0degC threshold (NCEP/NCAR, not shown) and melt has firmly set in. The Canadian sector (Baffin, Hudson, St Lawrence) continues a fall since mid March. Hudson Bay remains topped out, the decline in the Canadian sector is wholly due to Baffin and St Lawrence.
Temperatures are shown in a composite plot from NCEP/NCAR reanalysis below, which shows the region north of 65degN.
Baffin Bay and St Lawrence (not shown) have been cooler than average through April (upper right quarter), While Barents Sea has been warmer than average (upper left quarter), this may be due to less ice than average not a cause of less ice. However across the pack through to Beaufort and Chukchi temperatures have been above average. While warmer temperatures may be caused by reduced sea ice extent or thickness they will also have a feedback effect, reducing the thickening of ice.
Overall this pattern of warming has led to the region north of 80degN being amongst the warmer years for April (surface temperature). 80degN is chosen to be comparable to the DMI temperature data.
Surface temperatures in the first third of 2015 north of 80degN in the NCEP/NCAR dataset are also amongst the warmest on record. This is in line with global GISS data for the first quarter of the year, e.g. Tamino's Open Mind.
The post 2010 April thickness maps are shown in the following animated gif. In producing that the images were accidentally arranged in reverse order, which hasn't turned out too bad given that 2015 state is the focus.
2015 shows a large expanse of ice over 2m thick through from the Central Arctic to Beaufort, Chukchi and the East Siberian Sea. However, as shown below, the volume difference between April 2015 and April 2013 (after the 2012 crash) shows that 2015 has marginally less volume in Beaufort, Chukchi, and the East Siberia Sea. The bulk of the volume increase remains in the Central Arctic region, and the map plot above shows that this is from the Central Arctic close to the Canadian Arctic Archipelago.
The volume series for all regions within the Arctic Ocean in April shows that the main region with a convincing increase in volume is the Central Arctic (the scale for the Central Arctic is on the right and is 4.5 times as large as that for all other regions on the left vertical axis). The Canadian Arctic Archipelago also shows an increase that is not as proportionately large.
Turning now to the breakdown of volume with grid box effective thickness.
The volume increase is seen to be largely due to grid boxes reporting a thickness of 4m and above. These are grid boxes north of Canada.
Looking at some other data sources. The US Navy's HYCOM-c-ice model shows an intrusion of thick ice into Beaufort/Chukchi and the East Siberian Sea. This is also seen in the Drift Age Model.
Furthermore the ASCAT scatterometer satellite reveals it, as seen in the following adapted image with contrast adjusted to bring out the white thicker multi year ice.
However the numerical data from PIOMAS do not suggest that this is a sufficient volume of multi year / thicker deformed ice for it to be a major factor in this seasons melt.
With the increase in volume largely restricted to the Central Arctic, conditions in the peripheral seas of the Arctic Ocean are similar to other recent years, including April 2012. I will detail this with more graphs in my next post. However looking at the PIOMAS sub grid thickness distribution numerically, if I use the demarcation of above 3.3m thick, there is an April 2015 volume of 1.48k km^3 (for ice over 3.3m thick) for the peripheral seas of the Arctic Basin (Beaufort round to Laptev), this is the second lowest volume in the post 2007 period. Whereas for the Central Arctic region, sub grid thickness volume over 3.3m thick is 5.87k km^3, that is the highest in the post 2007 period and is 3.6 standard deviations above the 2007 to 2014 average April volume for sub grid thickness ice over 3.3m thick.
HYCOM, the Drift Age Model and ASCAT all may be taken to indicate survival of ice extent long into the summer, but they don't give hard numbers. If PIOMAS is right, and it's been reliable so far, this raises the prospect of a more exciting season than I had anticipated, much of what happens will be in the weather. However it is a matter of judgement as to what importance one assigns to each of the data sources, and I need to think some more before commenting further.