What caused the volume loss in 2010: Part 2.

Back in September 2012 I wrote some posts on the 2010 volume loss in PIOMAS, why it was important, and what caused it. I'm now able to add another layer of detail into the explanation of what caused the 2010 volume loss and consider whether it has any reality outside of the PIOMAS model. I'm more confident that the 2010 volume loss was real, not just a blip in a model.

I'm not going to re-hash the earlier posts, but I will write a few words to save busy people the bother of reading those posts.

Basically the 2010 volume loss is important because it was one of a series of five large volume loss events, three of which occurred at a time when the ice pack had much more volume, so were proportionately not such large events. The volume loss event preceding 2010 was the 2007 sea ice crash, this puts the scale of 2010 in what I consider to be its correct context.

 

This loss of volume in PIOMAS has caused a change to the seasonal cycle, in that a more pronounced and aggressive spring melt (April to June) is occurring as shown in the following graphic (PIOMAS anomalies, baseline 1980 to 1999).

In my post on what caused the 2010 volume loss I argued that warmer temperatures had retarded ice thickening over the Arctic coast of the Canadian Arctic Archipelago (CAA). Not having access to a model, and with certain gridded data only running to 2004, I am unable to further confirm this view point. However the failure to thicken still seems to me to be likely to have a thermodynamic component, and the additional factor I outline in this post doesn't seem to me to explain the loss of thick ice along the coast of the CAA.

I've recently broken down the PIOMAS volume/thickness breakdowns into three regions, see here. This further detail has enabled me to examine what was going on within the sub regions in 2010. I've neglected the Atlantic sector because this didn't play a large role.

The following image is from a spreadsheet, showing years from 1998 to 2012 for the Siberian region. The table of numbers are volumes for different thicknesses of sea ice in June, the actual thicknesses aren't important right now. Clicking on it will enlarge to full screen, but the notable feature is visible anyway: 2007 cleared out the thickest bands of ice from the Siberian region, except for 2010 when the thickest ice made a reappearance.

So where did this ice come from? Applying the regional boundaries to images from the Drift Age Model, source, shows what happened.

Week 20 is in late May, close enough to June not to make any substantive difference. What can be seen in that after 2006 there is a marked drop in old ice in the Siberian (SIB) sector, however in 2010 a large swathe of old ice enters the Siberian sector. This transport of ice won't all have occurred between April and June, the ice just doesn't move that fast. Trying to compare volume for June brings in the complication of ice thinning and volumes cascading down to thinner categories. However the difference between 2009 and 2010 in April for the Siberian sector (SIB) was +1.35k km^3, and for the American sector (AME) was -2.22 k km^3. So the gain in the Siberian sector accounts for over half the drop in the American, which is one reason why I still suspect a role for the warm anomalies over the CAA.

 

The Siberian sector shows marked gain in volume for ice over 2.5m thick, at a sum of 3.93k km^2, post 2007 years show nowhere near that sum total of volume of ice in grid cells over 2.5m thick. While in the American sector ice over 3.25m thick shows a notable deficit, with grid cells from 2.5m thick up to 3.25m thick showing a notable gain in volume, which I suspect is the result of grid cells that would have reported thicker ice falling into lower thickness groups due to less thermodynamic thickening and possibly less ridging as ice was freely exported from the region, rather than being crushed up against the CAA.

What caused this transport was a strong anticyclonic set up over the Arctic, for example this is shown in the following plot of SLP from NCEP/NCAR, note the Dipole formed between the North Pacific low and the Arctic high pressure centres.

This will have led to clockwise winds around this Arctic which drove the clockwise ice movement from Beaufort towards Siberia seen between week 1 and week 20 in the DAM: DAM Week 1, DAM Week 20.

The transport of thick ice shown in the DAM, and implied by PIOMAS volume changes and the presence of a strong SLP dipole, can therefore be accepted as not just an artefact of PIOMAS. This movement of ice really happened and this fact is confirmed by the ASCAT scatterometer, 365/2009, 140/2010.

Then we have the thermodynamics issue. NCEP/NCAR shows average absolute temperature was around -25degC for February to April 2010, this being some 4 degrees warmer than average, that's a factor of 1.16. As the heat flux through the ice is inversely directly proportional to the difference in temperature between the ocean at the base of the ice and the air at the surface, this 4 degC warm anomaly corresponds to a heat flux of 86% of the climatological baseline, based on temperature and neglecting issues such as snow cover. This heat flux is crucial because it is that flux of heat that causes new ice to freeze onto the base of the ice pack at the ice/ocean interface. However, although 2010 was the warmest year for surface temperatures north of 70degN, other recent years were also warm.

Against this broad average however there are the very warm surface temperature anomalies over the Canadian Arctic Archipelago and the nearby Arctic Ocean.

That right next to this large warm region there was a failure in winter thickening of the thick old ice off the CAA seems to me to beg too much of a coincidence. The following graphic shows the deviation from the 1980 to 1999 average thickness for each grid point in April. The dark area off the CAA shows the deficit in thickness in April 2010.

However without further data I can't be sure, so I'm not putting the atmospheric conditions and implications for thickening forward as strong evidence in support of the reality of the 2010 event.

So it seems to me that we have only one strong line of evidence that suggests that the volume loss of 2010 may not have been just an artefact of PIOMAS - the large export of old ice into the Siberian sector. But is there any further corroboration?

The PIOMAS volume loss of 2010 was followed by a change in the seasonal cycle due to a more aggressive spring melt. So is there anything not derived from PIOMAS that stands out about the years following 2010? Cryosphere Today anomalies (my anomaly calculations; baseline 1980 to 1999) show what I think is a result of the volume loss of 2010. Here's the 1980s, the 1990s, and below are the periods 2000 to 2006 and the post 2007 years, since that latter period needs to be considered as a separate phase of behaviour.

2000 to 2006.

Post 2007.

What stands out here is that 2007, 2010, 2011 and 2012 are common in that they all exhibit a massive drop around early June, days 160 to 170. Other years such as 1999 and 2005 also show such a drop, but there is no similar grouping of behaviour since 1979. However beyond this early June drop 2007, 2011 and 2012 are common having a steady drop thereafter. I think the reason for 2010's anomalies not dropping like those years is that the process of thick ice loss had not completed, there was still the substantial tongue of old ice in the Siberian sector that had to be melted out, i.e. Bremen plot from August here ( - was the spur towards Chukchi due to this older ice?). By 2011 and 2012 this process was complete, so area was free to fall unimpeded by thicker old ice. The fall in 2007 was driven by the unusual weather and preconditioning of the ice. So what has caused the CT Area anomalies for 2011 and 2012 to behave so atypically, if not the preceding volume loss of 2010, with the attendant aggressive spring melt in following years that PIOMAS reveals.

 

The resultant state of the pack can be appreciated from the following June thickness plot from PIOMAS gridded data, 2011 and 2012 stand out from the rest. Note the reduced size of the blue region (over 2m thick) and the lack of dark blue off the CAA post 2010.

So the 2010 volume loss, an event as large as 2007, seems to me to be a real event which has had real world repercussions.

As the Arctic ice pack transitions to a seasonally sea ice free state the spring volume loss has important implications. Until the Arctic winter warms substantially conditions will still be cold enough to grow ice substantially thicker than 1m thick, up to somewhat over 2m thick. This means that to get a repeated sea ice free state each year as much ice must be lost in the melt season as grows in the freeze season. As growth of new ice will be vigorous due to the growth/thickness feedback (thin ice grows faster than thick), commensurately increased loss of ice during the melt season will be a pre-requisite for a seasonally sea ice free state, resulting in a greatly increased seasonal cycle of area/extent and volume. The spring volume loss will play an increasingly important part in the attainment of this state, losing volume and preconditioning the ice for losses during the summer which will usher in a largely sea ice free ocean state probably by the end of this decade, possibly within a few years.