I'm currently ploughing through my stack of Arctic related papers. It seems to me that in order to fully appreciate the spectacle I think the coming years will deliver, knowing the scientific literature is a pre-requisite. So over the autumn and winter I'll be making more of an effort to post commentaries on what I've been reading. One aspect of the fallout of this year is that there will be a series of papers by various researchers who've been following this year. I can't wait!
However along the way I'll be distracted from my campaign of re-reading and catching up with my 'to be read' folder by research I've not been aware of.
Like these papers...
Thanks to Kevin O'Neill for bringing my attention to these two papers. The first is Rosel & Kaleschke, 2012, "Exceptional melt pond occurrence in the years 2007 and 2011 on the Arctic sea ice revealed from MODIS satellite data." The second is Perovich & Polashenski, 2012, "Albedo evolution of seasonal Arctic sea ice." Rosel is pronounced Roesel - there should be an umlaut over the 'o', but I'm too lazy/busy to go through the post cutting and pasting the appropriate character - call it anglo-saxon linguistic arrogance if you like.
As the title implies, Rosel and Kaleschke find exceptional melt pond areas in 2007 and 2011. This might partly explain the CT Area anomalies I refer to in preference to the other extent measures. The reason I prefer CT Area is that it seems to hold information lost in the calculation of extent.
That's from a previous post, so the August cyclone isn't of relevance here. What I need to draw attention to is that in 2012, 2011, and 2007 early June anomalies fall off a cliff. What these years have in common is that they were all area records. Also all the post 2007 years show drops around this time of year.
The significance of Rosel & Kaleschke is in their figure 3b, shown below.
The horizontal axis are dates - 18 May, 1 June, 15 June, etc. This shows the area of melt ponds for 2007 (red) and 2011 (magenta) against the average for 2000 to 2011 (black with one sigma bounds in dashed lines). So the area of melt in 2007 and 2011 rose rapidly in early June, exactly when the CT area anomalies drop off a cliff. More than that, the difference between the recent average and peak area of melt ponds is around 0.5M km^2, which is a substantial fraction of the drop in anomalies in 2007 and 2011 shown above.
The overall path of the CT area anomalies through the summer isn't due to melt ponds, the end of season state of the ice shows this. However one reason why the professionals tend to prefer extent to area is that area is known to be prone to error from counting melt ponds as open ocean. Is there something about the processing behind CT area that amplifies this effect in early June? I don't know enough about their processing technique to say so, but it's a possibility that has to be borne in mind. I've interpreted the rapid drops in CT area anomalies as rapid retreat of ice as the melt edge enters to basin: In 2007 due to fast early melt as a result of weather conditions (Arctic Dipole). In 2011 due to fast retreat due to first year ice prevalance and previous thinning. However part of this drop in anomalies could be due to melt ponds. Against that however, It is worth bearing in mind that ice extent series, such as JAXA also show a deviation from previous periods in recent years from early June. JAXA extent anomalies also show a drop in June, but not as precipitous as CT area.
2011 didn't have the open skies that 2007 had, in general the weather wasn't as conducive to ice melt. Rosel & Kaleschke note a role of weather, but add that in 2011 the location of open skies in July ties in with the melt ponds, which were concentrated in the Beaufort Sea and Canadian Arctic Archipelago (CAA). This feeds in with another obsession of mine - the role of the atmosphere in hastening the loss of ice, but I'll get on to that later in this post. However all of this doesn't mean that first year ice is 'off the hook' as a likely candidate factor.
In Perovitch & Poshenski the authors find that there is a significant difference in albedo, and seasonal evolution of albedo changes between seasonal or first year ice (FYI) and perennial or multi-year ice (MYI). They use insolation data from 1998, the year of the SHEBA experiment, and apply the difference they have found between FYI and MYI albedo. Over a square meter covered by MYI they find that by September cumulative solar heat input amounts to around 900MJ/M^2, however for a square meter of FYI the gain over the same period, with the same solar input is almost 1200MJ/M^2. That's almost 1/3 more energy gain. Which is what I call a feedback!
Figure 3 of Perovitch & Poshenski shows the annotated time evolution of albedo for seasonal ice (FYI) and MYI.
Above I noted that all years since 2007 show drops in CT area anomalies in early June, I don't think this is weather, it happens in such a short time window at a specific period of the seasonal cycle that interannual variation in weather argues against a weather driven process. The common factor seems to be the ice itself. The transition to thin FYI for large swathes of the Arctic post 2007, combined with the implications of the above graph, a large early deviation in albedo between FYI and MYI, and a possible role for melt ponds due to decreased albedo of FYI during early June seems to me to make a better explanation.
Going back to Rosel & Kaleschke, they provide a graph of the fraction of ice area covered by melt ponds.
If 2007 was due to extreme weather, what happened in 2010 and 2011. As I've discussed previously PIOMAS suggests massive losses of MYI in 2010 starting with a failure of thickening during the winter of 2009/10 and then massive losses off the Canadian Arctic Archipelago. Melt pond area doesn't show such an increase for area, but area has been dropping significantly overall, so I suspect that the best way to consider melt pond changes is using the relative measure of fraction, which takes into account overall decline in sea ice area. Is the increase in 2010 and 2011 due to larger amounts of FYI following the volume loss attributable to MYI in early 2010? I don't know, but I think it could have a role.
Recently I've posted about the late summer Arctic Dipole (AD) and have asked how much of a role this has played in the post 2007 behaviour, a question prompted by the anomalous low index AD following that year. There must be a role for it. However as other studies (Comiso 2011 and Polyavkov 2011) show, after 2007 there was a substantial loss of MYI. Due to my work on PIOMAS gridded data I'm convinced that the transition to a mainly FYI pack can now be considered largely complete. Even for those who aren't persuaded by this, the consistently lower sea ice areas post 2007 automatically imply about 1M km^2 more FYI at the end of each season w.r.t pre-2007 years.
So whilst the early summer AD must have an impact; equally, as Perovitch & Poshenski show, a transition to a largely FYI pack must also have an impact. As for their relative importance, I think that can only be solved by modelling, and the best model for that remains PIOMAS.
Here's a graph of CT area loss from 1 June to minimum of each year in the record.
There are at least two reasons for the jump seen in the years after 2007, changes in ice albedo, and the late summer AD. What other factors are at play? And are other factors as important as those two?
Rosel & Kaleschke, 2012, "Exceptional melt pond occurrence in the years 2007 and 2011 on the Arctic sea ice revealed from MODIS satellite data."
Perovich & Polashenski, 2012, "Albedo evolution of seasonal Arctic sea ice."