Using the G01235 Sea Ice Index the maximum was 14.595M km^2 on 22 February 2015, why the difference with the NSIDC Sea Ice News article? I do not know because it is not clear which index they are using. There has been a major revision to NSIDC sea ice data with reduction of the pole hole and new masks, the pole hole being the gap in coverage due to polar orbiting satellite tracks. Checking G01235 shows that there has indeed been a change in the finalised data, so that is not the issue. Which leaves me puzzled as to what the difference is between Sea Ice News and G01235. However as I have used G01235 throughout the history of this blog, I will continue with that dataset.
Extent is seen to be the lowest for the peak period in post 2007 years, 2015 having a flat maximum period. This is in part due to climate change, however the role of low sea ice in Bering and Okhotsk does not have a clear climate change cause, so it is not possible in my opinion to hold up this year's record minimum as wholly due to climate change. I discussed Bering back in January and the wider state around maximum in my previous post.
However this year has seen the third lowest winter extent on record, and the long term decline is clear and is very likely due to climate change. Current data takes us up to 20 March, just before the equinox, so 1 January to 20 March for 1979 to 2015 makes a reasonably consistent set of data, it is plotted below.
The post 2003 decline is marked, that years may be significant in view of Zhang et al 2008, discussed here, and Wang et al 2009, discussed here. My interpretation being that the dipole mode of atmospheric circulation was found by Zhang to become prominent after 2002, and Wang demonstrates a role for the Arctic Dipole in sea ice loss years. I suspect changes in the atmosphere associated with the winter dipole pattern may play a role in the post 2003 decline of winter sea ice. However I haven't had the time to look into this using regional data.
Note that for late February neither Eurasia nor North America anomalies of snow cover using Rutger's Snowlab data show a step change around 2003, suggesting land warming is not a likely cause.
Now that the maximum has been reached attention turns to the coming melt season, what does the record low of this year, and very early (though not the earliest) date of maximum mean?
I have often said that there is no relationship between the maximum and the minimum extent, this is indeed the case, but may not be the whole story. Comparing maximum and minimum extents is complicated by the fact that both show strong downward trends. To get around this I re-phrased the question of a relationship to: What is the relationship between the difference in successive year's maximum and minimum extents?
The above scatter plot answers the question, there is no relationship between the interannual difference in maximum extent and the interannual difference of the minimum extents for the following melt season minimum. So this year's record low extent tells us nothing about the coming season, which makes sense because maximum extent is set by conditions outside the Arctic Basin, whereas minimum extent is mainly set by conditions within the Arctic Basin.
But this year is a very early maximum, what does that mean?
The above scatter plot shows the interannual difference of minimum extent and the date of maximum, and it reveals an intriguing relationship with a weak R2. In plain English this suggests that: the later the date of maximum the more likely it is that the following summer's minimum extent will be lower than the last year's minimum extent. The low R2 indicates this is merely a suggestion, and it may not be significant.
To examine this further I have calculated the new records in the 1979 to 2014 data. I have then tabled the years of new records with the date of maximum.
|Date of Max
All of these are mid to late March, that may or may not be significant. What counts here is the distribution of all years of date of maximum, that data has been calculated as a histogram with a bin period of 7 days, and the histogram of record years has likewise been calculated.
The dataset is small, however I find the clustering of record years to after 9 March quite persuasive. It seems to me that while the early maximum of 2015 doesn't rule out a record this year it suggests it is not likely, but this data does no more than suggest that as being the case.
My problem now is that I find myself unable to hazard even a guess as to the mechanism that might underlie this possible relationship. One interpretation here is that within a small dataset this pattern is just happening by chance. However I still think it is a real pattern, and the problem lies with my grasping the mechanism.