UPDATE: Dr. Walt Meier of NSIDC writes in with some information, seethe end of the article.
I’m getting weary of answering this question in comments, so here it is front page. Note the little bump right about June 1st.
Rick W asks:
Can anyone explain the upward bump in the sea ice extent that seems to occur each June? Apologies if previously covered.
This is a seasonal adjustment to compensate for meltwater on top of the ice, which would ordinarily be viewed as “open water”. Right about now, the Arctic sea ice gets melt pools forming on the surface. If these are not compensated for, sea ice extent will read artificially low.
That being said, I wonder why we don’t see the same adjustment at NSIDC:
I don’t know the answer, but it could be in the difference between SSMI and AMSR-E satellite sensors (NSIDC uses SSMI, JAXA uses AMSR-E).
We also don’t see an adjustment at Cryosphere Today, and they also use SSMI:
Nor does NANSEN:
Click for larger images
If anyone knows why JAXA does the adjustment but the others do not, I’m all ears. My theory is that it is sensor related, but we should find out for sure. I’m swamped today, so I’ll leave this puzzle for WUWT readers to solve.
Dr. Walt Meir writes in with this:
Since you mentioned it on your blog, I can fill in at least some info:
You are correct. When the melt season kicks in the surface water changes
the contrast between ice and water. To more accurately measure the
area/extent, you should adjust coefficients to account for this.
This is done for SSM/I. However, because the SSM/I algorithm is
different from the AMSR-E algorithm (and other differences between the
sensors) the adjustment is different. In SSM/I, the adjustment is
smoother and thus there isn’t that “bump”.
You have to remember that AMSR-E is a research sensor and the algorithms
are still being refined. That is one reason we don’t use AMSR-E for the
long-term timeseries (though the more important reason is the
inconsistency between the two sensors and algorithms).