We are entering the final phase of the melting season. Extent decrease rates are getting lower and lower and soon we will start seeing the first days of extent growth, followed soon after by minimum extent. I figured it might be interesting to have a look at what happened in other years, specifically the years of the 'new Arctic era': 2007, 2008 and 2009. Perhaps we'll see some parallels that give us an idea of when and how the 2010 melting season will end.
I have downloaded a lot of images from different sources and combined them to make animations of the period August 25th-September 24th. The final date is based on when 2007 reached minimum extent.
The first End Zone instalment compared air temperatures, this one is about ice displacement. The arrows show the movement of the ice, and their size reflect the magnitude of displacement. I have retrieved the images from the PIPS ice displacement archive:
Whereas 2007 keeps showing (relatively) big arrows pointing more or less in the same direction all the way until the date of minimum extent (September 24th), 2008 has almost no big arrows on September 10th, the date it reaches its minimum extent. In the days after that small arrows are pointing every which way, and by the time some bigger arrows show up again it is probably too late to battle the onset of freezing temperatures.
The battle takes 12 days in which total extent gains only 66K square km, but after that it's finished. There is no such battle in 2007 after the late date it has reached minimum extent. But as soon as the arrows get smaller, low temperatures and insolation take over the baton immediately and the extent increases very fast.
We see the same thing happening with 2009. One day before minimum extent is reached, September 13th, the arrows get divided and lose their unity and strength. By the time they regroup, it is too late to drive the minimum extent further down.
By the way, we saw the same thing happening during July when low-pressure areas dominated the Arctic and the PIPS ice displacement arrows remained dazed and confused for weeks on end. That is what killed the July and first half of August extent decrease rates. You can read back my SIE updates, such as this one where I compare July 2010 ice displacement with that of July 2007.
So my advice for the remainder of the season is this: Keep an eye on the PIPS ice displacement maps. As long as there are big arrows, the last day of extent decrease is postponed. Small arrows pointing in all directions after the 10th of September and it's game over.
Of course SLP forecasts are also a good indicator, as the distribution and strength of low and high-pressure areas determine whether winds are strong and whether sea ice is pushed in the right direction. That's what we'll look at in the next End Zone instalment.