HadISST and rate of change Ice Extent

[Ice Cool Kim]

Following crandles helpful comments and criticism on my look at question of accelerating Arctic melting here:

https://forum.arctic-sea-ice.net/index.php/topic,232.0.html

I ended up plotting the rate of change arctic ice extent (anomaly) here:
https://forum.arctic-sea-ice.net/index.php/topic,232.msg4211.html#msg4211

I thought it would useful to see how this ties in with temperature. I took Met Office HadISST which is an extrapolated, filled in, continuous coverage dataset.  There may be caveats as will all these datasets but it seemed like a good place to start.

I have inverted the rate of change in ice extent so that it has the same polarity as temperature. The correlation is quite remarkable both in long term variation and in short term detail.

OK, so hot water melts ice, so what ?  Well let's see what else it can tell us.

[Ice Cool Kim]

A couple of points of explanation.

Note the rate of change of ice extent has been lagged by about six months in order to match the short term detail in temp. This means the change in ice extent happens first. Unless I'm misreading this, that would seem a little surprising.  This may be due to the fact that this was from _anomaly_ data, not the actual ice extent.

The swing around 1995 seems to verify that I have lagged in the right direction.

I have not rescaled either vertically but the units of ice area and temp are unrelated, so this should be regarded as arbitrary scaling.  I retained this scaling since the long term trend seems to match quite well. It does not scale the short term for the best visual comparison. I will do another plot to look at that.

Observations:

Short term pattern matches quite well too until 2008.

There is a significant divergence at the end of the record with the two heading off quite strongly in opposite directions.

This is similar to the deviation I found when comparing melting season length to AO, here:

https://forum.arctic-sea-ice.net/index.php/topic,174.msg3291.html#msg3291

The offset applied to align the two plots (ISST+0.6K) indicates an approx "neutral" value of ISST  close to the freezing point of sea water, though I thought that was nearer to -0.4 , this difference may reflect higher salinity just below the ice or some other factor.