The Autumn Inflection, and a Digression into Early Summer

Last week while updating my spreadsheets for the first time since September* and generally catching up I realised that there is a feature that I've not addressed before, the change of rate of sea ice gain in Autumn. *The PIOMAS data is now updated, last updated in August, see top box on the right of the page, top link.


Looking at the charts from the University of Bremen the inflection can be seen.

This is also apparent in NSIDC derived data, so it is not an artefact in one dataset, I have said for some years that it is due to the transition from ice extent growth within the Arctic Ocean to growth outside becoming dominant as the Arctic Ocean fills with ice. But I have never shown that this is the case.

I have taken regional sea ice data from Wipneus and calculated the average seasonal cycles for two 9 year periods, Pre 2007 is 1998 to 2006, Post 2007 is 2007 to 2015. The data for 2015 is only updated to 14 November, but that has little impact. The following graphs show the seasonal cycles of extent for regions outside the Arctic Ocean (green), the Arctic Ocean (red), and total Northern Hemisphere extent (black line). Sea ice extent is given in million kmsq.

The deeper 'cut' into extent is visible during summer, but what is clear from the above is that for both periods the inflection happens about the same time and is at the point at which Arctic Ocean extent starts to level out towards its peak extent and gains outside the Arctic Ocean start to take over.

What is also clear is the steepness of extent gains over the autumn after 2007. To look at this I have calculated and graphed the difference between the two periods for the Peripheral Seas (Beaufort round to Laptev - blue) and All Arctic (Northern Hemisphere - red) extent. The difference between the two (Delta - green) is also plotted.

The All Arctic plot shows the same behaviour as anomalies from the 1981 to 2010 baseline do, a steep decline in early summer, a levelling in the rest of the summer into early autumn, then a very steep rise in the late autumn. The green plot is the difference between the red and blue plots, so it shows the effect of taking out the extent in the Peripheral Seas.

What is clear is that when one takes out the Peripheral Seas (blue) autumn's rapid rise in extent is far more muted (green), therefore the increased rate of sea ice extent formation in the 2007 to 2015 period is due to activity in the peripheral seas. This is a result of rapid icing over after the sun sets for the winter across the larger tracts of open water that have dominated the Post 2007 period as compared to the Pre 2007 period.

However, whilst I have this graph it is worth turning to early summer. The same set of calculations also reveals that when one takes away the earlier melt of the peripheral seas, as shown by the drop in the blue plot from late May to early July, the difference plot (green) loses the steep drop shown in All Arctic extent. This is for extent, so can be taken to be indicative of the area behind the ice edge, excluding the impact of melt ponds. So since 2007 there has been a massively more aggressive retreat of the ice edge in early summer within the Peripheral Seas, something we probably all knew but I haven't explicitly shown like this before.

So how fast is this early melt proceeding in the Peripheral Seas? I have calculated the extent loss from 1 June to 31 July and plotted below as a timeseries.

The overall impression from this is of a fast transition of sorts from around 2004 until 2007 when a new steady state seems to commence. I started writing this blog post a week ago but still have no straightforward answer for this behaviour. However it looks more like a new regime from 2007 onwards, not a slide to greater early summer extent loss, and given that we now have nearly 1/4 of the data representing the post 2007 period this is significant.