July Status: How low can it go?

Rather then do a proper run down of July conditions I have decided to mostly look forward, although in doing so I cover July. In my previous post on sea ice conditions I covered up to mid July, and essentially not much has changed, the Arctic Dipole has dominated through July and sets the scene for August.

Before I begin, apologies for the stupid 'Cookies' message, this is a result of European Union legislation.

NSIDC Extent is now in the middle of the post 2007 group, having gained ground from being at the top in June.

However this hides detail that I think indicates how August will progress. Looking at the above plot of extent it would be easy to conclude that we're headed for a fairly average August, this overall view neglects conditions on a regional basis.

In the plot below triangles show conditions on 31 July 2015, the vertical bars show the historic range from 1979 to 2015 for that date, results are expressed as the difference from the long term average.

As is to be expected, no region is above average, Beaufort extent is about average, but Chukchi round to Kara are all significantly below average, and it is ice state in these regions that plays a critical role going into August.

Turning now to compactness (the ratio area/extent), Laptev Sea has average compactness, while Beaufort through to the East Siberian Sea (ESS) all show very low compactness. Note that both Barents and Kara are virtually ice free. Baffin and Hudson Bay can likely be ignored in considerations of September extent since they will probably melt out this month.

I will return to the issue of compactness and how it develops over August later in the post.

I have previously said I thought there was a good chance of seeing the Chukchi Sea ice free in July, however for the last 11 days there has been a stall in extent loss in Chukchi.

I have been unable to come up with a reasonable atmospheric explanation, and suspect that what has happened is that the ice edge retreat has stalled as multi year ice has been hit. As I have mentioned in several posts, this is also the explanation for the lack of extent loss in Beaufort.

The export of multi year ice into both Chukchi and Beaufort can be seen in ASCAT for early spring, which is shown below in a blink comparison with 2012.

This is also seen in the Drift Age Model for week 27 (latest available data).

As a final comparison with 2012 I have used extracts from University of Bremen AMSR-2 maps for 31 July. The red line shows the extent on 15 September 2012. 2015 has higher concentration towards the Siberian coast than 2012 outside the red line. Concentration is lower within the 2007 extent line poleward of Beaufort and Chukchi, however the multi year ice export over winter into that region seems to have stalled Chukchi and be leading to a maintenance of extent in Beaufort.

It is for these reasons that I am very sceptical about the possibility of 2015 reaching or beating 2012. There is another reason, in a few days 2012 should start racing ahead due to the August Cyclone of 2012.

However in terms of the atmosphere, 2015 finds a revealing analogue in 2012. Data source NCEP/NCAR.

June 2012 and July 2015 were months of a dominant dipole pattern, while July 2012 and June 2015 were also very similar. If my expectation for the September minimum is correct, and August weather may prevent that, this similarity may reveal that ideal conditions for a low September extent involve a prolonged period of dipole anomaly during the June/July time around the peak of insolation (21 June, the summer solstice) .

Vector winds for July reveal a strong flow across the region from Bering Straits to the Barents Sea, the result of the dipole between low pressure over the Eurasian Arctic coast, and high pressure over the central Arctic Ocean.

The impact of the development of the dipole set up over July is clear and marked. Using anomalies of extent and area over the Peripheral Seas of the Arctic Ocean Basin (Beaufort round to Laptev), the change from the long term average seasonal cycle reveals a shift to much greater than average loss of extent and area from the 1 July. It was around this date that the dipole set up firmly developed.

This has resulted in July extent losses for the whole northern hemisphere (NSIDC Extent) being the largest on record.

For comparison, here are the losses for June.

Since 2010, as a result of the volume loss of that year, June losses have been historically high. Despite warmer temperatures in June 2015 than in 2013 and 2014 the June extent loss this year was more typical of 1998 to 2009.

So the change in behaviour of 2015's summer happened in July with the emergence of a strong Arctic Dipole, without that factor it is hard to see how conditions would have ended up as they are now.

This then takes us to the point of understanding conditions that have set the stage for August. Now to consider conditions from now leading to the September minimum. I had said previously that my guess was that the dipole would continue, that now looks unlikely, as low pressure is taking over across the Arctic Ocean and the GFS Arctic Oscillation ensemble is showing signs of a shift to a high index Arctic Oscillation (i.e. low pressure dominance). But what may favour continued melt is actually such a shift.

The following graphic is adapted from figure 2 of Dong et al, "Critical mechanisms for the formation of extreme arctic sea-ice extent in the summers of 2007 and 1996" PDF (hat tip to Andreas T at the Sea Ice Forum). The top panel shows the downwards energy flux from sunlight (insolation), the bottom panel shows infra-red radiation from clear sky and clouds, both averaged for each month from 2000 to 2010.

What is clear is that by August the energy from sunlight is declining strongly, while downwelling infra-red is still near peak summer levels. This means that for maximum melt one needs cloudy skies with strong downwelling infra-red, not clear skies, clear skies will increase net heat loss by letting more heat out of the atmosphere (to space). However the behaviour of compactness suggests that the main loss of ice through August is not driven by radiative processes but is driven by melt from the ice edge.

Taking Arctic Ocean compactness, through May, June, July compactness falls, compactness being the ratio of area and extent. Falling compactness means that area is decreasing faster than extent, this means that there is thinning within the sea ice pack behind the ice edge. What drives the general decline in compactness through the summer is the absorption of sunlight (and infra-red) within the ice pack behind the ice edge.

By August, as suggested by the graphic above from Dong et al, the sunlight falls off, and sunlight and infra-red mainly offsets the loss of heat by radiation from the sea ice. So radiative melt behind the ice edge largely stalls and the melt of the ice edge takes over. This is why after about the first week of August compactness ceases to fall, then rises as extent retreats faster than the loss of area behind the ice edge. A major factor behind this is is that the seas that have warmed as open water develops through June and July continue to melt ice at the ice edge.

This year through July the open water around the pack has warmed, Neven gives a comparison with 2012 in his recent post Late Momentum.

So how low can this year go?

For the purpose of voting in a poll at the sea ice forum I have outlined a chain of reasoning here. In qualitative terms my expectation is that 2015 September NSIDC Extent will secure a place in the lowest four years, and that as a consequence my SIPN prediction for this year is likely to overshoot*. However I consider it very unlikely that 2012 will be beaten, and equally a repeat of the last two year's increased extent is very unlikely. A caveat here is that I may have underestimated the 'stalling' potential of multi-year ice in Chukch and Beaufort.

Later this week PIOMAS data will be released, with that data it should be possible to see how much of the volume gain of the last two years will have been destroyed by this summer. What will count for that is the Central Arctic, particularly off the Canadian Arctic Archipelago, because the volume increase was predominantly due to ice over 3.3m thick for the PIOMAS gice sub grid thickness distribution in this region as shown in May.