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Nightvid Cole

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When is the typical volume minimum for Arctic sea ice?
« on: July 11, 2015, 03:17:46 PM »
DMI puts it in mid-August, while PIOMAS puts it in September! Do we have clues from submarine data or other sources as to which one is right and which is wrong?

Vergent

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ktonine

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Re: When is the typical volume minimum for Arctic sea ice?
« Reply #2 on: July 11, 2015, 05:17:32 PM »
It would take a grid cell by grid cell comparison of the DMI and PIOMAS during selected time periods in the fall to see where each shows the ice thickening and by how much. And then real world observations to see which is correct.

At higher latitudes regrowth could start in August, but we're still seeing area and extent decline until September. I would tend to side with PIOMAS until proven otherwise.

ChrisReynolds

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Re: When is the typical volume minimum for Arctic sea ice?
« Reply #3 on: July 11, 2015, 06:17:19 PM »
DMI puts it in mid-August, while PIOMAS puts it in September! Do we have clues from submarine data or other sources as to which one is right and which is wrong?

During what period (years) does DMI show this behaviour?

The main process at work in summer is melt, heat flux into the ice/ocean system is positive, therefore there is no net growth of ice and no net increase in volume from mid August...

At least in recent years!

It is feasible that in the old Arctic sea ice system there might be some late season growth within the core of the Central Arctic, as the sun's angle drops near the horizon and the large expanse of ice acts to cool the atmopshere over the ice, with low insolation, low angle of incidence it is just feasible that some growth might begin in August. But I doubt it. NCEP/NCAR shows temperatures for 20 to 30 August dropping to just below zero in the early years i have checked, and a bit warmer in recent years, although 2014 had around or below zero temperatures for that period. This could cause some ice growth, but whether this would offset peripheral melt - that seems unlikely to me. Taking freezing point of sea water as -1.8degC, with say a -3.8degC air temperature, that would give a 2degC temperature difference. This would not drive much ice growth.

Note that PIOMAS Central Arctic region has a negative August September difference every year from 1978 to 2014, so it never shows this behaviour of an August start to melt, even in the Central Arctic, the same goes for the CAA. Looking at daily PIOMAS data for the Central Arctic volume, which only goes back to 2000, in 2000 and 2001 the turnaround to growth starts mid September, not august.

None of the submarine based papers I have read indicates the turn around of the seasonal cycle. I doubt if there is enough data to addres this using such data. Which is why I started in my first paragraph with the physics.

Tor Bejnar

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Re: When is the typical volume minimum for Arctic sea ice?
« Reply #4 on: July 11, 2015, 07:32:51 PM »
If (30 years ago, say) a high arctic ice floe was 4m thick and at the end of winter had ice that was -20C [is this realistic?] covered with snow that didn't entirely melt/sublimate during the summer [insulation from solar gain], heat from both above and below migrate to that coldest ice.  In July and August would any heat still be migrating? [or would the ice column warm to at least -1.7C by then?]  If so, wouldn't some sea water freeze?
Arctic ice is healthy for children and other living things.

oren

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Re: When is the typical volume minimum for Arctic sea ice?
« Reply #5 on: July 11, 2015, 08:21:12 PM »
DMI puts it in mid-August, while PIOMAS puts it in September! Do we have clues from submarine data or other sources as to which one is right and which is wrong?

During what period (years) does DMI show this behaviour?

...


DMI shows this for almost all recent years in this link
ocean.dmi.dk/arctic/icethickness/thk.uk.php

I think it only serves to bring down their model's reliability regarding volume, which has already been doubted in this forum when compared to PIOMAS.

ChrisReynolds

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Re: When is the typical volume minimum for Arctic sea ice?
« Reply #6 on: July 11, 2015, 10:16:37 PM »
Tor,

Ice of many metres thick would not exhibit a large heat flux due to insulation by the ice, so that would argue against ice growth. My main thought expressed above was towards say frozen leads in the centre of the pack that would thin through the spring/summer. Such thinner ice could have sufficient heat coveyance to acrete new ice underneath with only slightly sub zero (~ -5degC) aloft. But the more I think about it the less convinced I am that it could be a major factor.

Oren,

Thanks for the link. I have superimposed a black line tracing over the range of post 2007 PIOMAS tracks, with the scales lined up, time and volume.

When I had first looked at the DMI data I had thought it looked more 'sawtooth' than PIOMAS. Comparing the two plots like this and it is clear that the DMI model has a far more agressive ice loss in the summer. Even with the more agressive losses in most post 2010 years PIOMAS spring/summer losses don't match DMI for aggressiveness.

I don't know which is more accurate regards ice within a season and I lack the numeric data to analyse against other sources.