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Topics - Rob Dekker

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Arctic sea ice / Bottom melt in Central Arctic ?
« on: June 25, 2013, 09:36:03 AM »
So far, measuring by SIE and SIA, it looks like 2013 is following a path of 'recovery' from the devastating 2012 melting season.

However, as several posters on Neven's main pages have pointed out, the ice in the Central Arctic is in dire straights, after continued pounding and dispersion due to persistent lows over the Central Basin.

It remains to be seen if the dispersion of ice and the many polynias currently present in the Central Basin will be persistent and will collect enough heat to basically "burn a hole in the North Pole" area this year, or if the ice there will compact, and still has enough resilience to withstand the remainder of the summer, to prevent an ice-free North Pole come September.

Interesting in the discussion about the resilience or vulnerability of the Arctic Basin is the observation of Ice Mass Balance buoy 2012J, which seems to show significant bottom melt over the past month :

This buoy is currently located at 87.75 N, 24.30 E, which puts it smack in the middle of the "damage zone" inflicted by the persistent lows which dominated late spring and early summer 2013, and that Neven rightfully names, the Persistent Arctic Cyclone (PAC) of 2013.

There has been some questions about the integrity of the data from this buoy (for starters, it's thermometer went haywire earlier this month, and it's bottom sounder is unreliable) but the data from the thermistor string insists significant bottom melt is occurring there, at least 30 cm over the past month, even though atmospheric temperatures are still close to salt-water freezing (-2 C).

Purpose of this post is to investigate if the bottom melt recorded by 2012J is real, and if so, what is causing it.

For that, I consulted the Ice Tethered Profiler buoys from the good people at Woods Hole Institute. These buoys record temperature and salinity below the ice, from the surface down to 700 meter below the ice.

There are three ITP buoys in the NP area (Nansen basin) which may shed light on 2012J's melting profile. ITP 57, 58 and 61.

ITP 57, at 86.9271° N, 88.2188° E, is closest to 2012J, and just like 2012J also smack in the middle of the slush zone that was affected by the PAC 2013.

The buoy status graphs shows that ITP 57 has been rocking-and-rolling over the past month, presumably as a result of PAC 2013 :

But here is the interesting part :
There area where 2012J, ITP 57, 58 and 61 hang out has a rather shallow halocline.
For example, the halocline under ITP57 is only 25 meters below the surface.
And now it seems that with the Ekman pumping and general turbulence created by the PAC 2013, the salinity right under the ice has increased quite dramatically. For ITP57, salinity increased from 31.5 psu a few months ago to close to 34 psu currently. ITP58 shows an increase from 31.5 to 34 and ITP61 shows increased salinity from 28.5 to 33 psu over just the past month.

Now of course, with increased salinity under the ice, we can expect water temperature below the ice to drop, and the heat to go to bottom melt, since the melting point of ice will be lower. But how much bottom melt does this increased salinity cause ? That depends on how fast and how deep the cold fresh melting water will get mixed with the lower salty waters.

In the case of all three ITPs (57,58 and 61) it seems that the upper 100 meter layer of water has cooled something like 0.2 C over the past month. That suggests that some 82 MJ of energy was transferred from the saltier water to the ice, which would represent something like 25cm bottom melt.

Incidentally that is pretty close to what we see has happened with 2012J.

Also, some posters suggested that maybe 2012J is close to a polynia, and thus may absorb solar energy that causes bottom melt as well. However, if solar energy caused the bottom-melt, then we would expect the water to "freshen" right below the ice. Instead, ITP 57, 58 and 61 all show increased salinity at the surface, which suggests the bottom melt is dominated by salinity increase, not insolation into nearby polynia.

So the hypothesis I propose is that currently bottom melt is occurring in the Central Basin, due to salinity increase below the ice, caused by increased turbulence by the PAC 2013, which stirs up the shallow halocline in the region.

In favor of this hypothesis is data from buoys in the area, notably 2012J, ITP 57, 58 and 61.
Notably also : Against this hypothesis is data from IMB buoy 2012E and 2012B in the area, which show not much bottom melt (yet).

I'm curious to read your thoughts.

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