Support the Arctic Sea Ice Forum and Blog

Author Topic: Ice Gain and Loss - Key Mechanisms  (Read 3538 times)

logicmanPatrick

  • Frazil ice
  • Posts: 244
    • View Profile
    • The Chatter Box
  • Liked: 0
  • Likes Given: 0
Ice Gain and Loss - Key Mechanisms
« on: December 12, 2016, 02:17:16 AM »
This is not an article so much as a short list of some key mechanisms in the profit and loss account of Arctic ice.

Taking the Arctic as an area which includes 'the twilight zone' - i.e. the northern area which never sees total darkness in winter - it is possible to understand some of the key features and mechanisms which control the balance between ice formation and ice export.

In the distant past, ice would circulate for many years in the Beaufort gyre, becoming even as much as (iirc) 30 year ice.  Old ice was exported mainly through the Nares Strait and the Fram Strait and was replaced by new ice formed for the most part in the Laptev Sea.

The Odden Ice Tongue on the Atlantic side, the Aleutians and other islands on the Pacific side formed ice barriers which slowed down ice export.  Year-on-year shore-bound ice in the various North-West passages prevented export by those routes.

The Laptev factory would create new ice by the effect of wind blowing from land out over the sea.  The new ice was driven by the same wind into the central pack where it would be trapped and spend some years rejecting salt and compacting.


What has changed.

The loss of the Odden, the frequent loss of a solid Aleutian chain ice barrier, the frequent loss of an ice bridge in the Nares Strait and the frequently open North-West passages all add up to a loss of or dramatic weakening of barriers to export.

A reduced size of central pack means the new Laptev ice must travel further to join it, meanwhile being exposed to melting from warm winds and warm currents.

Reduced mass  of floes has three predictable effects. 

1 - loss of keel.   Really old and ridged ice has a deep keel underwater.  Such ice tends to go with the current.  Young ice tends to go with the wind.

2 - loss of mass.   Lighter objects are more readily blown around than heavier objects and, given enough distance travelled, attain higher velocities.

3 - impact velocity.  A fast moving floe coming against land or a stalled mass of ice is more likely to disintegrate than to be merged into thick pack.

Wave and tide action.

In former times, leads would open as thick shore-fast ice was lifted by tidal currents.  These leads would either close again or freeze over.  Thinner ice is shattered by tidal currents.

Waves, whether tide or wind, can readily shatter even exceedingly large floes.  The laws of physics show that the wave energy is converted into the work done in breaking the ice, and this is a heat equivalent.


A smaller pack of younger ice will tend to move more with the wind, less with the currents.

Warm Atlantic and Pacific surface water can now intrude further into the Arctic basin before being forced under the ice.

More open water means a greater fetch for the wind and greater Ekmann transport, so more mixing of water layers.

Ice is lost through transport into warm waters.  The warm water fronts have advanced as the ice has retreated.

So much has changed - even ignoring the meteorology - that we need a new Arctic ice model.
si hoc legere scis nimium eruditionis habes

Anne

  • Grease ice
  • Posts: 527
    • View Profile
  • Liked: 8
  • Likes Given: 2
Re: Ice Gain and Loss - Key Mechanisms
« Reply #1 on: December 12, 2016, 07:17:59 AM »
Really clear and helpful, thank you. It's great to have a summary like this to link to.

Jim Hunt

  • Young ice
  • Posts: 4289
    • View Profile
    • The Arctic sea ice Great White Con
  • Liked: 274
  • Likes Given: 28
Re: Ice Gain and Loss - Key Mechanisms
« Reply #2 on: December 12, 2016, 10:00:43 AM »
More open water means a greater fetch for the wind and greater Ekmann transport, so more mixing of water layers.

See: Importance of waves in the Arctic

Quote
So much has changed - even ignoring the meteorology - that we need a new Arctic ice model.

See for example:

http://forum.arctic-sea-ice.net/index.php/topic,108.msg3352.html#msg3352

and:

neXtSIM: a new Lagrangian sea ice model
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

logicmanPatrick

  • Frazil ice
  • Posts: 244
    • View Profile
    • The Chatter Box
  • Liked: 0
  • Likes Given: 0
Re: Ice Gain and Loss - Key Mechanisms
« Reply #3 on: December 15, 2016, 11:43:46 AM »
Thanks for the comments, Anne and Jim.

Quote
2 - loss of mass.   Lighter objects are more readily blown around than heavier objects and, given enough distance travelled, attain higher velocities.

A new report in Nature.com -

Quote
Rare winter expedition near northern Norway finds weak ice that is increasingly vulnerable to storms.

“This thinner and younger ice in the Arctic today works very differently than the ice we knew,” says Mats Granskog, a sea-ice researcher at the Norwegian Polar Institute in Tromsø and chief scientist on the expedition, called the Norwegian Young Sea Ice (N-ICE2015) project. “It moves much faster. It breaks up more easily. It’s way more vulnerable to storms and winds.”

http://www.nature.com/news/incredibly-thin-arctic-sea-ice-shocks-researchers-1.21163
si hoc legere scis nimium eruditionis habes

logicmanPatrick

  • Frazil ice
  • Posts: 244
    • View Profile
    • The Chatter Box
  • Liked: 0
  • Likes Given: 0
Re: Ice Gain and Loss - Key Mechanisms
« Reply #4 on: December 15, 2016, 12:56:28 PM »
The following links may be of interest.

https://www.researchgate.net/publication/230892132_The_Polar_Oceans_and_Their_Role_in_Shaping_the_Global_Environment

https://www.researchgate.net/publication/235945642_Extracting_tidal_variability_of_sea_ice_concentration_from_AMSR-E_passive_microwave_single-swath_data_A_case_study_of_the_Ross_Sea

http://www.ims.uaf.edu/tide/

Investigation of the ice-tide interaction in the Arctic Ocean

Z. Kowalik and A. Yu. Proshutinsky, 1994. The Arctic Ocean Tides, In: The Polar Oceans and Their Role in Shaping the Global Environment: Nansen Centennial Volume, Geoph. Monograph 85, AGU, 137--158.

http://legacy.sfos.uaf.edu/directory/faculty/kowalik/


si hoc legere scis nimium eruditionis habes

Jim Hunt

  • Young ice
  • Posts: 4289
    • View Profile
    • The Arctic sea ice Great White Con
  • Liked: 274
  • Likes Given: 28
Re: Ice Gain and Loss - Key Mechanisms
« Reply #5 on: December 15, 2016, 04:53:47 PM »
Thanks for the comments, Anne and Jim.

My pleasure Patrick!

Quote
A new report in Nature.com

A slightly older report by yours truly:

Arctic Sea Ice News from AGU
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

logicmanPatrick

  • Frazil ice
  • Posts: 244
    • View Profile
    • The Chatter Box
  • Liked: 0
  • Likes Given: 0
Re: Ice Gain and Loss - Key Mechanisms
« Reply #6 on: December 16, 2016, 05:21:14 AM »
Jim: you beat me to it by a long nose.   :)

A couple more points about mechanisms.

Snow melts faster when you apply salt.  When snow on ice presses the ice down so that it is awash, the snow tends to melt faster unless the air temperature is cold enough to freeze the mush and thicken the ice.

We can learn a lot about sea ice as a floating material by studying basic marine architecture.  In particular, the stresses that apply to a ship in waves apply equally to an ice floe.  The probability that a floe will be broken by a wave varies with the area of the floe and inversely with both thickness and wavelength.

Hogging is the flexing which occurs when only the middle of an object is supported on the crest of a wave.  Sagging is the flexing when the extremities are supported, leaving the middle unsupported.  Hogging and sagging are often described as static events.  In reality, there is a dynamic interchange of buoyancy and gravity forces which rolls along an object such as a ship or ice floe.  This means that a wave which is only just too weak to break an ice floe can, as a wave train, inject so much energy as to entirely shatter the floe.

https://en.wikipedia.org/wiki/Hogging_and_sagging
si hoc legere scis nimium eruditionis habes

Jim Hunt

  • Young ice
  • Posts: 4289
    • View Profile
    • The Arctic sea ice Great White Con
  • Liked: 274
  • Likes Given: 28
Re: Ice Gain and Loss - Key Mechanisms
« Reply #7 on: December 16, 2016, 12:47:07 PM »
The probability that a floe will be broken by a wave varies with the area of the floe and inversely with both thickness and wavelength.

Are you quite sure about that? Sea ice != steel! Gotta reference or three?
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

logicmanPatrick

  • Frazil ice
  • Posts: 244
    • View Profile
    • The Chatter Box
  • Liked: 0
  • Likes Given: 0
Re: Ice Gain and Loss - Key Mechanisms
« Reply #8 on: December 16, 2016, 02:24:17 PM »
Quote
Are you quite sure about that? Sea ice != steel! Gotta reference or three?

No reference needed: it's basic materials science.

Ice, steel, paper, glass, wood etc. - all are materials and all have the mechanical and physical properties of materials to one degree or another.

Steel and ice can both bend, just not to the same degree.  A sheet of steel supported at two widely separated points will bend.  A sheet of ice of equal thickness subjected to the same conditions will break.

That said, it sometimes happens that in a contest between steel and ice the ice wins.  Think Titanic.

Properties of materials -

http://www.virginia.edu/bohr/mse209/chapter6.htm

http://www.science.uwaterloo.ca/~cchieh/cact/applychem/propertyp.html
si hoc legere scis nimium eruditionis habes

Jim Hunt

  • Young ice
  • Posts: 4289
    • View Profile
    • The Arctic sea ice Great White Con
  • Liked: 274
  • Likes Given: 28
Re: Ice Gain and Loss - Key Mechanisms
« Reply #9 on: December 17, 2016, 12:32:52 AM »
What do you make of this Patrick?

Reality is merely an illusion, albeit a very persistent one - Albert Einstein

logicmanPatrick

  • Frazil ice
  • Posts: 244
    • View Profile
    • The Chatter Box
  • Liked: 0
  • Likes Given: 0
Re: Ice Gain and Loss - Key Mechanisms
« Reply #10 on: December 17, 2016, 09:25:07 AM »
Jim: that short video reminds me of the puzzles that Lewis Carroll was fond of.  At first sight there appears to be little information from which to derive a meaningful analysis.  However -

Tidal waves are of very slow and of long wavelength and tend to lift ice floes bodily, so we are looking at a wind-blown wave.

Some patches have snow cover, and are being trodden on, so are reasonably thick.

Other patches have no snow cover so are very young, hence thin ice.

This whole area is likely to be very loosely compacted ice with frozen leads.

The ice, not being homogenous, flexes readily when a wave is driven into it.

The cloud shadows indicate wind motion (at an unknown altitude) towards the camera, which reinforces the idea of wind-blown waves coming across open water.

One could work out the approximate latitude from the length of shadows if given the date.

Summary: the video shows a wave flexing fringe ice near the edge of the pack in the open sea: a common observation.

si hoc legere scis nimium eruditionis habes