Sea Ice

1
Sea Ice
Source Wikipedia
2
Sea Ice

• Definition Sea ice is a thin layer of ice
  floating on the sea surface that forms when the
  temperature of the water falls below its freezing
  point.
• The salinity of sea water depresses the freezing
  point to about -1.9oC.
• Sea ice is a predominant feature of the polar
  oceans that has dramatic effects on the physical
  characteristics of the ocean surface.

3

• The normal exchange of heat and mass between the
  atmosphere and ocean is strongly modulated by sea
  ice, which insulates the sea surface from the
  usual atmospheric forcing.
• Sea ice also affects albedo, exchange of heat and
  moisture with the atmosphere, the thermohaline
  circulation of the ocean, and the habitats of
  marine life.

4
Formation of an ice cover

• The simplest case is that of a shallow pond of
  calm water on which a thin and often transparent
  layer of ice cover forms.
• It is a continuous sheet of ice called skim and
  can be seen on pools, reservoirs, lakes and
  sluggish streams (velocities lt0.5 m s-1).

5

• Deeply supercooled aqueous solutions generate
  downward growing dendrites, commonly known as
  candle ice.
• More dramatic changes occur when the water is not
  calm turbulent exchange interrupts the
  undisturbed crystal growth, and multiplies the
  effective nucleation rate, thus creating a
  greater number of ice crystals known as frazil
  ice.
• Over time, floes of frazil generate slush
  patches.

6

• The increasingly congealed slush then forms into
  pancake ice and, in turn, pancakes may collect
  and freeze together to produce ice floes.
• In the ocean, the consolidation of pancakes into
  floes eventually leads to a continuous sheet of
  primary sea ice.
• Further growth may take two forms secondary ice
  that is produced on the bottom of the ice cover
  either by direct freezing or by the accretion of
  frazil or superimposed ice that forms on top of
  the ice cover when it is inundated with water.

7
Pancake Ice
8
Source Slaymaker and Kelly (2007)
9

• The other major source of superimposed ice is
  snow.
• The additional weight of the snow causes the ice
  to submerge and water is added to the ice by
  capillarity.
• Bottom ice growth by addition is strongly
  influenced by the temperature and velocity fields
  below.

10

• Once a continuous sheet of ice covers a huge area
  of sea, lake, or river, the process of secondary
  growth will depend on the removal of latent heat
  upwards through the cover this is thermal growth
  or congelation.
• Open fracture lines or leads commonly occur in
  sea ice and are often sites of high biological
  activity.
• A polynya is an ice clearing or area of open
  water, possibly containing some thin ice, in the
  ice pack.

11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15

• A polynya is distinguished from a lead by being a
  broad opening rather than a long, narrow
  fracture.
• Land-fast ice or simply fast ice is sea ice that
  has frozen along coasts (fastened to them) and
  extends out from land into sea.
• Drift ice consists of ice that floats on the
  surface of the water, as distinguished from the
  fast ice, attached to the coast.

16
(No Transcript)
17
(No Transcript)
18

• When packed together in large masses, drift ice
  is called pack ice that may either be freely
  floating or blocked by fast ice while drifting
  past.
• Sea-ice draft is the thickness of the ice that is
  submerged under water, whereas the sea ice
  freeboard is the thickness of the ice that is
  above water.
• The marginal ice zone (MIZ) is an interfacial
  region that forms at the boundary between open
  and frozen oceans.

19
Source Wikipedia
20
Source Wikipedia
21
Distribution

• Sea ice is a key element of the global energy and
  mass transfer system since it has a strong
  insulation capacity that restricts energy, mass,
  and momentum transfer between the atmosphere and
  ocean.
• Through brine rejection during formation, it also
  influences the salinity content, and, therefore,
  the ocean density of the upper ocean layers
  which, in turn, can influence ocean circulation
  and bottom water formation.

22

• Sea ice extent changes seasonally in the polar
  regions at its minimum extent in the northern
  and the southern hemisphere, sea ice covers 6-8
  106 km2 and 2-3 106 km2 respectively and at its
  maximum, it covers 15-16 106 km2 and 18-19
  106 km2 respectively.
• The key variables of interest for sea ice
  characterization are extent, concentration, ice
  type, and thickness.

23

• In the Arctic, some sea ice persists year after
  year, termed multiyear ice (MYI) and some as
  first year ice (FYI) which is seasonal ice
  meaning it melts away and reforms annually.
• Almost all Southern Ocean or Antarctic sea ice is
  FYI in character.
• Characterization of sea ice variations is
  challenging because not only does the extent of
  sea ice change seasonally, but also sea ice does
  not remain in one place it drifts and moves as
  the total extent expands and shrinks seasonally.

24

• Atmospheric synoptic-scale and larger atmospheric
  weather systems tend to drive the dynamics of sea
  ice extent and concentration at temporal scales
  of a few days to weeks.
• Hence, remote sensing observations of sea ice
  need to be frequent and consistent to effectively
  characterize sea ice variables.
• Attempts at measuring sea ice variables using in
  situ techniques are limited at best because the
  hostile polar climate tends to constrain human
  activity.

25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
The egg code
Source Canadian Ice Service
36
(No Transcript)
37
(No Transcript)
38
Annual cycle of sea ice extent in Antarctica
39
Source Wikipedia
40
Source Wikipedia
41
Energetics of sea ice

• The energy balances of snowpacks and glaciers are
  comparatively simple when compared with those of
  sea ice because of its high spatial variability,
  including its ability to be transported by ocean
  currents.
• There are many varieties of sea ice and each of
  them have different spectral albedos.
• Large changes can occur rapidly over the course
  of a summer melt season and also during the fall
  freeze back.

42

• Perhaps the most important aspect of the temporal
  behavior of the spectral albedos of sea ice is
  the general decrease that takes place with the
  onset of the melt season.

43
Mass balance of sea ice

• Sea ice's relatively straightforward (compared to
  land snow cover) and rapid (compared to land ice
  sheets and glaciers) response to atmospheric
  forcing suggests that observations of sea ice
  cover may provide early strong evidence of
  warming in the Arctic.
• Moreover, the sea ice cover is a spatially
  integrated indicator of environmental change by
  contrast with the spotty temperature records
  available for the Arctic.

44

• On the other hand, the dynamic response of sea
  ice to environmental change depends on a complex
  interplay of mechanical and thermodynamic
  processes. Because of ice deformation, a typical
  100 km2 patch of sea ice will contain a variety
  of ice thicknesses.
• These thicknesses range from open water to very
  thick ice, including pressure ridges extending
  possibly 30 m or more below the surface.

45

• On top of this matrix there is often a relatively
  thin snow cover that can cause substantial
  insulation of the ice and reduce its growth rate.
  
• This spatial heterogeneity, especially around the
  edges of the sea ice, makes the mass balance of
  sea ice the most complex in the cryosphere.
• Local growth and melt and horizontal transport
  and deformation alter the local mean thickness
  (ice volume per unit area) and involves exchanges
  of mass (fresh water) with the atmosphere and
  ocean.

46
Source Dery and Tremblay (2004)
47
Climate change and sea ice

• Warming is expected to cause a reduction in the
  area covered by sea ice, which allows increased
  absorption or solar radiation and a further
  increase in temperature.
• This sea ice-albedo positive feedback has the
  potential to produce an ice-free Arctic Ocean
  that is irreversible.

48

• Arctic sea ice extent decreased by approximately
  3 per decade from 1978 to 1996.
• Summer sea ice extent has shrunk by 20 over the
  past 30 years in the Atlantic part of the Arctic
  Ocean.
• Models predict sea ice extent in the Arctic Ocean
  to be reduced by 20 by 2050 (Vinnikov et al.
  1999).

49
(No Transcript)
50
Source Serreze et al. (2000)
51
Source NSIDC
52
(No Transcript)
53
Source NSIDC
54
(No Transcript)
55
Source Rothrock et al. (1999)
56
(No Transcript)
57
(No Transcript)
58
Source Vinnikov et al. (1999)
59
(No Transcript)
60
(2006)
61
(No Transcript)
62
Ecological impacts

• Changes in sea ice distribution will alter the
  seasonal distribution, geographic ranges,
  patterns of migration, nutritional status,
  reproductive success, and abundance and balance
  of species.
• There will be major ecological implications for
  the Arctic, especially habitat loss for certain
  species of seal, walrus, and polar bears.

63

• A shift in the global thermohaline circulation
  would be serious in both the Arctic and Antarctic
  oceans.

64
(No Transcript)