How Does Arctic Sea Ice Form and
Decay?
Peter Wadhams
Professor of Ocean Physics, Scott Polar Research Institute, University
of Cambridge, UK
(from 1 Jan 2003: Dept. of Applied Mathematics and Theoretical Physics)
|
Sea ice in Winter
in the Beaufort Sea |
Sea ice occupies about 7% of the area of the world ocean, and is
of enormous importance climatically because it reflects most of
the solar radiation that falls on it, affecting the average albedo
of the earth, and also because it interposes a solid layer between
the ocean and the atmosphere which reduces the free transfer of
heat and moisture between the two. Observational evidence at the
moment tells us that the sea ice in the Arctic (although not in
the Antarctic) is retreating and thinning, and computer models predict
that by the 2080s the ice cover will completely disappear in summer,
so it is important for us to understand the mechanisms by which
sea ice forms and decays.
Why does ice float?
We first have to account for the fact that ice floats on water at
all, since ice is one of very few substances where the solid is
less dense than its molten form. This is because the structure
of normal ice, called ice I, is an open hexagonal structure.
Each oxygen atom is at the centre of a tetrahedron with four other
O atoms at the apices. The O atoms are concentrated close to a series
of parallel planes that are known as the basal planes. The principal
axis, or c-axis, of the crystal unit cell lies perpendicular to
the basal plane. The whole structure looks much like a beehive,
composed of layers of slightly crumpled hexagons. The net of O atoms
is held together by hydrogen bonds. The H atoms lie along these
bonds. It is the length of the hydrogen bond that creates the open
structure of ice; when ice melts, some of the bonds are broken,
causing a disordered structure with a higher density. But even in
liquid water some short-range order remains, with a few water molecules
retaining the crystal-like bonded structure until this is destroyed
by thermal motion; this causes a curious density behaviour in fresh
water, where there is a maximum density at 4°C.
Cooling the water down
Consider a fresh water body being cooled from above, for instance
a lake at the end of summer experiencing subzero air temperatures.
As the water cools the density increases so the surface water sinks,
to be replaced by warmer water from below, which is in its turn
cooled. This creates a pattern of convection through which the whole
water body gradually cools. When the temperature reaches 4°C,
the lake reaches its maximum density. Further cooling results in
the colder water becoming less dense and staying at the surface.
This thin cold layer can then be rapidly cooled down to the freezing
point, and ice can form on the surface even though the temperature
of the underlying water may still be close to 4°C. Thus a lake
can experience ice formation while considerable heat still remains
in the deeper parts.
This does not apply to sea water. The addition of salt to the water
lowers the temperature of maximum density, and once the salinity
exceeds 24.7 parts per thousand (most Arctic surface water is 30-35),
the temperature
of maximum density disappears. Cooling of the ocean surface
by a cold atmosphere will therefore always make the surface water
more dense and will continue to cause convection right down to the
freezing point - which itself is depressed by the addition of salt
to about -1.8°C for typical sea water. It may seem, then, that
the whole water column in an ocean has to be cooled to the freezing
point before freezing can begin at the surface, but in fact the
Arctic Ocean is composed of layers of water with different properties,
and at the base of the surface layer there is a big jump in density
(known as a pycnocline), so convection only involves the surface
layer down to that level (about 100-150 metres). Even so, it takes
some time to cool a heated summer water mass down to the freezing
point, and so new sea ice forms on a sea surface later in the autumn
than does lake ice in similar climatic conditions.
How ice forms in calm water
In quiet conditions the first sea ice to form on the surface is
a skim of separate crystals which initially are in the form of tiny
discs, floating flat on the surface and of diameter less than 2-3
mm. Each disc has its c-axis vertical and grows outwards laterally.
At a certain point such a disc shape becomes unstable, and the growing
isolated crystals take on a hexagonal, stellar form, with long fragile
arms stretching out over the surface. These crystals also have their
c-axis vertical. The dendritic arms are very fragile, and soon break
off, leaving a mixture of discs and arm fragments. With any kind
of turbulence in the water, these fragments break up further into
random-shaped small crystals which form a suspension of increasing
density in the surface water, an ice type called frazil
or grease
ice. In quiet conditions the frazil crystals soon freeze together
to form a continuous thin sheet of young ice; in its early stages,
when it is still transparent, it is called nilas.
When only a few centimetres thick this is transparent (dark nilas)
but as the ice grows thicker the nilas takes on a grey and finally
a white appearance. Once nilas has formed, a quite different growth
process occurs, in which water molecules freeze on to the bottom
of the existing ice sheet, a process called congelation
growth. This growth process yields first-year
ice, which in a single season in the Arctic reaches a thickness
of 1.5-2 m.
How ice forms in rough water
If the initial ice formation occurs in rough water, for instance
at the extreme ice edge in rough seas such as the Greenland or Bering
Seas, then the high energy and turbulence in the wave field maintains
the new ice as a dense suspension of frazil, rather than forming
nilas. This suspension undergoes cyclic compression because of the
particle orbits in the wave field, and during the compression phase
the crystals can freeze together to form small coherent cakes of
slush which grow larger by accretion from the frazil ice and more
solid through continued freezing between the crystals. This becomes
known as pancake
ice because collisions between the cakes pump frazil ice suspension
onto the edges of the cakes, then the water drains away to leave
a raised rim of ice which gives each cake the appearance of a pancake.
At the ice edge the pancakes are only a few cm in diameter, but
they gradually grow in diameter and thickness with increasing distance
from the ice edge, until they may reach 3-5 m diameter and 50-70
cm thickness. The surrounding frazil continues to grow and supply
material to the growing pancakes.
At greater distances inside the ice edge, where the wave field
is calmed, the pancakes may begin to freeze together in groups and
eventually coalesce to form first large floes,
then finally a continuous sheet of first-year ice known as consolidated
pancake ice. Such ice has a different bottom morphology from
normal sea ice. The pancakes at the time of consolidation are jumbled
together and rafted over one another, and freeze together in this
way with the frazil acting as "glue". The result is a
very rough, jagged bottom, with rafted cakes doubling or tripling
the normal ice thickness, and with the edges of pancakes protruding
upwards to give a surface topography resembling a "stony field".The
rough bottom is an excellent substrate for algal growth and a refuge
for krill. The thin ice permits much light to penetrate, and the
result is a fertile winter ice ecosystem.
In the Arctic, a key area where pancake ice forms the dominant
ice type over an entire region is the so-called Odden
ice tongue in the Greenland Sea. The Odden (the word is Norwegian
for headland) grows eastward from the main East Greenland ice edge
in the vicinity of 72-74°N during the winter because of the
presence of very cold polar surface water in the Jan Mayen Current,
which diverts some water eastward from the East Greenland Current
at that latitude. Most of the old ice continues south, driven by
the wind, so a cold open water surface is exposed on which new ice
forms as frazil and pancake in the rough seas. The salt rejected
back into the ocean from this ice formation causes the surface water
to become more dense and sink, sometimes to great depths (2500 m
or more), making this one of the few regions of the ocean where
winter convection occurs, which helps drive the entire worldwide
system of surface and deep currents known as the thermohaline circulation
(or "Great
Ocean Conveyor Belt").
Growth of the ice
Once a continuous sheet of nilas has formed, the individual crystals
which are in contact with the ice-water interface grow downwards
by freezing of water molecules onto the crystal face. This freezing
process is easier for crystals with horizontal c-axes than for those
with c-axes vertical. The crystals with c-axis horizontal grow at
the expense of the others, and as the ice sheet grows thicker crowd
them out in a form of crystalline Darwinism . Thus the crystals
near the top of a first-year ice sheet are small and randomly oriented,
then there is a transition to a fabric composed of long vertical
columnar crystals with horizontal c-axes. This columnar structure
is a key identifier of congelation ice (i.e., ice which has grown thermodynamically by freezing onto an
existing ice bottom), and is a striking feature of first-year ice
even when viewed by the naked eye.The ions of the salts in sea water
cannot enter the crystal structure despite its open nature. One
might expect all salt to be rejected, therefore, leading to a sea
ice cover composed of pure ice. Such is not the case, however. If
you suck on a piece of first-year sea ice it will taste distinctly
salty. The water from young sea ice may have a salinity of about 10 parts per thousand, dropping to 1-3 in old ice. How
does this salt get into the ice?
The answer lies in the way that the ice sheet grows. The ice-water
interface advances in the form of parallel rows of cellular projections
called dendrites. Brine rejected from the growing ice sheet accumulates
in the grooves between rows of dendrites. As the dendrites advance,
ice bridges develop across the narrow grooves that contain the rejected
brine, leaving the brine trapped and isolated. The walls of the
"prison" close in through freezing, until the salt is
contained in a very
small cell of highly concentrated brine, concentrated enough
to lower the freezing point to a level where the surrounding walls
can close in no further. The cell then remains, a tiny inclusion.
They eventually drain out of the ice, by way of a network of brine
drainage channels which they create, and as the ice sheet ages
the brine concentration drops. These channels have a biological
role. Phytoplankton have been observed to live on their walls,
and even larger zooplankton such as amphipods have been observed
to crawl up the larger channels. Within a channel there is possibly
a higher light level than on the ice bottom, because of the waveguide
effect of the channel for light penetrating from above, while the
oscillating water flow brings nutrients and oxygen to the resident
biological community. In addition the tube provides security from
larger browsers.
The summer melt period
|
Melting
sea ice in Spring in the
Beaufort Sea |
In the Arctic, the overlying snow layer typically begins to melt
in mid-June and is gone by early July. The meltwater from the snow
gathers to form a network of meltwater
pools over the surface of the ice. On first year ice, which
has a smooth upper surface at the end of winter (except where ridged),
the pools are initially very shallow, forming in minor depressions
in the ice surface, or simply being retained within surviving snow
pack as a layer of slush. As summer proceeds, however, this initial
random structure becomes more fixed as the pools melt their way
down into the ice through preferential absorption of solar radiation
by the water, which reflects only 15-40% of the radiation falling
on it compared to 40-70% for bare ice.
As the melt pools grow deeper and wider they may eventually drain
off into the sea, over the side of floes, through existing cracks,
or by melting a thaw hole right through
the ice at its thinnest point or at the melt pool's deepest point.
The downrush of water when a thaw hole opens may be quite violent,
and on very level ice, such as fast ice, a single thaw hole may
drain a large area of ice surface. From the air such thaw holes
give the appearance of "giant spiders", with the "body"
being the thaw hole and the "legs" channels of melt water
draining laterally towards the hole.
The underside of the ice cover also responds to the surface melt.
Directly underneath melt pools the ice is thinner and is absorbing
more incoming radiation. This causes an enhanced rate of bottom
melt so that the ice bottom develops a topography of depressions
to mirror the melt pool distribution on the top side. In this way
an initially smooth first-year ice sheet acquires by the end of
summer an undulating topography both on its top and bottom sides.
Some of the drained melt water may in fact gather in the underside
depressions to form under-ice melt pools,
which refreeze in autumn and partially smooth off the underside,
leaving it with bulges but not depressions.
A final and most important role of the melt water is that some
of it works its way down through the ice fabric through minor pores,
veins and channels, and in doing so drives out much of the remaining
brine. This process, called flushing,
is the most efficient and rapid form of brine drainage mechanism,
and it operates to remove nearly all of the remaining brine from
the first-year ice. The hydrostatic head of the surface meltwater
provides the driving force, but an interconnecting network of pores
is necessary for the flushing process to operate. Given that the
strength properties of sea ice depend on the brine volume, this
implies that the flushing mechanism creates a surviving ice sheet
which during its second winter of existence has much greater strength
than in its first winter.
What happens to the ice that survives?
Ice which has survived one or more summer seasons of partial melt
is called multi-year
ice. In the Arctic, sea ice commonly takes several years to
either make a circuit within the closed Beaufort Gyre surface current
system (7-10 years) or else be transported across the Arctic Basin
and expelled in the East Greenland Current (3-4 years). More than
half of the ice in the Arctic is therefore multi-year ice. Growth
continues from year to year until the ice thickness reaches a maximum
of about 3 metres, at which point summer melt matches winter growth
and the thickness oscillates through an annual cycle. This old,
multi-year ice is much fresher than first-year ice; it has a lower
conductivity and a rougher surface. The low salinity of multi-year
ice makes it much stronger than first-year ice and a formidable
barrier to icebreakers.
Ice doesn't just grow and melt
In this essay we have been concerned with the way in which sea ice
forms and changes under thermal processes alone. Yet we know that
pack ice is constantly in motion, driven by the wind, and that this
produces many important changes to its appearance and development.
The two most obvious features that this creates are leads and pressure
ridges.
The wind stress which drives the sea ice through frictional drag
is integrated over a large area - it has been estimated that in
concentrated pack ice a piece of sea ice responds to wind fields
integrated over a distance of 400 km upwind. Therefore a large-scale
divergent wind field, created by an appropriate pressure pattern,
can also create a divergent stress over a large area of icefield.
Since ice has little strength under tension, this divergence can
open up cracks which widen to form leads.
In winter leads rapidly refreeze because of the enormous temperature
difference between the atmosphere (typically -30°C) and the
ocean (-1.8°C). The heat loss from a newly-opened lead can be
so violent (more than 1000 W m-2) that the lead steams
with frost
smoke from the evaporation and condensation of the surface water.
A young ice cover rapidly forms, within hours, as nilas if the surface
is calm, and this cuts out the evaporation. When a subsequent wind
stress field becomes convergent, the young ice in the refrozen leads
forms the weakest part of the ice cover and is the first part to
be crushed, building up heaps of broken ice blocks above and below
the water line. Such a linear deformation feature is called a pressure
ridge, the above-water part being the sail and the below-water part (more extensive) being called the keel.
Keels in the Arctic can reach down to 50 m, although most are about
10-25 m deep. Ridged ice in the Arctic makes a major contribution
to the overall mass of sea ice; probably about 40% on average and
more than 60% in coastal regions.
Because of leads and ridges the "landscape" of the Arctic
Ocean is an ever-changing panorama of white fields separated by
white hedges and walls (ridges) , and with rivers and streams (leads)
appearing at unpredictable locations. The whole array passes through
a seasonal cycle of growth and decay which, if present trends continue,
will eventually lead to the disappearance of the ice and an open
Arctic Ocean.
Illustrations
The crystal structure of ice
The temperature of the density maximum (Tpmax) and the freezing
point (Tf) of seawater as a function of salinity S, showing how
they meet at 27.4 psu. Density contours of sea water are also shown.
Pancake ice
Consolidated pancake ice in the Antarctic (continuous sheet)
The
Odden ice tongue in the Greenland Sea on February 12, 1993, shown
by satellite passive microwave imagery. The different colours represent
different percentage sea ice covers.
Sheets of brine cells in sea ice, seen under a microscope. Spacing
of cells about 0.6 mm.
Geometry of a typical brine drainage channel
Frost smoke from a freshly opened lead in winter
The sail of an Arctic pressure ridge
The keel (underwater part) of an Arctic pressure ridge
Ice Photographs
Ice photographs
from a ship in the Ross Sea (Antarctica): Grease ice, pancake ice,
and pack ice from the Virginia Institute of Marine Science at the
College of William and Mary
KidSat: Sea
Ice images from space
Alfred
Wegenener Institute - Sea ice formation
Pancake
ice in Bering Sea
Closeup
of pancake ice in the Bering Sea
Ice floes
in the Bering Sea
Closeup
of ice floes in the Bering Sea
Spring
melt
Winter
sea ice in the Beaufort Sea
Sea ice
off of Tigvariak Island
Melting
ice on Beaufort Sea
Jelly
fish floating under arctic ice
Other Essays on Sea Ice
Changes in Arctic sea ice over
the past 50 years: Bridging the knowledge gap between the scientific
community and the Alaska Native community, Rob Mattlin and Henry
Huntington
What changes have occurred in
Arctic sea ice volume and dynamics over the past 50 years? Norbert
Untersteiner
How might sea level be affected by changes
in the Arctic land ice? Roger Barry
What do we know about organisms
which thrive in the Arctic sea ice? Christopher Krembs and Jody
Deming
|