ALBEDO
Sea ice is much more reflective than the open ocean and its presence causes
most of the incoming solar radiation to bounce off the Earth's surface.
The reflection of solar energy by sea ice has important effects for climate
patterns by restricting energy inputs into regional heat and mass budgets.
Polar ice cover exhibits a wide variety of albedo values depending upon
the presence of particular ice types and phenomena. It usually has an albedo
of ~80%, but dirty sea ice covered with melt ponds can have an albedo as
low as 20% while ice with freshly fallen snow on its surface may reflect
as much as 98% of incoming solar radiation [Gloersen, et al, 1992].
Sea ice albedos almost always fall within
the wide range of values between 50% and 95%; whatever it may be, it is
always more reflective than open water, which has an albedo of 10-15% [Parkinson
lecture, 1998].
The growth and decay of sea ice are positive feedback processes: freezing
and melting processes are self-promoting. When ice melts, for example,
more solar energy is absorbed into the ocean surface waters, thereby promoting
further melting. Alternately, more energy is reflected as ice growth occurs,
restricting the solar radiation that warms the surface of the ocean [Kidsat
Explorations, 1997]. Less sea ice during one season may allow surface
waters to store enough energy to inhibit sea ice formation the following
season.
INSULATING PROPERTIES
The insulating properties of sea ice restrict the exchanges of heat, mass, and momentum that occur across the air-sea contact in open ocean conditions. Local and regional oceanographic and atmospheric characteristics are subsequently altered, changing their influences upon the climate.
heat
Sea ice greatly inhibits the radiation of heat from the relatively warm
ocean to the cold polar atmosphere. There exists a two orders of
magnitude difference between heat released from the open ocean and heat
released by an ice-covered ocean [Parkinson, et al.,1987].
Thick sea ice (>1 meter), can prevent better than 95% of the heat loss
from an otherwise open ocean. Heat
fluxes from the open sea surface range between 100-1000 W/m2
while sea ice thicker than 1 meter limits the heat flux to 5-20 W/m2
[Parkinson lecture, 1998].
Heat transfer is primarily, therefore, an open water phenomenon and the
distributions of thin ice, leads, and polynyas have important implications
upon regional heat budgets [Gloersen, et al, 1992]. Open water within the
ice pack is often the location of very high heat flow. Thermal plumes
of warm, moist air from a recurring polynya off of Bennett Island (eastern
Arctic Ocean) produce cloud formations that can achieve great heights;
some plumes have reached 7 kilometers in altitude [Debneth, 1994]
, the height of the polar tropopause.
mass
Ice cover also inhibits direct exchanges of matter between the ocean and
atmosphere. It becomes a physical barrier, impeding the normal exchange
rates of gases and particulates (like water, oxygen, and salt).
momentum
Momentum exchanges between ocean and atmosphere are also greatly inhibited
by the presence of sea ice. Waves, for example, are dampened rapidly
by ice floes, creating calm conditions within the ice pack [Parkinson,
1997]. Higher frequency waves are dampened rapidly while only the
lowest frequency waves penetrate deeply into the ice pack. Ice-breaking
ships can seek safety within the ice cover when encountering stormy seas
[Parkinson, 1997]. Ice cover also prevents the wind from generating
waves, limiting the transfer of momentum from the atmosphere to the
ocean.
CLIMATIC IMPORTANCE OF ICE FORMATION AND DECAY
Two major changes occur during the transformation of seawater into ice: salt is rejected and heat is expelled. Salt rejection promotes convection of the surface waters and latent heat release is added to the immediate environment.
processes related
to salt rejection
The density of the water directly underneath the ice forming region increases
as salt is expelled from the freezing seawater. The cold saline waters
become denser and possibly unstable enough to initiate overturn,
mixing deeper into the water column. This mixing gets progressively deeper
as the mixed layer becomes denser as a whole [Aagard and Carmack, 1989].
When sea ice formation occurs over continental shelves, well-ventilated
and dense saline waters develop. The high density waters may sink and ventilate
deeper water, potentially becoming contributors to the circulation of the
deep ocean.
Salt
rejection during the formation of sea ice is a distillation process.
Since evaporation and precipitation rates in the polar regions are very
small, the formation and redistribution of sea ice is the only effective
component of the hydrologic cycle at high latitudes [Aagard and Carmack,
1989].
exchanges of latent
heat
Ice formation and decay have mediating effects on local temperatures. As
water changes its state to less energetic, solid ice, it releases its latent
heat into the cold. Conversely, it absorbs heat as it melts because the
liquid state requires more energy to be maintained. Sea ice formation releases
heat under freezing conditions, and sea ice decay absorbs heat under melting
conditions. This results in a moderation of temperature extremes [Parkinson,
et al.,1987].
The distribution of open water within the ice pack therefore tends to represent
large heat fluxes from ocean to atmosphere [Hibler, 1989]. This energy
is released both as sensible heat flow from the warm ocean and as latent
heat as surface water freezes. Most of this heat exchange occurs near the
margins of the ice pack where ice formation processes are most active.
These pronounced energy exchanges at the ice margin have been known to
trigger the genesis of violent weather associated with polar cyclones [Gloersen,
et al, 1992].
TRANSPORT OF SEA ICE
When sea ice drifts away from its region of formation, heat and
freshwater are redistributed, resulting in regional scale exchanges.
Since heat is released at regions of ice formation and heat is absorbed
at the location where the ice melts, the transport of heat is in a direction
opposite to the ice drift. This redistribution of heat has been termed
"negative heat transport" and influences regional temperatures [Parkinson,
et al.,1987].
Similarly, a "negative salt transport" [Parkinson, et al.,1987] redistributes
freshwater as the drifting ice melts at a new location. Sea ice decay adds
water that is about 30 o/oo less
saline than the surrounding seawater [Hibler, 1989].
When sea ice melts, the salinity decrease in the surface water creates
a strong vertical salinity gradient [Parkinson, et al.,1987]. This
halocline acts like a "lid", limiting the depth of the mixed layer and
exchanges with deeper waters. Deep penetration of surface water is
possibleince the fresher, less dense waters require even colder temperatures
to stimulate convection in the water column. Once it does sink, its
colder temperature gives it a higher compressibility than the surrounding
warmer, more saline water. Its density continues to increase due
to this enhanced compressibility and it may penetrate to great depths,
potentially becoming part of deep water circulation [Aagard and Carmack,
1989].
If surface waters are freshened too much, however, even cooling to the
freezing point will be insufficient to induce convection. Such an event
in the North Atlantic can shut off deep circulation and therefore the Gulf
Stream transport of heat [Aagard and Carmack, 1989]. This would send
the Northern Hemisphere into a cold spell, an event that may have been
responsible for the Younger Dryas [Aagard and Carmack, 1989].
The redistribution of heat and freshwater obviously influence local and
regional circulation, but it attains a global character due to the significant
volume of drifting sea ice. The freshwater discharge into the North
Atlantic, for example, is second in volume only to the Amazon River [Aagard
and Carmack, 1989]. Since the ventilation of the global ocean is partly
controlled by the water properties of the Arctic-North Atlantic connection,
the flux of sea ice into it plays an important role in ocean circulation.