18
P. Wadhams et al.
One particular type of Antarctic ice shows promise of being mapped by a
remote sensing technique. During the Winter Weddell Sea Project cruise of FS
“Polarstern” in 1986 it was discovered [1, 2] that the outermost zone of the
advancing circumpolar sea ice cover in early winter is composed of pancake ice.
This is a type of young ice which forms in turbulent waters. The first stage in its
production is freezing at the sea surface to produce a suspension of small ice crystals called frazil ice. These crystals cannot freeze together to form a continuous
sheet because of the high energy and turbulence in the Southern Océan wave
field, so instead an increasingly dense suspension of frazil ice forms out of which
pancake ice congeals by wave-induced compression of the suspension, a mechanism described by Martin and Kauffman [3]. At the ice edge the pancakes are only
a few centimètres in diameter, but they gradually grow in diameter and thickness
with increasing distance from the ice edge, until they reach 3 to 5 m diameter and
50-cm thickness. The growth occurs by accretion from the frazil, which continues
to form because the open water surface permits maximum ocean-atmosphere
heat flux. The pancakes begin to freeze together in groups, but the wave field is
strong enough to prevent overall freezing until a pénétration of some 270 km
(observed in 1986) is reached. Here the pancakes coalesce to form a continuous
sheet of first-year ice. At this point, with the open water surface eut off, the growth
rate drops to a very low level (estimated at 0.4 cm per day by Wadhams et al. [1])
and the ultimate thickness reached by first-year ice is only a few centimètres more
than the thickness attained at the time of consolidation of the pancakes. If the
observed width of 270 km is a circumpolar phenomenon in early to mid-winter,
this would correspond to an area of 6 000 000 km2 occupied by pancake ice, making this a significant component of the planet’s surface.
Ice formed in this way has a different bottom morphology from Arctic 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 resuit
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” [1]. The rafted bottom provides a large
surface area per unit area of sea surface, providing an excellent substrate for algal
growth and a refuge for krill. The thin ice permits much light to penetrate, and the
resuit is a winter ice ecosystem which is more fertile than that seen in the Arctic.
Because of the large amount of free water on and around the pancakes, the zone
containing this type of ice had formerly been characterised from passive
microwave data as consisting of first-year ice at low concentration [4].
It has been found that in pancake icefields océan waves can be clearly imaged
by satellite-borne synthetic aperture radar (SAR), and these observations show
that the dominant waves often appear to change in wavelength (and direction) as
they enter the ice. The change in dispersion is related to ice thickness, and can be
used as a means of estimating the thickness of frazil and pancake icefields.
Existing expérience has been in the Arctic, but is now being applied to Antarctic
ice.
P. Wadhams et al.
One particular type of Antarctic ice shows promise of being mapped by a
remote sensing technique. During the Winter Weddell Sea Project cruise of FS
“Polarstern” in 1986 it was discovered [1, 2] that the outermost zone of the
advancing circumpolar sea ice cover in early winter is composed of pancake ice.
This is a type of young ice which forms in turbulent waters. The first stage in its
production is freezing at the sea surface to produce a suspension of small ice crystals called frazil ice. These crystals cannot freeze together to form a continuous
sheet because of the high energy and turbulence in the Southern Océan wave
field, so instead an increasingly dense suspension of frazil ice forms out of which
pancake ice congeals by wave-induced compression of the suspension, a mechanism described by Martin and Kauffman [3]. At the ice edge the pancakes are only
a few centimètres in diameter, but they gradually grow in diameter and thickness
with increasing distance from the ice edge, until they reach 3 to 5 m diameter and
50-cm thickness. The growth occurs by accretion from the frazil, which continues
to form because the open water surface permits maximum ocean-atmosphere
heat flux. The pancakes begin to freeze together in groups, but the wave field is
strong enough to prevent overall freezing until a pénétration of some 270 km
(observed in 1986) is reached. Here the pancakes coalesce to form a continuous
sheet of first-year ice. At this point, with the open water surface eut off, the growth
rate drops to a very low level (estimated at 0.4 cm per day by Wadhams et al. [1])
and the ultimate thickness reached by first-year ice is only a few centimètres more
than the thickness attained at the time of consolidation of the pancakes. If the
observed width of 270 km is a circumpolar phenomenon in early to mid-winter,
this would correspond to an area of 6 000 000 km2 occupied by pancake ice, making this a significant component of the planet’s surface.
Ice formed in this way has a different bottom morphology from Arctic 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 resuit
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” [1]. The rafted bottom provides a large
surface area per unit area of sea surface, providing an excellent substrate for algal
growth and a refuge for krill. The thin ice permits much light to penetrate, and the
resuit is a winter ice ecosystem which is more fertile than that seen in the Arctic.
Because of the large amount of free water on and around the pancakes, the zone
containing this type of ice had formerly been characterised from passive
microwave data as consisting of first-year ice at low concentration [4].
It has been found that in pancake icefields océan waves can be clearly imaged
by satellite-borne synthetic aperture radar (SAR), and these observations show
that the dominant waves often appear to change in wavelength (and direction) as
they enter the ice. The change in dispersion is related to ice thickness, and can be
used as a means of estimating the thickness of frazil and pancake icefields.
Existing expérience has been in the Arctic, but is now being applied to Antarctic
ice.