Geology Reference
In-Depth Information
the polar leads begins to freeze almost immediately when
exposed to the cold atmosphere and becomes completely
frozen within a day or a few days. Accordingly, the sur-
face of leads normally freezes quickly and becomes cov-
ered with thin ice types (e.g., Nilas or gray ice). This
causes a significant increase in upward turbulent flux of
sensible and latent heat. In the summer, the lower albedo
of the water surface in leads causes more absorption of
solar energy than the surrounding ice, which speeds up
the melting of the ice. Leads often branch or intersect,
creating a complex network at the surface of the ice
cover. Open leads release moisture to the atmosphere
while their freezing releases heat.
Lead identification and their spatial statistics in polar
regions are important for a few reason. Leads, as men-
tioned, are major sources of heat and moisture fluxes to
the atmosphere [
Maykut
, 1986]. Since they have much less
insulating capacity and a warmer surface temperature
than the surrounding ice, they trigger ocean‐atmosphere
heat exchange with rates of two orders of magnitude
greater than ice [
Maykut
, 1978]. Identification of leads
and their distributions within the regional sea ice cover
is therefore critical for weather and climate modeling
[
Worby and Allison
, 1991]. Leads also serve as navigable
marine routes even if they are covered with thin ice.
Moreover, they are important for wildlife. Seals, whales,
polar bears, Antarctic penguins, and other animals rely
on leads for access to oxygen and food. Recently, the
remote sensing community has developed more interest
in identification of leads in the Arctic ice since they
become more likely to occur as a result of ice thinning.
Moreover, leads have become the primary means for
determining the sea ice freeboard needed to estimate ice
thickness from airborne and space‐borne altimeters
[
Kwok and Cunningham
, 2008;
Farrell et al
., 2009,
Onana
et al
., 2013].
On a basin‐wide scale, leads form under geostrophic
wind forcing in bands of approximately uniform width,
running parallel to the direction of the major stress in the
ice sheet [
Mellor
, 1986). Leads are more observed in the
western region of the Beaufort Sea in winter. They are
triggered by high‐pressure systems that produce relatively
warm atmospheric temperatures and a series of high
southwesterly winds that drove an atmospheric circula-
tion known as the Beaufort Sea Gyre [
McLaren et al
.,
1987]. The Gyre is a large wind‐driven ocean circulation
pattern that transports the ice in a prevailing clockwise
direction (anticyclonic) from the North American coast
toward Siberia in winter. A study of atmospheric forcing
of the Gyre on ice motion is presented in
Asplin et al
.
[2009]. Winds are quite variable in speed and direction
over the wide spatial scale of the Beaufort Sea ice regime,
particularly when migratory cyclones move through the
region. Given the volume of the ice in the Beaufort Sea,
a wide‐scale wind forcing is needed to drive and sustain
the ice Gyre (which can reverse its direction due to
the passage of an intense storm). Clearly, the required
momentum transfer depends on the wind speed and
roughness of ice surface. The Gyre would be activated
under a minimum sustained wind speed of
x
meters/
second over
y
hours or days, across a minimum area of
z
(M. Asplin, personal communication). The wind speed
range that activates the Beaufort Sea Gyre is still a
research subject. It is worth mentioning here that
Kwok
et al
. [2013] studied the Arctic‐wide sea ice circulation
using a 28 year record and found that positive trends in
drift speed were common in regions with reduced MY ice
coverage. This positive trend in ice drift speed (23.6% and
17.7% per decade in winter and summer, respectively)
could not be explained by the much smaller trends in
wind speed (1.46% and −3.42% per decade in winter and
summer, respectively).
Cracks can only be detected in remote sensing data
using images from very high resolution sensors (<1 m)
such as Ikonos (0.82 m), QuickBird (0.61 m), Worldview‐1
(0.46 m), and GeoEye‐1 (0.41 m). These sources, however,
are not commonly used mainly because they are not
readily available and also because they are very expensive.
Another more commonly used source is aerial photogra-
phy. Detection of cracks is not practically important
except for the fact that it can serve as an indicator of a
possible lead formation. Recently, a team of NASA
researchers discovered that vigorous mixing in the air
above cracks in sea ice in the Arctic pumps atmospheric
mercury down to the exposed water surface [
Moore et al
.,
2014]. This causes toxic pollutants entering marine food
system, which may affect the health of the fish and ulti-
mately humans.
Detection of leads using remote sensing data is more
common as they have more impact on operational marine
navigation as well as energy exchange between ocean and
atmosphere. The rest of this section addresses tools and
techniques of leads detection from remote sensing
observations.
To allow its detection, a lead's width should be at least
double the spatial resolution or the footprint of the sen-
sor. Width of leads varies over a wide range from a few
meters to a few kilometers. Their length extends from a
few hundred meters to a few tens of kilometers in length.
Yet, some synoptic‐scale leads may extend for hundreds
of kilometers.
Kwok et al
. [2009] present a cumulative
distribution of lead width using data from the Arctic Ice
Mapping (AIM) moorings and an average of five sub-
marine cruises during the Scientific Ice Expeditions
(SCICEX) in 1993, 1996, 1997, 1998, and 1999
(Figure 9.7). While the frequency of occurrence of wide
leads (hundreds of meters or a few kilometers wide) is
low, a considerable fraction of the leads are smaller than
