Geology Reference
In-Depth Information
the Beaufort Sea Gyre (BSG) and the Transpolar Drift
Stream (TDS) (Figure 2.52). The BSG is a slowly clock-
wise circulation system centered north of Alaska that
turns the polar ice cap one complete cycle every 4 years or
so. The TDS, on the other hand, transports the ice from
the Laptev Sea and Eastern Siberian Sea across the east-
ern Arctic region and down the east coast of Greenland.
Ice motion in the Antarctic is also influenced by wind,
especially katabatic winds, which are prevalent due to the
large number of glaciers and high sloping topography
fields near the coastline. Katabatic winds are created
from air that is cooled in the evening in mountainous
regions near the coast and that becomes so heavy that it
sinks downward along the sloping mountains and glaciers.
The effect is typically limited to within 40 km of the coast.
Holland and Kwok
[2012] present a remarkable data set of
satellite-tracked sea ice motion in the Antarctic for the
period 1992-2010. It links the statistically significant trends
in Antarctic ice drifts (in most sectors) to local winds.
The three forms of ocean current that contribute to the
ice motion are (1) permanent currents such as the East
Greenland current (moves the ice from the Arctic basin
southward) and the Labrador current (moves the ice
along the east coast of the Baffin Bay to the Labrador
Sea, (2) periodic currents and tides, and (3) temporary
ocean surface currents triggered by local winds. Ocean
currents exert force (water stress) on floating ice, and this
determines long‐term trends of ice motion at monthly or
annual averages. However, in some instances in certain
regions currents are important in determining ice motion
over short periods of time. When triggered by ocean cur-
rents, ice motion is proportional to the roughness of the
ice bottom and the differential velocity between ice and
ocean current. Ocean currents usually act opposite to the
wind force direction, i.e., it becomes a drag force that
resists the wind‐driven ice motion. For example, in order
for ice to move at 2% of the wind speed in the presence of
an adverse ocean current moving at 1 km/h, the wind
speed must be greater than 50 km/h. Since the speed of
the ocean current decreases with water depth, then ice
with greater draft is expected to move slower. However,
this can only be noticeable in the case of very thick MYI
(thicker than, say, 8 m). It is more noticeable in the case of
icebergs, which move slower than the surrounding sea ice.
Internal stress is a resistance force to the ice motion
caused by the compactness and the strength of the ice
cover. The internal stress of the pack ice is directly pro-
portional to the ice concentration and mechanical
strength. The latter depends on ice thickness, tempera-
ture, density, and porosity (as measured by bulk salinity
and brine pockets' geometrical characteristics). This is
the most variable force among the five forces that influ-
ence the ice motion. It represents the loss of momentum
when ice floes collide during their motion. This force
affects the motion of small floes more than it affects big
floes, but it also causes deformation of the ice cover. The
probability of collisions of ice floes increases at higher ice
concentration of smaller floes. This reduces the overall
speed of the ice pack. For example, at the same wind
speed and ocean current, open drift ice (concentration
between 0.4 and 0.6) will move at a speed approximately
three times that of very close/compact pack ice (concen-
tration between 0.9 and 1.0). Consequently, ice motion
caused by onshore wind is expected to be significant
because the wind acts on low concentration ice fields at
the outer boundary of the ice pack. Conversely, offshore
wind is expected to produce less ice motion because the
wind in this case has to act on a compacted ice regime
near the shoreline.
Because of the rotation of the Earth, the Coriolis force
emerges as the force causing objects to alter their direc-
tion of motion. In the Northern Hemisphere, it causes
objects to deflect to the right, and in the Southern
Hemisphere, objects deflect to the left. The effect increases
toward the poles. The Coriolis force affects very large
scale dynamic phenomena such as synoptic‐scale wind
pattern, ocean currents, and polar ice. For sea ice, the
Coriolis force causes ice motion to be between 5° and 15°
to the right of the geostrophic wind in the Arctic region.
The fifth factor that affects ice motion is the sea surface
tilt. This is a minor force caused mainly by the fact that
the ocean surface is not perfectly flat even when it is com-
pletely at rest. Undulating surface is a manifestation of
variation of the gravity field, known as the Earth's geoid.
Other phenomena that contribute to differences in the
ocean surface level include uneven heating, spatial varia-
tion of salinity, and ocean currents especially near coastal
regions or ice shelves. Surface tilt may have a noticeable
effect on ice motion over longer periods measured in
months or years.
10.7.1. Ice Motion from Remote Sensing
Ice motion was first observed in animation of wide
swath coarse resolution satellite data such as AVHRR,
SMMR, and scatterometer imagery. Yet that was merely
qualitative observation with no attempt to quantify the
velocity field. In the early 1990s researchers started to
develop methods for the quantitative tracking of ice
motion in sequential images from thermal infrared,
passive microwave, and radar imagery. Passive microwave
data (SMMR, SSM/I, and AMSR‐E) were particularly
useful because of their excellent temporal resolution
(as manifested in the daily mapping of ice in the polar
regions) as well as their longer records (starting in 1979).
The coarse resolution of the data, however, inhibits
motion tracking in confined areas such as polynyas and
the Canadian Arctic Archipelago. Scatterometer data
