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S. LI, Z. CHENG, AND W.E WEEKS
Leads, pressure ridges, and near-shore shear zones form as a result of sea ice deformation. Leads are of particular geophysical interest because more than half of the
turbulent energy loss from the ocean to the atmosphere in the central Arctic during
the winter is believed to occur from the surfaces of recently formed leads (Badgley
1966; Andreas 1980; Maykut 1982; Makshtas 1991). The heat loss from leads results in
the formation and thickening of the thin, new ice types in these features. In turn, the
salt rejection and brine drainage from this new ice helps to maintain the arctic ocean
halo cline and also drives secondary circulation patterns in the upper ocean (Morison
et al. 1992).
Ridges playa key role in determining the aerodynamic and hydrodynamic coupling
between the ice, the atmosphere, and the ocean. The ridging process is not only a deformational energy sink, but in some regions ridges account for 20% or more of the total
ice volume (Koerner 1973). In addition, the ridging process also results in brine expulsion simply because of the effect of warm temperatures in the resulting keels. Finally,
both lead formation and ridging are important factors contributing to the overall rheological response of sea ice to the imposed external forcings of winds and currents.
Clearly, a better understanding of leads and ridges will be useful in the analysis of the
complex interactions between the ocean, the sea ice, and the atmosphere in the Arctic - interactions that are believed to playa significant role in the climate riddle.
In sea ice deformation studies, the direct quantity that needs to be measured is the
relative motion of a number of regions or floes in the ice pack. Historically, sea ice deformation in the Arctic has been difficult to study because of the remoteness, the harsh
environment, and the expense of carrying out such observations using surface-based
techniques. The first successful regional field campaign was the Arctic Ice Dynamics
Joint Experiment (AIDJEX), which utilized three manned camps separated by distances
of roughly 100 km as well as several data buoys to characterize ice motions over a roughly circular region with a diameter of 500 km (Thorndike and Colony 1977; Pritchard
1980). AIDJEX also included a limited study of smaller-scale ice floe motions near one
of the main camps (Hibler et al.1973,1974). One of the more important long-term results
of AIDJEX has been that it was instrumental in starting the activities that ultimately
led to establishing the Arctic Buoy Program, a program that has continued to this day
and that has contributed significantly to our understanding of the movement of arctic
pack ice (Colony and Thorndike 1984, 1985; Thorndike 1986).
Although data collected from buoy arrays is adequate for basin-scale sea ice models
based on isotropic ice rheology, even the buoy program, in which typically 20 buoys are
deployed over the Arctic Ocean at a given time, does not provide the detailed specification of the motion field that is desirable for studies of regional deformation processes. Overland et al. (1992) have also argued that this type of approach does not appropriately take into account the pertinent scale effects. Some attempts have been made to
apply an anisotropic constitutive model to sea ice (Coon et al.1993). For regional models, including mesoscale models, an examination of the state variables necessary to adequately describe the variation of sea ice (e.g., the principal directions and orientation
of fracture lines) has not been performed. Overland proposed to examine these
processes on a 1-20 km grid that could be used to predict regional ice responses on
100-500 km scales. For such types of study, the Advanced Very High Resolution
Radiometer (AVHRR) aboard the NOAA satellite can be tremendously useful (Overland et al.1992). However, on the other hand, persistent cloud cover during summer and
S. LI, Z. CHENG, AND W.E WEEKS
Leads, pressure ridges, and near-shore shear zones form as a result of sea ice deformation. Leads are of particular geophysical interest because more than half of the
turbulent energy loss from the ocean to the atmosphere in the central Arctic during
the winter is believed to occur from the surfaces of recently formed leads (Badgley
1966; Andreas 1980; Maykut 1982; Makshtas 1991). The heat loss from leads results in
the formation and thickening of the thin, new ice types in these features. In turn, the
salt rejection and brine drainage from this new ice helps to maintain the arctic ocean
halo cline and also drives secondary circulation patterns in the upper ocean (Morison
et al. 1992).
Ridges playa key role in determining the aerodynamic and hydrodynamic coupling
between the ice, the atmosphere, and the ocean. The ridging process is not only a deformational energy sink, but in some regions ridges account for 20% or more of the total
ice volume (Koerner 1973). In addition, the ridging process also results in brine expulsion simply because of the effect of warm temperatures in the resulting keels. Finally,
both lead formation and ridging are important factors contributing to the overall rheological response of sea ice to the imposed external forcings of winds and currents.
Clearly, a better understanding of leads and ridges will be useful in the analysis of the
complex interactions between the ocean, the sea ice, and the atmosphere in the Arctic - interactions that are believed to playa significant role in the climate riddle.
In sea ice deformation studies, the direct quantity that needs to be measured is the
relative motion of a number of regions or floes in the ice pack. Historically, sea ice deformation in the Arctic has been difficult to study because of the remoteness, the harsh
environment, and the expense of carrying out such observations using surface-based
techniques. The first successful regional field campaign was the Arctic Ice Dynamics
Joint Experiment (AIDJEX), which utilized three manned camps separated by distances
of roughly 100 km as well as several data buoys to characterize ice motions over a roughly circular region with a diameter of 500 km (Thorndike and Colony 1977; Pritchard
1980). AIDJEX also included a limited study of smaller-scale ice floe motions near one
of the main camps (Hibler et al.1973,1974). One of the more important long-term results
of AIDJEX has been that it was instrumental in starting the activities that ultimately
led to establishing the Arctic Buoy Program, a program that has continued to this day
and that has contributed significantly to our understanding of the movement of arctic
pack ice (Colony and Thorndike 1984, 1985; Thorndike 1986).
Although data collected from buoy arrays is adequate for basin-scale sea ice models
based on isotropic ice rheology, even the buoy program, in which typically 20 buoys are
deployed over the Arctic Ocean at a given time, does not provide the detailed specification of the motion field that is desirable for studies of regional deformation processes. Overland et al. (1992) have also argued that this type of approach does not appropriately take into account the pertinent scale effects. Some attempts have been made to
apply an anisotropic constitutive model to sea ice (Coon et al.1993). For regional models, including mesoscale models, an examination of the state variables necessary to adequately describe the variation of sea ice (e.g., the principal directions and orientation
of fracture lines) has not been performed. Overland proposed to examine these
processes on a 1-20 km grid that could be used to predict regional ice responses on
100-500 km scales. For such types of study, the Advanced Very High Resolution
Radiometer (AVHRR) aboard the NOAA satellite can be tremendously useful (Overland et al.1992). However, on the other hand, persistent cloud cover during summer and