Sevestre and Benn 2015 (7) Flashcards
What are surges defined as?
Quasi-periodic advances or increases in flow speeds unrelated to external triggers that generally occur occur in response to internally driven oscillations in basal conditions.
Surging glaciers exhibit a very wide range of observed characteristics and behaviours…
Encompassing land- terminating and tidewater glaciers, cirque glaciers, valley glaciers and ice streams, as well as both temperate and polythermal regimes (Murray, 2003).
The length of the surge cycle is…
commonly almost constant for a single glacier (Meier and Post, 1969), and tends to be proportional to the length of the surge phase (Dowdeswell and others, 1991).
To maintain a steady state (i.e. to have constant geometry, and velocity and temperature distributions in equilibrium with a hypothetical static climate), a glacier must simultaneously meet two conditions.
First, mass flux through the system must be in equilibrium with surface mass balance.
Second, energy fluxes into and out of the system must balance to maintain constant enthalpy.
In a glaciological context, enthalpy is defined as…
the internal energy of the glacier system, a function of ice temperature and water content (Aschwanden and others, 2012).
Changes in enthalpy result from…
(1) energy exchanges at the glacier surface (radiative and turbulent fluxes, runoff) and the bed (geothermal heat flux, runoff).
(2) frictional heating associated with ice flow.
As the glacier flows downslope, gravitational potential energy is converted into sensible or latent heat, warming and melting ice, respectively. If equilibrium is to be maintained…
these enthalpy gains must be dissipated from the glacier by heat conduction through the ice to the surface, heat advection with flowing ice, and/ or runoff of meltwater.
How can the glacier will decelerate and ice will accumulate within the system?
Conversely, if heat is lost more rapidly than generated by ice flow and geothermal heating, the glacier will decelerate and ice will accumulate within the system.
How can mass be evacuated faster than it can be replaced?
If strain heating near the bed increases ice temperature and/or creates basal meltwater faster than enthalpy can be dissipated from the glacier, positive feedbacks between ice flow processes (creep and sliding) and warming/melting will cause the glacier to accelerate above the balance velocity.
For small glaciers in cold, arid environments, balance fluxes and frictional heating are…
low, increasing the likelihood that enthalpy production can be balanced by conductive losses.
For longer and larger glaciers, balance fluxes and frictional heating…
will have higher balance velocities and therefore higher steady-state basal enthalpy production.
Larger glaciers will also tend to be thicker, reducing conductive heat losses to the atmosphere. Therefore, in cold, dry environments larger glaciers (e.g. the surge-type glaciers of Arctic Canada) are less likely to find a balance between enthalpy production and dissipation than small glaciers.
Surge-type glaciers are also essentially absent at the warm end of the climatic spectrum, why?
In this zone, glaciers are likely entirely temperate due to high summer air temperatures, warming of the snowpack by melting and refreezing, and high mass turnover.
Conductive losses are zero in the temperate case, so enthalpy dissipation can only occur via runoff of meltwater.
We hypothesise that high runoff will be encouraged by
(1) high basal melt rates maintaining efficient drainage systems
(2) seasonal development of efficient basal drainage systems in response to high surface melt and rainfall, and surface-to-bed drainage via crevasses and moulins.
Therefore, although enthalpy production is high in high-turnover glaciers, it may be easy for glaciers within the warm–temperate zone to evacuate excess energy by high water discharge.
