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Melt vs Slowdown (Winter - Pre-melt season (Water exists in…
Melt vs Slowdown
Winter -
Pre-melt
season
Water source: subglacial melt
No surface melt reaches bed
Water exists in distributed system
i.e. small pockets/ lakes water
with hydraulic transfers btwn them
∴
Water flows through small streams under v high pressure
(ensures do not collapse)
Facilitates ice motion ~ tens m / year
Cumulative melt season effect
Goldberg hypothesises
Channel persists ~4-5 years
Long enough for subglacial network to drain ?
i.e. filling timescale for network
Such strong winter slowdown after 4-5 years
--> perhaps reflects that all water drains away on this timescale ?
3. During
melt season
1. Efficient pathways form
as meltwater self-organises into channels
= Preferential growth of select few channels at expense of others
2. Frictional heat generate by fast flow through channel
--> melts channel walls
Big channel forms - lower pressure
4. End Melt Season
(winter)
Big incised channel remains as a cavity
--> as insufficient meltwater to fill
∴ Even lower pressure
Draws in water from elsewhere in system
(low hydraulic potential)
3. Winter slowdown
as water drawn away from distributed cavity network
Tidewater
glaciers
speedup
Marine terminating glaciers sped up in face of increased melt production
Subglacial discharge flows out @ g.l. into fjord
--> Imposes enhanced submarine melt rates
∴ Undercutting calving front potentially
Melt Volume vs Ice Motion
Annual timescale
no significant relation i.e. for given year
Strongest correlation melt & ice motion
Ice motion & cumulative melt volume from prior 4 years
Tedstone extended study period to include prior years melt volumes
2. Early Melt Season
1. Surface meltwater accesses bed via moulins
2. Meltwater floods the bed
--> overwhelms existing distributed hydrological network
3. Transient hydraulic jacking ∴ fast ice motion
meltwater overpressurises subglacial envmt
ice existing essentially glides over sheet of water
Slowdown Mechanism?
Bigger melt season speedup --> larger incised channels
∴ larger cavity remains @ end melt season
∴ greater potential to draw water away
from elsewhere in subglacial cavity system
Summary:
Over past 3 decades
50% incraese meltwater production
yet no ice speed-up observed
(along land terminating region)
Slowdown
15% below 800m asl
5% < 1100m asl
only seen in annual averaged velocities
*Proposed cause
=increased subglacial drainage efficiency*
Extension of alpine glacier hydrology model?
greater melt - greater winter slowdown
--> compensates summer speedup?
∴ Interannual cumulative effect of melt volume on subglacial envmt contributes to slowdown