Melt vs Slowdown
- Winter -
Pre-melt
season
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
∴ Interannual cumulative effect of melt volume on subglacial envmt contributes to slowdown
Water source: subglacial melt
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
2. Early Melt Season
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
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 ?
Extension of alpine glacier hydrology model?
greater melt - greater winter slowdown
--> compensates summer speedup?
No surface melt reaches bed
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
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
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*
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