Melt vs Slowdown

  1. 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

  1. Big channel forms - lower pressure

4. End Melt Season (winter)

  1. Big incised channel remains as a cavity
    --> as insufficient meltwater to fill

∴ Even lower pressure

  1. 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