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Mechanism Ice Stream Acceleration (Grounding zone (= region of bed which…
Mechanism Ice Stream Acceleration
Grounded Ice Velocity ∆
Ice speed-up observed due to ice shelf thinning
in past few decades
e.g. Pine Island, Thwaites E. Ice Shelf/Ice Tongue
How does ice shelf thinning cause ice stream acceleration?
Grounding
zone
= region of bed which 'feels' effect of ice shelf pulling on it
i.e. ice shelf 'yanks' on the grounding zone
∴ more pulling force --> higher velocity across grounding zone
= bed region behind grounding line supporting all driving stress from ice shelf
Horiz Stress Train Analogy
for transfer of forces in ice sheet/stream
Ice shelf = train car with no breaks
∴ yanks on upstream ice section
exerts greater force on other cars
Longitudinal stress limit
Obviously a limit exists
wrt how far upstream stress propagates
i.e. from ice shelf
Ice flux across grounding line
(Qgl)
Ice flux = velocity x depth of g.l.
Qgl = u gl * hgl
All else equal
Pulling force increases with depth across the g.l.
∴ Deeper g.l. --> Faster ice flow just upstream of g.l.
Ice flux into grounding zone
(Qint)
Interior conditions dictate Qint
(ice flow into grounding zone)
Ability of Qint to balance Qgl dictates g.l. stability/position
Grounding
line mvmt
Changes to grounding zone flux (Qint)
causes imbalance of flux/mass balance
May induce thinning/thickening @ g.l.
to satisfy mass continuity
∴ Grounding line moves
Higher g.l. velocity
Higher velocity at
grounding line vs interior ice
despite hydrology change (channelisation usually slows ice)
Pulling force = important
Ice shelf ' yanks ' on grounding zone
Marine Ice Sheet Instability
If g.l. retreats due to flux imbalance
thinning occurs @ coast
1. Greater g.l. flux
since bed now deeper @ g.l.
∴ velocity & thickness both greater
-->twofold effect enhancing Qgl
2. Lower interior flux
Less upstream accumualtion are ∴ Qint likely decrease
Possible positive feedback - imbalance exacerbated
Negative Balance
Negative balance following g.l. retreat
Thinning at coast leads to further retreat
∴ positive feedback with further retreat possible
2D world - no limit to retreat