complex antarctic ice-shelf height changes revealed by eighteen years of satellite radar altimetry
TRANSCRIPT
Complex Antarctic ice shelf thickness change revealed by 18 years of satellite radar altimetry
Fernando S. Paolo*Helen A. FrickerLaurie Padman*[email protected]
AGU Fall Meeting 2014
18 years of continuous satellite radar altimetry over the ice shelves
1994 2012
ERS-1 ERS-2 Envisat
How and what satellite radar altimetry measures
ocean tides atm pressuresea-level risesurf. densificationsurf. penetration ...thickness change
∂h/∂t :
High crossover density3-month time bins30-km spatial cells
We averaged many observations in time and space
Averaging data points
35 days of crossovers
# of crossovers per cell per 3 months
We constructed several records and stacked them to enhance the signal-to-noise ratio
Incoherent
The spatial distribution of crossing points changes with time due to “repeat” satellite tracks not being exact, miss-pointing errors, etc.
Averaging time series
1994 2012
18 years of volume change in East Antarctic ice shelvesVo
lum
e
1994 2012
Is the trend a
Line?
Quadratic?
Cubic?
Volu
meHow do we represent trends?
Tibshirani, 1996
misfit
bias-variance tradeoffWe select λ by
Cross-Validation
min || h − t β ||2 + λ || β ||1Using Lasso:
constrains bias
constrains variance
We fit polynomials: ĥ(t) = ∑ βn tn + ε
Regional time series of thickness change
Sustained ice loss since 1994
East Antarctica ice shelves in phase?
Ice-loss acceleration
5-year straight-line fit (ICESat period)
18-year polynomial fit
We derived average rate of change from the18-year polynomial fits
%-Thickness change 1994-2012
5%loss
5%gain
500 km
25 10 0 10Rate of thickness change
(m/decade)
19941997
20002003
20062009
2012
¡1500
0
+1500
Volume change (km3 ) West East All
Queen Maud
Amery
WilkesAmun
dsen
Bellin
gsha
usen
Larsen
Filchner-Ronne
RossROE
ROW
WIT
SUL
NICGET
DOT
CRO
THW
PINCOS
ABBVEN
STA
BAC
WIL
GEO
LAB
LACLAD
RONFIL
BRU
RII
FIM LAZBAU
PRI
AME
WES
SHA
DIBMERCOO
REN
MAR
DRY
WestAntarctica
EastAntarctica
TOT
MOS
HOL
Some ice shelves have lost up to 18% of their thickness in 18 years
%-Thickness change 1994-2012
5%loss
5%gain
500 km
25 10 0 10Rate of thickness change
(m/decade)
19941997
20002003
20062009
2012
¡1500
0
+1500
Volume change (km3 ) West East All
Queen Maud
Amery
WilkesAmun
dsen
Bellin
gsha
usen
Larsen
Filchner-Ronne
RossROE
ROW
WIT
SUL
NICGET
DOT
CRO
THW
PINCOS
ABBVEN
STA
BAC
WIL
GEO
LAB
LACLAD
RONFIL
BRU
RII
FIM LAZBAU
PRI
AME
WES
SHA
DIBMERCOO
REN
MAR
DRY
WestAntarctica
EastAntarctica
TOT
MOS
HOL
West Antarctic ice shelves:rate of volume loss increasedby ~70% in the past decade East Antarctic ice shelves:
earlier increase in volume stopped in the past decade
All Antarctic ice shelves: rate of volume change -25 km3/yr (pre-2003) ➔ -310 km3/yr (post-2003)
Average rate for different
4.5-year intervals
Short-term ratesare highly variable, and depend on the chosen time interval!
Volume loss from Antarctic ice shelves accelerated in the last decade
Rate of volume loss from WAIS increased by ~70% in the 2000s
West Antarctic ice shelves have lost up to 18% of their thickness in less than 2 decades
Single satellite missions are insufficient to infer the long-term state of the ice shelves
Summary
-9 deg isotherm moving southward
Cook & Vaughan, 2010
Regional atmospheric warming trend?
Different forcings within each environmental setting?
Pritchard et al., 2012
Characteristic signature of ocean-induced basal melting
Faster thinning rates near the grounding lines ➔ increased CDW inflow
A reliable and complete ice shelf mask is a problem.So we (Geir Moholdt) created our own using all data available: MOA (Scambos et al. 2007), ASAID (Bindschadler et al. 2011), InSAR (Rignot et al. 2011), ICESat (Fricker/Brunt et al. 2006-10)
We explore all possible time combinations
These are elevation changes with respect to
different epochs
D J F M A M J J A D J Fsummer autumn winter spring
h(t0) h(t1) h(t2) h(t3) h(tN)
Δh0,1 Δh0,2 Δh0,3 Δh0,... Δh0,N
Δh2,3 Δh2,... Δh2,N
Δh1,2 Δh1,3 Δh1,... Δh1,N
Δh(t,t0)
Δh(t,t1)
Δh(t,t2)
...
Why can we do this? The spatial distribution of crossovers changes with time
As a consequence of hydrostatic balance...
This limits detection of changes in the vertical component
≈