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Interannual and decadal variations of ice shelves
using multi-mission satellite radar altimetry,
and links with oceanic and atmospheric forcings
Fernando S. Paolo
Scripps Oceanography
Committee
PhD defense Sep 2, 2015
University of California, San Diego
Helen A. Fricker, Chair
Sarah Gille
Falko Kuester
Jean-Bernard Minster
Laurie Padman
David T. Sandwell
Dissertation structure
Ch1. Introduction
Ch3. Trend analysis
Ch4. Variability analysis
Ch2. Time-series construction
In revision for Remote Sens. Environ.
Published in Science 2015
In preparation for publication
The Antarctic ice sheet interacts
with the ocean through the ice shelves
Modified from NASA
The ice in Antarctica flows
just like rivers do on continents
NASA, Rignot et al. 2011
East
West
Circumpolar Deep Water melts
the ice shelves from below
Ice shelves restrain the flow
from the ice sheet interior to the ocean
Modified from Hughes 2011
gravitational
driving stress
Plan view of an ice shelf
highs
Large portions of the Antarctic ice sheet
are prone to instability
Vaughan and Arthern 2007, Schoof 2007
Grounded below sea level
(prone to instability)
q
H, grounding-line thickness
∝ 
ax
ax-δ
q
q+Δ
Future ice-sheet contribution
to sea-level rise is (highly) uncertain
IPCC AR5, Velicogna andWahr 2013
Total SLR
Thermal
expansion
Mass
increase
Greenland
Antarctica
Summary
The ice sheets are currently
loosing mass at an accelerated rate
Large portions of the Antarctic
ice sheet are prone to instability
Ice-shelf loss ➔ increased ice
discharge ➔ sea-level rise
Scientific questions
How have ice shelves varied over time?
What are the spatial patterns?
What are the links to climate variability?
How widespread are the changes?
Three satellite radar altimetry missions:
18 years of continuous data (1994-2012)
= − ( − )
ERS-1
ERS-2
Envisat
Detecting changes in the vertical component
is challenging over floating ice
∂
∂
= ∂ ∆ − ∂ ρ−
+ ∂ ρ−
+ (ρ−
− ρ−
) ( ˙ + ˙ + ∇ · v)
changes in
ocean height
changes in
ocean density
changes in
firn density
changes in
surface mass
changes in
basal mass
changes in
velocity field
ice-ocean
density contrast
Shepherd et al. 2004; Padman et al. 2012
We averaged height measurements
over 30-km grid cells and 3-month bins
Paolo et al. 2015b
We stack several time series to
smooth out incoherent signal
We estimated trends using
polynomial regularized regression
ˆ( ) = β + β + β + β
Minimizing
Subject to
Fit
(bias)
(variance)

|β | 

( − ˆ )
Lasso regularization:
Tibshirani 1996
Cross validation:
Efron andTibshirani 1993
We estimated uncertainties by
bootstrapping the residuals of the fit
∗
( ) = ˆ( ) + ε∗
( )
ε( ) = ( ) − ˆ( )Residual of the fit
Bootstrap sample
We performed a total of
1,330,000 sets of calculations
Combined error σ =

(σ∗) + (σ )
Trend fit
Derivative
95% CI of the fit
Paolo et al. 2015b
Over 1600 time series
We constructed time series and maps
of ice-shelf height change and acceleration
Paolo et al. 2015a
18 years of changes
show a clear spatial pattern:
West ice shelves are thinning fast
East ice shelves not so much
18% volume loss
in less than
2 decades
Time
There is large variability in the ice-shelf system
Paolo et al. 2015a
Paolo et al. 2015a, 2015b
West Antarctic ice shelves:
Volume-loss rate increased by
~70% from the 1990s to 2000s
East Antarctic ice shelves:
Earlier increase in volume
ceased in the 2000s
All Antarctic ice shelves:
Volume-loss rate accelerated
-25 km3/yr ➔ -310 km3/yr
Schoof et al. 2010, Paolo et al. 2015a
We observe faster ice-shelf melt rates
near the grounding lines
CDW
Summary
At current rates some ice shelves
may disappear within this century
Ice shelves are decaying fast,
leading to Antarctic mass-loss increase
Enhanced inflow of warm CDW
is melting West Antarctica
Short observational records
with different scales in time
with large errors (noisy)
and many simultaneous time series
Motivation
Given
Can we distinguish between regular deterministic
behavior (cycles) and irregular behavior (noise)?
Question
Multivariate Singular Spectrum Analysis
identifies common oscillatory modes
Vautard et al. 1992, Golyandina et al. 2001, Ghil et al. 2002,
Time
Multivariatedataset
Time
Reconstructedcomponent
Window
Rank
EigenvectorEigenvalue
Signal
Noise
Paolo et al. in prep.
140 time series
There is statistically significant energy
at the interannual band in AS
f = 0.22 ➞ T ≈ 4.5 years
Paolo et al. in prep.
Time window
9 years
Time span
18 years
We identified an interannual oscillation
in Amundsen Sea ice-shelf height
NOAA
?
ENSO is the strongest natural climate
fluctuation at interannual time scales
Southern Oscillation Index (SOI)
Paolo et al. in prep.
Time window
6 years
Time span
18 years
Low and hight frequency modes of ENSO
are identified in the SOI series
T ≈ 4.5 years
T ≈ 2.5 years
f = 0.22, 0.40
Paolo et al. in prep.
Low-freq mode of ENSO
Ice-shelf height variability
El Nino events
Interannual ice-shelf height in Amundsen
is strongly correlated with ENSO
Ok, but does this make sense?
ρ  0
(+∆SST)
ρ  0
(−∆SST)
SOI−SST Correlation (ρ)
Riffenburgh 2007, Kwok and Comiso 2002, Cullather et al. 1996
Higher
moisture
convergence
Higher
snowfall
along the coast
Higher
cyclonic
activity
During an El Nino event:
Lower
temperature
along the coast
Summary
There is statistically significant
interannual variability in AS height
This variability is strongly correlated
with El Nino-Southern Oscillation
First direct observational evidence
of the ENSO-AIS teleconnection?
Thank you.
Circumpolar Deep Water melts
the ice shelves from below
Jenkins et al. 2010, Jacobs et al. 2011
Temperature SalinityPine Island
Ice Shelf
As the ice shelves thin,
so does the adjacent grounded ice
Pritchard et al. 2012
Grounded-ice
thinning Ice-shelf thinning
As the ice shelves thin,
so does the adjacent grounded ice
Pritchard et al. 2012
Grounded-ice
thinning Ice-shelf thinning
As the ice shelf is removed
the glaciers behind speed up
Rignot et al. 2004, Scambos et al. 2004
Before collapse After collapse
Horizontal velocity (InSAR) Horizontal velocity (InSAR)Flow rate (Landsat)
The geometry of the bed constrains
the stability of a marine ice sheet
Vaughan and Arthern 2007, Joughin and Alley 2011, Schoof 2007
=
Stable Unstable
Retrograde bed slope
q
ax
Grounding-line thickness H
(steady state)
(at the GL)∝
time 1 time 2
H1
H2
∆ = ( − ρ /ρ ) ∆ ≈ ∆
ρ
ρ
∆
Paolo et al. 2015a
x is time (1994 to 2012)
y is thickness change (m)
rates are in (m/decade)
Short records do not capture the trend
Morris andVaughan 2003, Paolo et al. 2015a
−9℃ isotherm
moving southward
Limit of ice-shelf viability
appears to be moving southward?
Paolo et al. 2015a
CDW
CDW
Paolo et al. in prep.
There is statistically significant energy
at the interannual band
The interannual component explains
a larger portion of the total variance
Interannual
Annual
Riffenburgh 2007
PhD Defense
PhD Defense

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PhD Defense

  • 1. Interannual and decadal variations of ice shelves using multi-mission satellite radar altimetry, and links with oceanic and atmospheric forcings Fernando S. Paolo Scripps Oceanography Committee PhD defense Sep 2, 2015 University of California, San Diego Helen A. Fricker, Chair Sarah Gille Falko Kuester Jean-Bernard Minster Laurie Padman David T. Sandwell
  • 2. Dissertation structure Ch1. Introduction Ch3. Trend analysis Ch4. Variability analysis Ch2. Time-series construction In revision for Remote Sens. Environ. Published in Science 2015 In preparation for publication
  • 3. The Antarctic ice sheet interacts with the ocean through the ice shelves Modified from NASA
  • 4. The ice in Antarctica flows just like rivers do on continents NASA, Rignot et al. 2011 East West
  • 5. Circumpolar Deep Water melts the ice shelves from below
  • 6. Ice shelves restrain the flow from the ice sheet interior to the ocean Modified from Hughes 2011 gravitational driving stress Plan view of an ice shelf highs
  • 7. Large portions of the Antarctic ice sheet are prone to instability Vaughan and Arthern 2007, Schoof 2007 Grounded below sea level (prone to instability) q H, grounding-line thickness ∝ ax ax-δ q q+Δ
  • 8. Future ice-sheet contribution to sea-level rise is (highly) uncertain IPCC AR5, Velicogna andWahr 2013 Total SLR Thermal expansion Mass increase Greenland Antarctica
  • 9. Summary The ice sheets are currently loosing mass at an accelerated rate Large portions of the Antarctic ice sheet are prone to instability Ice-shelf loss ➔ increased ice discharge ➔ sea-level rise
  • 10. Scientific questions How have ice shelves varied over time? What are the spatial patterns? What are the links to climate variability? How widespread are the changes?
  • 11. Three satellite radar altimetry missions: 18 years of continuous data (1994-2012) = − ( − ) ERS-1 ERS-2 Envisat
  • 12. Detecting changes in the vertical component is challenging over floating ice ∂ ∂ = ∂ ∆ − ∂ ρ− + ∂ ρ− + (ρ− − ρ− ) ( ˙ + ˙ + ∇ · v) changes in ocean height changes in ocean density changes in firn density changes in surface mass changes in basal mass changes in velocity field ice-ocean density contrast Shepherd et al. 2004; Padman et al. 2012
  • 13. We averaged height measurements over 30-km grid cells and 3-month bins Paolo et al. 2015b We stack several time series to smooth out incoherent signal
  • 14. We estimated trends using polynomial regularized regression ˆ( ) = β + β + β + β Minimizing Subject to Fit (bias) (variance) |β | ( − ˆ ) Lasso regularization: Tibshirani 1996 Cross validation:
  • 15. Efron andTibshirani 1993 We estimated uncertainties by bootstrapping the residuals of the fit ∗ ( ) = ˆ( ) + ε∗ ( ) ε( ) = ( ) − ˆ( )Residual of the fit Bootstrap sample We performed a total of 1,330,000 sets of calculations Combined error σ = (σ∗) + (σ ) Trend fit Derivative 95% CI of the fit
  • 16. Paolo et al. 2015b Over 1600 time series We constructed time series and maps of ice-shelf height change and acceleration
  • 17. Paolo et al. 2015a 18 years of changes show a clear spatial pattern: West ice shelves are thinning fast East ice shelves not so much 18% volume loss in less than 2 decades
  • 18. Time There is large variability in the ice-shelf system Paolo et al. 2015a
  • 19. Paolo et al. 2015a, 2015b West Antarctic ice shelves: Volume-loss rate increased by ~70% from the 1990s to 2000s East Antarctic ice shelves: Earlier increase in volume ceased in the 2000s All Antarctic ice shelves: Volume-loss rate accelerated -25 km3/yr ➔ -310 km3/yr
  • 20. Schoof et al. 2010, Paolo et al. 2015a We observe faster ice-shelf melt rates near the grounding lines CDW
  • 21. Summary At current rates some ice shelves may disappear within this century Ice shelves are decaying fast, leading to Antarctic mass-loss increase Enhanced inflow of warm CDW is melting West Antarctica
  • 22. Short observational records with different scales in time with large errors (noisy) and many simultaneous time series Motivation Given Can we distinguish between regular deterministic behavior (cycles) and irregular behavior (noise)? Question
  • 23. Multivariate Singular Spectrum Analysis identifies common oscillatory modes Vautard et al. 1992, Golyandina et al. 2001, Ghil et al. 2002, Time Multivariatedataset Time Reconstructedcomponent Window Rank EigenvectorEigenvalue Signal Noise
  • 24. Paolo et al. in prep. 140 time series There is statistically significant energy at the interannual band in AS
  • 25. f = 0.22 ➞ T ≈ 4.5 years Paolo et al. in prep. Time window 9 years Time span 18 years We identified an interannual oscillation in Amundsen Sea ice-shelf height
  • 26. NOAA ? ENSO is the strongest natural climate fluctuation at interannual time scales Southern Oscillation Index (SOI)
  • 27. Paolo et al. in prep. Time window 6 years Time span 18 years Low and hight frequency modes of ENSO are identified in the SOI series T ≈ 4.5 years T ≈ 2.5 years f = 0.22, 0.40
  • 28. Paolo et al. in prep. Low-freq mode of ENSO Ice-shelf height variability El Nino events Interannual ice-shelf height in Amundsen is strongly correlated with ENSO
  • 29. Ok, but does this make sense? ρ 0 (+∆SST) ρ 0 (−∆SST) SOI−SST Correlation (ρ) Riffenburgh 2007, Kwok and Comiso 2002, Cullather et al. 1996 Higher moisture convergence Higher snowfall along the coast Higher cyclonic activity During an El Nino event: Lower temperature along the coast
  • 30. Summary There is statistically significant interannual variability in AS height This variability is strongly correlated with El Nino-Southern Oscillation First direct observational evidence of the ENSO-AIS teleconnection?
  • 32.
  • 33. Circumpolar Deep Water melts the ice shelves from below Jenkins et al. 2010, Jacobs et al. 2011 Temperature SalinityPine Island Ice Shelf
  • 34. As the ice shelves thin, so does the adjacent grounded ice Pritchard et al. 2012 Grounded-ice thinning Ice-shelf thinning
  • 35. As the ice shelves thin, so does the adjacent grounded ice Pritchard et al. 2012 Grounded-ice thinning Ice-shelf thinning
  • 36. As the ice shelf is removed the glaciers behind speed up Rignot et al. 2004, Scambos et al. 2004 Before collapse After collapse Horizontal velocity (InSAR) Horizontal velocity (InSAR)Flow rate (Landsat)
  • 37. The geometry of the bed constrains the stability of a marine ice sheet Vaughan and Arthern 2007, Joughin and Alley 2011, Schoof 2007 = Stable Unstable Retrograde bed slope q ax Grounding-line thickness H (steady state) (at the GL)∝
  • 38. time 1 time 2 H1 H2 ∆ = ( − ρ /ρ ) ∆ ≈ ∆ ρ ρ ∆
  • 39. Paolo et al. 2015a x is time (1994 to 2012) y is thickness change (m) rates are in (m/decade) Short records do not capture the trend
  • 40. Morris andVaughan 2003, Paolo et al. 2015a −9℃ isotherm moving southward Limit of ice-shelf viability appears to be moving southward?
  • 41. Paolo et al. 2015a CDW CDW
  • 42. Paolo et al. in prep. There is statistically significant energy at the interannual band The interannual component explains a larger portion of the total variance Interannual Annual