Interannual and decadal variations of ice shelvesusing 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, ChairSarah GilleFalko Kuester

Jean-Bernard MinsterLaurie PadmanDavid T. Sandwell

Dissertation structure

Ch1. Introduction

Ch3. Trend analysis

Ch4. Variability analysis

Ch2. Time-series constructionIn 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 flowsjust like rivers do on continents

NASA, Rignot et al. 2011

East

West

Circumpolar Deep Water meltsthe ice shelves from below

Ice shelves restrain the flowfrom the ice sheet interior to the ocean

Modified from Hughes 2011

gravitationaldriving stress

Plan view of an ice shelf

highs

Large portions of the Antarctic ice sheetare 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 and Wahr 2013

Total SLR

Thermalexpansion

Massincrease

Greenland

Antarctica

Summary

The ice sheets are currentlyloosing mass at an accelerated rate

Large portions of the Antarcticice 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 componentis challenging over floating ice

∂

∂= ∂ ∆− ∂ ρ− + ∂ ρ− + (ρ− − ρ− ) ( ˙ + ˙ +∇ · v)

changes inocean height

changes inocean density

changes infirn density

changes insurface mass

changes inbasal mass

changes invelocity field

ice-oceandensity contrast

Shepherd et al. 2004; Padman et al. 2012

We averaged height measurementsover 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 usingpolynomial regularized regression

ˆ( ) = β + β + β + β

Minimizing

Subject to

Fit

(bias)

(variance)�

|β | �

�( − ˆ )

Lasso regularization:

Tibshirani 1996

Cross validation:

Efron and Tibshirani 1993

We estimated uncertainties bybootstrapping the residuals of the fit

∗( ) = ˆ( ) + ε∗( )

ε( ) = ( )− ˆ( )Residual of the fit

Bootstrap sample

We performed a total of1,330,000 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 changesshow a clear spatial pattern:West ice shelves are thinning fastEast ice shelves not so much

18% volume lossin less than2 decades

Tim

e

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 ratesnear the grounding lines

CDW

Summary

At current rates some ice shelvesmay disappear within this century

Ice shelves are decaying fast,leading to Antarctic mass-loss increase

Enhanced inflow of warm CDWis 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 Analysisidentifies common oscillatory modes

Vautard et al. 1992, Golyandina et al. 2001, Ghil et al. 2002,

Time

Mul

tivar

iate

dat

aset

Time

Rec

onst

ruct

ed c

ompo

nent

Window

Rank

Eige

nvec

tor

Eige

nval

ue

Signal

Noise

Paolo et al. in prep.

140 time series

There is statistically significant energyat the interannual band in AS

f = 0.22 ➞ T ≈ 4.5 years

Paolo et al. in prep.

Time window9 years

Time span18 years

We identified an interannual oscillationin Amundsen Sea ice-shelf height

NOAA

?

ENSO is the strongest natural climatefluctuation at interannual time scales

Southern Oscillation Index (SOI)

Paolo et al. in prep.

Time window6 years

Time span18 years

Low and hight frequency modes of ENSOare 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 Amundsenis 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

Highermoistureconvergence

Highersnowfallalong the coast

Highercyclonicactivity

During an El Nino event:

Lowertemperaturealong the coast

Summary

There is statistically significantinterannual variability in AS height

This variability is strongly correlatedwith El Nino-Southern Oscillation

First direct observational evidenceof the ENSO-AIS teleconnection?

Thank you.

Circumpolar Deep Water meltsthe ice shelves from below

Jenkins et al. 2010, Jacobs et al. 2011

Temperature SalinityPine IslandIce 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 removedthe 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 constrainsthe 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

H1H2

∆ = ( − ρ /ρ )∆ ≈ ∆

ρρ

∆

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 and Vaughan 2003, Paolo et al. 2015a

−9℃ isotherm moving southward

Limit of ice-shelf viabilityappears to be moving southward?

Paolo et al. 2015a

CDW

CDW

Paolo et al. in prep.

There is statistically significant energyat the interannual band

The interannual component explainsa larger portion of the total variance

Interannual

Annual

Riffenburgh 2007