Don P. ChambersDon P. ChambersCenter for Space ResearchCenter for Space Research

The University of Texas at AustinThe University of Texas at Austin

Understanding Sea-Level Rise and VariabilityUnderstanding Sea-Level Rise and Variability

6-9 June, 20066-9 June, 2006

Paris, FranceParis, France

The Potential to Estimate The Potential to Estimate Ocean Thermal Expansion by Ocean Thermal Expansion by

Combining GRACE and Satellite Combining GRACE and Satellite AltimetryAltimetry

GOALSGOALS

• Computing mean ocean mass component of sea level from GRACE

• Potential for combining with altimetry to determine long-term trend in steric sea level

» Steric SL = Altimeter SL - GRACE SL

• Sources of uncertainty in rate estimate for GRACE

» Glacial Isostatic Adjustment (GIA) correction

» Degree 1 gravity coefficients (geocenter)

» Interannual variations in ocean mass and a short record

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Gravity Recovery & Climate Experiment Gravity Recovery & Climate Experiment (GRACE)(GRACE)

Science GoalsMeasure time variable gravity field to detect changes in the water storage and movement from reservoir to another (e.g., from ice sheets to ocean)

MissionJoint NASA/German mission implemented by NASA and DLR (Deutschen Zentrum für Luft-und Raumfahrt) under the NASA Earth System Science Pathfinder Program.Science data processing by University of Texas Center for Space Research (UTCSR) and GeoForschungsZentrum (GFZ)

OrbitLaunched: March 17, 2002Regular Science Data: August, 2002Original Lifetime: 5 yearsRecently NASA/DLR extended mission through 2009

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GRACE ErrorsGRACE Errors

long wavelength short

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•GRACE project produces a set of global gravity coefficients (Clm, Slm) every month

•Can convert these to a time-series of monthly average water level (sea level) over a basin by

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ηba sin =QlΩba sinl,m

∑ W lmCΔClm +W lm

SΔSlm( )

Ql =aρE3ρW

2l +1( )1+ kl( )

Ocean kernel

•Ocean kernel designed to minimize error from GRACE noise AND aliasing of hydrology signals [Swenson and Wahr, JGR, 2002]

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• From CSR_RL01 GRACE coefficients

» Replacing C20 with values from SLR analysis and using seasonal model of C10, C11, and S11 terms (Chambers et al., GRL, 2004)

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Glacial Isostatic AdjustmentGlacial Isostatic Adjustment

• GRACE will measure:

» The long-term gravitational change due to glacial isostatic adjustment (GIA)

» Gravitational changes due to water mass transfer from melting of ice sheets

» Shorter period exchanges of water mass with continents

• Can we model GIA adequately over the ocean to remove this signal?

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GIA in GRACE ObservationGIA in GRACE Observation

• GRACE will observe a fall in SL related to GIA

• Part of drop in sea level measured by GRACE since 2002 is this GIA signal

• M. Tamisiea has calculated that the GIA signal in the GRACE observations ranges from -0.6 to -2 mm/year

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• Adding maximum GIA correction to GRACE changes interpretation of trend significantly

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Degree 1 Gravity VariationsDegree 1 Gravity Variations

• GRACE satellites orbit instantaneous mass center of Earth

• Degree 1 gravity coefficients are zero in a reference frame that is centered on the instantaneous mass center

• Terrestrial reference frame has a fixed center not at instantaneous mass center

• Water mass flux in a terrestrial reference frame will have variations in degree 1 terms

• To use GRACE to measure mass flux in an Earth-fixed frame, we have to model/measure these degree 1 variations

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• Previously demonstrated importance of modeling degree 1 variations for seasonal ocean mass studies [Chambers et al., GRL, 2004; Chambers, JGR, 2006].

• Seasonal models of degree 1 variations have some level of consistency

• Trends are completely unknown

GRACE w/o degree 1 coefficients GRACE w/ degree 1 coefficients

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• Convert simulated water level changes into gravity field coefficients (to degree/order 180)

• Compute ocean mass with and without degree 1 terms

• Result: trend is 0.1 mm/year lower if degree 1 not used.

Greenland: 22.0 cm/m2 water mass lost per year (0.75 mm SL)

Antarctica: 4.1 cm/m2 water mass lost per year (0.75 mm SL)

Oceans: 1.5 mm/year increase in SL

Land: No change

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• We have limited knowledge of interannual variations in ocean mass

• Some evidence of ± 4-5 mm variations at ENSO periods

With 1-year smoothing

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Trend removed from Altimeter - TSL

• Simulate interannual ocean mass by scaling SOI to estimate from J. Willis in 1997-1998

» 55 yr. trend set to zero

• Estimate 95% confidence interval based on standard deviation of trends over 3-15 yr intervals

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Rate Uncertainty for Ocean Mass from Rate Uncertainty for Ocean Mass from GRACE with 3-years of ObservationsGRACE with 3-years of Observations

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Source Uncertainty (mm/year)

Formal 0.3

Knowledge of GIA correction 0.71

Knowledge of degree 1 rates 0.22

3-year period & ENSO-like variability 2.8

RSS (3-year rate)3 0.8

RSS (long-term)4 2.9

1 - from range of GIA corrections2 - doubled simulation estimate to be conservative; systematic!3 - without interannual uncertainty4 - all sources of trend uncertainty

• Why the big difference between in situ TSL and space-based estimates?

» Unknown error in one or more of the systems?

» Changes in deep ocean heat storage not measured by Argo floats?

Yearly averages, maximum GIA correction added to GRACE

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