Sea Level Change Using GRACE and Implications for Continental Hydrology

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Sea Level Change Using GRACEand Implications for
Continental Hydrology
R. S. Nerem Colorado Center for Astrodynamics
Research University of Colorado at Boulder John
Wahr CIRES, University of Colorado Don
Chambers Center for Space Research University of
Texas at Austin
GRACE Hydrology Workshop March 22, 2004
UC Irvine
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TOPEX/Poseidonand Jason
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Global Mean Sea Level from Altimetry
(3.1 mm/year after correcting for PGR)
http//sealevel.colorado.edu
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Tide Gauge Estimates of Global Sea Level Rise

• Investigation
  Rate (mm/year) Note
• Peltier and Tushingham 1989 2.4 0.9 Global
• Barnett 1990 1-2 Global
• Nakiboglu and Lambeck 1990 1.15 0.38 Global
• Trupin and Wahr 1990 1.75 0.13 Global
• Douglas 1991 1.8 0.1 Global
• Shennan and Woodworth 1992 1.0 0.15 Europe
• Unal and Ghil 1995 1.62 0.38 Global
• Davis and Mitrovica 1996 1.5 0.3 U.S. East
  Coast
• Peltier and Jiang 1997 1.8 0.2 U.S. East
  Coast
• Douglas 1997 1.8 0.1 Global

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Steric Sea Level Trends 1955-1996
Cabanes et al., 2001
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Tide Gauge Sea Level versus Dynamic Height
2.0 mm/yr
2.0 mm/yr
0.5 mm/yr
0.2 mm/yr
Atlantic
Pacific
Miller and Douglas, 2004
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Large Eustatic Contribution?
Antonov et al., 2002 (figure from Munk 2003)
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Current Global Sea Level Change Debate
Tide Gauges 1.8 mm/year e.g. Douglas
et al., 2001 Steric Component 0.5 mm/year
Levitus et al., 2000 How to explain the
difference? Theory 1 The tide gauges are
located in regions of greater warming, and thus
are biased high by a factor of
2-3 Cabanes et al., 2001. Holgate and
Woodworth 2004 provide some additional
support for this theory by showing
tide gauges have measured 1 mm/year higher
sea level rise than TOPEX over
1993-2003. Theory 2 There is a large eustatic
contribution of 1.3 mm/year, as observed by
Antonov et al., 2002. This theory
is also supported by Miller and Douglas 2004,
who performed an alternative
analysis of the hydrographic data to Cabanes et
al, and concluded the tide
gauges are not biased.
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TOPEX/Jason EOFs 1-5
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TOPEX/Jason EOFs 1-5
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Tide Gauge Locations
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Acceleration of Sea Level Change?
0.5 mm/year
1.1 mm/year
2.7 mm/year
Rate 1.0 mm/yr (1930-2000) Acceleration
0.0056 mm/yr2 (1930-2000)
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TGR MSL versus Levitus Steric MSL
TGR MSL Observed Steric Change
after Cabanes et al. 2001
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GRACE Global Ocean Mass

• Optimal ocean averaging kernel applied to 11
  GRACE monthly fields (no difference seen when
  averaging from 300 km maps).
• Degree 1 terms are not included.
• Barotropic ocean model contributes no significant
  global ocean mass signal.
• No significant changes seen when averaging is
  restricted to the TOPEX latitudes (66).
• GRACE global ocean mass errors are roughly 3.8 mm
  each month (2 mm when J2 is not included),
  computed two different ways
• A) GRACE pre-launch errors x 40 estimate of
  leakage of land hydrology signal
• B) Using GRACE calibrated error covariance

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Global Ocean Averaging Kernel
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GRACE Global Ocean Mass Variations
Chambers, D. P., R. S. Nerem, and J. Wahr, Global
Ocean Mass Variations from GRACE Gravity Fields,
AGU-CGU Joint Assembly, Montreal, Canada, 2004.
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GRACE Global Ocean Mass Variations
Chambers, D. P., R. S. Nerem, and J. Wahr, Global
Ocean Mass Variations from GRACE Gravity Fields,
AGU-CGU Joint Assembly, Montreal, Canada, 2004.
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Global Water Storage Variations
Chen, J. L., C. R. Wilson, D. P. Chambers, R. S.
Nerem, and B. D. Tapley, Seasonal Global Water
Mass Budget and Mean Sea Level Variations,
Geophys. Res. Lett., 25(19), 3555-3558, 1998.
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Ocean Mass from Hydrologic Variables
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Global Mean Ocean Mass Variations
Chen, J. L., C. R. Wilson, D. P. Chambers, R. S.
Nerem, and B. D. Tapley, Seasonal Global Water
Mass Budget and Mean Sea Level Variations,
Geophys. Res. Lett., 25(19), 3555-3558, 1998.
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Annual Global Ocean Mass Variations
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Runoff Versus Latitude
Dai and Trenberth, 2002
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Regional Runoff
Dai and Trenberth, 2002
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Conclusions

• With only 11 months of data, GRACE has observed a
  clear annual variation in global mean ocean mass,
  with an amplitude of 7.5 mm and a maximum in
  October (280).
• Estimates of global mean ocean mass from TOPEX
  corrected for steric variations show similar
  amplitude (8.7 mm) and phase (288).
• The hydrologic models also reproduce the annual
  ocean mass variations quite well.
• An estimate of global ocean mass from an analysis
  of loading at GPS sites shows a similar amplitude
  (7.6 mm), but an earlier phase (234).
• GRACE clearly has the precision to monitor
  long-term changes in global ocean mass, and
  determine the continental sources of that water
  mass.

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Annual Global Ocean Mass Variations
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Global Mean Sea Level Estimates
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Tide Gauge Sea Level Reconstruction

• TOPEX/Jason EOFs 1-10 covering 1993-2003 (11
  years)
• Testing done different numbers of EOFs, 10
  appears to be optimum at this time
• PSMSL Tide Gauge Data covering 1930-2001
• Revised Level Record (RLR) data used
• Tide gauge must have record length gt 25 years
  between 1970 and 1998
• Record must be 90 complete
• Tide gauge data corrected for land motion using
  ICE-4G post-glacial rebound (PGR) model.
• Both data sets have annual and semi-annual
  signals removed.
• Neither data set has an inverted-barometer
  correction applied.

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Enhanced Coastal Sea Level Rise?
Holgate and Woodworth, 2004
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TOPEX/Jason EOFs 6-10
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TOPEX/Jason MSL EOF 3
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TOPEX/Jason MSL EOF 3
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Tide Gauge Sea Level Reconstruction
Tide Gauge Measurements
From TOPEX and Jason
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Number of Gauges Available (PSMSL, RLR)
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Tide Gauge Locations
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TGR Error Analysis
7 mm
6 mm
3 mm
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EOF Contributions to the TGR MSL
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EOF Contributions to the TGR MSL
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Comparison to Surface Temperatures
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TGR MSL versus PDO
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TGR Secular Sea Level Change 1930-2000
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TGR MSL versus Latitude
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Comparison to Levitus Global Heat Content
Levitus et al. 2001
TGR MSL Observed Heat Content
Ocean-Atm Model Heat Content (GHGs, sulfate
aerosols, solar irradiance, volcanic
aerosols) Ocean-Atm Model Heat Content (same,
minus solar irradiance, volcanic aerosols)
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TOPEX/Jason MSL versus PDO
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TOPEX/Jason Global Sea Level Change Versus
Latitude
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Spatial Pattern of Sea Level Rise (1993-2003)