-10

0

10

20

30

40

1994 1996 1998 2000 2002 2004 2006

20

30

40Sea Level

Change Measurements:Estimates from Altimeters

Understanding Sea Level Rise and Variability June 6-9, 2006 Paris, France

R. S. Nerem, University of ColoradoR. S. Nerem, University of Coloradoand a host of othersand a host of others

What Are We Trying to Do?What Are We Trying to Do?

• We would like to measure changes in the We would like to measure changes in the volume of the ocean water over timevolume of the ocean water over time

• This is difficult to do, so instead we This is difficult to do, so instead we measure changes in the level of the top measure changes in the level of the top surfacesurface

• When averaged globally, gives us When averaged globally, gives us changes in global mean sea levelchanges in global mean sea level

• If possible, correct this for changes If possible, correct this for changes in the volume of the ocean basins (GIA, in the volume of the ocean basins (GIA, etc.)etc.)

Why T/P and Jason?Why T/P and Jason?• Every component of these measurement Every component of these measurement systems are of the highest possible systems are of the highest possible fidelity (dual-frequency, POD, atmospheric fidelity (dual-frequency, POD, atmospheric delays (ionosphere, troposphere), tidal delays (ionosphere, troposphere), tidal aliasing, etc.)aliasing, etc.)

• Altimeters in sun-synchronous orbits have Altimeters in sun-synchronous orbits have undesirable tidal aliasing characteristics, undesirable tidal aliasing characteristics, especially for detecting climate signalsespecially for detecting climate signals

• Nevertheless, combinations of sun-Nevertheless, combinations of sun-synchronous altimeters (ERS, etc.) with T/P synchronous altimeters (ERS, etc.) with T/P and Jason have been used successfully to and Jason have been used successfully to study sea level change in the polar regions study sea level change in the polar regions above ±66° latitude.above ±66° latitude.

TOPEX/Poseidon and JasonTOPEX/Poseidon and Jason10-day Groundtrack10-day Groundtrack

TOPEX Sea Level: January 1998TOPEX Sea Level: January 1998

Global Mean Sea Level VariationsGlobal Mean Sea Level Variations

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006

TOPEX/PoseidonJason

Year

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

Global Mean Sea Level VariationsGlobal Mean Sea Level Variations

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

Global Mean Sea Level VariationsGlobal Mean Sea Level Variations

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

Seasonal variations removed

Global Mean Sea Level VariationsGlobal Mean Sea Level Variations

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

Seasonal variations removedGIA correction applied (0.3 mm/year)

Rate = 3.2 ± 0.4 mm/year

http://sealevel.colorado.edu

Glacial Isostatic AdjustmentGlacial Isostatic Adjustment

-7.0 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 25.0 Present-Day Radial Deformation (mm/year)

[Milne, 2005]

Effect of High Latitude Altimeter Effect of High Latitude Altimeter DataData

-20

-10

0

10

20

30

40

1992 1994 1996 1998 2000 2002 2004 2006

55° Cutoff66° Cutoff

Year

T/P/Jason GMSL TrendsT/P/Jason GMSL Trends

0

0.5

1

1.5

2

2.5

3

3.5

4

1996 1998 2000 2002 2004 2006Year

Total versus Thermosteric Sea Level Total versus Thermosteric Sea Level ChangeChange

[Willis et al., 2005]

-5

0

5

10

15

20

25

30

35

1992 1994 1996 1998 2000 2002 2004 2006

Total GMSLThermosteric (Willis)

Year

3.2 /mm year

1.1 /mm year

Altimeter - Thermosteric Sea LevelAltimeter - Thermosteric Sea Level

Sea Level Trend versus LatitudeSea Level Trend versus Latitude(1993-2005)(1993-2005)

-1

0

1

2

3

4

5

-60 -40 -20 0 20 40 60

TotalThermostericDifference

Latitude

Sea Level Variations versus Sea Level Variations versus LatitudeLatitude

Global Sea Level Trends: 1993-2005Global Sea Level Trends: 1993-2005

Global Mean Sea Level VariationsGlobal Mean Sea Level Variations

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

-20

-10

0

10

20

30

1992 1994 1996 1998 2000 2002 2004 2006Year

Seasonal variations removedGIA correction applied (0.3 mm/year)

Rate = 3.2 ± 0.4 mm/year

http://sealevel.colorado.edu

GPS ANTENNA (JPL)WVR (JPL)

BUBBLER (NOAA)

ACOUSTIC (NOAA)

EQUIP SHED

MET SENSORS

Point Arguello, CA

+ 54 m

+ 27 m

+ 6 m

LASERSENSOR (CU)

Platform Harvest Calibration SitePlatform Harvest Calibration Site

Harvest Altimeter Bias ResultsHarvest Altimeter Bias Results

-200

-100

0

100

200

300

400

1993 1995 1997 1999 2001 2003 2005

Bias (mm)

LAD Drift(mm/yr)

LSQ Drift(mm/yr)

LAD Bias(mm)

LSQ Bias(mm)EpochAltimeter

+4.0+2.7–0.4+4.31993.0TOPEX-A–3.1–1.0+2.0–6.41993.0POSEIDON-1–5.2–3.0–1.2+3.12002.0TOPEX-B–9.2–12.0+136.0+141.82002.0JASON

[Haines et al., 2006]

Comparison of T/P and Jason-1Comparison of T/P and Jason-1

Map of Jason-1 relative bias (relative to global mean Map of Jason-1 relative bias (relative to global mean of 15.0 cm)of 15.0 cm)– A single calibration site can never accurately determine the A single calibration site can never accurately determine the true global averagetrue global average

– SSH bias affected by multiple sources of geographically SSH bias affected by multiple sources of geographically correlated errors (SSB, orbit, troposphere, etc)correlated errors (SSB, orbit, troposphere, etc) [Chambers, 2006]

Tide Gauges Available for Altimeter Tide Gauges Available for Altimeter CalibrationCalibration

Tide Gauges versus Satellite Tide Gauges versus Satellite AltimetryAltimetry

TOPEX and Jason Tide Gauge TOPEX and Jason Tide Gauge CalibrationsCalibrations

TOPEX

Jason

Error Analysis of the TrendError Analysis of the Trend

• Formal error = 0.05 mm/yearFormal error = 0.05 mm/year

• After accounting for serial correlation of After accounting for serial correlation of residuals, the error scales to 0.2 mm/yearresiduals, the error scales to 0.2 mm/year

• Tide gauge calibration error = 0.4 mm/year Tide gauge calibration error = 0.4 mm/year (dominated by errors in the land motion (dominated by errors in the land motion corrections)corrections)

• This gives a total error in the trend of This gives a total error in the trend of 0.44 mm/year0.44 mm/year

• Thus our final trend estimate is +3.2 ± Thus our final trend estimate is +3.2 ± 0.4 mm/year over 1993-20050.4 mm/year over 1993-2005

• Ignores effects of decadal variability, Ignores effects of decadal variability, other smaller error sourcesother smaller error sources

Estimating Biases Between Estimating Biases Between InstrumentsInstruments

-20

-10

0

10

20

30

40

1992 1994 1996 1998 2000 2002 2004 2006

TOPEX-A/B/J-1Fit (a + b + c + dt)

Year

Rate = 2.6 mm/year

What Have We Learned from What Have We Learned from Altimetry?Altimetry?

A great leap forward has occurred in sea level A great leap forward has occurred in sea level change science in the last decade, largely due to change science in the last decade, largely due to the influx of satellite altimeter measurements.the influx of satellite altimeter measurements.

• Satellite altimetry has observed an average Satellite altimetry has observed an average increase in sea level of 3.2 mm/year over 1993-increase in sea level of 3.2 mm/year over 1993-2005, which probably reflects an acceleration 2005, which probably reflects an acceleration versus the 20th century rate (~1.8 mm/year).versus the 20th century rate (~1.8 mm/year).

• Roughly half the current rise may be attributed to Roughly half the current rise may be attributed to thermal expansion, leaving half to the addition of thermal expansion, leaving half to the addition of water to the oceans (likely from melting of water to the oceans (likely from melting of mountain glaciers and polar ice).mountain glaciers and polar ice).

• Large spatial variability in the sea level rise Large spatial variability in the sea level rise signal has been observed.signal has been observed.

• Together, altimetry and satellite gravity Together, altimetry and satellite gravity measurements provide a powerful means of measurements provide a powerful means of determining the amount of sea level change and the determining the amount of sea level change and the causes of the change.causes of the change.

Sea Level Change Satellite MissionsSea Level Change Satellite Missions

Pending approval

   

In orbit Approved

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 

JASON-2/OSTM

TOPEX/POSEIDON

JASON-1

Envisat RA-2

Medium accuracy RA (SSH) from high-inclination orbit

High accuracy RA (SSH) from mid-inclination orbit

CRYOSAT-2

OceanSat-3 AltiKaERS-2 RA

GMES S-3

ICESAT

Geosat Follow-On NPOESS

GRACE

Ice Altimetry

Temporal Gravity (ocean mass)

GRACE follow-on ?

ICESAT-2 ?

Dreams

Jason-3 ?

Jason-3: New Orbit?Jason-3: New Orbit?

• The current T/P/J-1/J-2 orbit has a high The current T/P/J-1/J-2 orbit has a high 1336 km altitude (increased s/c costs), a 1336 km altitude (increased s/c costs), a relatively low 66° inclination, and relatively low 66° inclination, and moderate spatial resolution (10-day moderate spatial resolution (10-day repeating orbit).repeating orbit).

• Lower altitude (800-1000 km) orbits exist Lower altitude (800-1000 km) orbits exist that have better latitude coverage (~78°), that have better latitude coverage (~78°), good tidal aliasing characteristics, and good tidal aliasing characteristics, and variable repeat periods (10-21 days), variable repeat periods (10-21 days), which would provide a lower-cost mission which would provide a lower-cost mission with broader scientific applications.with broader scientific applications.

• While continuing with Jason-3 in the T/P While continuing with Jason-3 in the T/P orbit is highly desirable for sea level orbit is highly desirable for sea level change applications, alternative orbits change applications, alternative orbits could be considered.could be considered.

Jason-3: New Orbit ConsiderationsJason-3: New Orbit Considerations• Sun-synchronous orbits undesirableSun-synchronous orbits undesirable• Non-repeating orbits undesirableNon-repeating orbits undesirable• Continuation of the T/P/J-1/J-2 sea Continuation of the T/P/J-1/J-2 sea level time serieslevel time series

• Tide gauge calibration of data from a Tide gauge calibration of data from a new orbit (offset with Jason-2)new orbit (offset with Jason-2)

• Reference mean sea surface, geoid Reference mean sea surface, geoid gradientsgradients

• Tidal aliasing characteristicsTidal aliasing characteristics• Tide modeling errorsTide modeling errors• Tradeoffs of temporal/spatial sampling Tradeoffs of temporal/spatial sampling (repeat period)(repeat period)

• Latitudinal coverage (inclination)Latitudinal coverage (inclination)• AltitudeAltitude

ConclusionsConclusions• We have made some great progress in sea level We have made some great progress in sea level change science using satellite altimetry, but change science using satellite altimetry, but longer datasets are needed to study climate longer datasets are needed to study climate signals.signals.

• The 13-year continuous satellite record of sea The 13-year continuous satellite record of sea level change is at significant risk, with the level change is at significant risk, with the possibility of gaps in the future, and no possibility of gaps in the future, and no concrete plans for ensuring the record after concrete plans for ensuring the record after Jason-2 (20 years).Jason-2 (20 years).

• While it is highly desirable to continue in the While it is highly desirable to continue in the T/P orbit, alternative orbits with lower T/P orbit, alternative orbits with lower altitude and higher inclination exist that altitude and higher inclination exist that could be considered to continue the T/P/J-1/J-2 could be considered to continue the T/P/J-1/J-2 sea level time series.sea level time series.

• Calibration of the altimeter instruments using Calibration of the altimeter instruments using tide gauges is an indispensable component of tide gauges is an indispensable component of any altimeter-based sea level monitoring any altimeter-based sea level monitoring program.program.