Transcript How do we observe sea level variations

Sea Level Change Observation
Status on the elements of the puzzle
Christian Le Provost
LEGOS / CNRS
Toulouse, France
year mean records
3.1 mm / year
1.5 mm / year
0.8 mm / year
0.9 mm / year
2.0 mm / year
1.3 mm / year
From Woodworth
Sea Level recorded by tide gauges is rising in many places
but not at the same rate everywhere,
and even is going down in several areas
Sea Level observed
by high precision
altimetry has been
globally rising over
the last decade of
the ninetees
Why to worry about sea level change?
Major environmental question
to predict flood risks in coastal regions
mean sea level rise
increase frequency of extremes
Why to worry about sea level change?
long term sea level change is linked to climate change,
and SLC is an « easy ? » parameter to monitor,
which can help to validate climate change predictions
IPCC predict a global sea level rise 5 time more rapid than over the 20th century
How do we observe sea level variations ?
• Tide gauges, of different technologies
Float or acoustic stations
coastal pressure gauges
How do we observe sea level variations ?
• Bottom pressure gauges in the deep ocean
How do we observe sea level variations ?
The challenge in term of climate change
Observe
• on the long term
• mm / year sea level trends
• hidden behind a large variability of sea level signal
- several orders of magnitude larger
- ranging from high frequency
to decadal and larger time scales
What governs sea level variations?
What governs sea level variations?
Sea Level variations are an index of many ocean processes
at the different time and space scales (eustatic, steric, and dynamic)
+ crustal motions
From Pugh
The global ocean state
Seasonnal cycle, interannual variations, pluriannual to decadal oscillations
including slow baroclinic planetary waves, thermohaline circulation rapid changes
Order of magnitude: a few cm to a few tens of cm
Dynamic topography observed by Topex/ Poseidon
ENSO
Order of magnitude: tens of cm
Avril 1999
El Nino
La Nina
North Atlantic Oscillation
NAO
mm
50
0
- 50
Sea Level Variation
over the North Atlantic
1993
2001
Qualities and weakness of each system
• Tide gauges
+ High frequency sampling
- but poor space coverage
+ long term records (for some tide gauges)
- but highly demanding in term of quality
control on the long term
(instrument drift, monitoring of the reference)
(cf Woodworth, Woppelmann, Merrifield, Bevis)
One example of the impact of tide gauge sampling: the
thermosteric contribution to SLC
From IPCC
Location of 287 GLOSS tide gauges
From Cabanes
Sampling of the thermosteric contribution to sea level trend
Qualities and weakness of each system
• high precision satellite altimetry
+ quasi global coverage
TOPEX/Poseidon Sampling
Qualities and weakness of each system
• high precision satellite altimetry
+ quasi global coverage
- but aliasing problems of the HF signals
+ homogeneity of the quality control,
- but only a decade of high precision altimetry,
• Note that we are at the extreme limit of the technology
- need for careful calibration and drift control,
- need for very careful cross-calibration of the different mission
(ex: T/P and JASON)
(cf Mitchum)
only a decade of high precision altimetry
From Cabanes
Qualities and weakness of each system
• high precision satellite altimetry
+ quasi global coverage
- but aliasing problems of the HF signals
+ homogeneity of the quality control,
- but only a decade of high precision altimetry,
• Note that we are at the extreme limit of the technology
- need for careful calibration and drift control,
- need for very careful cross-calibration of the different mission
(ex: T/P and JASON)
(cf Mitchum)
Synergy between tide gauges and satellite altimetry
• The two systems are totally independent
• We need thus to study their level of agreement
• This is not an easy task : they do not measure the same quantity
- altimetry gives absolute measurement of the sea level
by reference to the center of mass of the earth
while tide gauges measure sea level by reference to land
NEED for CGPS@TG
- tide gauge measurement is very local, including coastal processes
while altimetry, up to now, is not able to measure close to the coast
NEED for local studies at each site (observation and modeling)
Conclusions
• Measuring sea level change is very demanding
• We have now two independent observing systems:
They need to be maintained BOTH hopefully on the long term
- GLOSS high quality network
with CGPS@TG and leveling
- High precision satellite altimetry,
with high quality calibration (drift free)
and intercalibration (T/P, JASON, ENVISAT…)
They need to be cross-calibrated:
- GLOSS alt subnetwork has proven its efficiency
- All the GLOSS stations are need for a good SLC monitoring
Further work is needed at each tide station to understand the disagreements
(if any) between tide gauge measurements and altimetry
 high quality maintenance
of the GLOSS tide gauges,
including high precision leveling
The GLOSS Core Network (280 stations)
 high quality maintenance
of the GLOSS tide gauges,
including high precision leveling
CGPS@TG
 high quality maintenance
of the GLOSS tide gauges,
including high precision leveling
CGPS@TG
high precision altimetry
Error Budget for altimetric missions
Centimeters
100
90
orbit error
RA error
Ionosphere
Troposphere
EM Bias
80
70
60
50
40
ATSR
30
TMR
PRARE
20
Oceanic signal
GPS/DORIS
10
0
Geos 3
843 km
115°
various repeat
cycles
SEASAT
GEOSAT
ERSI
800 km
108°
3 days
800 km
108°
17 days
(ERM)
780 km
98.5°
35 days
(3/168)
T/P
(before launch)
T/P
(after launch)
1336 km
66°
9.95 days
 high quality maintenance
of the GLOSS tide gauges,
including high precision leveling
CGPS@TG
high precision altimetry
in situ measurements and regional modeling