Sea Level Change Observation Status on the elements of the puzzle Christian Le Provost LEGOS / CNRS Toulouse, France

From Woodworth 3.1 mm / year 1.5 mm / year 0.8 mm / year 0.9 mm / year 2.0 mm / year 1.3 mm / year 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 year mean records

Sea Level observed by high precision altimetry has been globally rising over the last decade of the ninetees

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?

IPCC predict a global sea level rise 5 time more rapid than over the 20th century 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 Why to worry about sea level change?

Tide gauges, of different technologies Float or acoustic stationscoastal pressure gauges How do we observe sea level variations ?

Bottom pressure gauges in the deep ocean How do we observe sea level variations ?

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 The challenge in term of climate change

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 What governs sea level variations?

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 The global ocean state

Avril 1999 ENSO La Nina Order of magnitude: tens of cm El Nino

North Atlantic Oscillation NAO Sea Level Variation over the North Atlantic mm

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) Qualities and weakness of each system

From IPCC One example of the impact of tide gauge sampling: the thermosteric contribution to SLC

Sampling of the thermosteric contribution to sea level trend From Cabanes

high precision satellite altimetry + quasi global coverage Qualities and weakness of each system

TOPEX/Poseidon Sampling

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) Qualities and weakness of each system

only a decade of high precision altimetry From Cabanes

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) Qualities and weakness of each system

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 - 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) Synergy between tide gauges and satellite altimetry

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 and - 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 Conclusions

 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 

 high quality maintenance of the GLOSS tide gauges, including high precision leveling   high precision altimetry

Error Budget for altimetric missions Oceanic signal orbit error RA error Ionosphere Troposphere EM Bias 100 Centimeters Geos 3SEASATGEOSAT ERSIT/P (before launch) T/P (after launch) 843 km 115° various repeat cycles 800 km 108° 3 days 800 km 108° 17 days (ERM) 780 km 98.5° 35 days (3/168) 1336 km 66° 9.95 days ATSR PRARE TMR GPS/DORIS

 high quality maintenance of the GLOSS tide gauges, including high precision leveling   high precision altimetry  in situ measurements and regional modeling