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Carla Braitenberg Dipartimento Scienze della Terra,

Evoluzione Sistema Terra 2003/2004 Stima della variazione di livello marino medio da osservazioni satellitari . Carla Braitenberg Dipartimento Scienze della Terra, Università di Trieste, Via Weiss 1, 34100 Trieste Berg@units.it Tel +39-040-5582258 fax +39-040-575519.

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Carla Braitenberg Dipartimento Scienze della Terra,

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  1. Evoluzione Sistema Terra2003/2004Stima della variazione di livello marino medio da osservazioni satellitari Carla Braitenberg Dipartimento Scienze della Terra, Università di Trieste, Via Weiss 1, 34100 Trieste Berg@units.it Tel +39-040-5582258 fax +39-040-575519

  2. Sea level changes observed with satellite altimetry • Sea level observed from satellite: global observation • First approaches: Seasat (1978) and Geosat(1985-1989) • Major problems: errors on orbit and ionospheric correction • Topex/Poseidon: started 1992. Useful for sea, lakes, rivers • Measurement: Range estimated by return time of radar pulse reflected on sea surface

  3. Sea level changes observed with satellite altimetry • Reference system anchored to c.m. of earth and satellite tracking system positioned on earth. • Errors which affect height measurement: • Satellite orbit • Corrections to delay due to atmosphere • Corrections due to state of sea • Instrumental errors, drifts

  4. Sea level changes observed with satellite altimetry • Spatial resolution of T/P • Inclination of orbit=66°: latitude range sampled: 66° • Satellite height: 1336 km • After N=127 revolutions the tracks on earth surface repeat. In degrees the distance between tracks is: • Time resolution: time to complete N revolutions=10 days

  5. (Chelton et al., 2001) Relations between range R, orbit height H and height h of sea surface relative to ellipsoid. Hd is dynamic surface elevation R: Range, h: height of sea surface on ellipsoid, H: orbit height, hd: dynamic surface elevation, hg: geoid height hT: ocean tidal height, ha: atmospheric pressure loading,

  6. Altimetric corrections Principle: emission of micro-wave impulse with known power. Some of the signal is reflected by the sea surface. range Range computed neglecting refraction. c= speed of light. t= return time Corrections for:a) various components of atmospheric refraction b) bias between scattering surface and mean sea level. All corrections are positive.

  7. Radar frequency Choice of frequency dictated by reflectivity of water (maximized) and the atmospheric attenuation (minimized) T/P: dual frequency emission: primary frequency13.6 GHz and secondary frequency 5.6 GHz Secondary frequency is used to correct for ionospheric refraction

  8. Reflectivity of water Reflectivity at normal incidence angle for fresh water at 25°C for a smooth surface

  9. Transmittance Transmittance at normal incidence for cloud-free subpolar (dotted), midlatitude (solid) and tropical (dashed) atmosphere.

  10. Elements of elliptical orbit a: semimajor axis, i: inclination, : longitude (or right ascension) of ascending node, : argument of perigee, : true anomaly, e: eccentricity

  11. Groundtracks of T/P Ground track for a single orbit and ground track pattern traced out in one day for the T/P prograde orbit with 66° inclination.

  12. Groundtracks of T/P over Atlantic ocan for the 10-day exact repeat orbit configuration. Top L.: full 10-day period Bot. L.: 3-day period. Solid, dashed, dot: day 1,2,3 Top R.: 9-day period. Solid, dashed, dot: days 1-3, 4-6, 7-9

  13. Zonal separation of tracks Zonal separation of ground tracks in km as a function of latitude for the T/P, ERS and Geosat orbit configurations

  14. Using T/P data for global sea level change • Data along track: sea level every 10 days. A) find mean sea surface in every sampled point averaged in time (1992-2002) B) Find deviations from mean in time for every sampled point. Calculate trends C) Make global average as function of time Corrections made: dry and wet tropophere, ionosphere, drift of micro-wave radiometer (1.2 mm/yr) used in the wet tropospheric correction (integrated amount of water vapor estimated from microwave radiation). Drift estimated from calibration with tide gauges

  15. Comparison T/P and tide gauge Cheney et al. 2002

  16. Result regarding global average For the years 1992-2001: average increase of 2.5±0.2 mm/a (Cabanes et al., 2001) The average for each cycle (10 day sampling) has 7 mm standard deviation with respect to average curve This value is systematically greater than the variation estimated from tide gauges over last 60 yrs: 1.8±0.1 mm/a (Nerem and Mitchum, 2001). This values is obtained with corrections for: post-glacial movements, stations with subsidence or tectonic movements have been eliminated, tide gauges have been grouped to represent areas.

  17. Global mean sea level for every T/P cycle 10 day estimates of global mean sea level from the T/P mission and after smoothing using a 60-day boxcar filter. Data corrected for instrument effects using the tide gauge calibration time series and removing annual and semi-annual variations (Nerem and Mitchum, 2001).

  18. Gloabl sea level change for T/P 10-day estimates of global mean sea level from T/P mission and after smoothing using a 60-day boxcar filter (Nerem andMitchum, 2001)

  19. Spatial variations of sea level rise • The global observations with satellites allows to determine spatial distribution of sea level variations. • Necessary also the variation of sea-temperature: some of the sea level change due to thermal expansion • Availability: • hydrographic measurements of temperature on water column • Surface temperature observed by satellite (through infrared or micro-wave observation)

  20. Local variations are strong. Reveal that global coverage of observation is necessary (Cabanes et al., 2001).

  21. Model spatial variations of sea level rise by temperature variations (Cabanes et al., 2001) • Available global temperature variations: • Yearly mean temperature data for upper 500 m of water for 1993-1998 • Mean temperature for upper 3000m of water at 5 yr sampling for 1945-1998. • Temperature variation converted to “thermosteric sea level variation” by applying thermal expansion coefficient.

  22. Model spatial variations of sea level rise by temperature variations (Cabanes et al., 2001) • Result: • model (500 m temperature) predicts 95% of observed (T/P) sea level change for 1993-1998. • Other causes : • salinity changes (halosteric changes) • Water exchange with continent and atmosphere: glaciers, precipitation, evaporation, river run-off

  23. STERIC SEA LEVEL reference density with To: reference temperature, and So: reference salinity F is a non linear, empirical function. (Cabanes et al., 2001)

  24. Model: thermal expansion considering upper 500 m (Cabanes et al., 2001)

  25. Global sea level rise 1993-2001(Topex/Poseidon): 2.5±0.2 mm/a observation Thermosteric component 500m Mean on 1 yr Cabanes et al., 2001

  26. Thermosteric sea level trends with temperature data down to 3000 m depth • The temperature values of upper 3000 m were used to compute expected thermosteric variations for 1955-1996 (5 yr sampling). This is compared with 25 tide gauge stations • Average over 60°S to 60°N gives mean thermosteric trend of 0.50  0.05 mm/yr

  27. Thermal expansion contribution to sea level rise for 1955-1994 0.5 +/- 0.05 mm/yr Cazenave, 2003

  28. Thermosteric sea level trends with temperature data down to 3000 m depth • To compare with tide gauge estimate, the thermostaeric variation was sampled at the 25 tide gauge stations: global average gives 1.4  0.1 mm/yr (1955-1996). It shows that the particular distribution of the 25 tide gauges over-estimates sea level rise. Dashed: global thermosteric Dotted: subsampled at 25 st. Solid: average of tide gauges

  29. Thermosteric sea level trends with temperature data down to 3000 m depth. Black triangles show the locations of the25 tide gauges (Cabanes et al., 2001).

  30. Steric Sea Level Global mean ‘pseudo’ global mean at tide gauge sites Cazenave, 2003

  31. Steric sea level ---- Tide gauge-derived sea level ---- Cazenave, 2003

  32. Summary of the tide gauge observations and predictions SEA LEVEL RISE (1955-1995) Steric average at the 25 tide gauge sites: 1.4 +/- 0.1 mm/yr  Observed by tide gauges (25 sites) : 1.6 +/- 0.15 mm/yr Steric global mean : 0.5 +/- 0.05 mm/yr Cazenave, 2003

  33. Map of steric sea level trends and GLOSS tide gauge network Cazenave, 2003 Steric sea level: - global mean (in blue) -subsampled at GLOSS sites (in red)

  34. STERIC SEA LEVEL --------- OBSERVED BY TOPEX/POSEIDON ------ 1950 1990 Cazenave, 2003

  35. Bottari A., Carveni P., Lo Giudice E., Nikonov A., Rasà R. (1992) Anomalous crustal movements prior to great earthquakes as derived from tide-gage records: the messina 1908, I=XI, earthquake case history, Tectonophysics, 202, 269-275. Bruun, P. (1962) Sea-level rise as a cause of shore erosion. Am Soc. Civ. Eng. Proc., J. Waterways Division, 88, 117-130. Cabanes C., Cazenave A., Le Provost C.(2001) Sea level rise during past 40 years determined from satellite and in situ observations, Science, 294, 840-842. Chelton D.B., Ries J.C., Haines B.J., Fu L.-L., Callahan P. (2001) Satellite altimetry, in: , Fu L.-L. And Cazenave A., Satellite altimetry and earth sciences, Academic Press, San Diego, 1-463. Douglas B.C. (2001) An introduction to sea level Sea level change in the era of the recording tide gauge, in: Douglas B.C., Kearney M.S. And Leatherman S.P., edts. Sea level rise, Academic Press, San Diego, 1-233. Douglas B.C. (2001) An introduction to sea level, in: Douglas B.C., Kearney M.S. And Leatherman S.P., edts. Sea level rise, Academic Press, San Diego, 1-233. Nerem R.S. And Mitchum G.T. (2001) Sea level change in: Fu L.-L. And Cazenave A., Satellite altimetry and earth sciences, Academic Press, San Diego, 1-463. References

  36. References • Lambeck K. , Chappell J. (2001) Sea level change through the last glacial cycle, Science, 292, 679-686 • Lambeck K., E. Bard (2000) Sea-level change along the French Mediterranean coast for the past 30000 years , Earth Planet. Sci. Let., 175, 203-222 • Bard E., Antonioli F., Silenzi S. (2002) Sea-level during the penultimate interglacial period based on a submerged stalagmite from Argentarola Cave (Italy), Earth Planet. Sci. Let., in press • Cazenave (2003) Present day sea level observations from satellite altimetry and tide gauges and causes, Proceedings of Int. Workshop on Satellite Altimetry for Geodesy, Geophysics and Oceanography: summer lecture series and scientific applications, Wuhan, 2002 (in press)

  37. Books Douglas B.C., Kearney M.S. And Leatherman S.P. (2001) Sea level rise, Academic Press, San Diego, 1-233. Fu L.-L. And Cazenave A., (2001) Satellite altimetry and earth sciences, Academic Press, San Diego, 1-463.

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