ODINAFRICA/GLOSS Sea Level Training Course TIDE GAUGES AND ALTIMETRY IN THE GULF OF GUINEA 13-24 November 2006, Oostende Angora AMAN

Contents The system - Principles of altimetry - sampling characteristics Application - Mean sea surfaces - Sea Level variability Why TG in the age of Altimetry? - Comparison SL derived from T/P signal and TG records (Pointe Noire, Sao Tome, San Pedro) - Propagation of coastal upwelling in the GG using T/P signal and TG records Conclusion

Tide gauges limits 2 fundamental problems Tide gauges have limited spatial distribution and suboptimal coastal locations and thus provide poor sampling of the open ocean Tide gauge measures sea level relative to a crustal reference point, which may be moving vertically at rates comparable to the true sea level signals

What is satellite altimetry? By means of a nadir looking radar we measure the reflection of short pulse in the footprint. This footprint is about 4 to 8 kilometer in diameter. Source: JPL

Principles satellite altimetry Orbit Determination The position of the radar altimeter satellite is derived from observations acquired from a network of ground stations Newer satellites carry their own GPS receiver, but in principle the method remains the same Radar data processing The radar observes a waveform samples As scientists we get: range, significant wave height and a radar backscatter value, and scalar wind speed estimates Great effort are made to calibrate/validate this data Geophysical corrections are applied the sea surface to remove all unwanted effects

Satellite range: It characterizes the distance from the satellite to the sea surface 2. Orbital height of the satellite: a distance from the satellite to a reference ellipsoid 3. Conversion from time delay to distance. The system requires an accurate measurements necessary to estimate the index of refraction of the atmosphere (troposphere and ionosphere)

The satellite transmits a radar pulse toward the ocean surface After passing through the atmosphere, the pulse arrives at the Atmosphere/ocean boundary, interacts with the ocean, and is then reflected back toward the satellite, again through the atmosphere.

H= (Ta-Tt)C/2 Tt: time of the pulse transmission Ta: time the pulse arrival back at the satellite C : speed of light What the altimeter measures is the average waveform of thousands of returned pulses as function of time.

Error Sources in Satellite Altimetry Error due to the orbit determination The estimate of the index of refraction is bit complicated with regard to the wet tropospheric correction (0.5 cm for the ionospheric correction and 1.1 cm for the wet tropospheric contribution) Surface errors The tide model error is ~1-2 cm (Shum et al.,1997) in the open ocean

Past and Current altimeter satellites Satellite Years Organisation Accuracy SKYLAB 1972 NASA 20 m GEOS-3 1975-1978 NASA 3 m SEASAT 1978 NASA 2 m GEOSAT 1985-1990 US Navy 30 cm ERS-1 1991-1996 ESA 4-10 cm ERS-2 1995-2006 ESA 4 cm T/P 1992-2005 NASA/CNES 2 .. 3 cm GFO 2000- US Navy 2 .. 5 cm JASON 2001- NASA/CNES 2 .. 3 cm ENVISAT 2002- ESA 2 .. 3 cm

Calibrating the measurement One way to make an overall assessment of the precision and accuracy of the satellite altimetry system for producing sea surface heights is to compare these heights to sea level measurements from tide gauges. However, it is not easily to attribute any errors so observed to a particular component of the altimetric system. It provides an important end – to- end assessment of all the system.

APPLICATION Sea Level variability Mean Sea Level variability

in the “Age of Altimetry”? Why Tide Gauges in the “Age of Altimetry”? Principle of continuity, relative low cost of gauges Long records for secular trend/acceleration studies (e.g. for input to IPCC) Higher frequency sampling important in straits and other areas High latitude regions of ice coverage Altimeter calibrations (‘absolute’ and ‘relative’) Coastal applications (GOOS Coastal Module) Acoustic Gauge in Australia

ALTIMETRY AND TG RECORDS IN THE GULF OF GUINEA POINTE NOIRE SAO TOME SAN PEDRO

POINTE NOIRE The first attempt to estimate sea level using altimetric data was made by Menard (1988), Arnault et al. (1994) with GEOSAT altimeter. RMS difference of 7.1 cm (1988) and 5.4 cm (1994)

The appearance of the upwelling event is detected by a drop of MSL starting May. This occurs 2 weeks prior to the drop of SST

Seasonal upwelling at Pointe Noire and San Pedro using SST in situ measurements

PROPAGATION OF COASTAL UPWELLING SIGNAL USING - SST DERIVED FROM SATELLITE - SEA LEVEL ANOMALIES FROM SATELLITE ALTIMETER

Conclusion

Conclusion - Satellite altimeters could detect correctly the spatio-temporal variability of SL in the GG with a great confidence (RMS~ 2 cm). Description of the seasonal upwelling variability with great confidence Analysis of the propagation of the upwelling signal along the coast

Conclusion However, in spite of their accuracy, satellite observations must be carefully processed and supported by in situ measurements The combination of altimetric signal and TG measurements and numerical models will offer an interesting way for climate study. PB??? Most of the tide gauges along the Gulf of Guinea cost are abandoned or provide poor quality records. ODINAFRICA !!!!!!

Estimated Global Sea Level Rise Using Tide Gauges and Satellite Altimetry (1948–2003) Plot present the sum of all forcing factors. Sea level drop near Greenland. Higher sea level trends appear in equatorial area. CK Shum Estimated Sea Level Rise = 1.74±0.24 mm/yr 585 selected tide gauges, multiple satellite altimetry used

PRODUCTS EXPECTED FROM ODINAFRICA Two types of products can be generated: - Real time products such as detecting upwelling, forecasting storm surges, - Delay mode data product such as tidal analysis, detecting extreme tide, developing tide tables. Long term climate studies High quality data for satellite calibration …..