EO Information Services in support of West Africa Coastal vulnerability Service 2 : Sea Level Height & currents. Vinca Rosmorduc, CLS

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1 EO Information Services in support of West Africa Coastal vulnerability Service 2 : Sea Level Height & currents Vinca Rosmorduc, CLS World Bank HQ, Washington DC Date : 23 February 2012

2 West Africa coastal vulnerability Project context : Requirements estimate coastal changes and urban coastal vulnerability in critical coastal areas of Africa focus on Sao Tome and Principe islands, coast of Gambia and Senegal, and the coast of Cotonou (Benin) and Lagos (Nigeria) Project Information requirements : key requirements: Altimetry sea level height maps (last 20 years) + chart showing the sea levels over time Map of coastal ocean currents that dominate the area additional further requirements Maps with the water level variations related to tidal cycles Time series of raster images related to tidal current speed and direction in the Gulf of Guinea and Senegal/Gambia coast. Reason / Driver for this information : understand the coastal degradation and erosion along the last 20 years inputs to the generation of a hazard map in the coastal areas

3 The EO Information Products : What they bring Sea level height & ocean currents Service - Benefits / Advantages - Altimetry a high-precision, highly validated, global technique - 20 years of background at CLS in this technique, use of the most widely used datasets, supported by the Space agencies and European commission - In situ (tide gauges, currrentmeters) the only alternative, BUT not global, question of intercalibration, availability... - Limitations / Constraints - Altimetry is not a very high-resolution measurement (1/3 basic gridding scheme, 1/8 possible), - Not optimized for coastal areas at this stage (several studies ongoing; but no data very close) - Not as immediately understandable as e.g. visible imagery can be (or can seem to be).

4 Altimetry Sea Level Anomalies = Sea surface height variations (Temperature, Salinity, Currents variations, El Niño, Eddies and ocean circulation variations, Seasons ) SLA Absolute Dynamic Topography = Total Sea Surface height due to ocean dynamics (major currents, total transport, includes variability) Mean Sea Surface ADT Mean Sea Level = an average of the sea level heights. What is interesting is the trend on long period

5 Two main type of maps: constant maps: Service 2 Products: What they are Sea level height corresponding to the major currents ( Mean Dynamic Topography ) Sea level change rates during the period (sea level trends) Ocean Tide and Tidal currents Annual means (as per request of World bank during SRR) ( ) of: the variations of sea level height with respect to a several-year average (Sea level anomalies) the variations of surface currents (Geostrophic Velocity Anomalies) the sea level heights due to the ocean dynamics, i.e. the currents, variability, changes etc. (Absolute Dynamic Topography) the ocean currents directly due to winds (Wind-driven currents (Ekman)) Spatial resolution: 1/3 degrees lat/lon Accuracy: geometric 0.05 m;

6 grid of measurements Missions used: ERS-1 & 2 (yellow), Topex/Poseidon (pink), GFO (green), Jason-1 (red), Envisat (yellow), Jason-2 (red) Measurements are not larger (rather thinner) than shown here.

7 Sea level trends The sea level is rising over all the area. The elevation rate over the area is between 2.3 and 3.6 mm/year between late 1993 and 2010, with an average of 3.1 mm/year (to compare, the global rate is at 2.9±0.6 mm/year; North Atlantic 2.5±0.6 mm/year, South Atlantic 3.2±0.6 mm/year).

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9 Examples of Previous Use Monitoring of mean sea level by altimetry - CNES project + ESA climate change initiative - Development of a Global Mean Sea Level indicator; first as a demonstrator of what altimetry can bring to this domain. Then the indicator was adopted by IPCC, CEOS and NASA, as well as by the French Office for climate change studies - Data compared with other techniques - Tide gauges - Temperature/salinity in situ measurements (Argo floats) - Gravimetry data - Ocean models outputs - Results show very good agreement between all data sources

10 Mean Dynamic topography Major currents in the Senegal area. Mean Dynamic Topography (in cm) and the main surface current system (in cm/m). NEC= North Equatorial Current, NECC= North Equatorial Countercurrent, CC= Canary current, GD= Guinea Dome, MC= Mauritania current.

11 Tidal component and current M2 tidal component (main one) due to an ideal Moon; occuring twice daily (1/18 model)

12 Sea level anomalies & derived currents cm cm

13 Wind-driven currents Annual mean of wind-driven currents for 2001 (typical of the annual mean of wind-driven currents for the period). Amplitude ~2% of the geostrophic current one

14 Absolute Dynamic topography cm Sea level anomalies (1994 Mean yearly and deseasonalized maps of Sea Level Anomalies - in cm - superimposed to the geostrophic velocities anomalies - in cm/s - cm 1994 Absolute Dynamic Topography - in cm. Note the ocean eddy close to the coast in both maps; On the contrary, in the South of the mpa, the Absolute Dynamic map is reflecting mostly the mean topography rather than its variability

15 Validation Data validation done globally, at several steps of the processing Eliminating spurious values (too high, too low, or completely out of range) Analysing measurements statistically Analysing measurements from one satellite with the others Analysing measurements with respect to in situ networks of instruments (tide gauges, Argo floats)