Sea Level anomalies and mesoscale activity using altimetry along the African coats in the eastern tropical Atlantic ocean (OSTST Alti-ETAO project)

Habib Boubacar Dieng (LEGOS, France)

CoAuthors

Isabelle Dadou (LEGOS, France); Fabien Léger (LEGOS, France); Florence Birol (LEGOS, France); Yves Morel (LEGOS, France); Alexis Chaigneau (LEGOS/IRHOB/CIPMA, Cotonou, Benin)

Event: 2017 Ocean Surface Topography Science Team Meeting

Session: Advances in coastal altimetry: measurement techniques, science applications and synergy with in situ and models

Presentation type: Type Poster

The equatorial region, near the coast, represents a major contributor to the ocean/atmosphere/land heat and water fluxes, controlled by SST and the oceanic dynamics. The eastern tropical Atlantic ocean (ETAO, 35°S-20°N ; 25°W - African coast) region remains little studied. This region also encompasses a large-range of peculiar dynamics: large-scale zonal equatorial currents, strong coastal currents, equatorial and coastal trapped waves, the presence of both equatorial and near-coastal upwelling cells, gyre-like structures with the presence of the Guinea and Angola domes (Schott et al, 2004).
In this area, there are few in-situ measurements and the time coverage of these data is not better. Since 1993 sea level anomaly (SLA) are routinely measured using high-precision satellite altimetry (Topex/Poseidon, Jason-1/2, …) with the 25th anniversary of the TOPEX/Poseidon launch, this year. While spatial altimetry has enabled us to highlight the regional variability of mesoscale dynamics, it still provides incomplete information in coastal areas in the first 10 km from the coasts, especially due to the perturbation of radar echoes by the continents (land, island, etc.). In the OSTST Alti-ETAO project, we studied the meso-scale dynamics using different altimetry sea level anomaly (SLA) products: AVISO gridded product (1/4°) and the coastal X-TRACK product from CTOH (LEGOS) based on Jason1-2 altimeters (Birol et al, 2016). We used also the tide gauge data available in the ETAO region for the validation of the altimetry SLA along the coast.
The comparison between the coastal altimetry along-track data (X-TRACK last reprocessing) and tide gauge data using different statistical criteria depends on the geographical position along the ETAO coasts. Near the coasts of Senegal and the Gulf of Guinea, we note a good agreement, in terms of correlation and quadratic errors (RMS), between the X-TRACK coastal altimetry data (closed to the tide gauge position and not in the first 10 km from the coast) and tide gauge data. On the other hand, near the Namibian and South African coasts, where tide gauge data appear to be better in terms of time cover, low correlations and more significant quadratic errors are found between the X-TRACK coastal data and tide gauge data. This weak agreement could be related to the higher oceanic and atmospheric variability in the Benguela upwelling system and the geographic location of the altimetry data relative to the tide gauges. Studies are also underway at LEGOS to estimate the influence of the tide correction in this region.
 

Poster show times:

RoomStart DateEnd Date
Concerto Ballroom Thu, Oct 26 2017,14:00 Thu, Oct 26 2017,18:00
Habib Boubacar Dieng
LEGOS
France
habib.dieng@legos.obs-mip.fr