Sea level anomalies using altimetry, model and tide gauge along the African coasts in the Eastern Tropical Atlantic Ocean: inter-comparison and temporal variability

Habib Boubacar Dieng (LEGOS: CNRS/CNES/IRD/UT3-PS, France)

CoAuthors

Isabelle Dadou (LEGOS: CNRS/CNES/IRD/UT3-PS, France); Fabien Léger (LEGOS: CNRS/CNES/IRD/UT3-PS, France); Julien Jouanno (LEGOS: CNRS/CNES/IRD/UT3-PS, France); Yves Morel (LEGOS: CNRS/CNES/IRD/UT3-PS, France)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: Coastal Altimetry

Presentation type: Type Poster

Because of sparse in situ measurements, the use of altimeter and ocean models are currently the only options to understand the coastal variability of sea level anomalies (SLA) near the African coasts of the Eastern Tropical Atlantic Ocean (ETAO: 35°S–25°N; 25°W–African coasts). In this study, SLA products derived from altimetry (X-TRACK and CMEMS) and from a regional ocean model (NEMO) are validated near the coast using the 14 tide gauges (TGs) available in the region. We used statistical criteria (correlation, standard deviation and root mean square) to inter-compare our three different products with the TG. We then analyzed the sub-seasonal to inter-annual variability of SLAs (i.e. from 20 days up to 2 years) over the period January 2008 – December 2014.
We found a very good agreement between altimetry, model and TGs near the coasts of Senegal (10°N – 25°N) and Gulf of Guinea (10°S – 10°N). This is not the case near the coast of Benguela region (south of 10°S) and is mainly explained by the combined effects of the position of TGs (located in semi-enclosed bays), the geophysical corrections used in the computations of the SLA derived from altimetry (i.e. tide, dynamical atmospheric correction -DAC- and sea state bias -SSB-) and the still low spatial resolution of the CMEMS and model data in connection with the greater oceanic and atmospheric variability in the Benguela upwelling system. We also show, with all products, that the time signal of SLA is mainly seasonal (annual and semi-annual) throughout the ETAO region. The altimetry data also show some inter-annual (15 to 24 months) variability in the equatorial band, which can be related to the equatorial Kelvin waves very present in this zone. This is less marked with the NEMO model and does not appear on the whole equatorial band.
We show that high spatial resolution and improved altimeter geophysical corrections near the coast can reduce near-shore data errors by up to 10% for each improvement. However, despite these improvements, agreements between SLA products are still weak in the Benguela region, suggesting that efforts need to be intensified on geophysical corrections and in increasing the temporal and spatial resolution of data near the coast. Moreover, the TGs network should be completed where measurement gaps remain along the West African coast, especially outside the bays for the Benguela region.
 

Poster show times:

Room Start Date End Date
The Gallery Tue, Oct 22 2019,16:15 Tue, Oct 22 2019,18:00
The Gallery Thu, Oct 24 2019,14:00 Thu, Oct 24 2019,15:45
Habib Boubacar Dieng
LEGOS: CNRS/CNES/IRD/UT3-PS
France
habib.dieng@legos.obs-mip.fr