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

Abstract

Abstract 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, three 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. Statistical analysis (correlation, standard deviation and root mean square) are performed to compare our three products with the TG. We then analyze 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 (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-). In addition, the low spatial resolution of the CMEMS and model data do not allow a good description of the small scale oceanic and atmospheric variability, which dominates in the Benguela upwelling system. We then show, with all products, that the temporal variability of SLA is mainly seasonal (annual and semi-annual) throughout the ETAO region. The altimetry data also show some inter-annual (15–24 months) variability in the equatorial band, which can be related to equatorial Kelvin waves. This is less pronounced with the NEMO model and does not appear over the whole equatorial band. We show that high spatial resolution and improved altimetric geophysical corrections near the coast can each reduce near-shore data errors by up to 10%. However, despite these improvements, agreements between SLA products are still unsatisfactory 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, there exist gaps in TG measurements, along the West African coast, especially in the Benguela region. The TG network should thus be completed.