Coastal surface circulation of the Gulf of Cadiz from satellite altimetry measurements

Abstract

The main objective of this thesis is to contribute to the understanding of coastal zone dynamics using high-resolution altimetry data. The study evaluates the possibility of studying mesoscale surface circulation in coastal areas, particularly in the Gulf of Cadiz, Spain, using high-resolution altimetry data with a posting rate of 20 Hz from CryoSat-2 (CS2). For this purpose, a specific filtering strategy for coastal areas was developed along with wind and bottom friction ageostrophic corrections to enhance the accuracy and realism of the outputs. The benefits of such methodology and corrections were assessed by comparison with measurements from high-frequency radar (HFR). The correction of these ageostrophic effects substantially enhanced the correlation between altimetry and HFR data, increasing from 0.61 (no correction) to 0.72 (corrected).

Furthermore, the root mean square error (RMSE) decreases from 12.54 cm/s to 7.35 cm/s. Additionally, the research evaluates the quality and capabilities of altimetry- derived wind speed (WS) retrievals for validation/calibration of numerical weather prediction models (NWPM), specifically the Weather Research and Forecasting (WRF) model over the complex area of the Gulf of Cadiz. To assess the applicability of altimetry data for this purpose, the study compares Sentinel-3A/B (S3A/B) WS data with in-situ measurements and validates the WRF model with data from multiple stations in the area. The results show that S3A/B WS data align well with in-situ measurements and are trustworthy for model calibration/validation. The spatial variability of WS derived from the WRF model is compared with the along-track altimetry-derived WS under different wind conditions, demonstrating good agreement between the two datasets even under varying wind conditions. This illustrates that the spatial coverage of satellite altimetry can effectively validate high-resolution numerical weather prediction models in complex coastal areas. Based on the previously validated methodologies to obtain high-resolution satellite altimetry estimates of current velocities, along with high-resolution wind data from the WRF model, the research provides a comprehensive characterization of surface circulation over the northern shelf of the Gulf of Cadiz. The study compares high-resolution satellite altimetry-based surface zonal currents, corrected for bottom-drag and wind effects, with a publicly available altimetry dataset (generic CMEMS product) and validates them with in-situ measurements from Acoustic Doppler Current Profilers (ADCP). The findings indicate that the corrected altimetry product outperforms the generic CMEMS product in terms of statistical metrics, accurately capturing surface circulation direction when compared with in-situ measurements. The research emphasizes the importance of considering both wind-driven and geostrophic components of the circulation in different sectors of the Gulf of Cadiz. It reveals that in the western basin, positive (eastward) surface currents are mainly driven by westerly winds, while occasional westward flows are mostly dominated by geostrophic forces. In contrast, in the eastern basin, both eastward and westward flows are primarily driven by favourable winds. Additionally, the analysis of Absolute Dynamic Topography (ADT) values along the entire basin highlights the presence of ADT gradients both cross and along-shore, with a greater significance attributed to the along-shore gradients. The research also conducts a seasonal analysis, showing that eastward circulation dominates during the spring and summer months, associated with upwelling and westerly winds. Conversely, westward flows prevail during the winter months, related to easterly winds and the effect of the along-shore sea level gradient during unfavourable upwelling conditions. While the research highlights the effectiveness of satellite altimetry data for studying coastal dynamics, it also acknowledges certain limitations, such as the inability to measure wind direction and the inability to capture variability at scales smaller than the across-track footprint length. Nonetheless, the results suggest that improved methodologies and higher- resolution altimetry measurements can significantly enhance our understanding of coastal zone dynamics, bringing it closer to that of the open ocean