Monthly mean sea level variability in the eastern sub-basin of the Gulf of Cadiz from altimetry and coastal tide gauge data sets

In this work, the main forcing factors affecting the monthly mean sea level in the eastern sub-basin of the Gulf of Cadiz (south-western Iberian Peninsula) are analysed in detail. Monthly mean sea level (MMSL) anomaly time series (30 years:1993-2022) from altimetry and tide gauges are used for this purpose. Validation of altimeter monthly MSL using two tide gauges (Huelva and Bonanza) gives high correlation coefficients (> 0.85) and low RMSE (< 3.5 cm). The comparison against Huelva was performed in the period: 1997 – 2022 due to data unavailability. A good level of accuracy is also observed for all the altimeter grid points analysed in the study area. The different local hydrodynamic conditions in the location of the tide gauges explains the differences in the validation results of altimeter data. The atmospheric pressure explains more than 40% of the variance in the monthly mean sea level, indicating that this forcing factor is the main contributor to the sea level variability in our study area on long (> 1 month) timescales. Taking into account the seasonal cycle, the variance ranges between 65% (western sub-basin) and 74% (eastern sub-basin). This percentage reaches up to 86–90 % of the variance considering the sea level trend. The isostatic assumption (-1 cm/hPa) is only valid on the western side of the sub-basin, reaching -1.3 cm/hPa on the eastern side with a marked E-W variation. For this reason, removing the atmospheric effect using inverse barometer correction can lead to misinterpretations of the contribution of this effect to the sea level variability. No significant differences were observed in the variability of the MMSL between coastal areas and the open ocean, while there was a slight spatial variation in the sea level trend, with minimum values in coastal areas close to the Strait of Gibraltar. Atmospheric pressure, although the main driver of MSL variability, does not affect the value of the sea level trend, but only reduces the uncertainty This is also the case for the seasonal cycle.