Mediterranean coastal sea level reconstruction based on tide gauge observations

In order to carry out a proper coastal management it is compulsory to have oceanographic databases that accurately characterize the spatiotemporal variability of sea level along the coast. A first source of sea level observations are tide gauges, which cover different time periods, some of them dating back to the 17th century. Whereas tide gauges generally provide very accurate measurements, their main limitation is that they are point-wise measurements with a heterogeneous spatial distribution and temporal coverage. Therefore, it is difficult to represent the complexity of sea level variability at the coast directly from tide gauge observations. Since 1992, sea level measurements provided by satellite altimetry are also available. This technique has a quasi-global coverage, and by minimising all sources of error affecting the measurements, an accuracy close to 1 cm can be achieved. However, altimetric products have a limited spatial and temporal resolution due to the separation between adjacent satellite ground tracks and to the revisiting time of the satellites. Most important, the accuracy of altimetry observations decreases very rapidly near the coast; despite the advances reached in recent years, standard altimetric data are only available from 5-10 km offshore.

As an alternative to coastal altimetric products, in this work we develop a new methodology to reconstruct coastal sea level from a number of tide gauge observations, which in our case is applied to the western basin of the Mediterranean sea. The reconstruction covers all coastal regions and has the spatial and temporal resolution required to characterise coastal processes. The sea level reconstruction is based on a multiscale optimal interpolation where the spatial correlations between tide gauges and all the coastal points has to be determined prior to the interpolation. In our case, these correlations are computed from the outputs of a high-resolution numerical model. As for observations, for the monthly reconstruction we use PSMSL tide gauge records, which cover the period from 1884 to 2015. For the daily reconstruction we use the series of the GESLA-2 data set, which cover from 1980 to 2015.

A cross-validation test developed to validate the skills of the method shows that our reconstruction clearly outperforms altimetric and modelling products at different time scales, and therefore represents a valuable contribution to the attempts of recovering coastal sea level. Thus, the obtained reconstruction has been used to complement the characterization of open sea level variability in the western Mediterranean previously done by other authors, allowing us to estimate coastal sea level trends, and to examine the correlation between Western Mediterranean coastal sea level and the main North Atlantic climate indices. The limitations and applicability of the method to other regions will also be discussed in the presentation.