The VANI2-ERA hindcast of sea-level residuals: atmospheric forcing of sea-level variability in the Mediterranean Sea (1958-2008)

The hindcasts of residual sea surface are very useful tools for the design of infrastructures and coastal management applications, as well as for scientific studies on sea-level variability. In this paper we present VANI2-ERA, a new hindcast of residual sea surface covering the Mediterranean Sea and a sector of the NE Atlantic Ocean with a spatial resolution of 1/6 degrees x1/4 degrees. VANI2-ERA was obtained with a barotropic version of the HAMSOM model forced with a dynamical down-scaling of the ERA40 reanalysis spanning the period 1958-2008. This new hindcast represents an update of the well-known HIPOCAS hindcast, which has been the reference dataset for the last few years. VANI2-ERA has been extensively validated with tide gauge observations distributed all around Southern Europe and compared with four other hindcasts, including HIPOCAS. The new hindcast shows smaller root mean square errors, higher correlation and higher variance reduction than any previous hindcasts at most stations. However, its performance is poorer than HIPOCAS in reproducing the magnitude of extreme events. Residual sea surface trends were also obtained and their uncertainty was assessed by means of a comparison between VANI2-ERA and HIPOCAS for the common period 1958-2001. Differences of up to 0.4 +/- 0.1 mm yr(-1) between the two datasets are derived from the differences between the ERA40 and NCEP atmospheric reanalyses. The VANI2-ERA hindcast was also used to compare the variability of the period 1998-2008 with past decades, obtaining that the standard deviation of sea-level residuals has been 5-10% lower than for the period 1958-2008. The period 1998-2008 also shows a 20% reduction in the magnitude of positive extreme events, while negative extreme events have not changed significantly with respect to past decades. Finally, the trends computed for the period 1998-2008 are slightly positive (+0.11 +/- 0.04 mm yr(-1)) compared with the clearly negative trends obtained for the period 1958-2008.