Sea level variations in the Mediterranean Sea and Black Sea from satellite altimetry and tide gauges

Present-day sea level changes in the Mediterranean Sea and Black Sea are studied using satellite altimetry. Analysis of altimetry data from Topex–Poseidon (T/P) between January 1993 and December 1998, and from ERS-1/2 between October 1992 and June 1996 shows that the mean rate of sea level rise is 7±1.5 mm/year over the Mediterranean Sea and 27±2.5 mm/year over the Black Sea. The geographical distribution of the observed trends is rather uniform in the Black Sea unlike the Mediterranean Sea. There we observe, over the 6 years of analysis, a quite large (20–30 mm/year) sea level rise in the Levantine basin. In the Ionian Sea, on the other hand, a negative sea level trend is reported during that period. In the western basin, sea level trends are significantly lower, some regions rising and others falling. An Empirical Orthogonal Function (EOF) analysis of the sea level data is presented which confirms the main features reported above. Analysis of sea surface temperature (SST) data over the two seas and over the same time span indicates that basin-scale trends are correlated with the altimetry-derived sea level trends; however, the spatial variations of SST trends are smoother than sea level trends, the latter presenting subbasin fluctuations. The spatial correlation between sea level trends and SST trends suggests that at least part of the sea level change reported during the few years over the Mediterranean Sea and Black Sea is due to heating of surface layers. Moreover, the temperature and salinity increase reported since the early 1960s in the deep waters of the western Mediterranean basin and more recently since the early 1990s in the eastern basin may contribute to the observed sea level trend. However, the observed trends in the Mediterranean sea level for 1993–1998 may also result from the interannual/decadal variability of the upper ocean circulation that is predicted by theoretical circulation models. In the Black Sea, apart from a possible steric contribution, change in regional hydrology, in particular, a decrease in river runoff, could be responsible for observed sea level changes. Finally, we also present results of long-term sea level trends (multidecadal time scale) using tide gauge records.