a b s t r a c tSixteen years of satellite radar altimeter data are analyzed to investigate the sea-level variation (SLV) of the Mediterranean Sea. The time evolution of the overall mean sea level of the Mediterranean Sea follows its own regional dynamics. The geographical distribution of the seasonal signal (annual and semi-annual) indicates that the major features of the Mediterranean Sea circulation are driving the highest seasonal variability, and that an eastward propagation exists between the western and eastern basins. While in previous studies the trend of SLV has been modeled as linear, in this study with a longer record of observations we found that a quadratic acceleration term is statistically significant for practically the whole basin, especially in those regions where the trend provides a significant contribution to the SLV. The inclusion of the quadratic acceleration term accounts better for the Mediterranean SLV trend, as the residual low frequency SLV in wintertime is highly correlated with NAO at zero time lag in almost the whole basin. The residual high-frequency signal variability, on the other hand, can be explained by mesoscale phenomena, such as eddies and gyres. Our comprehensive analysis of the Mediterranean SLV provides source observations for monitoring and understanding of both regular and transient phenomena.
The surface geostrophic currents (SGC) can be derived via the principle of geostrophy from the dynamic height of the ocean, or the deviation of the true, variable sea surface height with respect to the Earth's static geoid, both of which can be measured by geodetic means. Here we calculate the Mean Dynamic Topography (MDT) by subtracting the geoid height determined by the GOCE satellite mission from the Mean Sea Surface Topography (MSST) derived from multi-satellite ocean altimetry (T/P, Jason 1/2, ERS-1/2, GEOSAT). Results for SGC are compared with those obtained from a GRACE-based mean geoid, as well as with the mean circulation patterns from measurements done by in situ drifter buoys and from simulations of the ECCO Ocean General Circulation Model. We found GOCE-based geoid solution clearly leads to significant improvements in the spatial resolution of SGC globally except in the Equatorial band where special filtering may be needed, with current velocities and spatial patterns closest to the in situ measurements of currents, compared with the GRACE-based results or ECCO model simulations that give significantly weaker Values with lower spatial resolution.
Sea level variations (SLV) can be measured by tide gauges (TG) at the coast and by altimeters onboard satellites. The former measures the SLV relative to the coast, whereas altimetry provides the SLV with respect to a geocentric reference frame. The differences between SLV measurements from these two techniques can be used as an indirect assessment of vertical crustal motions at the TG sites. In this study, we exploit this idea, analyzing differences between sea level signals as measured by altimetric missions (TOPEX/Poseidon and Jason-1) and by 47 TG stations along northern Mediterranean coasts for the period 1993-2007. This allows us to estimate the vertical land motion along these coasts at the TG sites in this time window. For those sites where the TG is co-located or has a nearby global positioning system (GPS) station, these estimates are compared with the vertical rates derived from GPS measurements. Our results on vertical ground motion along the Mediterranean coast provide a useful source of data for studying, contrasting, and constraining tectonic models for the region.
We have studied, for the first time, variations in absolute surface geostrophic currents (SGC) using satellite data only. The proposed approach combines 18 years' altimetry data, which provide reliable measurements of absolute sea level (ASL), with a gravity field and steady-state ocean circulation explorer geoid model to obtain dynamic topography, and achieves unprecedented precision and accuracy. Our proposal overcomes the main limitations of existing approaches based solely on altimetry data (which suffer from lack of an independent reference for derivation of ASL maps), and approximations based on in-situ data (which are characterized by a sparse and inhomogeneous coverage in time and space). Features of annual variations of SGC are also addressed. As a result of our study we provide new absolute SGC climatology in the form of a 52-week data set of surface current fields, gridded at quarter degree longitude and latitude resolution and resolving spatial scales as short as 140 km. For presentation, this data set is averaged monthly and the results, presented as monthly climatology, are compared with climatology based on in-situ observations from drifter data.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.