Interannual and annual variations in the Mediterranean Sea from satellite
David Garcia(1) , Isabel Vigo(1) , Benjamin F. Chao(2) , and Jorge Del Rio(3)
University of Alicante,
PO Box 99,
(2) NASA Goddard Space Flight Center, ., 20771 Greenbelt, MD, United States
(3) ESRIN/European Space Agency, Via Galileo Galilei, Casella Postale 64 , 00044 Frascati (Rome) , Italy
Since its launch in 1992, TOPEX/Poseidon altimeter satellite has collected more than a decade of high-quality Sea Level Variation (SLV) measurements. We have used these SLV data to study the inter-annual variation in the Mediterranean Sea, whose behavior is found to be rather different from that of the global ocean. An interesting abrupt change of linear trend in mid-1999 is observed in the Ionian, Adriatic and Aegean seas and in the Levantine basin. This event coincides in time with the restoration of the Adriatic Sea as the main source of deep water in the Eastern Mediterranean, that is, the end of the Eastern Mediterranean Transient (EMT), although the possible connection is still unclear. On longer timescales we estimated the linear trends of the altimetric SLV versus that of the tide gauges along the northern coast of the Mediterranean. The difference of the two slopes is an estimate of the vertical motion (in mm/year) of the ground at each tide gauge site, presumably due to tectonic movements modified by any post-glacial rebound signals. Interesting regional patterns of such vertical movements emerge as a result, and can be used as data source for regional tectonic studies.
We have also examined the annual SLV cycle in the Mediterranean. Thus, the SLV was corrected for the steric height variations, or the dilatation/contraction of the column of water due to salinity and temperature changes, for which we adopted the profiles from ECCO ocean circulation model. The resultant non-steric altimetry is an indirect measurement accounting for water mass variations. In parallel with this estimation, we use the GRACE data for a direct measurement of the Mediterranean water mass budget variation. The GRACE satellite mission was launch in 2002 to measure time-variation gravity (TVG) of the Earth system, which can be converted into surface mass variations. The comparison between those direct and indirect estimations of water mass variations in the Mediterranean shows a good agreement and a similar annual signal with an amplitude between ~40 and ~50 cm, which is lagged ~4 months with the apparent SLV from altimetry. We can conclude that the Mediterranean SLV is mainly driven by steric height variations which are offset by water mass variations.