A description of the Mediterranean surface variable circulation from combined ERS-1 and TOPEX/POSEIDON altimetric data

The ERS-1 and TOPEX/POSEIDON (T/P) data have been combined to study the sur face circulation variability in the Mediterranean Sea, from October 1992 to December 1993. The ERS-1 orbit error is corrected using T/P data as a refe rence which yields two consistent datasets. We combine them via a space-tim e objective analysis method. Comparison of sea level anomaly (SLA) maps all ows us to check that the specific contribution of ERS-1 consists of an impr oved mesoscale circulation description with respect to the analysis of T/P only. The Mediterranean circulation variability, as observed by T/P and ERS -1, is characterised by a wide range of temporal and spatial scales. As the various signals are superimposed and very likely to interact with each oth er, it is difficult to isolate them. Moreover, as shown by the comparison o f fall 1992 and fall 1993 maps, strong interannual signals are suspected to affect the seasonal circulation. This makes the reference to previous obse rvations almost impossible, especially in the Eastern Basin. However, sever al well-known signals have been recovered and new interesting features are observed. The variability in the Western Basin consists of a wintertime int ensification of the basin-scale cyclonic cell, with the acceleration of coa stal narrow currents, and in mesoscale activity all year round in the south ern part of the basin and in the Tyrrhenian Sea. The Alboran gyres temporal variations and eastward and seaward propagations of Algerian Current eddie s are detected. The analysis of ECMWF monthly averages suggests that wind s tress curl variations are responsible for the large-scale seasonal variabil ity. In the Ionian Basin, the signals are more complex. In winter 1993, the eastward current along the coast of Africa is strongly intensified; in sum mer 1993, it is shifted to the north and forms a large anticyclonic meander extending up to the Otranto Strait. From April to December 1993, an anticy clone is detected around 17 degrees E-34 degrees N. In the Levantine Basin, the strongest signal reflects the seasonal variations of the Ierapetra gyr e, southeast of Crete. No basin-scale features are detected there. On the c ontrary, strong mesoscale activity appears throughout the year, in the form of transient anticyclonic 'eddies'; their development seems to be correlat ed with the basin topography. However, we cannot identify distinct, isolate d structures in the south (such as the expected Mersa-Matruh or the Shikmon a gyres). It appears instead that the circulation in the southern part of t he basin is composed of multi-centered anticyclonic systems with high tempo ral variability. (C) 1998 Elsevier Science B.V. All rights reserved.