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 surface 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 reference which yields two consistent datasets. We combine them via a space-time objective analysis method. Comparison of sea level anomaly (SLA) maps allows us to check that the specific contribution of ERS-1 consists of an improved 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 other, it is difficult to isolate them. Moreover, as shown by the comparison of fall 1992 and fall 1993 maps, strong interannual signals are suspected to affect the seasonal circulation. This makes the reference to previous observations almost impossible, especially in the Eastern Basin. However, several well-known signals have been recovered and new interesting features are observed. The variability in the Western Basin consists of a wintertime intensification of the basin-scale cyclonic cell, with the acceleration of coastal narrow currents, and in mesoscale activity all year round in the southern part of the basin and in the Tyrrhenian Sea. The Alboran gyres temporal variations and eastward and seaward propagations of Algerian Current eddies are detected. The analysis of ECMWF monthly averages suggests that wind stress curl variations are responsible for the large-scale seasonal variability. In the Ionian Basin, the signals are more complex. In winter 1993, the eastward current along the coast of Africa is strongly intensified; in summer 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 anticyclone is detected around 17 degrees E-34 degrees N. In the Levantine Basin, the strongest signal reflects the seasonal variations of the Ierapetra gyre, southeast of Crete. No basin-scale features are detected there. On the contrary, strong mesoscale activity appears throughout the year, in the form of transient anticyclonic 'eddies'; their development seems to be correlated with the basin topography. However, we cannot identify distinct, isolated structures in the south (such as the expected Mersa-Matruh or the Shikmona gyres). It appears instead that the circulation in the southern part of the basin is composed of multi-centered anticyclonic systems with high temporal variability. (C) 1998 Elsevier Science B.V. All rights reserved.