RESPONSE OF THE MEDITERRANEAN MEAN SEA-LEVEL TO ATMOSPHERIC-PRESSURE FORCING

The response of the Mediterranean mean sea level to atmospheric pressu re forcing is analyzed using 3 years of TOPEX/POSEIDON data. Coherence analysis between mean sea level and atmospheric pressure shows a sign ificant departure from a standard inverse barometer effect at frequenc ies higher than 30 days(-1). At high frequencies the phase difference between sea level and pressure is about 100 degrees, while it should b e 180 degrees for a perfect inverse barometer response. This result is in agreement with previous findings and confirms the role of the Stra its of Gibraltar and Sicily in limiting the water exchange (and thus t he response to atmospheric pressure forcing) at high frequencies. The response of the Mediterranean mean sea level is then investigated usin g the Candela [1991] analytical model which takes account of friction in the Straits of Gibraltar and Sicily. The model explains a large par t of the variance in TOPEX/POSEIDON mean sea level variations (50% for the western basin and 38% for the eastern basin). Compared to an inve rse barometer correction, it gives a smoother response with a phase de lay at high frequencies. It also explains more variance in TOPEX/POSEI DON mean sea level variations (5 cm(2) and 7 cm(2) for the western and eastern basins, respectively). This demonstrates that this simple mod el provides an improved correction of atmospheric pressure effects in TOPEX/POSEIDON data. As the two corrections have an rms difference of 2-3 cm with maximum differences of up to 10 cm, the impact on the mapp ing of oceanic circulation is not negligible. This is exemplified thro ugh the comparison of sea level anomaly derived from the two correctio ns.