Response of the Black Sea mean level to atmospheric pressure and wind forcing

The response of the Black Sea mean level to atmospheric pressure (AP) and w ind forcing is investigated using 5 years of TOPEX/POSEIDON (T/P hereafter) data. A coherence analysis is first applied to mean sea level and pressure to examine the validity of the inverse barometer (IB) approximation over t his area. As expected, it reveals very significant deviations from an IB re sponse attributed to the narrowness of the Bosphorus Strait and its limitin g role in water exchanges. A comparison is drawn with the Mediterranean Sea case. A single basin version of the Candela analytical model [Candela, J., 1991. The Gibraltar Strait and its role in the dynamics of the Mediterrane an Sea. Dyn. Atmos. Oceans 15, 267-300], which takes linear friction at the strait into account, is then used. The model explains a significant part o f the T/P mean sea level variance (about 30%, while the IB correction only explains 5% of its variance) and provides a means to correct the altimeter data for the pressure effect much better than the standard IB effect. The r esponse of the mean sea level to wind forcing is then analysed. Coherence a nalysis between sea level and along-strait wind stress (WS) reveals a signi ficant coherence at periods ranging from 40 to 100 days, with an almost ste ady phase of 270 degrees. This result is confirmed with a multiple coherenc e analysis (mean sea level vs. WS and AP). A plausible mechanism is a pilin g-up of water at the northern or southern end of the strait due to along-st rait wind forcing. The associated along-strait pressure gradient would modi fy the barotropic flow in the strait and then the mean sea level. Using an extension of the Candela model, we show that this mechanism is consistent w ith T/P mean sea level observations. (C) 1999 Elsevier Science B.V. All rig hts reserved.