THERMOHALINE CIRCULATION IN THE MEDITERRANEAN <B>THERMOHALINE CIRCULATION IN THE MEDITERRANEAN</B> <HR><P> Roether</B><SUP>1</SUP><B>, W., Beitzel</B><SUP>1</SUP><B>, V., Lascaratos</B><SUP>2</SUP><B>, A., Haines</B><SUP>3</SUP><B>, K. & Wu</B><SUP>3</SUP><B>, P. </B><I><P><P> 1Institut für Umweltphysik, Universität Bremen, Bremen, Germany<BR> 2Univ. of Athens, Athens, Greece<BR> 3Univ. of Edinburgh, Edinburgh, UK</I><P><P> A major topic of the MERMAIDS project have been studies of the formation and recirculation of Mediterranean intermediate and deep waters and their representation in models. These studies have served to elucidate relevant physical mechanisms, and have improved our predictive capability with respect to thermohaline circulation, for example concerning interannual variability, using modern ocean circulation models. <P><P> Reproduction of Eastern Mediterranean deep and bottom waters in the MERMAIDS MOM model [1] (Cox-Bryan model, code MOM 1.0, 1/4 degrees resolution, 31 levels, annually periodic forcing of temperature and salinity in the surface layer) was investigated by comparing model-simulated distributions of the transient tracer CFC-12 with observations for this tracer [2]. Adequate dense water production in the S Adriatic and overflow into the Ionian Basin was only obtained after increased forcing and a special convection scheme were introduced into the model. After considerable tuning, the large-scale CFC-12 distribution in the deep and bottom waters was reproduced by the model about realistically. However, the extra forcing required is excessively large, which is related to the model's limited capability to reproduce an overflow process. As an example Fig. 1 shows the model circulation at mid-depth for the tuned model; the strong cyclonic circulation in the north-west of the Ionian Basin is among the features the model resorts to in order to achieve downward transfer of the waters overflowing from the Adriatic. Further detail of the model performance is under study currently. A conclusion is that fundamental model developments are still required before a satisfactory operational capability for the deep thermohaline circulation can be expected. <P><P> The formation and spreading of the Levantine Intermediate Water (LIW) were studied using the Princeton Ocean Model [3] (resolution 5.5 km) initialised and forced by realistic data [4, 5]. Heat flux and evaporation were calculated in an interactive mode. Both climatological forcing and annually variable forcing were employed, the latter to study the temporal variability of LIW formation rates and characteristics and the response to synoptic time scale variability. The climatological experiments clearly outlined the Rhodes gyre area as the main LIW formation site, with a maximum convection down to 300 m and a mixed layer density of 29.05 in late February. The total amount of LIW produced annually is 1.2 Sv, in agreement with earlier estimates obtained by other methods (e. g. [6]). Using a lower model resolution (11 km) gave less favourable results. In the interannual variability experiments, one mild winter (1984) two typical ones (1985 & 1986) and one heavy winter (1987) were compared. Winter time heat loss reached from 108 W/m<SUP>2</SUP> (1984) to 171 W/m<SUP>2</SUP> (1987), and the rates of LIW formed from 0.64 Sv (1984) to 1.38 Sv (1985). The latter rate is high due to a special event in February 1984, indicating that such events are extremely important. The 1987 value was less (1.3 Sv) despite the large heat loss in that year; LIW formation, however, was shifted toward the north Levantine coast, with simultaneous production of deep water in the Rhodes gyre. The study was able to reproduce various recent observations in the Levantine. <P><P> Levantine Intermediate water dispersal across the entire Mediterranean was studied by running the MOM model (same as above, but 19 levels) for 100 years to study the thermohaline circulation throughout the basin. LIW formed between Crete and Cyprus is dispersed in baroclinic eddies reaching both the Adriatic in the East and the Gulf of Lions in the west where it helps to form deep waters. Deep waters are formed throughout the run with a higher salinity and density than is possible on the basis of local surface forcing alone therefore demonstrating the role of the subsurface salinity [7]. A good vertical water column structure is maintained in all areas throughout. Eastern Mediterranean deep waters are excessively mixed at the Otranto sill and do not reach the bottom of the Ionian basin. Further model improvements such as the Gent and McWilliams advective parametrization scheme may improve this. Western deep waters are formed in more open water and circulate throughout the western basin, overflowing into the Tyrrhenian basin consistent with observations, after about 50 years. The model can now be used in a variety of work where Lagrangian transport is important, such as nutrient modelling and modelling of transient tracer data. <B><P><P> References</B><P><P> [1] V. Roussenov, E. Stanev, V. Artale, N. Pinardi, J. Geophys. Res., 100, 13.515-13538, 1995.<BR> [2] V. Roussenov, W. Roether, V. Beitzel, J. Geophys. Res., submitted.<BR> [3] Blumberg A. F. and G. L. Mellor, 1987, in: Three- Dimensional Coastal Ocean Circulation Models, Coastal Estuarine Sci., 4, edited by N.S. Heaps, pp 1-16, AGU, Wahington D.C. 1987.<BR> [4] Brasseur P., J.M.Beckers, J.M.Brankart and R.Schoenauen, Deep Sea Research, submitted.<BR> [5] Castellari, S., N. Pinardi, and A. Navara, IMGA-CNR Tech. Rep. 4-90, Modena, Italy, 1990.<BR> [6] Lascaratos A., R.Williams and E.Tragou, J. Geophys. Res., 98, 739-749, 1993<BR> [7] Wu and Haines (1995), J. Geophys. Res., submitted<P><P>