EUROMODEL EXPERIMENTAL STUDIES 1990-1995: CONTRIBUTION TO THE PRIMO-0 AND PRIMO-1 OPERATIONS <B>EUROMODEL EXPERIMENTAL STUDIES 1990-1995: CONTRIBUTION TO THE PRIMO-0 AND PRIMO-1 OPERATIONS</B> <HR><P> Astraldi<SUP>1</SUP>, M., Font<SUP>2</SUP>, J., Millot<SUP>3</SUP>, C. & the EUROMODEL Group</B><I><P><P> 1 Stazione Oceanografica CNR, C.P. 316, 19100, La Spezia, Italy.<BR> 2 Institut de Ciències del Mar, CSIC, 08039 Barcelona, Spain.<BR> 3 Centre dOcéanologie de Marseille, B.P. 330, 83507 La Seyne, France.</I><B><P><P> EUROMODEL experimental strategy</B><P><P> The key characteristic of EUROMODEL has been the synergy between observations (in situ and satellite), theoretical, physical and numerical models in studying the different processes responsible for the circulation in the western Mediterranean sub-basins, as well as their dynamic interactions. Field experiments have been organised to identify and quantify specific phenomena, some of them as a continuation to previous observational programs. Fig. 1 shows the 1990-1995 EUROMODEL observational network, mainly focused to investigate the dynamics of the incoming Atlantic water (Alboran sea), mesoscale frontal structures and topography interaction (Balearic basin), and the exchanges and variability in several straits and passages. Remote sensing, including microwave sensors from the European ERS-1 satellite, has also been used to analyse the long-term variability or the interactions between adjacent sub-basins, as well as to test new techniques to observe mesoscale structures in the ocean surface.<I><P><P> PRIMO-0</I><BR> The major EUROMODEL joint operations, with the participation of other European teams, have been the PRIMO-0 and PRIMO-1 experiments. PRIMO-0 took place in 1990-91 in the Liguro-Provencal basin, when 60 current meters were moored to investigate, together with regular CTD surveys, the seasonal and mesoscale variability of the Mediterranean Northern Current (Albérola <I>et al</I>., 1995). In summer, the Northern Current is relatively wide and shallow, and displays a reduced mesoscale variability. In winter, it becomes thicker and narrower while it tends to flow closer to the slope. At this time, it develops relatively intense mesoscale meanders (with amplitude and wavelength of a few tens of km), as evidenced by very characteristic features during PRIMO-0: the semi-major axes of the dispersion ellipses are perpendicular to the coast in the core of the current and display a clockwise (resp. anticlockwise) rotation of the current on its inner (resp. outer) side. This mesoscale variability, with a homogeneous vertical structure in the whole surface layer, is associated with a turbulence which clearly spreads seawards over large distances (Fig. 2). Some aspects of the winter distribution of the intermediate waters have been clearly evidenced. WIW appears to result from an overlaying of cooled and mixed MAW by less cooled one (a process which can occur everywhere in the sea) while LIW no more flows like a vein along the continental slope, but spreads seawards where, as well known, it participates to the formation of WMDW. In a several-km coastal zone (between the inner edge of the current and the coast), it seems that most of the time, except maybe in the deep winter, a relatively small scale turbulence precludes from clearly evidencing any significant mean circulation. In such a case, it would be extremely difficult to make a significant forecast of the circulation there. <P><P> The existence of a clear seasonality in the mesoscale motion in the whole Northern Current has been confirmed by the analysis of a long time series (1987-1992) of current meter data in the shelf break off the Ebro river (Font <I>et al</I>. 1995). The main observed feature is a rapid and strong autumn increase in mesoscale activity, that results to be always present and with very small interannual variability on its temporal location. The maximum of mesoscale activity is followed by a rapid decrease in winter and then by a continuous lowering until the end of the summer. A secondary maximum appears by the end of the spring.<P><P> Parallel to PRIMO-0, a current meter chain has been continuously working since 1985 (with the exception of 1989-1990) in the Corsica channel close to the beginning of the Northern Current. The time series indicates that the seasonal oscillation of the current represents the most important feature of the regional circulation. Nevertheless, it shows a relevant interannual variability and its energy has been seen to strongly decrease in the most recent years. This is interpreted (Astraldi <I>et al</I>., 1994) as a significant change sustained by the atmospheric-climatic conditions over the basin. As these conditions have significantly evolved (increase of the winter air temperature over the basin and regression of the Alps ice cover), the annual variability of the transport in the Corsica Channel could be indicative of the response of the whole Mediterranean circulation to the atmospheric forcing.<P><P> Even if the major characteristics of the Mediterranean Northern Current itself as well as those of its associated mesoscale meanders can now be considered as pretty well described, we are not yet able to clearly account for an accurate seasonal variation of the transport of this current which might be indicative of its relationships with the WMDW formation. Nevertheless, data analysis is still in progress, especially the comparison of the PRIMO-0 data in the Ligurian Sea and the channel of Corsica.<I><P><P> PRIMO-1</I><BR> The PRIMO-1 experiment was organised to monitor the seasonal variability of the fluxes in the Western/Eastern Mediterranean connection region, and the evolution of anticyclonic eddies in the Algerian basin near the Sardinia Channel. Deployments of current meter moorings and CTD samplings by the EUROMODEL and GEODYME teams in the Sicily-Sardinia-Tunisia (SST) region were initiated in November 1993. The data collected until now in these observations give a new insight into the circulation of the different Mediterranean water masses in such a crucial area.<BR> LIW outflowing from the eastern basin through the two parallel sills of the Sicily Strait has been seen to directly enter the Tyrrhenian Sea following the western and northern Sicilian slopes (Fig. 3). After a recirculation within the basin, which strongly contributes to change the original properties of the incoming water, LIW leaves the Tyrrhenian Sea by the deepest canyon of the Sicily-Sardinia passage, and then flows toward the central part of the Sardinia Channel. Current meters off the Sardinia slope indicate during one year an almost permanent flow to the NW in intermediate and deep layers.<P><P> Another vein of IW has been observed to enter the SST region from the west. Currents recorded off the African slope in the Sardinia channel are always directed to the E down to 2500 m. The signal of this vein was observed in the Sicily-Sardinia passage at the edge of the eastern side of the same canyon, where it appears as a vein having specific properties which differentiate it from the surrounding waters. Its characteristics are those of an IW flowing from the western Mediterranean and driven by topography toward the Tyrrhenian Sea.<P><P> A bottom water vein was seen for the first time to flow in the deepest parts of both the Sardinia channel (Fig. 4) and Sicily-Sardinia passage. In contrast with the previous water types, that were observed year round, the presence of this water flowing northwards seems to be intermittent, and its hydrographic properties, along with the period of its observation, are consistent with the episodes of deep water formation occurring in winter in the Gulf of Lions. <P><P> The analysis of the data is still going on, even taking into account new data recently obtained in the region, however new hypotheses have emerged concerning the intermediate and deep circulation. We are now thinking that WMDW formed in the Gulf of Lions first fills the Algero-Provençal basin (max depth ~ 2900 m) before flowing through the Channel of Sardinia (depth of ~ 2000 m) and sinking into the Tyrrhenian Sea (max depth of ~ 3900 m) where it mixes with the waters issuing from the Channel of Sicily to form the Tyrrhenian Deep Water. TDW would be the sole Mediterranean deep water issuing through the Strait of Gibraltar together with LIW.<B><P><P> Future Studies</B><P><P> There are now evidences that operational oceanography is feasible in a Mediterranean context. However, important problems are still to be solved, mainly related to the processes driving intermediate and deep flows constrained by topography. It appears that present simulations may underestimate water velocities at intermediate layers, what influences the fluxes of LIW involved in winter dense water formation. The hypotheses raised from the PRIMO-1 results concerning the formation of TDW and its role in the Gibraltar outflow have to be investigated. A satisfactory estimation of the dynamic conditions governing the water fluxes through the strait of Sicily is far from being reached, as past values spanned from 0.65 to more than 3 Sv.<P><P> In general the southern basin of the western Mediterranean is, compared to the north regions, very poorly understood. Mean paths of the water masses in the Algerian basin are still debated, and the fundamental role of large mesoscale phenomena there for the Mediterranean dynamics is, although generally accepted, far from being quantified. The origin of LIW lenses found in the interior of the basin and their role in the distribution of biogeochemical parameters has still to be established.<P><P> Taking into account the present situation, we consider that future experimental efforts in the western Mediterranean should be focused in two main aspects: long time monitoring of fluxes and dynamic conditions in straits and passages, and intensive multidisciplinary studies in the southern region to solve specific problems. The use of Lagrangian drifters and tracers should be recommended. And these in situ observations have to be complemented by spatial and temporal variability investigations from satellite remote sensing, using the possibility of merging altimetric measurements from different platforms and the expected new colour sensors. This is the core of the experimental program submitted by the EUROMODEL group, together with many other teams, to the Mediterranean Targeted Project for MAST 3.<B><P><P> References</B><P><P> Albérola C., Millot C. and Font J. 1995. On the seasonal and mesoscale variabilities of the Northern current during the PRIMO-0 experiment in the Western Mediterranean Sea. <I>Oceanologica Acta, PRIMO-0 special issue</I>, 18(2) in press.<BR> Astraldi M., Gasparini G.P. and Sparnocchia S. 1994. The seasonal and interannual variability in the Ligurian-Provencal basin. In : <I>The Seasonal and Interannual Variability of the Western Mediterranean Sea</I>, P.E. La Violette edt., <I>AGU Coastal and Estuarine Studies</I>, 46, 93-113.<BR> Font J., García Ladona E. and Górriz E.G. 1995. The seasonality of mesoscale motion in the Northern current of the western Mediterranean Sea: several years of evidence. <I>Oceanologica Acta, PRIMO-0 special issue</I>, 18(2) in press.<I><P><P> Fig.1. EUROMODEL observational network: CTD stations (*), AXBT/XBT (+) and moorings (x).<P><P> Fig.2. Monthly mean alongslope current and variance in the upper layer at four points 8 km (1),15 km (2), 20 km (3) and 30 km (4) off Nice during PRIMO-0.<P><P> Fig.3. (Above) Salinity section from Sardinia to Sicily during PRIMO-1 (RV Urania, November 1993).<BR> Fig.4. (Below) Salinity section from Tunisia to Sardinia during PRIMO-1 (RV Garcia del Cid, November 1993).</I><P><P>