VARIABILITY OF THERMOHALINE PROPERTIES AND CIRCULATION IN THE AREA OF THE OTRANTO STRAIT <B>VARIABILITY OF THERMOHALINE PROPERTIES AND CIRCULATION IN THE AREA OF THE OTRANTO STRAIT</B> <HR><P> Manca</B><SUP>1</SUP><B>, B., Papageorgiou</B><SUP>2</SUP><B>, E. & Scarazzato</B><SUP>1</SUP><B>, P. <I></B><P><P> 1Osservatorio Geofisico Sperimentale, Trieste, Italy<BR> 2National Center of Marine Research, Athens, Greece</I><P><P> From February 1994 to May 1995 six seasonal oceanographic cruises were performed in the Otranto Strait area. The synoptic shipboard measurements included CTD casts as well as water samples collection for the geochemical determinations. The main station network (Fig. 1) was designed with the aim of investigating the pathways of the water masses, the mechanisms responsible of the mixing processes and the modification of the major water masses when they cross the Otranto Strait. The main station CTD data has been integrated with other data collected in the same area or adjacent zones, that may be considered synoptic with those collected in the framework of the Otranto project.<P><P> At least four water masses take part in the processes of the water exchange between the Adriatic and Ionian seas. However the hydrography in the Otranto Strait can be schematically represented as a three-layer system. The surface layer (0 - 100 m) is dominated by intense dynamic features under the influence of the seasonal climate variability of the Adriatic and Ionian seas. The flow through the Strait appears to be subjected to seasonal fluctuations. During winter the computed geopotential anomalies revealed a sub-basin scale cyclonic circulation (Fig. 2) reinforcing the inflow of the Ionian Surface Water (ISW) into the Adriatic along the Albanian coast and the outflow of the less saline and colder Adriatic Surface Water (ASW) along the western side. The density driven currents can be modified by intense local phenomena such as eddies travelling from the Ionian to the Adriatic sea and wind forcing which are responsible for current reversal as documented by direct current measurements (Gacic <I>et al</I>., this issue; Ferentinos and Kastanos, 1988; Michelato and Kovacevic, 1991). During summer the entire area South of the Strait was dominated by a large anticyclone (Fig. 3) mainly determined by the less saline and warmer ASW which, under the influence of favourable meteorological conditions, outflows along the Italian coast and in the central part of the Strait.<P><P> The intermediate layer (100 - 400 m) is almost completely occupied by the inflow of Levantine Intermediate Water (LIW). The LIW has relatively high temperature and the highest salinity of all water masses present in the Strait. The main core of the LIW is centred at a depth of about 200 m. During summer the LIW extends its influence at the interior of the Strait from the Albanian continental slope westward up to the Italian one. However mainly during winter an outflow of less saline and colder Adriatic water takes place at the same intermediate layer along the Italian continental slope. In same circumstance, particularly favourable during the spreading southward of this water mass, the Adriatic water presses the LIW against the eastern side of the Strait reducing the LIW intrusion into the Adriatic.<P><P> The deep layer (400 m - down to the bottom) is interested by direct mixing of the LIW with the Adriatic Deep Water (ADW) which overflows the sill of the Otranto Strait. The main feature in the bottom layer is represented by the southward flow of the ADW from the South Adriatic pit. The thickness of the bottom layer occupied by the ADW varies from 150 m during winter to few tens of meters during summer.<P><P> Average temperature-salinity relationship for the two most important clearly identifiable water masses, LIW and ADW, and their presence in the area are studied in more details. LIW can be identified by a salinity maximum located between 150 and 400 m depth. Its average temperature-salinity relationship is calculated in each transect considering all the data for which S >= 38.75 PSU. On the other hand it is know that the ADW is the densest water in the Strait and its average temperature salinity characteristics are computed considering all the data for which gf >=29.2 kg/m<SUP>3</SUP>. Table 1 summarises the average characteristics of the temperature-salinity relationship for the LIW (upper panel ) and ADW (lower panel) as defined above.<P><P> Looking at temperature and salinity distribution measured along the five transects allows one to resolve the section areas occupied by LIW and ADW. Fig. 4 shows the results of these calculations for all the seasonal surveys, indicating separately the spatial variations in each of the five transects. Fig. 5 shows the temporal variations occurring at the three transect located at the interior of the Otranto Strait; the averaged values are also reported in Table 1, right column. The section area occupied by LIW appears larger than the area occupied by ADW. The former exhibits a reduction as it moves from the southern transect to the northern one. One sees that ADW has the largest area during winter season while its minimum is in summer. This can be explained in terms of the filling up and subsequently emptying of the Southern Adriatic deep water reservoir. However in autumn the ADW volume maximum occurs again. The only possible explanation for the occurrence of the increased volume of ADW that outflows the sill of the Otranto Strait (800 m) can be found in an increasing contribution of much colder Northern Adriatic Deep Water that sinks in the Southern Adriatic pit south of Palagruza sill or during its long journey along the Italian continental slope (Zoccolotti and Salusti, 1987) after passing the Gargano section. <P><P> Estimates of the volume transports of the different water masses through the Otranto Strait are obtained from density field and long-term current measurements. The monthly average values of the current vectors has been calculated from the current meters data deployed in correspondence of section areas occupied by the different water masses (i.e. ASW, ISW, LIW and ADW). The geostrophic transports estimates are discussed with special emphasis on the surface layer, taking into account the bottom topography and the station distributions.<B><P><P> References</B><P><P> Gacic, M., Kovacevic, V., Vetrano, A., this issue, Sub-inertial current variability in the Otranto Strait.<BR> Ferentinos, G., N. Kastanos, 1988, Water circulation patterns in Otranto Straits, Eastern Mediterranean. Continent. Shelf Res., 8, 1025-1041.<BR> Michelato, A., V. Kovacevic, 1991, Some dynamic features of the flow through the Otranto Strait. Boll. Oceanol. Teor. Appl., IX, 1, 39-51.<BR> Zoccolotti, L., E. Salusti, 1987, Observations of a vein of very dense marine water in the southern Adriatic Sea. Continent. Shelf Res., 7, 535-551.<P><P>