Dept. of Oceanology and Environmental Geophysics

The area of the cyclonic gyre in the South Adriatic Pit was identified as a site where the Adriatic Deep Water (ADW) forms through open-ocean convection processes (Ovchinnikov et al., 1985). The pre-conditioning and the convection phases take place in the period December - March.

Daily net heat fluxes for December, January, February and March in the Southern Adriatic are calculated using ECMWF (Reading, UK) meteorological data for the period from 1982 through 1998. Then the interannnual variability of the winter climatic conditions is studied analysing the integrated heat loss at one location in the South Adriatic Pit for the last decade, when within the framework of the various national and international projects (PRISMA, POEM, OTRANTO, MATER) hydrographic surveys in the area of the Southern Adriatic have been carried out almost every year towards the end of winter. The total winter heat losses are obtained integrating daily net fluxes in the time interval December-March for each year. The heat loss ranges from the minimum value in winter 1988/89 (4.86 × 10⁸ J/m²) to a maximum of 13.5 × 10⁸ J/m² in the winter 1991/92. Rather good correlation was found between the winter integrated heat losses and the North Atlantic Oscillation index (Hurell, 1995) only in the period 1991 - 1998 when the ECMWF data have a high spatial resolution of .5 degree lat/long grid (Fig. 1). The hydrographic data show a prominent interannual variability in the vertical pattern of thermohaline properties across the South Adriatic Pit. The IR satellite imagery reveals also the important year-to-year changes in both the SST spatial pattern and the absolute values of SST (Gacic et al., 1987). Here an attempt has been made to associate these variations with the interannual changes of the local climatic conditions as represented by the integrated winter heat losses.

The hydrographic data show the strongest vertical convection reaching a depth of 800 m in winter 1991/92 (Fig. 2) , when the maximum heat loss was recorded. On the other hand, the water column in the South Adriatic Pit remains highly stratified in winter 1996/97 when the heat loss is rather low. The last winter 1997/98 shows an interesting structure where within an isolated chimney (Fig. 2) , only an intermediate layer convection up to 300 m depth took place splitting the subsurface oxygen minimum layer associated to the Levantine Intermediate Water (LIW) into two separated low-oxygen cores (Fig.3). The dissolved oxygen data in the absence of the open-ocean convection during winter 1996/97, show a clear signature of the oxygen poor LIW layer at a depth of about 200 m. On the other hand, in years with a strong vertical convection like in winter 1991/92, the whole water column becomes highly oxygenated and the low-oxygen intermediate layer is completely absent.

In addition to the interannual variability of the vertical thermohaline pattern, there are variations in the absolute values of the water column temperature, salinity and hence density. Appreciable variations have also been noticed in the nitrate distribution which show an almost continuous increase in the analysed period.

The variability in thermohaline properties, however, cannot be explained only in terms of the heat flux variations associated with local climatic conditions, but in addition the hydrographic conditions of the Ionian Sea and of the Eastern Mediterranean as a whole must also be taken into account since they determine the pre-conditioning state of the water column. One of the main contributions to the ADW is the inflowing LIW from the Ionian Sea, the characteristics of which may have been altered due to the recently observed changes in the deep circulation of the Eastern Mediterranean and Ionian Seas (Roether et al., 1986).

-Gacic, M., S. Marullo, R. Santoleri and A. Bergamasco, 1997: Analysis of the seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea from AVHRR data (1984-1992). Journal of Geophysical Research, 102, C10, 22937-22946.

-Hurell, J. W. and H. van Loon, 1997: Decadal variations in climate associated with the North Atlantic oscillation. Climate Change, 36, 301-326.

-Ovchinnikov I.M., V.I. Zats, V.G. Krivosheya and A.I. Udodov, 1985: Formation of deep Eastern Mediterranean waters in the Adriatic Sea. Oceanology, 25, 6, 704-707.

-Roether, W., B.B. Manca, B. Klein, D. Bregant, D. Georgopoulos, V. Beitzel, V. Kovacevic and A. Lucchetta, 1996: Recent changes in the Eastern Mediterranean deep water. Science, 271, 333-335.

Fig.1 .- Integrated winter heat loss for the South Adriatic Pit (42°N;17°E) and North Atlantic Oscillation Index (NAO)



Fig.2 .- Vertical distribution of water temperature (°C) of the South Adriatic Pit



Fig.3 .- Vertical distribution of dissolved oxygen content M/l) in the South Adriatic Pit

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