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Archive

3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Subsurface temperature and salinity variability in the Aegean-Black Sea system during 1950-2000 and relation to meteorologic forcing


Haris Kontoyiannis (Hellenic Center for Marine Research); V. Papadopoulos

Subsurface temperature and salinity time-series have been constructed for the period 1950-2000 for two areas in the Aegean (one in the southwest and one in the northeast) and two areas in the Black Sea (one in the southwest and one in the northeast) with dense hydrographic data coverage in the MEDAR/MEDATLAS data base. The focus is on air-sea interaction in climatic time scales. In order to capture the climatic response to the atmospheric forcing with as little bias as possible we selected a sub-thermocline layer, 80-120 m in the Aegean and σθ ~14.4-15.5 (~40-~90 m) in the Black Sea for the temperatures, and constructed winter and summer averages for each year and each area. The salinity is also examined in the entire upper ~80 m. In order to examine the variability in all time scales involved, from 3-to-4 year periods to ~30-year trends, the resulting time series were not smoothed.
Well-defined trends are evident in the temperature time series, that have already appeared in the literature in basin and year-round averages. A warming period common to all time-series from early-mid 90's until at least 2000, a cooling period common to all time series from mid-late 60's until early-mid 90's, and the period from 1950 until mid-late 60's, in which there is a differentiation in the trend behavior of the time series. 'Warm' time series characterized by higher temperatures, i.e., both Aegean areas in summer, show no trend in this period, whereas 'cold' time series characterized by lower temperatures, i.e., both Black Sea areas in winter, show a warming trend. Similar trend behavior exists in the NAO (North Atlantic Oscillation) and EAWR (East Atlantic West Russian) atmospheric indexes and explains partly but not entirely the temperature trends, indicating that a detailed examination of the climatic atmospheric circulation is necessary. The periods of the turning points or regime shifts (~1965-1967 and ~1992-1993) seem to be associated with extreme oceanic and/or weather events, reported in the literature, since they are also concurrent with extreme values in the atmospheric indexes.
The salinity time-series have different variability characteristics from the temperature time series. There are no apparent trends in the periods ~1950-1965, ~1965-1995, ~1995-... as in temperature. The only possible long-term trend appears in the salinity of the Aegean during summer where there is an increasing trend after ~1973/1975 and until ~2000, on which there are superimposed ~10-year cycles, i.e, ~1973-1983, ~1983-1990, ~1990-1998. The upper-layer salinity characteristics in the Black Sea are different from the corresponding characteristics of the Aegean. The 'Evaporation-Precipitationr' timeseries obtained from NCEP reanalysis data for the particular areas are visually correlated with the Aegean salinity oscillations of ~1982-1990 and ~1990-1998 and possibly there is a weaker correlation with the Black Sea salinity oscillations in ~1984-1989 and ~1989-1995. In the Black Sea, apparently the rivers have a considerable contribution in the upper-layer long-term salinity variability. Its is obvious that in the period ~1950-1970 the 'E-P' reanalysis timeseries show in overall no correlation with the salinity timeseries indicating possibly a weakness in estimating and predicting the weather conditions in those years 4-6 decades ago.