Least-squares tracer analysis of water masses in the South Adriatic (1967–1990)

Abstract

The paper deals mainly with the temperature and salinity data collected during 30 cruises along the South Adriatic hydrographic section Bari-Dubrovnik but also uses data collected along the sections Palagruža Sill, Brindisi-Bar and Otranto between 1967 and 1990, most of which have never been described before. The analysis reveals the fractions of Middle Adriatic Deep Water (MADW), South Adriatic Deep Water (SADW) and Modified Levantine Intermediate Water (MLIW) computed by least-squares tracer analysis. The major feature found by analysis of the time series is the shift of water properties in the early 1980s, giving an increase in density (about 0.1 kg m −3 on average), due to exceptionally cold and dry winters over the whole Mediterranean. At the section Bari-Dubrovnik the fraction of SADW is the highest and the fractions of MADW and MLIW are the lowest during winter, because of the deep convection occurring in the South Adriatic Pit in winter, usually reaching the depth of about 600–800 m. Subsurface southeastward outflow of dense water usually occurs during spring, 4 months after the weakening of the Po River discharge and 2 months after the high surface buoyancy losses in the North Adriatic, respectively. Surface buoyancy losses occurring 6 and 8 months prior to the appearance of the subsurface outflow are in general favourable events for generation of dense water in the North Adriatic. The increase of MADW fraction at the bottom of the South Adriatic Pit lags for up to 2 years behind dense water formation in the North Adriatic. The case study of November 1989 reveals the renewal of bottom water in the South Adriatic Pit due to intrusion of MLIW, whereas in April 1987 strong mixing and sinking of extremely dense water, originating in the North Adriatic, was observed in the area of the Bari Canyon on the Italian shelf break. Near-bottom outflow of the Adriatic deep water towards the Ionian Sea was observed in the Otranto Strait in both cases. The case study of March 1975 revealed the maximum of SADW fraction.