Dense water formation in the Southern Adriatic Sea and spreading into the Ionian Sea in the period 1997–1999

Abstract Hydrographic and current measurements conducted during the period from March 1997 to March 1999 in the Southern Adriatic and in the Otranto Strait provide clues to the mechanisms of dense water formation and its spreading in the Ionian Sea. The hydrographic surveys covered the successive phases of preconditioning, convection, vertical mixing and spreading. The sub-basin scale cyclonic circulation, the presence of highly saline and dense water in the intermediate layer and winter outbreaks of cold and dry continental air are verified as necessary prerequisites to open-ocean deep convection. However, in two winters, surface cooling and surface buoyancy fluxes were not of sufficient intensity, and the convective mixing reached only intermediate depth of ∼400 m in 1998 and ∼700 m in 1999. The ventilated convective cell in the centre of the gyre had density of ∼29.16 kg m −3 , which was lower than the typical density ( σ θ ∼29.24 kg m −3 ) of the water that resides in the bottom layer of the southern basin (Adriatic Deep Water–ADW). In addition, dense waters from the northern shelf region contributed in filling up the Southern Adriatic deep reservoir. During more severe winter in 1999, two concomitant processes were observed: convection in the centre of the gyre and advection along the shelf of much denser waters ( σ θ ∼29.34 kg m −3 ), which, originating from the Northern Adriatic (NAdDW), sunk at the shelf-break and mixed with the resident bottom water in the Southern Adriatic. Finally, the fate of the ADW and its spreading into the Ionian has been investigated. Transport rates in the bottom layer across the Strait of Otranto have been estimated from long-term current-meter measurements and are related to the intensity of the ADW formation during the study period (1997–1999). Sub-inertial flow fluctuations reach as much as 1 Sv and occur during post-convection, presumably as a consequence of violent mixing processes and spreading. At longer time-scales (seasonal and yearly), the low-frequency water flux is seasonally modulated and exhibits a maximum during post-convection period. It ranges from 0.1 Sv in winter 1997 to 0.4 Sv in winter 1999, showing a fairly consistent interannual variability related to the intensity of winter convection.