Off-shelf fluxes across the southern Adriatic margin: Factors controlling dense-water-driven transport phenomena

Abstract

The northern Adriatic Sea is a basin where dense shelf waters form during winters if the following favorable conditions are attained: high salinities, mainly related to low river water discharge (Po river in primis) during fall season, and strong heat fluxes induced by cold and dry Bora wind events blowing from north-east. Extremely favorable conditions to Northern Adriatic Dense Waters (NAdDW) formation characterized winter 2012 when, following several months of low Po river runoff, a strong event of Cold Air Outbreak (CAO) occurred from the end of January until mid-February. Consequently, the mean temperature of northern Adriatic waters dropped to about 6°C and exceptional densities (potential density anomaly locally exceeding 30.0kgm−3) were reached.The production and spreading mechanisms of dense water in the Adriatic Sea have been modeled by means of the COAWST (Coupled-Ocean–Atmosphere-Wave-Sediment-Transport) modeling system. The model builds upon a high-resolution (1km spaced horizontal grid), fully 3-D primitive equations hydrodynamic model coupled with a phase-averaged wave model and sediment routines, and is driven by simulated atmospheric forcings.The dataset used to assess model outputs relies on the measurements acquired during the dedicated field campaigns “Operation Dense Water”, a set of two rapid response cruises carried out in southern Adriatic during winter 2012, and by five mooring arrays deployed in the Southern Adriatic Margin (SAM) that allowed the continuous acquisition of temperature, salinity, currents and suspended matter samples.Results from the integrated data-model approach suggest that the NAdDW propagates along the shelf to the southern basin following both a shallower vein and a deeper stream, with a process characterized by a strong variability (mean value of 0.31Sv with peaks rapidly growing in the first weeks after the CAO up to 2.19Sv). Additionally, COAWST capability to couple different numerical models allowed to disentangle the relative importance of aspects on dense water generation, mixing and spreading, demonstrating how coupled runs can lead to volumes up to 50% larger with respect to uncoupled simulations. Additional light is shed on the transport pathways off the shelf and on possible sediment transport phenomena in the area, in this benefiting from an unprecedented spatial resolution and a new bathymetry reflecting very high resolution data acquired via multi-beam techniques. Although dense water propagation appears as a relatively large-scale process involving the whole western side of the SAM, topographic local discontinuities and seabed slopes appear crucial in triggering descent and governing flow patterns. The presence of suspended sediment along the water column, despite not significantly influencing the overall fluxes across sections, is responsible of pulling part of the veins towards deeper zones.