Multi-Approach Analysis of Baroclinic Internal Tide Perturbation in the Ionian Sea Abyssal Layer (Mediterranean Sea)

Despite being widely recognized, the importance of deep layers thermohaline and mixing processes in the ocean circulation and variability is still poorly investigated, especially in the Mediterranean Sea. This limits understanding and parametrizing deep dynamics, which result in evident biases in the global circulation representation by observations and numerical ocean simulations. Having access to hydrological datasets, collected on a whole water column, we investigated the abyssal stratification and its variability of the Ionian Sea (Central Mediterranean). Applying multiple analyses, we found a tidal-period oscillation and the resulting activation of mixing, pointing out that the combined effect of stratification, morphology, and tides has a key role in enhancing local diapycnal diffusivity in the deepest layers, being a mechanism that connects the whole water column with a compelling impact on the vertical transport of heat and tracers. The presence of a quasi-homogeneous density layer in the deep sea made it possible to observe a periodic effect: the layer acts like a "cushion" that is pulled up and then readjusted in circa 12 hr, which is the period typical of tides. The fact that tides can propagate down to the sea interior impacting the deep processes, down to 3,000 m of depth, is not so straightforward to observe. The deep ocean has a very slow dynamic, and the physical-chemical phenomena involved are still mostly unknown. Near-full-depth Conductivity-Temperature-Depth (CTD) casts data collected in the Ionian Sea with an unorthodox methodology allowed resolving tidal scaleThe temperature and salinity diagram depicted typical water masses for the Ionian area including a dense abyssal water massThe analyses showed a periodic pulsation and an activation of mixing in the abyssal layer due to the baroclinic internal tide