Role of stratification in vorticity propagation throughout the entire water column: a Mediterranean example (Ionian Sea)

The Mediterranean is a climate change hot spot: it is warming 20% faster than the rest of the globe's oceans. Despite the importance of monitoring such dramatic changes and their consequences, the processes and the mechanisms involved in deep-sea variability are still unclear, due to a lack of long-term observations above 2000 m of depth. The availability of seafloor time series data and full-depth CTD profiles collected in the Ionian Sea allowed studying processes connecting the deep variability with the whole water column. The analysis of the in-situ seafloor time series showed a near-inertial peak in the current kinetic energy spectrum in the bottom layer, which hints at the presence of local vorticity. Moreover, the analysis of the CTD profiles revealed for the first time in this area the presence of variability at tidal periodicity in the deep layers (below 2000m), suggesting a connection with what was observed in the surface and subsurface layers. A unique opportunity to study and validate this mechanism was offered by the adjustment of the water stratification before and after the Eastern Mediterranean Transient, the major climate event occurred at the beginning of the 90s when warmer and saltier Aegean waters replaced colder and fresher Adriatic deep waters in the bottom layers of the Ionian basin. Data collected between 1999 and 2003 depict a stable water mass in the Ionian deep layers, identified as the Ionian Abyssal Water (IAW). The presence of the IAW layer could be a key condition for catching such variability in the deep. This allows studying the role that the stratification can have on the rapid propagation of the perturbation in depth, and also how the relative layer thicknesses and different densities can trigger the instability transport throughout the water column. The observed mean structure of the Ionian Sea stratification suggests that a 4-layer scheme should be enough to have a realistic yet straightforward theoretical representation. To study how much and under which conditions a potential vorticity input can propagate, a quasi-geostrophic equation has been considered, with 4 coupled layers of arbitrary thickness and density, simulated with a custom-designed algorithm. The relative stability of the coupled layers, and their response to external forcing, is crucial to understand how vorticity can propagate through the water column down/up and to/from the deepest layers. This case study aims to give more insight into how energy stored by the deep-sea layers can be released along the entire water column. This will enable a better parametrization of the deep processes also contributing to future Mediterranean climate numerical models.