Disentangling local processes at the Strait of Gibraltar and their climatic influence on Mediterranean hydrography

The Mediterranean Sea is a semi-enclosed basin losing heat and freshwater to the atmosphere. As such, Mediterranean volume, heat, and salt budgets are strongly constrained by water mass exchanges with the global Ocean. Most of these exchanges take place through a shallow and narrow channel: the Strait of Gibraltar (SoG), making local processes at stake in its vicinity impact on a larger scale. Among these, tidal and fine-scale dynamics are known to influence Mediterranean hydrography, but their combined effects remain unclear. In this study, we address this question by comparing a set of four 30 year-long fully-coupled hindcast simulations of the Mediterranean region (CNRM-RCSM6 regional climate model, ~ 7km ocean horizontal resolution) differing only by the inclusion or omission of explicit tidal forcing and refined resolution (~ 1.5km) at the SoG. We show that tidal and fine-scale processes are equally relevant to Mediterranean hydrography and that their interplay is essential over climatic scales. On the one hand, tides drive the water mass transformations across the strait and the zonal slope of the interface between the exchanged flows. On the other hand, fine-scale dynamics, shaped by the abrupt topography of the strait, drive the Atlantic layer thickness and the magnitude of its inflow.  These results highlight that kilometric-scale and tidal dynamics at the SoG are essential components of the physical system shaping the Mediterranean mean state and climatic trends. As such, they should be explicitly included in numerical models or parameterized when this is numerically unaffordable.