The characteristics and effects of tides on the general circulation of the Mediterranean Sea

Tidal forcing was not included in a large proportion of numerical models of the ocean from the past but is known to be necessary to forecast the ocean accurately, since tides dissipate around 3.5TW of energy in the global ocean, and are an important driver of mixing. The inclusion of tides in models affects not only the near-field dynamics such as local temperature and salinity, but also large-scale circulations that influence the entire global ocean. In the Mediterranean Sea, amplitudes of tides are typically low, but are known to have effects both at local scales where tidal amplitude is larger, and across the entire basin. However, their impact on processes such as circulation, vertical mixing, and internal tides at the basin scale are not well known. 

In this work, the characteristics of tides in the Mediterranean Sea were investigated using the hydrodynamic model NEMO (Nucleus for European Modelling of the Ocean) version 3.6, corresponding to the Copernicus Monitoring Environment Marine Service (CMEMS) system, a baroclinic forecasting model for the Mediterranean Sea, integrated over five years. 

Analysis of potential and kinetic energy due to tides showed that tides have impacts across a wide variety of spatial and temporal scales in the basin, both at the surface and at deeper levels. Increased kinetic energy at depth in the basin led to an exploration of internal tides, which have not been previously studied at the scale of the Mediterranean Sea using a numerical approach. Additionally, the effects of tides on salt transport through the Gibraltar Strait were analysed, and numerical results were compared to a two-layer box model of the Gibraltar Strait and Mediterranean Sea. This demonstrated the utility of simple theoretical frameworks to understand dynamics in the region, while highlighting the impact of increased vertical mixing in the Gibraltar Strait due to internal tides.

Our improved understanding of the impacts of tides across temporal and spatial scales lays out an argument for the inclusion of tides in ocean models ranging from local to global, and from short timescales to long term climate modelling. Our analysis also provides a novel understanding of dynamics such as internal tides and transport of salinity in the Mediterranean region.