Analysis of Mediterranean ocean variability using five numerical simulations

The oceans play a fundamental role on the slow evolution of climate and all life on earth, and must therefore be studied and understood. The increase in oceanographic research and available measurements over the past half century have greatly increased our knowledge about the ocean’s behaviour and variability, and highlighted its complexity and ubiquity over a wide range of space and time scales. However observational datasets still remain too short, too superficial or too dispersed in time and space to allow detailed studies of many of the physical processes governing the ocean’s variability. In order to continue advancing our understanding of how the oceans work, it is crucial to complement observational data with ocean numerical modelling studies. Throughout this thesis, we explore di↵erent aspects of the variability and circulation of the Mediterranean Sea, where many of the oceanic processes found throughout the worlds oceans can be studied in a reduced scale. We perform a comprehensive analysis through the validation of a series of ocean numerical models comparing them with observational datasets. The first part is an initial approach at the analysis in the Mediterranean Sea of a 1/4o global ocean simulation (ORCA025-G70). We focus on basin scale mean temperature (T) and salinity (S) temporal evolution at di↵erent depth layers by comparing to the MEDAR database, mean sea surface height temporal evolution and trends by comparing with altimetry, and transport through the Strait of Gibraltar and the Black Sea by comparing to the available literature. The results show that T variability at surface is well reproduced, but trends in intermediate and deep layers are positively biased. Poor results are obtained for S interannual variability, mainly due to the sea surface salinity (SSS) relaxation. Sea surface height interannual variability and seasonal cycle are well reproduced but climatological trends are too positive due to an imbalance in the freshwater fluxes from the atmospheric forcing. Water transport through the main straits is within the range of observed values. The second part is a continuation of the analysis with global models, this time focusing on the Western Mediterranean (WMED) and adding two more simulations, one with higher vertical resolution and improved atmospheric forcing (ORCA025-G85) and one with data assimilation (GLORYS). The results show that the combination of a higher vertical resolution and better time continuity of the atmospheric forcing in G85 o↵er better T trend results at intermediate and deep layers with respect to G70. On the other hand, G85 has a much weaker SSS restoring and displays unrealistic negative S trends. Two exceptional deep convection events in the Gulf of Lions are reproduced by G85 due to the extremity of the atmospheric conditions, while weaker convection events are not reproduced due to the coarse resolution of both the model and atmospheric forcing. GLORYS has a better T and S performance than the ORCA hindcasts. Temporal evolution of sea level interannual variability is well reproduced in all simulations, but the ORCA hindcasts, which do not maintain a water balance show exaggerated positive trends, especially G85 that has a weaker SSS restoring. Through EOF analysis, the simulations recreate the surface circulation variability patterns correctly within their resolution limits. Transports through the main straits appears to be