A high resolution reanalysis for the Mediterranean Sea

In order to be able to predict the future ocean climate and weather, we need to understand what happened in the past and the mechanisms responsible for the ocean variability. This is particularly true in a complex area such as the Mediterranean Sea with diverse dynamics such as deep convection and thermohaline circulation or coastal hydrodynamics. To this end, effective tools are reanalyses or reconstructions of the past ocean state. 

Here we present a new physical reanalysis of the Mediterranean Sea at high resolution, developed in the CMEMS Med-MFC framework. The hydrodynamic model is based on the Nucleus for European Modelling of the Ocean (NEMO) combined with a variational data assimilation scheme (OceanVAR). A series of system developments have been carried out to upgrade the current Med-MFC reanalysis to the new one with high resolution, including new NEMO version and configuration, the new version of atmospheric forcing (ERA-5) datasets and revised OceanVAR scheme.

The model has a horizontal resolution of 1/24° and 141 vertical z* levels and provides daily and monthly 3D values of temperature, salinity, sea level and currents. Hourly ERA-5 atmospheric fields force the model and daily boundary conditions in the Atlantic are taken from the global CMCC C-GLORS reanalysis. 39 rivers model the freshwater input to the basin plus the Dardanelles. The reanalysis covers 30-years, initialized from World Ocean Atlas climatology in January 1985, getting to a nominal state after a two years spin-up and ending in 2018. In-situ data from CTD, ARGO floats, XBT are assimilated into the model in combination with satellite altimetry data.

This reanalysis has been validated and assessed through comparison to in-situ and satellite observations as well as literature climatologies. The results show good agreement with observations and a better representation of the main dynamics of the region compared to the previous, lower resolution (1/16°) reanalysis. The new reanalysis will allow the study of physical processes at multi-scales, from the large scale to the transient small mesoscale structures.