Effect of vertical grid resolution and mixing schemes on mesoscale dynamics in coastal ocean models: case study in a mid-latitude marginal sea (northern Adriatic Sea)

The northern Adriatic Sea, due to the discharge from the Po and other rivers, can be described as a region of freshwater influence (ROFI). Together with fluvial inflows, other defining features are the strong atmospheric forcing, mainly Bora and Sirocco wind regimes, and tidal motions, most noticeably in peripheral environments such as the Venice Lagoon.
These elements contribute in shaping the vertical structure of the water column and the in making the northern Adriatic basin one of the dense water formation sites in the Mediterranean Sea. Furthermore, vertical processes such as stratification contribute in defining horizontal dynamics, for example Rossby radius of deformation variability, therefore affecting the mesoscale dynamical field.
Here we investigate and characterise the mesoscale variability of the northern Adriatic basin via numerical experiments, and assess how different vertical grid discretisations and turbulence parametrisations can affect it, using the MITgcm numerical ocean model.
We run a series of numerical experiments: a first batch of tests on periodic boundary “boxes”, to evaluate different mixing schemes and vertical grids; then, we select a subset of four among these setups and use them to run 5-year long simulations of the northern Adriatic hydrodynamics.
We find that increased vertical resolution results in better agreement with temperature and salinity observations, both remotely sensed (satellite SST) and sampled in situ (temperature and salinity profiles). As regards the vertical mixing shemes, GGL outperforms KPP, at times independently of the resolution under consideration.
By studying the mesoscale dynamical field we find that the Rossby radius of deformation responds mainly to the change in vertical grid resolution rather than to the mixing parametrisations. Instead, the summer stratification improved by the GGL scheme leads to more stable, wider eddies and more variability in the transport of fresh water of riverine origin from the coast towards the open sea, helping explain the observations of lower open sea salinity in summer with respect to winter.
We conclude that turbulent mixing parametrisations in ROFI ocean models can be as important as vertical resolution in determining the overall properties of both the water column and the basin-wide dynamics by shaping the mesoscale range of motion.