The advantage of a generic coordinate approach for ocean modelling

Abstract

A main concern in three-dimensional coastal and oceanic modelling is the type of coordinate used for the vertical direction. The appropriate coordinate system should guarantee an adequate resolution in the entire domain and must lead to a stable method with minimal numerical diffusion in order to preserve the vertical structure of the water column. The need for meshes with high aspect ratios of up to 1000:1 makes ocean models highly sensitive to vertical fluxes. A diversity of vertical coordinates has been used in ocean modelling; cartesian, sigma and isopycnic are now widespread. Sigma coordinates are more indicated when topography plays a main role in the flow, while isopycnic coordinates are the most indicated when density is the major forcing. Cartesian coordinates do not privilege any type of forcing and are a good compromise when both are important. The generic approach implemented in MOHID 2000 system using finite-volumes allows the use of different vertical discretizations simultaneously. The model is applied to the Western Iberian Margin. The objective of this study is to reproduce the poleward slope current. Results were obtained using cartesian, double-sigma and lagrangian type coordinates. Advantages and disadvantages of each coordinate system are discussed. The velocity field results are compared at 400 meters depth for different type of coordinates. Salinity and temperature profiles will be analysed to verify what type of coordinate is able to maintain the vertical density gradient for a longer period.