A study on boundary separation in an idealized ocean model

TL;DR

This paper evaluates how well boundary separation points are represented in idealized and global ocean models, revealing significant limitations in current criteria and their impact on boundary current simulations.

Contribution

It demonstrates that existing separation criteria often fail in numerical models, highlighting the need for improved methods to accurately capture boundary separation in ocean simulations.

Findings

Separation criteria often fail to detect boundary separation points in models.

Most criteria work well along no-slip boundaries but not along free-slip boundaries.

Advanced dynamical system criteria reveal limitations in current boundary separation representations.

Abstract

In numerical ocean models coast lines change the direction from one grid cell to its neighbor and the value for viscosity is set to be as small as possible. Therefore, model simulations are not converged with resolution and boundary separation points differ in essential properties from flow separation in continuous flow fields. In this paper, we investigate the quality of the representation of boundary separation points in global ocean models. To this end, we apply well established criteria for boundary separation within an idealized ocean model setup. We investigate an eddy-resolving as well as a steady test case with idealized and unstructured coast lines in a shallow water model that is based on a finite element discretization method. The results show that well established criteria for separation fail to detect boundary separation points due to an insufficient representation of ocean flows along free-slip boundaries. Along no-slip boundaries, most separation criteria provide adequate results. However, a very sophisticated criterion based on dynamical system theory reveals that the representation of boundary separation is limited for these flows as well. We conclude that the representation of boundary separation points in numerical ocean models is not satisfying. This will have an impact on the separation of boundary currents in global ocean models.