A Curvilinear Ocean Model Using a Grid Regionally Compressed in the South Indian Ocean

Abstract

It is shown that a global curvilinear grid with variable resolution is an efficient way of providing a high density of grid points in a particular region. In equilibrium experiments using asynchronous time steps, this type of grid has been found to allow a better representation of smaller-scale features in the high-resolution region while maintaining contact with the rest of the World Ocean, provided that lateral mixing coefficients be scaled with grid size so as to maintain marginal numerical stability. In this study, the region of interest is the southern Indian Ocean and, in particular, that of the South Indian Ocean Current. In all experiments, decreased viscosities and diffusivities generally led to increased currents and tracer gradients. In horizontal mixing simulations, maximum current speeds in the frontal region were mainly determined by local (i.e., high-resolution region) viscosities, while maximum temperature gradients were determined by local values of both lateral viscosity and diffusivity. With eddy-induced transport experiments, maximum values were analyzed on isopycnal surfaces. Isopycnal diffusivities were found to control tracer gradients on isopycnals but not isopycnal slopes, while thickness diffusivities controlled isopycnal slopes but only to a small degree tracer gradients. Changes to mixing coefficients in the coarse part of the grid had hardly any influence on the frontal properties examined, although they did affect currents in the Indian Ocean to some extent via their control on size of the Antarctic Circumpolar Current and the Pacific?Indian Throughflow.