A Thermodynamic Ocean Modelling System With Application to the Pacific and Indian Oceans
Tommy G. Jensen (International Pacific Research Center, School of Ocean, Earth Science & Technology, University of Hawaii, 1000 Pope Road, Honolulu, 96822, USA)
e-mail: jensen@soest.hawaii.edu
ABSTRACT
A multi-layer Thermodynamic Ocean Modelling System (TOMS) has been developed for upper ocean circulation studies. TOMS is designed for oceanic and coastal flows where the aspect ratio is small and for integrations up to a few decades. It can be applied as an upper ocean only model (reduced gravity mode), or it can be configured with finite depth, including bottom topography in the lowest layer. The model includes prognostic velocity, temperature, salinity and tracers as well as mixed layer physics. In addition, a simple data assimilation scheme, and a number of open boundary conditions, which increase the range of possible applications of TOMS, have been included.
Ocean models with a vertical coordinate fixed in time or models with a moving coordinate both have their problems and advantages. TOMS uses an Arbitrary Lagrangian Eulerian vertical coordinate, which predicts a new Lagrangian layer thickness each time step. Based on physical and numerical criteria, the layer thickness is adjusted (i.e. remapped) and the associated mass, heat and salt fluxes between layers are computed. This makes it a hybrid between an isopycnic model and a z-coordinate model.
A number of applications are presented, ranging from single column models to 3-dimensional circulation models. As one example, results from a 1/4°. resolution simulation with a 4.5 layer Pacific Ocean upper ocean, including prognostic temperature and salinity, are shown. High resolution (1/8°) results from a hydrodynamics only Indian Ocean model are also discussed. Both models are forced by monthly wind stress from the European Centre for Medium-range Weather Forecasts reanalysis.
INTRODUCTION
Large scale flow in the interior of the ocean is mainly along surfaces of constant density, i.e. isopycnals. This makes density a natural choice for a vertical coordinate in ocean models. However, the uppermost ocean is well mixed and varies continuously in density. In isopycnal circulation models (e.g. Bleck and Boudra, 1986; Oberhuber, 1992) the transfer between the mixed layer and isopycnal layers is rather complicated. In the model described here, a hybrid between an isopycnal and a z-coordinate model is used, allowing easier water mass transfer between layers. The method applied here usess an Arbitrary Lagrangian Eulerian (ALE) coordinate, and was developed by Hirt et al., (1974) for computational fluid dynamics. A recent review can be found in Benson, (1992). The Thermodynamic Ocean Modelling System (TOMS) is only applying this technique in the vertical, which makes it a layer ocean model with a generalized method of vertical flux calculations. The original design was for an upper ocean model, but it can be set up for a number of environmental flow problems in one to three dimensions.
