1. Overview

This document provides the reader with the information necessary to carry out numerical experiments using MITgcm. It gives a comprehensive description of the continuous equations on which the model is based, the numerical algorithms the model employs and a description of the associated program code. Along with the hydrodynamical kernel, physical and biogeochemical parameterizations of key atmospheric and oceanic processes are available. A number of examples illustrating the use of the model in both process and general circulation studies of the atmosphere and ocean are also presented.

1.1. Introduction

MITgcm has a number of novel aspects:

  • it can be used to study both atmospheric and oceanic phenomena; one hydrodynamical kernel is used to drive forward both atmospheric and oceanic models - see Figure 1.1
One model for atmospheric and oceanic simulations

Figure 1.1 MITgcm has a single dynamical kernel that can drive forward either oceanic or atmospheric simulations.

  • it has a non-hydrostatic capability and so can be used to study both small-scale and large scale processes - see Figure 1.2
MITgcm can simulate a wide range of scales

Figure 1.2 MITgcm has non-hydrostatic capabilities, allowing the model to address a wide range of phenomenon - from convection on the left, all the way through to global circulation patterns on the right.

  • finite volume techniques are employed yielding an intuitive discretization and support for the treatment of irregular geometries using orthogonal curvilinear grids and shaved cells - see Figure 1.3
Finit volume techniques

Figure 1.3 Finite volume techniques (bottom panel) are user, permitting a treatment of topography that rivals \(\sigma\) (terrain following) coordinates.

  • tangent linear and adjoint counterparts are automatically maintained along with the forward model, permitting sensitivity and optimization studies.
  • the model is developed to perform efficiently on a wide variety of computational platforms.

Key publications reporting on and charting the development of the model are [HM95][MHPA97][MAH+97][AHM97][MJH98][AM99][CHM99][MGZ+99][AC04][ACHM04][MAC+04] (an overview on the model formulation can also be found in [AHC+04]):

We begin by briefly showing some of the results of the model in action to give a feel for the wide range of problems that can be addressed using it.

1.2. Illustrations of the model in action

1.2.1. Convection and mixing over topography

Dense plumes generated by localized cooling on the continental shelf of the ocean may be influenced by rotation when the deformation radius is smaller than the width of the cooling region. Rather than gravity plumes, the mechanism for moving dense fluid down the shelf is then through geostrophic eddies. The simulation shown in Figure 1.4 (blue is cold dense fluid, red is warmer, lighter fluid) employs the non-hydrostatic capability of MITgcm to trigger convection by surface cooling. The cold, dense water falls down the slope but is deflected along the slope by rotation. It is found that entrainment in the vertical plane is reduced when rotational control is strong, and replaced by lateral entrainment due to the baroclinic instability of the along-slope current.

Finit volume techniques

Figure 1.4 MITgcm run in a non-hydrostatic configuration to study convection over a slope.

1.2.2. Boundary forced internal waves

The unique ability of MITgcm to treat non-hydrostatic dynamics in the presence of complex geometry makes it an ideal tool to study internal wave dynamics and mixing in oceanic canyons and ridges driven by large amplitude barotropic tidal currents imposed through open boundary conditions.