Ocean Circulation Models

Numerical ocean models approximate the real ocean by dividing the global ocean into finite sized grid boxes, and representing the exchange of ocean tracers and momentum between these boxes using the equations for fluid flow on a rotating sphere. This work at GFDL originates from the pioneering efforts of the 1960s, with the study of global climate remaining the key focus of GFDL ocean model applications.

While ocean models start with the same continuous equations, the discrete equations possess important distinctions that play a role in the simulation features. Different parts of the ocean are most naturally represented using different coordinate systems. Level-coordinate models consider each box is at the same level. These models are relatively easy to code and have been the basis for ocean climate modeling since the 1960s. Such models may also have particular advantages in representing the transition between the poorly stratified mixed layer and the interior ocean where flow is predominantly along density surfaces. In contrast, isopycnal models handle the interior ocean more naturally, and may also have significant advantages in representing thin overflows. In shallow coastal regions, where the top and bottom boundary layers are thought to be most important, many oceanographers use terrain-following coordinate systems. For more discussion, see Griffies et al.

In 2005, GFDL embarked on two parallel and complementary paths in ocean climate modeling. The first focused the numerical and physical enhancement of our workhorse ocean model, the Modular Ocean Model (MOM). The second effort prototyped a new generation of ocean model for the study of climate, whose Lagrangian vertical coordinate is based on density layers, rather than depth or pressure levels used in MOM. Being able to simulate the ocean climate system using two distinct and complementary ocean model classes is an exceptionally valuable capability, as exemplified in studies using GFDL’s ESM2M and ESM2G Earth System Models. It provides us with important tools to establish robust features of the climate simulations, such as the response of the Atlantic Meridional Overturning Circulation in a warming planet.

These two complementary but separate paths have now reached their successful conclusion with the climate simulations of GFDL’s two new Earth System Models, ESM2M and ESM2G, and the public release of MOM5 and the Generalized Ocean Layered Dynamics (GOLD) model. GFDL has now embarked upon a new effort to fully merge the best capabilities of these efforts in a new ocean model, which will be called MOM6.

Next Generation Ocean Model

While MOM4/5 and GOLD are distinct developments and fundamentally differ in formulation, the vision for the next generation modular ocean model will blend these two disparate approaches together over the next development cycle, in a new model to be called MOM6. MOM6 will incorporate all the new coordinate work represented by MOM4/5 and GOLD and multiple horizontal grid types (B and C grids). The model will likely have new physics (including non-hydrostatic terms) and the software may look more like a library than a single source code. It will be both a completely new model and yet be configurable to recover the capabilities of its predecessors.