Robert Hallberg, Stephen M. Griffies, Bonita Samuels, and J. Robert Toggweiler,
NOAA Geophysical Fluid Dynamics Laboratory
Thierry Huck and Geoffrey Vallis, Princeton University

Research Objectives

The Ocean Group at GFDL is embarking on an ambitious, multi-year effort to model the Southern Ocean with sufficient horizontal and vertical resolution to resolve the ubiquitous energetic mesoscale eddies. We are using a series of numerical simulations to determine the role of the Southern Ocean eddy dynamics in setting the structure and magnitude of the Antarctic Circumpolar Current and in determining the global mean density structure of the ocean.

Computational Approach

We have begun a sequence of experiments using two different primitive equation ocean models. At each resolution we spin up from a climatological state for 20 years using climatological wind stresses. At that point, we start three separate simulations with wind stresses that are stronger, weaker, and the same as the winds used in the spinup phase. These experiments will test the sensitivity to changing surface forcing of the mean circulation and the balance between diabatic flows and eddy fluxes. Com-parison between the various runs will provide a prognostic estimate of the sensitivity of the various terms in the momentum, potential vorticity, and heat and salt balances, in addition to the diagnostic description available from the Fine Resolution Antarctic Model (FRAM). The comparison will also directly test the hypothesis that the principal response to the changing wind stress is in the vigor of the eddy field, and not in the mean density structure.

Accomplishments

We have run a 20-year spinup simulation with the hybrid isopycnal model (HIM) at 1/2º resolution at NERSC. This run identified certain problems with the experimental setup, and we are refining it in shorter runs at GFDL. We have also performed the entire suite of experiments at 1º resolution with GFDL's Modular Ocean Model (MOM). While these preliminary experiments are not capable of exploring the extent to which eddies control the dynamics of the Southern Ocean, they are the necessary preliminary exercise to the higher resolution, eddy permitting and eddy resolving experiments.

The instantaneous sea surface height in a 1/2º resolution simulation of the Southern Hemisphere ocean circulation using an isopycnal coordinate ocean model. The near-surface flow is very nearly along contours of sea surface height. The mean path of the Antarctic Circumpolar Current is roughly given by the large-scale zonal contours in the Southern Ocean, while the small-small wiggles and extrema are caused by the rich transient eddy field.

Significance

The dynamical balance of the Southern Ocean may be the key to predicting the changes in the density structure of the deep ocean. Heretofore there have been no experiments which directly address this hypothesis. Our proposed experiments will test it directly. Understanding the response of the abyssal ocean to changing atmospheric forcing will be critical in determining the rate, extent, and satial distribution of climate change due to changing greenhouse gas concentrations. It is also anticipated that these experiments will lead to superior parameterizations of the effects of mesoscale eddies on the large-scale ocean circulation, thereby improving the reliability of predictions from coarse-resolution climate simulations.

Publications

A. Gnanadesikan, "A simple predictive model for the structure of the oceanic pycnocline," Science 283, 2077 (1999).

G. K. Vallis, "Large-scale circulation and production of stratification: Effects of wind, geometry and diffusion," J. Phys. Oceanogr. (in press).

J. R. Toggweiler and B. Samuels, "Effect of Drake Passage on the global thermohaline circulation," Deep-Sea Res. 42, 477 (1995).