Ocean Modeling of the North Atlantic 
Albert J. Semtner 
National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307 
A number of considerations make the North Atlantic Ocean a preferred part of the World 
Ocean to study. These are indicated in Table 1. The North Atlantic is especially attractive 
from a modeling viewpoint because a relatively complete set of processes are present in an 
ocean basin which is tractable for modelling and well-suited for satellite data acquisition. The 
understanding derived from studying the North Atlantic can be tested against present and past 
climates, generalized to the World Ocean, and applied to the prediction of future climates. 
Present modeling of the North Atlantic is inadequate and can be improved in a number of 
ways. Table 2 lists a number of important physical processes in five categories from the 
viewpoints of how they are treated in isolation, how they are usually represented in present 
ocean basin models, and how they may be better represented in future models. In the first two 
categories of vertical boundary processes and internal vertical mixing, parameterizations exist 
which can easily be incorporated into models and which will have important effects on the simu- 
lated structure of the North Atlantic. For the third category (mesoscale eddy effects), adequate 
parameterizations do not exist; but the order of magnitude of the effects is known from obser- 
vational and process-model studies. A horizontal grid spacing of 100 km or less is required to 
allow parameterizations with this order of magnitude, as well as to resolve the time-averaged 
ocean fields (Fig. 1). Existing simulations with this gridsize and constant eddy diffusion coef- 
ficients show some success in reproducing oceanic phenomena that have previously been 
misrepresented in coarse-grid models. However, improved parameterizations are needed, since 
eddy resolving studies of the North Atlantic are computationally unfeasible, except for a few key 
simulations. In the fourth category of Table 2, improvements are suggested by way of increased 
vertical resolution and by the requirement that lateral mixing due to eddies takes place on iso- 
pycnal surfaces. Model incorporation of the latter phenomena is underway. In the fifth cate- 
gory of miscellaneous high-latitude processes, formulations for the treatment of sea ice are 
available for use. However, the treatment of gravitational instability, which is crucial to deep- 
water formation in the Atlantic Ocean, will require additional refinements to account for the 
unresolved physics of chimney formations in the open ocean and buoyant plumes near ocean boun- 
daries. 
TABLE 1. Why Study the North Atlantic Ocean? 
I. Practical Considerations 
A. Focal point of climatic change 
B. Site of nuclear waste disposal 
C. Area of heavy shipping 
D. Strategic importance 
E. Biological productivity 
II. Logistical Considerations 
A. Much already known about the circulation 
B. Many investigations focused there 
C. Easier to observe than distant oceans 
D. Amenable to remote sensing 
E. Suitable for satellite data relay 
F. Small enough to model 
III. Physical Considerations 
A. Region of diverse oceanic processes 
1. Mixed layer physics 
2. Sea-ice dynamics and thermodynamics 
3. Bottom water formation in high-latitude seas 
4. Marginal input of highly saline water 
5. Seasonal production of intermediate water 
6. Mixing along isopycnals 
7. Constraints on potential vorticity 
8. Unstable western boundary current 
9. Unstable mid-ocean flows 
10. Western boundary undercurrent 
11. Heat transport across the equator 
12. Transient response to tracers 
B. Understanding generalizes to the global ocean 
