Ocean Mesoscale Eddies
Up until the late 1960s, oceanographers thought of the ocean circulation as consisting of slowly moving interior gyres and fast moving boundary currents. However, as illustrated in the animation to the left, the ocean circulation also contains vigorous eddies at spatial scales from roughly 100 km and smaller, evolving over time scales from weeks to months. These eddies are important in establishing the ocean’s circulation and tracer properties. In the animation above, much of mean transport through the Mozambique channel separating Southern Africa from Madagascar is actually carried by drifting eddies corresponding to deep lenses of lighter water. When the circulation is averaged over time, these time-varying flows actually result in significant net transport- a process often referred to as rectification.
The largest scale eddies emerge from instabilities of strongly horizontally sheared motions, particularly in western boundary currents, and are important in shaping the pathways and intensity of the most intense ocean currents ; they often take the form of well defined rings extending to great depth. At slightly smaller scales, of order tens of kilometers, eddies are generated by the slumping of horizontal density gradients. These baroclinic eddies with strongly rotating flows and are important in the movement of watermasses from near the surface into the ocean interior and for the dynamical balance governing the Antarctic Circumpolar Current. At yet smaller scales, often of less than a kilometer, submesoscale eddies within the near-surface well-mixed layer drive the shoaling of the mixed layer at fronts, and move water out of the surface planetary boundary layer.
Most ocean climate models are of too coarse a horizontal resolution (typically 1-degree) to explicitly represent any but the very largest eddies. In such models all of the important eddy effects must be parameterized- represented in terms of large-scale gradients of density or velocity. Higher resolution global ocean models (with resolutions of order 10 km) are able to explicitly capture many eddy effects, but at much greater computational expense and while also introducing new challenges for the numerical representation of the vigorous flows that emerge with scales similar to the model’s spatial grid.
Research at GFDL aims to understand and represent the net effects of eddies on the large-scale climate. We approach the problem in two ways.
- Parameterizing the effect of small-scale eddies on the large-scale structure, by relating the transport of key properties to the large-scale density field. Examples include work to develop a new representation of the effects of mesoscale eddies as part of the Eddy-Mixed Layer Climate Process Team, work to understand the net effect of eddies and wind on biogeochemical processes, and idealized studies comparing eddying and non-eddying solutions.
- Developing higher-resolution calculations that explicitly simulate many effects of the eddies. Examples include the Modeling Eddies in the Southern Ocean Project (MESO).