On the dynamical role of eddy transports in the Atlantic thermohaline circulation.
Abstract
We evaluate the role of the eddy transports in the Southern Ocean and the Northern Hemisphere in maintaining the Atlantic thermohaline circulation. The two main effects of eddies, to flatten isopycnals and to transport heat and salt across isopycnal surfaces, are illustrated by two eddy parameterization schemes in a numerical ocean general circulation model. Namely, we consider the Gent-McWilliams (GM) scheme, which has a strong flattening effect on isopycnals, and a horizontal diffusion scheme, which corresponds to large, sometimes overestimated, cross-isopycnal transports of heat and salt across sloping isopycnal surfaces. Two control runs, one with each scheme, exhibit very different circulations and density structures. In order to analyze the dynamical reasons for the differences between the control runs, we carry out a number of numerical experiments with regionally varying diffusion coefficients emphasizing the effects of different schemes in key regions. We identify an important dynamical role of the eddy transports in the Southern Ocean. Our experiments demonstrate that the eddies in the Southern Ocean strongly influence the meridional density structure in the Atlantic and effectively control the rate of formation of the North Atlantic Deep Water (NADW) by affecting density of the intermediate water that enters the Atlantic from the south. The main effect of the eddies in the Southern Ocean in nature is to shoal the subsurface isopycnal surfaces thus increasing density of the inflow of the intermediate water into the Atlantic - as we will see, this is more effectively done by the GM parameterization of the eddies. The resulting increase in the subsurface density at the low latitudes decreases the meridional density contrast with the high latitudes in the North Atlantic, and consequently weakens the meridional flow. The cross-isopycnal eddy transports across the western boundary current in the Northern Hemisphere also influence density and horizontal flow structure at the low and mid latitudes. These processes, however, are complex and the combined effect of the Northern Hemisphere eddies on the strength of the NADW formation is small in our model. The Northern Hemisphere upwelling is controlled by the local eddy transports, and the outflow of the NADW is very sensitive to the Northern Hemisphere eddy transports as a result. The eddies also affect the properties of the Antarctic Bottom Water, which influences the vertical penetration of the NADW overturning cell as well as the density of the deep ocean.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1488K