Mignot Juliette , Levermann A., Griesel A. (2006). A decomposition of the Atlantic meridional overturning circulation into physical components using Its sensitivity to vertical diffusivity. Journal of Physical Oceanography, American Meteorological Society, 36 (4), 636-650. ISSN 0022-3670.
Titre du document
A decomposition of the Atlantic meridional overturning circulation into physical components using Its sensitivity to vertical diffusivity
Journal of Physical Oceanography, American Meteorological Society, 2006,
36 (4), 636-650 ISSN 0022-3670
The sensitivity of the Atlantic Ocean meridional overturning circulation to the vertical diffusion coefficient K in the global coupled atmosphere-ocean-sea ice model CLIMBER-3 alpha is investigated. An important feature of the three-dimensional ocean model is its low-diffusive tracer advection scheme. The strength M-max of the Atlantic overturning is decomposed into three components: 1) the flow M-s exported southward at 30 degrees S, 2) the large-scale upward flow that balances vertical diffusion in the Atlantic, and 3) a wind-dependent upwelling flux W-bound along the Atlantic boundaries that is not due to vertical diffusion. The export of water at 30 degrees S varies only weakly with kappa, but is strongly correlated with the strength of the overflow over the Greenland-Scotland ridge. The location of deep convection is found to be mixing dependent such that a shift from the Nordic seas to the Irminger Sea is detected for high values of kappa. The ratio R = M-s/M-max gives a measure of the interhemispheric overturning efficiency and is found to decrease linearly with K. The diffusion-induced upwelling in the Atlantic is mostly due to the uniform background value of kappa while parameterization of enhanced mixing over rough topography and in stratified areas gives only a weak contribution to the overturning strength. It increases linearly with kappa. This is consistent with the classic 2/3 scaling law only when taking the linear variation of the density difference to kappa into account. The value of W-bound is roughly constant with kappa but depends linearly on the wind stress strength in the North Atlantic. The pycnocline depth is not sensitive to changes in kappa in the model used herein, and the results suggest that it is primarily set by the forcing of the Southern Ocean winds. The scaling of the total overturning strength with kappa depends on the combined sensitivity of each of the terms to kappa. In the range of background diffusivity values in which no switch in deep convection sites is detected, M-max scales linearly with the vertical diffusivity. It is argued that scalings have, in general, to be interpreted with care because of the generally very small range of kappa but also because of possible shifts in important physical processes such as deep convection location.
