Driving the ocean's overturning : an adjoint sensitivity study

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001.
Includes bibliographical references (p. 183-193).
The focus of this thesis is the sensitivity of the strength of the meridional overturning circulation to surface forcing and mixing on climatological time scales. An adjoint model is used to gain new insights into the spatial characteristics of the sensitivity patterns. Adjoint models provide the sensitivity of a diagnostic, often called cost function, to all model parameters in a single integration. In contrast, traditional sensitivity analyses are performed by repeated integrations of the so-called "forward" model, perturbing slightly the value of a single parameter at each integration. The results of the adjoint model allows us to calculate global maps of sensitivity. These maps provide a geographic picture of where on the ocean heat and freshwater flux, wind stress and diapycnal mixing perturbations in the North Atlantic are likely to have the greatest impact on the meridional overturning and its heat transport. The adjoint model provides clear identification of the physical mechanisms which can influence the meridional overturning on times scales of years to decades. Boundary and equatorial Kelvin waves and equatorially trapped Rossby waves carry information around the boundaries of the basin and across the equator in less than a decade for a basin of the size of the Atlantic. Advection of buoyancy perturbations has an important influence on the meridional overturning on the decadal time scale. Diffusion is important in determining the final equilibrated state of the meridional overturning on the centennial scale. The role of diapycnal mixing in determining the overturning's strength is confined to regions near the lateral boundaries in the Northern hemisphere and to the tropical region in both hemispheres. The important role played by the tropics in setting the overturning's strength seems to confirm the thermodynamic principles outlined by Sandstrom (1908), Jeffreys (1925) and Munk and Wunsch (1998): upward advection of heat is balanced by downward diffusion.
by Véronique Bugnion.
Ph.D.