"Toward a Theory of the North Atlantic Oscillation", AOS Seminar by M. Ghil, January 2009


Recent developments of observing technology, both space-borne and in situ, have greatly improved oceanic data sets, in quality as well as quantity. The study of the oceans' low-frequency variability, however, still has to rely to a large extent on models, due to the inhomogeneity and temporal shortness of the data sets; short-term predictability on synoptic time scales, on the other hand, can be studied using data assimilation.

Basin-scale oceanic motion is dominated by wind-driven (horizontal) and thermohaline (vertical) circulations. Their variability, independently and interactively, may play a significant role in climate changes, past and future. The wind-driven circulation plays a role mostly in the oceans' subannual-to-interannual variability, while the thermohaline circulation is most important in decadal-to-millenial variability. Our approach is to study the two governing circulations separately, and then in terms of their interaction, especially on the interannual-to-interdecadal time scale. Dynamical-systems methods, especially from bifurcation theory, provide a powerful tool to reveal the processes underlying the oceans' variability.

Wind-driven circulation

The horizontal circulation of the mid-latitude ocean is governed by its wind-driven, double-gyre structure. As shown in the bifurcation diagram (center) of a reduced-gravity, shallow-water model, its behavior can change, even for fixed wind stress, depending on the system's parameter values. Single and multiple equilibria, purely and quasi-periodical oscillations, and chaotically varying flow occur as the stress on the system increases or the constraints on its motion decrease. The variability has dominant peaks at subannual and interannual time scales; the exact periods of these peaks depend, in turn, on basin size. Other panels in the figure are snapshots of the model's upper-layer thickness for various parameter values (from Speich et al., 1995).

Thermohaline circulation (THC)

Buoyancy contrasts at the ocean surface result from exchanges of heat and fresh water with the overlying atmosphere, sea ice, or adjacent land through river run-off. These contrasts give rise to a sensitive balance: the ocean water density decreases with temperature (thermo) and increases with salinity (haline). Empirical evidence and model results indicate that its variability ranges from interdecadal to paleoclimatic time scales. Since the THC transfers heat and other quantities laterally, its impact on the climate system can be substantial. The figure shows two snapshots of the (zonally averaged) meridional streamfunction for an interdecadal oscillation using the box GFDL's modular-ocean model in a North Atlantic "box" geometry; the two panels show the strong (right) and weak (left) extremes of the overturning (from Chen and Ghil, 1995, 1996).

TCD Members:
M. Ghil, K. Ide, S. Kravtsov.
Chen, F., and M. Ghil, 1995: Interdecadal variability of the thermohaline circulation and high-latitude surface fluxes, J. Phys. Oceanogr., 25, 2547-2568.
Chen, F., and M. Ghil, 1996: Interdecadal variability in a hybrid coupled ocean-atmosphere model, J. Phys. Oceanogr., 26, 1561-1578.
Feliks, Y., and M. Ghil, 1996: Mixed barotropic-baroclinic eddies growing on an eastward midlatitude jet. Geophys. Astrophys. Fluid Dyn., 82, 137-171.
Feliks, Y., and M. Ghil, 1997: Stability of a front separating water masses with different stratifications. Geophys. Astrophys. Fluid Dyn., 84, 165-204.
Ghil, M., and J. McWilliams, 1994: Workshop tackles oceanic thermohaline circulation, Eos, Trans. AGU, 75, pp. 493, 498.
Ghil, M., and N. Paldor, 1994: A model equation for nonlinear wavelength selection and amplitude evolution of frontal waves, J. Nonlin. Sci., 4, 471-496.
Ghil, M., A. Mullhaupt and P. Pestiaux, 1987: Deep water formation and quaternary glaciations, Climate Dyn., 2, 1-10.
Jiang, S., and M. Ghil, 1993: Dynamical properties of error statistics in a shallow-water model, J. Phys. Oceanogr., 23, 2541-2566.
Jiang, S., and M. Ghil, 1997: Tracking nonlinear solutions with altimetric data in a shallow-water model. J. Phys. Oceanogr., 27, 72-95.
Jiang, S., F.-F. Jin, and M. Ghil, 1995: Multiple equilibria, periodic, and aperiodic solutions in a wind-driven, double-gyre, shallow-water model, J. Phys. Oceanogr., 25, 764-786.
Paldor, N., and M. Ghil, 1990: Finite-wavelength instabilities of a coupled density front, J. Phys. Oceanogr., 20, 114-123.
Paldor, N. and M. Ghil, 1991: Shortwave instabilities of coastal currents, Geophys. Astrophys. Fluid Dyn., 58, 225-241.
Quon, C., and M. Ghil, 1992: Multiple equilibria in thermosolutal convection due to salt-flux boundary conditions, J. Fluid Mech., 245, 449-483.
Quon, C., and M. Ghil, 1995: Multiple equilibria and stable oscillations in thermosolutal convection at small aspect ratio, J. Fluid. Mech., 291, 33-56.
Speich, S., and M. Ghil, 1994: Interannual variability of the mid-latitude oceans: a new source of climate variability? Sistema Terra, 3(3), 33-35.
Speich, S., H. Dijkstra, and M. Ghil, 1995: Successive bifurcations in a shallow-water model, applied to the wind-driven ocean circulation, Nonlin. Proc. Geophys., 2, 241-268.

Last modified: 2/23/03