



Recent developments of observing technology, both spaceborne and in situ, have greatly improved oceanic data sets, in quality as well as quantity. The study of the oceans' lowfrequency variability, however, still has to rely to a large extent on models, due to the inhomogeneity and temporal shortness of the data sets; shortterm predictability on synoptic time scales, on the other hand, can be studied using data assimilation.
Basinscale oceanic motion is dominated by winddriven (horizontal) and thermohaline (vertical) circulations. Their variability, independently and interactively, may play a significant role in climate changes, past and future. The winddriven circulation plays a role mostly in the oceans' subannualtointerannual variability, while the thermohaline circulation is most important in decadaltomillenial variability. Our approach is to study the two governing circulations separately, and then in terms of their interaction, especially on the interannualtointerdecadal time scale. Dynamicalsystems methods, especially from bifurcation theory, provide a powerful tool to reveal the processes underlying the oceans' variability.
The horizontal circulation of the midlatitude ocean is governed by its winddriven, doublegyre structure.
As shown in the bifurcation diagram (center) of a reducedgravity, shallowwater model, its behavior can change, even for fixed wind stress, depending on the system's parameter values. Single and multiple equilibria, purely and quasiperiodical 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 upperlayer thickness for various parameter values (from Speich et al., 1995).
TCD Members:  
M. Ghil, K. Ide, S. Kravtsov.  
References:  
Chen, F., and M. Ghil, 1995: Interdecadal variability of the thermohaline circulation and highlatitude surface fluxes, J. Phys. Oceanogr., 25, 25472568.  
Chen, F., and M. Ghil, 1996: Interdecadal variability in a hybrid coupled oceanatmosphere model, J. Phys. Oceanogr., 26, 15611578.  
Feliks, Y., and M. Ghil, 1996: Mixed barotropicbaroclinic eddies growing on an eastward midlatitude jet. Geophys. Astrophys. Fluid Dyn., 82, 137171.  
Feliks, Y., and M. Ghil, 1997: Stability of a front separating water masses with different stratifications. Geophys. Astrophys. Fluid Dyn., 84, 165204.  
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, 471496.  
Ghil, M., A. Mullhaupt and P. Pestiaux, 1987: Deep water formation and quaternary glaciations, Climate Dyn., 2, 110.  
Jiang, S., and M. Ghil, 1993: Dynamical properties of error statistics in a shallowwater model, J. Phys. Oceanogr., 23, 25412566.  
Jiang, S., and M. Ghil, 1997: Tracking nonlinear solutions with altimetric data in a shallowwater model. J. Phys. Oceanogr., 27, 7295.  
Jiang, S., F.F. Jin, and M. Ghil, 1995: Multiple equilibria, periodic, and aperiodic solutions in a winddriven, doublegyre, shallowwater model, J. Phys. Oceanogr., 25, 764786.  
Paldor, N., and M. Ghil, 1990: Finitewavelength instabilities of a coupled density front, J. Phys. Oceanogr., 20, 114123.  
Paldor, N. and M. Ghil, 1991: Shortwave instabilities of coastal currents, Geophys. Astrophys. Fluid Dyn., 58, 225241.  
Quon, C., and M. Ghil, 1992: Multiple equilibria in thermosolutal convection due to saltflux boundary conditions, J. Fluid Mech., 245, 449483.  
Quon, C., and M. Ghil, 1995: Multiple equilibria and stable oscillations in thermosolutal convection at small aspect ratio, J. Fluid. Mech., 291, 3356.  
Speich, S., and M. Ghil, 1994: Interannual variability of the midlatitude oceans: a new source of climate variability? Sistema Terra, 3(3), 3335.  
Speich, S., H. Dijkstra, and M. Ghil, 1995: Successive bifurcations in a shallowwater model, applied to the winddriven ocean circulation, Nonlin. Proc. Geophys., 2, 241268. 