In the central regions of the open ocean, the wind drives two large gyres that dominate shallow circulation patterns. In this seminar, Prof Ghil examines how dynamical systems theory can be used to predict variability in these patterns of circulation.
Abstract
The large-scale, near-surface flow of the mid-latitude oceans is dominated by a larger, anticyclonic and a smaller, cyclonic gyre. The two gyres share the eastward extension of western boundary currents, such as the Gulf Stream or Kuroshio, and are induced by the shear in the winds that cross the respective ocean basins. The boundary currents and eastward jets carry substantial amounts of heat and momentum. These currents also play a key role in the subpolar oceans of both hemispheres.
The low-frequency variability (LFV) of the wind-driven, double-gyre circulation in mid-latitude ocean basins has been studied extensively for time-constant wind stress. First, we summarize analytical and numerical results of applying concepts and methods of dynamical systems theory to this problem. The sequence of bifurcations, from steady to periodic and on to aperiodic flows, is followed across a hierarchy of models, up to eddy-resolving ones. The atmospheric effects of 7–8-year intrinsic ocean variability seem to provide a promising explanation of the North Atlantic Oscillation.
Recent work has focused on the application of non-autonomous and random forcing to double-gyre models. Such forcing is associated with both natural and anthropogenic effects. We discuss the associated pullback and random attractors and the non-uniqueness of the invariant measures that are obtained. Some results of a coupled mid-latitude model of ocean-atmospheric dynamics that suggest the presence of genuinely coupled decadal modes conclude the talk.
This talk reflects collaborative work with a large and still increasing number of people. Please visit http://www.atmos.ucla.edu/tcd/ for their names, affiliations, and respective contributions.
Reference
Ghil, M., 2016: The wind-driven ocean circulation: Applying dynamical systems theory to a climate problem, Discr. Cont. Dyn. Syst. – A, in press.
Biography
Michael Ghil obtained his Ph.D. from New York University’s Courant Institute of Mathematical Science with Peter D. Lax in 1975. He is a Distinguished Professor of Geosciences (emeritus) at the Ecole Normale Supérieure, Paris, past Head of its Geosciences Department (2003–2009) and founder of its Environmental Research and Teaching Institute. He is also a Distinguished Research Professor at the University of California, Los Angeles, where he was Chair of the Department of Atmospheric Sciences (1988–1992) and Director of the Institute of Geophysics and Planetary Physics (1992–2003).
Michael Ghil is a founder of theoretical climate dynamics, as presented in his Springer-Verlag (1987) book with Steve Childress, as well as of advanced data assimilation methodology, as presented in the Springer-Verlag (1981) book co-edited with Lennart Bengtsson and Erland Källén. He has applied systematically ideas and methods from nonlinear dynamics to planetary-scale flows, atmospheric and oceanic. He has used these methods to proceed from simple flows with high temporal regularity and spatial symmetry to the observed flows, with their complex behavior in space and time. His studies of climate variability on many time scales have used a full hierarchy of models, from the simplest ‘‘toy’’ models all the way to atmospheric, oceanic and coupled general circulation models.
Michael Ghil has worked on Climate Dynamics, Dynamical and Complex Systems, Extreme Events, Numerical and Statistical Methods, and (most recently) Mathematical Economics. He is the author or editor of a dozen books and author or co-author of nearly 300 research and review articles. Many of the latter can be found on the website of his research group at UCLA. His honors and awards include the L.F. Richardson Medal of the European Geosciences Union (EGU, 2004), the E.N. Lorenz Lecture of the American Geophysical Union (2005), a Plenary Lecture at the 7th International Congress on Industrial and Applied Mathematics (ICIAM 2011), the Alfred Wegener Medal of the EGU (2012), and Membership in the Academia Europaea (1998).