Understanding the low-frequency (f>1/10 days) variability (LFV) of large-scale (L>10,000 km) atmospheric flows is essential for extending the practical range of numerical weather prediction beyond its theoretical limit (T~10 days). The TCD group studies various aspects of this variability via observations, laboratory experiments and hierarchical numerical model studies, from the perspective of dynamical systems theory, combined with statistical data analysis.

The atmosphere exchanges angular momentum with the solid earth: as the winds speed up, the earth's rotation slows down, and vice-versa. The two stacks show a comparison of the midlatitude oscillation in angular momentum: i) in a 40- year data set, a barotropic, one-layer, and a ii) baroclinic, 5-layer model, and iii) the UCLA General Circulation Model. The period of the oscillation shown on each deck is close to 40 days and arises from the interaction of the meandering westerlies with the topography shown in the lowermost deck.

The two lower (round) panels show the oscillation's same two extreme phases of zonal and blocked flow [high and low atmospheric angular momentum (AAM) or "index," as shown on the harmonic AAM clock in the upper-left corner of either stack]. They are taken from an experiment carried out in collaboration with the team at University of Texas, Austin in the rapidly rotating tank. Wavenumber-two topography (shown as a black rim around the margin of the round panels) was introduced into the tank and the experimental results are being compared with a specially designed numerical model.

Please see the Department of Atmospheric Sciences brochure ( Theoretical Climate Dynamics) for other reserach areas in the low-frequency variability in the atmopshere.

TCD Members:
	M. Ghil, K. Ide, S. Kravtsov, A. Robertson.
References:
	Bernardet, P., A. Butet, M. Déqué , M. Ghil and R. L. Pfeffer, 1990: Low-frequency oscillations in a rotating annulus with topography, J. Atmos. Sci., 47, 3023-3043.
	Dickey, J. O., M. Ghil, and S. L. Marcus, 1991: Extratropical aspects of the 40-50 day oscillation in length-of-day and atmospheric angular momentum, J. Geophys. Res., 96, 22643-22658.
	Ghil, M., 1987: Dynamics, statistics and predictability of planetary flow regimes, Irreversible Phenomena and Dynamical Systems Analysis in the Geosciences, C. Nicolis and G. Nicolis (Eds.), D. Reidel, Dordrecht/Boston/Lancaster, pp. 241-283.
	Ghil, M., 1988: Nonlinear approaches to low-frequency atmospheric variability, Dynamics of Low-Frequency Phenomena in the Atmosphere, G. W. Branstator, R. A. Madden and J. J. Tribbia (Eds.), National Center for Atmospheric Research, Boulder, CO 80307, pp. 603-714; also in Proc. Summer School in Atmospheric Dynamics, Q.-C. Zeng (Ed.), Beijing, China, 1988.
	Ghil, M., and S. Childress, 1987: Topics in Geophysical Fluid Dynamics: Atmospheric Dynamics, Dynamo Theory and Climate Dynamics, Springer-Verlag, New York/Berlin/London/Paris/ Tokyo, 485 pp.
	Ghil, M., R. Benzi and G. Parisi (Eds.), l985: Turbulence and Predictability in Geophysical Fluid Dynamics and Climate Dynamics, North-Holland Publ. Co., Amsterdam/New York/Oxford/ Tokyo, 449 pp.
	Ghil, M., M. Kimoto, and J. D. Neelin, 1991: Nonlinear dynamics and predictability in the atmospheric sciences, Rev. Geophys., Supplement (U.S. Nat'l Rept. to Int'l Union of Geodesy & Geophys. 1987-1990), 36, 46-55.
	Jin, F-f., and M. Ghil, 1990: Intraseasonal oscillations in the extratropics: Hopf bifurcation and topographic instabilities, J. Atmos. Sci., 47, 3007-3022.
	Kimoto, M., and M. Ghil, 1993a: Multiple flow regimes in the Northern Hemisphere winter. Part I: Methodology and hemispheric regimes, J. Atmos. Sci., 50, 2625-2643.
	Kimoto, M., and M. Ghil, 1993b: Multiple flow regimes in the Northern Hemisphere winter. Part II: Sectorial regimes and preferred transitions, J. Atmos. Sci., 50, 2645-2673.
	Legras, B., and M. Ghil, l985: Persistent anomalies, blocking and variations in atmospheric predictability, J. Atmos. Sci., 42, 433-47l.
	Marcus, S. L., M. Ghil and J. O. Dickey, 1994: The extratropical 40-day oscillation in the UCLA general circulation model. Part I: Atmospheric angular momentum, J. Atmos. Sci., 51, 1431-1446.
	Marcus, S. L., M. Ghil and J. O. Dickey, 1996: The extratropical 40-day oscillation in the UCLA general circulation model. Part II: Spatial structure, J. Atmos. Sci., 53, 1993-2014.
	Mo, K., and M. Ghil, 1987: Statistics and dynamics of persistent anomalies, J. Atmos. Sci., 44, 877-901.
	Mo, K., and M. Ghil, 1988: Cluster analysis of multiple planetary flow regimes, J. Geophys. Res., 93D, 10927-10952.
	Strong, C. M., F.-f. Jin and M. Ghil, 1993: Intraseasonal variability in a barotropic model with seasonal forcing, J. Atmos. Sci., 50, 2965-2986.
	Strong, C. M., F.-f. Jin and M. Ghil, 1995: Intraseasonal oscillations in a barotropic model with annual cycle, and their predictability, J. Atmos. Sci., 52, 2627-2642.
	Weeks, E. R., Y. Tian, J. S. Urbach, K. Ide, H. L. Swinne y, and M. Ghil, 1997: Transitions between blocked and zonal flows in a rotating annulus with topography. Science, 278, 1598-1601.