# Publications

2013
Kondrashov, Dmitri, Mickaël D. Chekroun, Andrew W. Robertson, and Michael Ghil. 2013. “Low-order stochastic model and past-noise forecasting" of the Madden-Julian oscillation.” Geophysical Research Letters 40: 5305–5310.
Rousseau, D.-D., Michael Ghil, G. Kukla, A. Sima, P. Antoine, M. Fuchs, C. Hatté, F. Lagroix, M. Debret, and O. Moine. 2013. “Major dust events in Europe during marine isotope stage 5 (130–74 ka): a climatic interpretation of the" markers".” Climate of the Past 9 (5). Copernicus GmbH: 2213–2230.
Feliks, Yizhak, Andreas Groth, Andrew W. Robertson, and Michael Ghil. 2013. “Oscillatory Climate Modes in the Indian Monsoon, North Atlantic and Tropical Pacific.” Journal of Climate 26: 9528-–9544. Abstract

This paper explores the three-way interactions between the Indian monsoon, the North Atlantic and the Tropical Pacific. Four climate records were analyzed: the monsoon rainfall in two Indian regions, the Southern Oscillation Index for the Tropical Pacific, and the NAO index for the North Atlantic. The individual records exhibit highly significant oscillatory modes with spectral peaks at 7–8 yr and in the quasi-biennial and quasi-quadrennial bands. The interactions between the three regions were investigated in the light of the synchronization theory of chaotic oscillators. The theory was applied here by combining multichannel singular-spectrum analysis (M-SSA) with a recently introduced varimax rotation of the M-SSA eigenvectors. A key result is that the 7–8-yr and 2.7-yr oscillatory modes in all three regions are synchronized, at least in part. The energy-ratio analysis, as well as time-lag results, suggest that the NAO plays a leading role in the 7–8-yr mode. It was found therewith that the South Asian monsoon is not slaved to forcing from the equatorial Pacific, although it does interact strongly with it. The time-lag analysis pinpointed this to be the case in particular for the quasi-biennial oscillatory modes. Overall, these results confirm that the approach of synchronized oscillators, combined with varimax-rotated M-SSA, is a powerful tool in studying teleconnections between regional climate modes and that it helps identify the mechanisms that operate in various frequency bands. This approach should be readily applicable to ocean modes of variability and to the problems of air-sea interaction as well.

2012

Ghil, Michael. 2012. “The Complex Physics of Climate Change: Nonlinearity and Stochasticity.” Workshop on Critical Transitions in Complex Systems, Imperial College London, United Kingdom. Conference website Abstract

Ghil, Michael. 2012. “What is a Tipping Point and Why Do We Care?” EGU 2012. Abstract

Moron, Vincent, Andrew W. Robertson, and Michael Ghil. 2012. “Impact of the modulated annual cycle and intraseasonal oscillation on daily-to-interannual rainfall variability across monsoonal India.” Climate Dynamics 38 (11-12). Springer: 2409–2435.
Brachet, Sidonie, Francis Codron, Yizhak Feliks, Michael Ghil, Hervé Le Treut, and Eric Simonnet. 2012. “Atmospheric circulations induced by a midlatitude SST front: A GCM study.” Journal of Climate 25 (6): 1847–1853. Abstract

The atmospheric effects of sea surface temperature (SST) anomalies over and near western boundary currents are a matter of renewed interest. The general circulation model (GCM) of the Laboratoire de Météorologie Dynamique (LMD-Z) has a zooming capability that allows a regionally increased resolution. This GCM is used to analyze the impact of a sharp SST front in the North Atlantic Ocean: two simulations are compared, one with climatological SSTs and the other with an enhanced Gulf Stream front. The results corroborate the theory developed previously by the present team to explain the impact of oceanic fronts. In this theory, the vertical velocity at the top of the atmospheric boundary layer has two components: mechanical and thermal. It is the latter that is dominant in the tropics, while in midlatitudes both play a role in determining the wind convergence above the boundary layer. The strengthened SST front does generate the previously predicted stronger ascent above the warmer water south of the front and stronger descent above the colder waters to the north. In the GCM simulations, the ascent over the warm anomalies is deeper and more intense than the descent.

Deremble, Bruno, Guillaume Lapeyre, and Michael Ghil. 2012. “Atmospheric dynamics triggered by an oceanic SST front in a moist quasigeostrophic model.” Journal of the Atmospheric Sciences 69 (5): 1617–1632.
Ghil, Michael. 2012. “Climate variability: nonlinear and random effects.” Encyclopedia of Atmospheric Sciences. Elsevier, 1–6.
Chekroun, Mickaël D., and Jean Roux. 2012. “Homeomorphisms group of normed vector space: Conjugacy problems and the Koopman operator.” Discrete and Continuous Dynamical Systems - Series A 33: 3957–3980.
Chekroun, Mickaël D., and N. E. Glatt-Holtz. 2012. “Invariant measures for dissipative dynamical systems: Abstract results and applications.” Communications in Mathematical Physics 316: 723–761.
Deremble, B., E. Simonnet, and Michael Ghil. 2012. “Multiple equilibria and oscillatory modes in a mid-latitude ocean-forced atmospheric model.” Nonlinear Processes in Geophysics 19 (5): 479–499.
Alessio, Silvia, Gianna Vivaldo, Carla Taricco, and Michael Ghil. 2012. “Natural variability and anthropogenic effects in a Central Mediterranean core.” Climate of the Past 8 (2): 831–839.
Groth, Andreas, Michael Ghil, Stéphane Hallegatte, and Patrice Dumas. 2012. “The Role of Oscillatory Modes in U.S. Business Cycles.” Fondazione Eni Enrico Mattei (FEEM) 26. Fondazione Eni Enrico Mattei (FEEM): 1. Publisher's Version Abstract

We apply the advanced time-and-frequency-domain method of singular spectrum analysis to study business cycle dynamics in a set of nine U.S. macroeconomic indicators. This method provides a robust way to identify and reconstruct shared oscillations, whether intermittent or modulated. We address the problem of spurious cycles generated by the use of detrending filters and present a Monte Carlo test to extract significant oscillations. Finally, we demonstrate that the behavior of the U.S. economy changes significantly between episodes of growth and recession; these variations cannot be generated by random shocks alone, in the absence of endogenous variability.

Hannart, A., Michael Ghil, J. L. Dufresne, and P. Naveau. 2012. “The uncertain future of climate uncertainty.” Geophysical Research Letters.
2011
Daae, M., Y. Y. Shprits, B. Ni, J. Koller, Dmitri Kondrashov, and Y. Chen. 2011. “Reanalysis of radiation belt electron phase space density using various boundary conditions and loss models.” Advances in Space Research 48 (8): 1327 - 1334. Publisher's Version
Ghil, Michael. 2011. “Toward a Mathematical Theory of Climate Sensitivity.” International Congress on Industrial and Applied Mathematics (ICIAM), Vancouver. Abstract

Chekroun, Mickaël D., Eric Simonnet, and Michael Ghil. 2011. “Stochastic climate dynamics: Random attractors and time-dependent invariant measures.” Physica D 240 (21): 1685-–1700. Abstract
This article attempts a unification of the two approaches that have dominated theoretical climate dynamics since its inception in the 1960s: the nonlinear deterministic and the linear stochastic one. This unification, via the theory of random dynamical systems (RDS), allows one to consider the detailed geometric structure of the random attractors associated with nonlinear, stochastically perturbed systems. We report on high-resolution numerical studies of two idealized models of fundamental interest for climate dynamics. The first of the two is a stochastically forced version of the classical Lorenz model. The second one is a low-dimensional, nonlinear stochastic model of the El Niño-Southern Oscillation (ENSO). These studies provide a good approximation of the two models' global random attractors, as well as of the time-dependent invariant measures supported by these attractors; the latter are shown to have an intuitive physical interpretation as random versions of Sina\"ı-Ruelle-Bowen (SRB) measures.