École Polytechnique

About: École Polytechnique is a education organization based out in Palaiseau, France. It is known for research contribution in the topics: Laser & Plasma. The organization has 18995 authors who have published 39265 publications receiving 1225163 citations. The organization is also known as: Ecole Polytechnique & Polytechnique.

Scalable simulation of cellular signaling networks

Abstract: Given the combinatorial nature of cellular signalling pathways, where biological agents can bind and modify each other in a large number of ways, concurrent or agent-based languages seem particularly suitable for their representation and simulation [1,2,3,4]. Graphical modelling languages such as κ [5, 6, 7, 8], or the closely related BNG language [9,10,11,12,13,14], seem to afford particular ease of expression. It is unclear however how such models can be implemented. Even a simple model of the EGF receptor signalling network can generate more than 1023 non-isomorphic species [5], and therefore no approach to simulation based on enumerating species (beforehand, or even on-the-fly) can handle such models without sampling down the number of potential generated species. We present in this paper a radically different method which does not attempt to count species. The proposed algorothm uses a representation of the system together with a super-approximation of its 'event horizon' (all events that may happen next), and a specific correction scheme to obtain exact timings. Being completely local and not based on any kind of enumeration, this algorithm has a per event time cost which is independent of (i) the size of the set of generable species (which can even be infinite), and (ii) independent of the size of the system (ie, the number of agent instances). We show how to refine this algorithm, using concepts derived from the classical notion of causality, so that in addition to the above one also has that the even cost is depending (iii) only logarithmically on the size of the model (ie, the number of rules). Such complexity properties reflect in our implementation which, on a current computer, generates about 106 events per minute in the case of the simple EGF receptor model mentioned above, using a system with 105 agents.

Clustering in globally coupled phase oscillators.

Abstract: A model of many globally coupled phase oscillators is studied by analytical and numerical methods. Each oscillator is coupled to all the other oscillators via a global driving force that takes the form ${\mathit{tsum}}_{\mathit{j}}$ g(${\mathrm{\ensuremath{\varphi}}}_{\mathit{j}}$), where g(${\mathrm{\ensuremath{\varphi}}}_{\mathit{j}}$) is a periodic function of the jth phase. The spatiotemporal properties of the attractors in various regions of parameter space are analyzed. In addition to simple spatially uniform fixed points and limit cycles, the system also exhibits spatially nonuniform attractors of three kinds. First, there are cluster states in which the system breaks into a few macroscopically big clusters, each of which is fully synchronized. Second, there is a stationary state with full frequency locking but no phase locking. The distribution of phases is stationary in time. Third, in an extremely narrow regime of parameters, a nonperiodic attractor exists. It is found that the cluster state is stable to the addition of weak stochastic noise. Increasing the level of noise beyond a critical value generates a continuous transition to a stationary ergodic state. In the special case where the nonlinearities in the dynamics involve only first harmonics, marginal states are observed, characterized by a continuum of marginally stable limit trajectories. These states are unstable under the introduction of noise.

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

Abstract: The correlation between Sahel rainfall and El Nino–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200 hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Nino-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.