The dynamics of a low-order coupled ocean-atmosphere model

A system of five ordinary differential equations is studied which combines the Lorenz-84 model for the atmosphere and a box model for the ocean. The behaviour of this system is studied as a function of the coupling parameters. For most parameter values, the dynamics of the atmosphere model is dominant. For a range of parameter values, competing attractors exist. The Kaplan-Yorke dimension and the correlation dimension of the chaotic attractor are numerically calculated and compared to the values found in the uncoupled Lorenz model. In the transition from periodic behaviour to chaos intermittency is observed. The intermittent behaviour occurs near a Neimark-Sacker bifurcation at which a periodic solution loses its stability. The length of the periodic intervals is governed by the time scale of the ocean component. Thus, in this regime the ocean model has a considerable influence on the dynamics of the coupled system.Comment: 20 pages, 15 figures, uses AmsTex, Amssymb and epsfig package. Submitted to the Journal of Nonlinear Scienc

The dynamics of a low-order coupled ocean-atmosphere model

A system of ve ordinary dierential equations is studied which combines the Lorenz model for the atmosphere and a box model for the ocean The behaviour of this system is studied as a function of the coupling parameters For most parameter values the dynamics of the atmosphere model is dominant Stable equilibria are found as well as periodic solutions and chaotic attractors For a range of parameter values competing attractors exist The KaplanYorke dimension and the correlation dimension of the chaotic attractor are numerically calculated and compared to the values found in the uncoupled Lorenz model The correlation dimension diers much less than te KaplanYorke dimension indicating that there is little variability in the ocean model In the transition from periodic behaviour to chaos intermittency is observed This is explained by means of bifurcation analysis The intermittent behaviour occurs near a NeimarkSacker bifurcation at which a periodic solution loses its stability The average length of a periodic interval in the intermittent regime l is studied as a function of the bifurcation parameter Near the bifurcation point it shows a power law scaling It diverges as l where and is the distance from the bifurcation point in reasonable agreement with the results of Pomeau and Manneville Commun Math Phys The intermittent behaviour persists beyond the point where the unstable periodic solution disappears in a saddle node bifurcation The length of the periodic intervals is governed by the time scale of the ocean component Thus in this regime the ocean model has a considerable inuence on the dynamics of the coupled syste

Are we overusing IVF?

Peer reviewedPublisher PD

Interannual variability in net accumulation on the Greenland Ice Sheet: Observations and implications for mass balance measurements

This is the published version, also available here: http://dx.doi.org/10.1029/1998JD200082.Nine 24-year accumulation records from the Summit region in central Greenland are analyzed to separate the effects of spatial noise and interannual fluctuations on the variability in each core. The study shows that both processes are equally important, with standard deviations of 25 mm water equivalent per year and 24 mm water equivalent per year, respectively. A comparison with estimates of surface roughness based on high-resolution laser altimetry of the surface indicates that in the studied region the spatial noise can be reliably estimated from surface roughness. The response of the ice-sheet surface to the interannual fluctuations can be estimated using a simple zero-dimensional ice-sheet response model. For the Summit region of central Greenland, a change in surface elevation of ∼20 mm water equivalent per year measured over a 5-year period, can be attributed with 95% confidence to a trend in climate. This probability decreases rapidly as the observation period is shortened. For intervals greater than ∼5 year, the probability depends only weakly on the measurement interval. This suggests an optimum spacing of ∼5 years between repeat elevation measurements

Loop quantum gravity induced modifications to particle dynamics

The construction of effective Hamiltonians arising from Loop Quantum Gravity and incorporating Planck scale corrections to the dynamics of photons and spin 1/2 particles is summarized. The imposition of strict bounds upon some parameters of the model using already existing experimental data is also reviewed.Comment: 9 pages, 0 figures, talk presented at the X Mexican School of Particles and Fields, latex, aipproc style 6x

An assessment of a days off decomposition approach to personnel scheduling

This paper studies a two-phase decomposition approach to solve the personnel scheduling problem. The first phase creates a days off schedule, indicating working days and days off for each employee. The second phase assigns shifts to the working days in the days off schedule. This decomposition is motivated by the fact that personnel scheduling constraints are often divided in two categories: one specifies constraints on working days and days off, while the other specifies constraints on shift assignments. To assess the consequences of the decomposition approach, we apply it to public benchmark instances, and compare this to solving the personnel scheduling problem directly. In all steps we use mathematical programming. We also study the extension that includes night shifts in thefirst phase of the decomposition. We present a detailed results analysis, and analyze the effect of various instance parameters on the decompositions' results. In general, we observe that the decompositions significantly reduce the computation time, and that they produce good solutions for most instances