Synchronization and predictability in experimental fluids and climate dynamics

A thorough understanding of the environment and climate of the Earth, and the development of methods to predict its future behaviour and responses to changes in factors such as atmospheric composition and external forcing, requires a holistic consideration of the entire Earth system. Such an approach views the Earth in terms of a complicated collection of distinct sub-systems (the troposphere, stratosphere, oceans, cryosphere, land surface and biosphere, for example), all mutually interacting via complex feedback processes and subject to time-varying external forces (such as the diurnal and annual cycles, and other external processes on longer timescales), and leading to complex behaviour that is very difficult to predict. Such a view of the Earth System underpins modern approaches to modelling the Earth's present and past climates, and more recently also to evaluating socio-economic and ecological responses to such changes, such as in NERC's QUEST programme. In a system as complex as the Earth, interactions between sub-systems are likely themselves to be highly complex, intermittent and nonlinear, presenting enormous challenges to the modelling community to represent accurately and realistically. In this context, a knowledge and understanding of the kinds of interactions possible between dynamical systems in the presence of nonlinearity is vital to guide the future development of modelling strategies. In recent years, the study of synchronization phenomena in nonlinear systems has made a number of significant advances in various areas of physics, engineering and the life sciences. The first documented example of synchronization was reported as long ago as 1665 by Christiaan Huygens, who noted the tendency of a pair of pendulum clocks, mounted on a common support, eventually to swing together in synchronized motion, even if they would tend to swing at slightly different speeds if isolated from each other. More recently, the study of such nonlinear frequency entrainment and synchronization has been extended to a much more quantitative understanding of the nature of synchronization, the identification of various forms of imperfect synchronization phenomena (e.g. where synchronization happens for a short while and then breaks up, only to resynchronize a little later), and the generalisation to the study of synchronization effects manifest in coupled chaotic systems. In this project, we will study the range of complex forms of synchronization in a fluid dynamical analogue of the Earth's mid-latitude atmospheric circulation in the laboratory. A fluid placed in a cylindrical tank, and subject to differential heating between the inner and outer radius whilst being rotated about the axis of the cylinder, will spontaneously generate complicated jet streams and wave-like instabilities that are dynamically similar to the jet stream and cyclones that organize mid-latitude weather on the Earth. We have recently developed an apparatus that allows us to couple two of these experiments together in such a way that we can study their interaction and possible synchronization behaviour. This is analogous in some respects to certain kinds of feedback process in the climate system. We plan to carry out an extensive study of the range of possible behaviour of this system, measuring the form and strength of synchronization effects and developing new methods for analysing these effects from timeseries of measurements. These methods will then be applied to real climate data in an attempt to detect and quantify similar synchronization phenomena in the atmosphere and oceans during the past 50-100 years. For the latter part of the study we will concentrate on known cyclic phenomena on timescales ranging from 20-60 days to interannual periods (around 1-3 years).