Dynamical analysis of the atmospheric angular momentum short-term fluctuations. Comparison with the Earth

Abstract

The general behaviour of geophysical flows (atmosphere, oceans) is mainly nonlinear. Nevertheless, to date most of the studies concerning the atmospheric angular momentum functions and the Earth rotation time series have been devoted to their linear characterization and comparison, assuming their stochastic nature. In order to look for an alternative description of that time series, nonlinear time series analysis is applied to the effective atmospheric angular momentum short-term fluctuations (the pressure term of the equatorial functions and the wind term of the axial function) evaluated from the National Center for Environmental Prospect (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis Project, assuming a possible deterministic and nonlinear nature for these systems. The method consists of embedding the time series into a multidimensional space to recover the `attractor' of the system. It is shown that the dynamics of these processes are indeed nonlinear, deterministic and low-dimensional, thanks to their geometrical and topological description in the embedded space. The existence of a positive Lyapunov adds the property of chaoticity to the underlying system. It is found that the atmospheric processes observed through the angular momentum functions are chaotic and we are able to quantify the associated characterizations (D E=5, 4<D Lyap<5, variations of stability). We can also evaluate a theoretical horizon of prediction for the systems of about 6 days. The results suggest that the atmospheric angular momentum time series should no longer be described as purely stochastic processes. Additionally, the various nonlinear quantities evaluated are of the same order as those previously found for the short-term fluctuations of the Earth's rotation, indicating that the observed chaotic dynamics in the Earth's rotation is induced (through forcing processes) and belongs for a large part to the atmospheric dynamics, as expected.