Is the North Atlantic Oscillation just a pink noise?

In this paper the authors address the problem of predictability for the NAO index series. The spectral analysis, completed with a bootstrap procedure, shows a rather featureless structure of the index. In other words, the actual time series could be a realisation of many different stochastic processes. An analysis of the Hurst exponent does suggest a slightly red noise as a model for the index, which is interpreted as the NAO being driven by meteorological noise. A nonlinear study of the series (embedding dimension, fractal correlation dimension and leading Lyapunov exponent) shows little predictive performance as well.