[1] The analysis of univariate or multivariate time series provides crucial information to describe, understand, and predict climatic variability. The discovery and implementation of a number of novel methods for extracting useful information from time series has recently revitalized this classical field of study. Considerable progress has also been made in interpreting the information so obtained in terms of dynamical systems theory. In this review we describe the connections between time series analysis and nonlinear dynamics, discuss signal-to-noise enhancement, and present some of the novel methods for spectral analysis. The various steps, as well as the advantages and disadvantages of these methods, are illustrated by their application to an important climatic time series, the Southern Oscillation Index. This index captures major features of interannual climate variability and is used extensively in its prediction. Regional and global sea surface temperature data sets are used to illustrate multivariate spectral methods. Open questions and further prospects conclude the review.

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Decadal fluctuations in the climate of the North Atlantic-European region may be influenced by interactions between the atmosphere and the Atlantic Ocean, possibly as part of a coupled ocean-atmosphere mode of variability. For such a mode to exist, a consistent atmospheric response to fluctuations in North Atlantic sea surface temperatures (SST) is required. Furthermore, this response must provide feedbacks to the ocean. Whether a consistent response exists, and whether it yields the required feedbacks, are issues that remain controversial. Here, these issues are addressed using a novel approach to analyze an ensemble of six integrations of the Hadley Centre atmospheric general circulation model HadAM1, all forced with observed global SSTs and sea-ice extents for the period 1949-93. Characterizing the forced atmospheric response is complicated by the presence of internal variability. A generalization of principal component analysis is used to estimate the common forced response given the knowledge of internal variability provided by the ensemble. In the North Atlantic region a remote atmospheric response to El Niño-Southern Oscillation and a further response related to a tripole pattern in North Atlantic SST are identified. The latter, which is most consistent in spring, involves atmospheric circulation changes over the entire region, including a dipole pattern in sea level pressure often associated with the North Atlantic oscillation. Only over the tropical/subtropical Atlantic, however, does it account for a substantial fraction of the total variance. How the atmospheric response could feed back to affect the ocean, and in particular the SST tripole, is investigated. Several potential feedbacks are identified but it has to be concluded that, because of their marginal consistency between ensemble members, a coupled mode that relied on these feedbacks would be susceptible to disruption by internal atmospheric variability. Notwithstanding this conclusion, the authors' results suggest that predictions of SST evolution could be exploited to predict some aspects of atmospheric variability over the North Atlantic, including fluctuations in spring of the subtropical trade winds and the higher latitude westerlies.

Equatorial winds in the stratosphere are known to influence the frequency of stratospheric midwinter sudden warmings. Sudden warmings, in turn, influence the Earth's climate both through their direct influence on polar temperatures and through the temperature dependence of ozone depletion in the lower stratosphere. The conventional (Holton-Tan) explanation for the equatorial influence on sudden warmings is in terms of the equatorial winds in the lower stratosphere (-20-30 km) acting as a waveguide for midlatitude planetarywave propagation. This study employs stratospheric-temperature analyses and equatorial rocketsonde wind data extending to 58 km to diagnose the relationship between the northern-hemisphere polar temperatures and equatorial zonal winds at all height levels in the stratosphere. In addition to the recognized Holton-Tan relationship linking the polar temperatures to the quasi-biennial oscillation in equatorial winds in the lower stratosphere, a strong correlation of polar temperatures with equatorial winds in the upper stratosphere is found. We suggest that this may be associated with the strength and vertical extent of the westerly phase of the semi-annual oscillation in the upper stratosphere, although the observations alone cannot provide a conclusive, causal relationship. The main diagnostic tools employed are correlation studies and composite analysis. The results underline the need for continued high quality, equatorial wind measurements at all stratospheric levels.