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Annular patterns with a high degree of zonal symmetry play a prominent role in the natural variability of the atmospheric circulation and its response to external forcing. But despite their apparent importance for understanding climate variability, the processes that give rise to their marked zonally symmetric components remain largely unclear. Here the authors use simple stochastic models in conjunction with an atmospheric model and observational analyses to explore the conditions under which annular patterns arise from empirical orthogonal function (EOF) analysis of the flow. The results indicate that annular patterns arise not only from zonally coherent fluctuations in the circulation (i.e., "dynamical annularity") but also from zonally symmetric statistics of the circulation in the absence of zonally coherent fluctuations (i.e., "statistical annularity"). It is argued that the distinction between dynamical and statistical annular patterns derived from EOF analysis can be inferred from the associated variance spectrum: larger differences in the variance explained by an annular EOF and successive EOFs generally indicate underlying dynamical annularity. The authors provide a simple recipe for assessing the conditions that give rise to annular EOFs of the circulation.When applied to numerical models, the recipe indicates dynamical annularity in parameter regimes with strong feedbacks between eddies and the mean flow. When applied to observations, the recipe indicates that annular EOFs generally derive from statistical annularity of the flow in the midlatitude troposphere but from dynamical annularity in both the stratosphere and the mid-high-latitude Southern Hemisphere troposphere. [ABSTRACT FROM AUTHOR]

This article investigates the prominent features of the Southern Hemisphere (south of 20°S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the southern annular mode (SAM), the nature of its recent trend, and the zonal symmetry of associated spatial patterns. The study uses the NCEP-Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (NCEP-2) (period 1979-2009) to obtain robust patterns over the recent years and the Twentieth Century Reanalysis Project (period 1871-2008) to document decadal changes. Also presented is a comparison of these signals against a station-based reconstruction of the SAM index and a gridded interpolated dataset [Hadley Centre Sea Level Pressure dataset version 2 (HadSLP2)]. Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decadal time scale, the study shows that the trend toward the positive SAM phase (as inferred from the usual EOF-based index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis therefore allows for complement descriptions based on simple indexes or EOF decompositions, highlighting the nonlinear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature. [ABSTRACT FROM AUTHOR]