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Cegeon J. Chan, R. Alan Plumb, and Ivana Cerovecki

variability of the zonal jets in the model. At any instant in time, the mean zonal flow comprises a dominant jet, together with two or three secondary jets primarily on its poleward side. The main jet wobbles quasiperiodically, in a manner that appears similar to the “annular mode” behavior of atmospheric jets. At the same time, the secondary jets, poleward of the main jet, migrate systematically equatorward such that, once every period of the main jet’s fluctuation, one secondary jet merges with the main

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Mauro Dall’Amico and Joseph Egger

in surface weather ( Thompson and Wallace 2001 ; Thompson et al. 2002 ). The dynamical coupling of the troposphere with the stratosphere 1 in the NH is captured by the northern annular mode (NAM; see Thompson and Wallace 1998 ; Baldwin and Dunkerton 1999 ; Thompson and Wallace 2000) . In the winter stratosphere the annular mode is linked to the temperature and strength of the polar vortex. In the lowermost troposphere, the NAM appears as the Arctic Oscillation and, in particular, over the

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Edwin P. Gerber and Geoffrey K. Vallis

1. Introduction What determines the zonal structure of the North Atlantic Oscillation (NAO) and the annular modes, the dominant patterns of variability in the extratropical atmosphere on intraseasonal time scales of 10–100 days? Various authors, including Limpasuvan and Hartmann (2000) , DeWeaver and Nigam (2000) , and Benedict et al. (2004) show or suggest that these large-scale patterns are driven and maintained by eddy–mean flow interactions and that their decorrelation time scale is on

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Judah Cohen and Kazuyuki Saito

symmetry. Therefore there has been a growing acceptance to refer to these modes as annular ( DeWeaver and Nigam 2000 ). It has even been suggested that the dominant modes in each hemisphere bear a resemblance and should be referred to as the northern annular mode (NAM) and the southern annular mode ( Limpasuvan and Hartmann 2000 ). The AO or NAM is strongly correlated with the well-known teleconnection pattern, the North Atlantic Oscillation (NAO). Whether the dominant mode of Northern Hemisphere (NH

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Adam H. Monahan and John C. Fyfe

-mean zonal wind will be referred to as the zonal index mode. The leading EOF of Northern Hemisphere (NH) extratropical sea level pressure (SLP) was defined by Thompson and Wallace (1998) as the Arctic Oscillation, or Northern Annular Mode [respectively the Antarctic Oscillation or Southern Annular Mode for the Southern Hemisphere (SH) SLP]. The EOF spatial structures are equivalent barotropic and dipole-like, and are approximately zonally symmetric with oppositely signed anomalies in the mid- and high

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Varavut Limpasuvan and Dennis L. Hartmann

1. Introduction Recent observations demonstrate that month-to-month tropospheric variation in the Northern Hemisphere is dominated by a mode that has a strong zonally symmetric or annular component ( Baldwin and Dunkerton 1999 ; Thompson and Wallace 1998 , 2000 , hereafter TW). The zonal mean part of the Northern Hemisphere variability is remarkably similar to that of the annular atmospheric variability in the Southern Hemisphere (e.g., Trenberth 1984 ; Yoden et al. 1987 ; Kidson 1988

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Matthew A. H. Wittman, Andrew J. Charlton, and Lorenzo M. Polvani

1. Introduction Attempts to explain intraseasonal variability in the Northern Hemisphere have recently popularized a large-scale pattern known as the Arctic Oscillation (AO) and the related Northern Annular Mode (NAM). Diagnostics based upon the AO/NAM are now widely used in many fields of the geophysical sciences as a convenient way of describing the atmospheric variability. With respect to its temporal evolution, it is well established that no specific periodicity is associated with this

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Paul J. Kushner and Gabriel Lee

1. Introduction In this study, we pursue the ongoing question of the zonal structure of the annular modes. The annular modes (AMs) are usually defined as the first EOF of the hemisphere-wide streamflow in the Northern Hemisphere (NH) or Southern Hemisphere (SH) extratropics ( Thompson and Wallace 1998 , 2000 ). The hemisphere-wide EOF analysis produces hemispheric-scale streamflow patterns with a dipolar meridional structure and relatively little zonal structure. On the other hand

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Pedram Hassanzadeh and Zhiming Kuang

1. Introduction In the extratropical circulation of both hemispheres, a long-recognized dominant pattern of variability at the intraseasonal to interannual time scales is the “annular mode,” which is often derived from empirical orthogonal function (EOF) analysis of meteorological fields such as zonal-mean zonal wind or geopotential heights and has been known for decades ( Kidson 1988 ; Thompson and Wallace 1998 ; Feldstein 2000 ). The leading EOF (EOF1) of features an equivalent

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Steven B. Feldstein

. (2000 )]. These characteristics are consistent with the global warming scenario of many climate models. In a recent study, Thompson et al. (2000) placed this climate trend into a coherent, well-organized framework, by showing that the trend of many different climate variables is related to the trend in just one particular dominant mode of atmospheric variability, known as the NH annular mode ( Thompson and Wallace 2000 ). The spatial structure of the NH annular mode 1 corresponds to the first

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