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Michael J. Ring and R. Alan Plumb

substantial components of atmospheric variability, these patterns have come to be known as “modes.” But is this a meaningful description? If these patterns are truly mode-like, then not only will they appear as unforced natural variability, but also as a preferred response of the atmospheric circulation to external forcings. There are indeed suggestions of such a preferred response in the atmosphere to Antarctic ozone depletion ( Thompson and Solomon 2002 ) and greenhouse forcing (e.g., Shindell et al

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

al. 1997a , b ). Readers are referred to CPH for a complete description of the model setup and the equilibrated states. Here, we just provide a cursory description. This is a zonally reentrant, semihemispheric model ranging from 50.67° to 0.17°S and 0° to 10°E on a ⅙° × ⅙° latitude × longitude grid with 15 vertical levels. The model does not include salinity; density is simply a linear function of temperature. The model-imposed forcings are shown in Fig. 1 . The wind stress is eastward

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Francis Codron

1. Introduction Quasi-annular patterns, often called annular modes, dominate atmospheric extratropical low-frequency variability ( Thompson and Wallace 1998 ). For both hemispheres, these modes are characterized by pressure anomalies of one sign over the polar region, surrounded by a band of opposing polarity with peak amplitude in the midlatitudes. They also appear as the favored response to a wide range of climate forcings, such as the observed trend in the Southern Hemisphere ( Thompson and

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Adam P. Showman

. 1999 ; Porco et al. 2003 ). In contrast, most two-dimensional turbulence studies use random forcing that occurs everywhere simultaneously and is confined to a small range of wavenumbers. This shortcoming prevents a robust assessment of jet formation in the giant-planet context. Furthermore, most published turbulence investigations that focus on jets have been purely two dimensional, hence precluding the vortex stretching (and associated horizontal divergence) that can be crucial in atmospheres

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R. K. Scott and L. M. Polvani

(1996a , b ) showed that, in the absence of forcing, an initially random flow on the sphere spontaneously organizes itself into a banded configuration, with the observed number of bands roughly appearing for each of the four giant outer planets once the radius, rotation rate, and Rossby radius are specified. One notable shortcoming of that model, however, concerns the direction of the zonal winds at low latitudes: while Jupiter and Saturn have strong prograde jets at the equator, the freely

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Edwin P. Gerber and Geoffrey K. Vallis

1. Introduction In this paper we investigate processes that determine the persistence of the zonal index and North Atlantic Oscillation (NAO), focusing on intraseasonal time scales. By intraseasonal, we mean time scales of 10–100 days, longer than those associated with synoptic variability, but not so long as to allow for significant changes in the boundary conditions or forcing, such as sea surface temperature. On these shorter periods, then, the focus is on internal processes in the

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M. L. R. Liberato, J. M. Castanheira, L. de la Torre, C. C. DaCamara, and L. Gimeno

1. Introduction Several observational studies, performed over the last 10 years, suggest that the stratosphere does not respond passively to tropospheric forcing but does play instead an important role in driving climate and weather variability down to the surface (e.g., Baldwin and Dunkerton 1999 , 2001 ; Hartmann et al. 2000 ; Thompson et al. 2002 ; Perlwitz and Harnik 2004 ). Such studies have naturally generated an increasing interest for a better understanding of the dynamics of the

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John Marshall, David Ferreira, J-M. Campin, and Daniel Enderton

parameterizes eddies as an advective and stirring process, using the scheme of Gent and McWilliams (1990) with a transfer coefficient of 800 m 2 s −1 . Convective adjustment, implemented as an enhanced vertical mixing of temperature ( T ) and salt ( S ), is used to represent ocean convection. A thermodynamic ice model following Winton (2000) is also incorporated into the model. Orbital forcing and CO 2 levels are prescribed at present-day values. The seasonal cycle is represented: there is no diurnal

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Yasuko Hio and Shigeo Yoden

1. Introduction This paper considers nonlinear dynamics of an idealized winter polar vortex in the Southern Hemisphere (SH) stratosphere with a barotropic model on a spherical domain. The SH polar vortex is stronger and less disturbed compared to that of the Northern Hemisphere. In other words, the zonal-mean zonal flow is stronger and planetary waves are weaker in the SH due to weaker forcing of the planetary waves in the troposphere. As a result, a major stratospheric sudden warming event had

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Peter L. Read, Yasuhiro H. Yamazaki, Stephen R. Lewis, Paul D. Williams, Robin Wordsworth, Kuniko Miki-Yamazaki, Joël Sommeria, and Henri Didelle

. Convective forcing A key objective of this study was to investigate the properties of geostrophic turbulence maintained by a mechanism that was as “natural” and unconstrained as possible. In particular, it was considered important to apply forcing on a relatively small horizontal scale, but one that was not fixed in position relative to the rotating reference frame. To achieve this, convective forcing was applied by gently spraying relatively dense, salty water onto the main fluid layer from a reservoir

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