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Robert X. Black, Brent A. McDaniel, and Walter A. Robinson

subject of considerable debate in the recent literature (e.g., Manzini et al. 2003 ). A robust bidirectional dynamical coupling between the stratosphere and troposphere has been observed in the boreal extratropics during winter ( Thompson and Wallace 1998 ; Baldwin et al. 2003 ; McDaniel and Black 2005 ) in association with intraseasonal variability in the northern annular mode (NAM). The NAM is the primary mode of circulation variability in the Northern Hemisphere extratropics and its lower

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Judah Cohen, Mathew Barlow, Paul J. Kushner, and Kazuyuki Saito

well separated from the next EOF, which explains 7% of this variance. In the remainder of the article we refer to the principal component of the leading multivariate WAF/SLP EOF as the stratosphere–troposphere coupling index (STCI), or s ( t ), where t is a time index running from the 1948/49 to the 2004/05 December–January season. We discuss the characteristics of this mode in the next section. A third physically distinct field we employ is a measure of October-mean snow cover extent over

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Kevin M. Grise, David W. J. Thompson, and Piers M. Forster

1. Introduction Observations and numerical simulations both suggest that variability in the extratropical stratosphere has a demonstrable impact on the extratropical troposphere. The coupling between stratospheric and tropospheric flow is observed in the context of Northern Hemisphere (NH) sudden stratospheric warmings ( Baldwin and Dunkerton 1999 , 2001 ; Limpasuvan et al. 2004 ), Southern Hemisphere (SH) sudden stratospheric warmings ( Thompson et al. 2005 ), and recent trends in the SH

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Nili Harnik, Judith Perlwitz, and Tiffany A. Shaw

). When the upward propagating waves reach the stratosphere, they either dissipate and initiate zonal-mean stratosphere–troposphere coupling or they are reflected downward toward the troposphere, which results in downward wave coupling ( Perlwitz and Harnik 2004 ; Harnik 2009 ). Recently, Shaw et al. (2010) showed that climatological downward wave coupling is stronger than zonal-mean coupling on the intraseasonal time scale in the Southern Hemisphere, particularly during austral spring. They found

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Sandro W. Lubis, Katja Matthes, Nili Harnik, Nour-Eddine Omrani, and Sebastian Wahl

stratosphere to the troposphere, known as downward wave coupling (DWC; e.g., Perlwitz and Harnik 2003 ; Shaw et al. 2010 ; Shaw and Perlwitz 2013 ; Lubis et al. 2016a , 2017 ). DWC events occur when upward-propagating waves reach the stratosphere and then get reflected downward toward the troposphere, where they impact the wave and circulation there ( Perlwitz and Harnik 2003 ; Shaw et al. 2010 ; Lubis et al. 2016a , 2017 ). Many episodes of DWC are tied to the so-called bounded wave geometry of

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Hannah E. Attard and Andrea L. Lang

coupling and forcing the SSW. They concluded that synoptic-scale phenomena are important considerations when analyzing troposphere–stratosphere coupling and should not be ignored. In a case study of the January 2013 SSW, Coy and Pawson (2015) further emphasized this fact by showing that an extratropical cyclone in the North Atlantic perturbed the waveguide in such a way to promote a period of upward WAF during the initial period of the SSW. Though Polvani and Waugh (2004) showed that there is a

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Edwin P. Gerber and Lorenzo M. Polvani

1. Introduction Recent observational studies have demonstrated coupling between the stratosphere and troposphere in which stratospheric events originating as high as 10 hPa are linked to changes in surface weather. On intraseasonal time scales (10–100 days), coupling is observed primarily in the winter and early spring, preferentially in the Northern Hemisphere, when and where the stratospheric polar vortex, or polar night jet, is most variable ( Thompson and Wallace 2000 ; Charlton and

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Tiffany A. Shaw, Judith Perlwitz, and Nili Harnik

1. Introduction Planetary waves represent the most important source of dynamical coupling between the stratosphere and troposphere. They are generated in the troposphere by orography and continent–ocean heating asymmetries and propagate upward into the stratosphere where they either dissipate and initiate a downward-propagating zonal-mean response or they are reflected downward toward the troposphere. The focus of many recent studies has been on zonal-mean stratosphere–troposphere coupling (e

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Tiffany A. Shaw and Judith Perlwitz

1. Introduction Dynamical coupling between the stratosphere and troposphere is a key component of atmospheric variability in the winter hemisphere. Understanding the mechanisms involved in this coupling and its impact on tropospheric weather and climate is an important topic of current research ( Shaw and Shepherd 2008 ; Gerber et al. 2012 ). It is well known that stratosphere–troposphere coupling is driven by the upward propagation of planetary-scale waves generated in the troposphere. A

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Judith Perlwitz and Nili Harnik

1. Introduction There is increasing evidence that stratospheric dynamic processes play a significant role in tropospheric climate variability across a wide range of time scales. However, the dynamic mechanisms by which the stratosphere can influence the tropospheric circulation are not well understood. Many recent studies of the downward dynamic coupling between the stratosphere and troposphere have emphasized zonal mean dynamics, in relation to the annular modes ( Baldwin and Dunkerton 1999

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