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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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Chengyang Zhang
,
Wenshou Tian
,
Jiankai Zhang
,
Tuantuan Zhang
,
Wei Yu
,
Song Yang
, and
Tao Wang

temperature (SST) ( Cui et al. 2015 ; Yu et al. 2021 ), and the South Asian high ( Wang and Guo 2012 ; Liu et al. 2017 ; Ge et al. 2018 ). In addition to the abovementioned signals, recent studies showed that the downward-extending stratospheric signals can also regulate the onset of Indian summer monsoon ( Ren and Hu 2014 ; Hu and Ren 2018 ). It has been revealed that the dynamic coupling between the stratosphere and the troposphere is closely related to the Arctic Oscillation (AO) that has a quasi

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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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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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Xiaocen Shen
,
Lin Wang
,
Adam A. Scaife
,
Steven C. Hardiman
, and
Peiqiang Xu

extreme case of weak SPV ( Charlton and Polvani 2007 ; Mitchell et al. 2013 ; Seviour et al. 2013 ). These studies suggest the complexity of the SPV-associated stratosphere–troposphere coupling and highlight the importance of variation in location and geometry in addition to the strength of SPV. The complexity of time scales and spatial characteristics in stratosphere–troposphere dynamical coupling ( Perlwitz and Harnik 2003 ; Dunn-Sigouin and Shaw 2018 ; Kretschmer et al. 2018 ) further

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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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Zachary D. Lawrence
,
Dillon Elsbury
,
Amy H. Butler
,
Judith Perlwitz
,
John R. Albers
,
Laura M. Ciasto
, and
Eric Ray

coupling processes has not been methodically assessed and documented for NOAA S2S prediction systems. The CFSv2 has seen widespread use in studies focused on aspects of stratospheric predictability and stratosphere–troposphere coupling (see, e.g., Zhang et al. 2013 ; Riddle et al. 2013 ; Yu et al. 2019 ; Miller and Wang 2019 ; Rao and Garfinkel 2020 ), including in model intercomparison studies such as those by Domeisen et al. (2020a , b) and Lawrence et al. (2022) . Since the GEFSv12 is

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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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Verónica Martínez-Andradas
,
Alvaro de la Cámara
, and
Pablo Zurita-Gotor

−35 to −5 days and (c),(e) lag 15 to 45 days. Black thick contours in (b)–(e) and marks in (a) indicate significance at the 95% confidence level based on a Monte Carlo test. The number of events in the composites are shown in brackets. Next, we analyze the vertical evolution of the hemispheric NAM index for the SSW composite ( Fig. 3 ), which is a standard diagnostic of stratosphere–troposphere coupling. The composite for all SSWs is shown in Fig. 3a , and displays well-known features of the

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