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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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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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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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Masakazu Taguchi

troposphere, as lower-stratospheric anomalies, such as those after MSSWs, are suggested to be the key for downward coupling to the troposphere (e.g., Hitchcock et al. 2013 ). Figure 2a shows the yearly time series for the JRA-55 data (black) and HC ensemble mean (blue), together with results from all ensemble members (cyan). Figure 2b plots results from the probability forecasts of the same index for the three categories: lo, nt, and hi, as introduced in section 2b . The JRA-55 and HC ensemble

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Hua Lu, Lesley J. Gray, Patrick Martineau, John C. King, and Thomas J. Bracegirdle

). Extensive research has studied coupling between the stratosphere and the troposphere in association with major stratospheric sudden warmings (SSWs; Mitchell et al. 2013 ; Sigmond et al. 2013 ; Maycock et al. 2020 ), polar-night jet oscillations (PJO; Kuroda and Kodera 2004 ; Hitchcock and Shepherd 2013 ), downward wave coupling ( Perlwitz and Harnik 2003 ), lower-stratospheric control ( Scott and Polvani 2004 ; Martineau et al. 2018 ), the role of tropospheric eddies ( Simpson et al. 2009 ), and

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Bo Pang, Adam A. Scaife, Riyu Lu, and Rongcai Ren

pattern, are found to influence stratospheric variability ( Martius et al. 2009 ; Kolstad et al. 2010 ; Nishii et al. 2011 ; Karpechko et al. 2018 ; Peings 2019 ; White et al. 2019 ). The coupling between troposphere and stratosphere is established through the upward propagation of planetary wave activity (e.g., Scaife and James 2000 ; Chen et al. 2003 ; McDaniel and Black 2005 ; Plumb 2010 ; Bancalá et al. 2012 ; Attard et al. 2016 ; Huang et al. 2018 ). The planetary waves are mainly

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Aditi Sheshadri, R. Alan Plumb, and Edwin P. Gerber

of Northern Hemisphere stratospheric final warming events . J. Atmos. Sci. , 64 , 2932 – 2946 , doi: 10.1175/JAS3981.1 . Black , R. X. , and B. A. McDaniel , 2007b : Interannual variability in the Southern Hemisphere circulation organized by stratospheric final warming events . J. Atmos. Sci. , 64 , 2968 – 2974 , doi: 10.1175/JAS3979.1 . Black , R. X. , B. A. McDaniel , and W. A. Robinson , 2006 : Stratosphere–troposphere coupling during spring onset . J. Climate , 19

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