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Masakazu Taguchi

, 489 pp . Baldwin , M. P. , and T. J. Dunkerton , 1999 : Propagation of the Arctic Oscillation from the stratosphere to the troposphere . J. Geophys. Res. , 104 , 30 937 – 30 946 , doi: 10.1029/1999JD900445 . Baldwin , M. P. , and T. J. Dunkerton , 2001 : Stratospheric harbingers of anomalous weather regimes . Science , 294 , 581 – 584 , doi: 10.1126/science.1063315 . Baldwin , M. P. , and D. W. J. Thompson , 2009 : A critical comparison of stratosphere–troposphere coupling

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Kazuaki Nishii, Hisashi Nakamura, and Yvan J. Orsolini

1. Introduction It is well established that a stratospheric sudden warming (SSW), which is characterized by abrupt warming in the polar stratosphere, is induced by enhanced upward propagation of planetary waves (PWs) from the troposphere ( Matsuno 1971 ). Many previous case studies on SSWs have pointed out that a tropospheric blocking high (BH), which gives rise to persistent anomalous meanders of tropospheric jets and abnormal weather conditions, can contribute to the enhancement of upward PW

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Shellie M. Rowe and Matthew H. Hitchman

1. Introduction a. Motivation Stratosphere–troposphere exchange (STE) of air in the upper troposphere and lower stratosphere (UTLS) is an essential part of the Brewer–Dobson circulation, influencing the distribution of ozone, water vapor, and other constituents. An abrupt increase in static stability occurs at the tropopause because of the combined effects of stratospheric ozone heating and surface heating, with buoyant adjustment in the troposphere (e.g., Manabe and Wetherald 1967 ). The

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Gloria L. Manney, Michaela I. Hegglin, William H. Daffer, Michael J. Schwartz, Michelle L. Santee, and Steven Pawson

vortex that may extend into the lowermost stratosphere, McIntyre (1995) ] play an important role in coupling the stratospheric and tropospheric circulations. In addition to the upper tropospheric jets’ role in forcing stratospheric disturbances, the structure of the stratospheric vortex is critical to wave coupling between the stratosphere and troposphere (e.g., Harnik et al. 2011 ). Climate model simulations indicate that the effects of the upper tropospheric–lower stratospheric (UTLS) jets on

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Lawrence Coy and Steven Pawson

Oscillation ( Baldwin and Dunkerton 2001 ) and Pacific blocking ( Kodera et al. 2013 ). Stratospheric forecasts are especially intriguing as the stratosphere (with dynamics dominated by global-scale vorticity advection) tends to be more predictable than the troposphere ( Hoppel et al. 2008 ) so that, if the stratosphere has a significant influence on global modes, a realistic stratosphere may enhance their predictability. Stratospheric and tropospheric analyses are useful for dynamical studies of coupling

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Chaim I. Garfinkel, Luke D. Oman, Elizabeth A. Barnes, Darryn W. Waugh, Margaret H. Hurwitz, and Andrea M. Molod

. J. Climate , 23 , 3282–3299. Garfinkel , C. I. , A. M. Molod , L. Oman , and I.-S. Song , 2011 : Improvement of the GEOS-5 AGCM upon updating the air–sea roughness parameterization . Geophys. Res. Lett. , 38 , L18702 , doi:10.1029/2011GL048802 . Garfinkel , C. I. , D. W. Waugh , and E. Gerber , 2012 : Effect of tropospheric jet latitude on coupling between the stratospheric polar vortex and the troposphere . J. Climate , 26 , 2077 – 2095 . Held , I. M. , 1975

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Edwin P. Gerber

) showed that a very simple surface topography was sufficient to excite an active stratospheric circulation. Surface topography of wavenumber k and amplitude h 0 is added in the winter hemisphere between 25° and 65°N. Gerber and Polvani (2009) found that the most realistic stratosphere–troposphere coupling was found in simulations with γ = 4 and topographic wavenumber k = 2 and amplitude h 0 = 3 km. The Polvani and Kushner (2002) GCM includes a crude parameterization of mesospheric

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Torben Kunz and Richard J. Greatbatch

a noticable impact on the tropospheric circulation, and this coupling is manifested in terms of concurrent NAM anomalies in the stratosphere and troposphere (see, e.g. Baldwin and Dunkerton 1999 , 2001 ). Although the corresponding signal in the troposphere is small, it occurs on time scales of 1–2 months in association with slow variations in the stratosphere, and, hence, the downward effect of stratospheric variability enhances the predictability of the tropospheric NAM on intraseasonal time

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Ying Li and Ngar-Cheung Lau

studies (e.g., Garfinkel et al. 2010 ; Kolstad et al. 2010 ; Kolstad and Charlton-Perez 2011 ) and is shown to be almost identical to the NAM index based on the EOF analysis. The index used in our study is also more effective than zonal wind speed at 60°N for studying the stratosphere–troposphere coupling ( Baldwin and Thompson 2009 ). A negative anomaly in our vortex strength index corresponds to anomalously low heights and a strong vortex, whereas a positive anomaly implies a weak vortex. The

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N. Joss Matthewman and J. G. Esler

Abstract

The fundamental dynamics of “vortex splitting” stratospheric sudden warmings (SSWs), which are known to be predominantly barotropic in nature, are reexamined using an idealized single-layer f-plane model of the polar vortex. The aim is to elucidate the conditions under which a stationary topographic forcing causes the model vortex to split, and to express the splitting condition as a function of the model parameters determining the topography and circulation.

For a specified topographic forcing profile the model behavior is governed by two nondimensional parameters: the topographic forcing height M and a surf-zone potential vorticity parameter Ω. For relatively low M, vortex splits similar to observed SSWs occur only for a narrow range of Ω values. Further, a bifurcation in parameter space is observed: a small change in Ω (or M) beyond a critical value can lead to an abrupt transition between a state with low-amplitude vortex Rossby waves and a sudden vortex split. The model behavior can be fully understood using two nonlinear analytical reductions: the Kida model of elliptical vortex motion in a uniform strain flow and a forced nonlinear oscillator equation. The abrupt transition in behavior is a feature of both reductions and corresponds to the onset of a nonlinear (self-tuning) resonance. The results add an important new aspect to the “resonant excitation” theory of SSWs. Under this paradigm, it is not necessary to invoke an anomalous tropospheric planetary wave source, or unusually favorable conditions for upward wave propagation, in order to explain the occurrence of SSWs.

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