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Katrina S. Virts, John M. Wallace, Michael L. Hutchins, and Robert H. Holzworth

evening and a maximum during the morning. In contrast, wintertime lightning peaks around 1900 LT, at the time of minimum precipitation, and declines steadily through the night to a minimum in the hours after sunrise. Interestingly, the results in Fig. 2 indicate that lightning frequencies during early evening (from 1700 to 2000 LT) are of comparable magnitude during summer and winter. Seasonal-mean precipitation and lightning over the eastern United States and western Atlantic are shown in Figs. 3

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Joseph Egger and Klaus-Peter Hoinka

resides above the mountain. Further theories of topographic instability evolved quickly after the seminal contribution by CDV . The baroclinic case was first considered by Charney and Straus (1980) for channel flow. Pedlosky (1981) included nonlinear effects. The horizontal resolution of the models has been enhanced as well. Jin and Ghil (1990) investigated barotropic topographic instability for more general basic flows than admitted by CDV . They found a dipole topographic instability that

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Matthew H. Hitchman and Amihan S. Huesmann

statistics have been calculated for various locations and seasons by Baldwin and Holton (1988) , Peters and Waugh (1996 , 2003 ), Postel and Hitchman (1999 , 2001 ), Waugh and Polvani (2000) , Knox and Harvey (2005) , Berrisford et al. (2007) , and Martius et al. (2007) . Hitchman and Huesmann (2007 , hereafter HH07 ) provided a seasonal climatology of RWB statistics throughout the 330–2000-K layer. HH07 showed a striking structure at the equator: a strong PV gradient occurs in each season

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Hirohiko Masunaga

possibility that neighboring ridges of the interrupted Kelvin wave as a whole could account for the slow eastward propagation of the MJO. The analysis of Masunaga et al. (2006) , however, is limited to subjective diagnosis based on 10 MJO episodes. In this work, long-term outgoing longwave radiation (OLR) data are analyzed to seek climatological evidence (or its absence) of the Kelvin and ER waves as a driver of MJO propagation. Seasonality and regionality of the MJO, which were outside the scope of

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Sean Faulk, Jonathan Mitchell, and Simona Bordoni

cross-equatorial circulation and upper-level easterlies in the tropics and can lead to seasonal shifts in the Hadley cell’s dominant angular momentum budget ( Bordoni and Schneider 2008 ; Schneider and Bordoni 2008 ). Additionally, moisture greatly impacts the zonally averaged large-scale circulation owing to the effects of latent heat on static stability and meridional temperature gradients ( Frierson et al. 2006 ; Frierson 2008 ). We aim here to determine how sensitive the large

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Simona Bordoni and Tapio Schneider

-equatorial Hadley cell undergoes a transition from a regime in which its strength is strongly tied to eddy momentum fluxes to a regime in which the influence of the eddies is weaker and the angular-momentum-conserving limit is more closely approached. A similar transition also occurs in a simple single-layer axisymmetric model in which eddy momentum fluxes are parameterized ( Sobel and Schneider 2009 ). Schneider and Bordoni (2008 , hereafter SB08) showed that, over the course of seasonal cycles simulated

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Eli Tziperman, Stephen E. Zebiak, and Mark A. Cane

” reflecting the influence of the upwelling seasonality on the background stability. Predictability studies of ENSO also indicated the crucial role of the seasonal cycle. Blumenthal (1991) and Xue et al. (1994) showed that the stability of the background state of the equatorial Pacific is strongly seasonal and that this has implications on the seasonality of ENSO predictability. Related seasonal effects were also investigated by Goswami and Shukla (1991) , Battisti and Sarachik (1995) , Latif et al

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V. Ramanathan and W. L. Grose

Januaw 1977)ABSTRACT This paper examines the effects on the seasonal stratospheric circulation due to the following two longwave radiative transfer processes: 1) radiative coupling between the troposphere and lower stratosphereand 2) latitudinal and seasonal variation in the radiative response time h-i, where h is the Newtonian coolingcoefficient. Two numerical experiments have been performed with a quasi-geostrophic nine-level globalcirculation model, in which one includes the two longwave

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Edmund K. M. Chang and Pablo Zurita-Gotor

filter after Blackmon (1976) . Our results suggest that the midwinter minimum is unlikely to be simply explained by seasonal changes in the mean flow structure. What other mechanisms can contribute to this phenomenon? Nakamura (1992) speculated about a number of possible mechanisms, including wave saturation, excessive deformation, excessive advection, trapping of waves near the surface, the effect of moisture, and change in upstream seeding. Most of these, apart from the effects of moisture, are

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Heng Xiao and Carlos R. Mechoso

for effects by seasonal variations in other features of the upper-ocean circulation (e.g., the thermocline) and GCMs capable of producing a realistic seasonal cycle in the upper ocean have been used to explore the interactions between ENSO and the seasonal cycle in the upper ocean (e.g., Chang et al. 1995 ; Guilyardi 2006 ). An and Wang (2001) , for example, working with a modified Cane–Zebiak model, found that allowing the basic state thermocline depth (upper-layer depth) to vary seasonally

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