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Richard T. Wetherald

of time mean state and variability of a number of hydrologic variables. Any change in the occurrence of floods and droughts is a topic of agricultural and societal importance. In a general way, this study attempts to estimate the future likelihood of changes in these events as greenhouse gas concentrations increase in the atmosphere. It has been noted many times that changes in precipitation and many other hydrological variables have smaller signal-to-noise ratios than surface air temperature

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Aixue Hu, Luke Van Roekel, Wilbert Weijer, Oluwayemi A. Garuba, Wei Cheng, and Balu T. Nadiga

et al. 2005 ; Stouffer et al. 2006 ). Given the current upward trend of atmospheric greenhouse gas concentrations, coupled climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) have projected a decline of AMOC in the twenty-first century and beyond, and the rate of the AMOC’s decline depends on the different climate forcing pathways (e.g., Cheng et al. 2013 ). Most CMIP6 models also projected a significant weakening of the AMOC ( Weijer et al. 2020 ). Here

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Daniel Hernández-Deckers and Jin-Song von Storch

1. Introduction The study of the energetics of the atmosphere provides a fundamental approach to the understanding of the dynamics of the atmosphere. Here we use it to investigate the dynamical responses of the atmosphere to higher greenhouse gas concentrations simulated by an ocean–atmosphere coupled GCM. It is clear that higher greenhouse gas concentrations imply an increase in global mean temperature, but it is not clear how this may affect the energetics of the atmosphere. One way of

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Shinji Morimoto, Takashi Yamanouchi, Hideyuki Honda, Issei Iijima, Tetsuya Yoshida, Shuji Aoki, Takakiyo Nakazawa, Shigeyuki Ishidoya, and Satoshi Sugawara

1. Introduction To delineate the temporal and spatial variations of greenhouse gases in the troposphere and to elucidate sources of the variations, a large number of observations have been carried out around the world using surface stations, aircraft, and ships (e.g., WMO 2008 ). We have also carried out continuous measurements of atmospheric CO 2 and CH 4 concentrations at Syowa Station (69.0°S, 39.5°E), Antarctica, since 1984 and 1988, respectively ( Aoki et al. 1992 ; Morimoto et al

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Natalia Calvo and Rolando R. Garcia

1. Introduction Several recent studies have pointed out the effects of climate change induced by greenhouse gases (GHGs) on the stratospheric mean meridional, or Brewer–Dobson (BD), circulation (e.g., Butchart and Scaife 2001 ; Rind et al. 2001 ; Shindell et al. 2001 ; Sigmond et al. 2004 ; Butchart et al. 2006 ; Eichelberger and Hartmann 2005 ; Olsen et al. 2007 ; Oman et al. 2009 ). In particular, Butchart et al. compared simulations from several general circulation models and found

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Richard S. Stolarski, Anne R. Douglass, Paul A. Newman, Steven Pawson, and Mark R. Schoeberl

conclusions are based mainly on the results reported in Shine et al. (2003) and Langematz et al. (2003) . The assessment report further states that there may be a less certain contribution to stratospheric cooling from changes in stratospheric water vapor. Over the next two decades, ozone recovery should begin to occur while greenhouse gases (GHGs) continue to increase. These two trends will have opposite effects on the temperature of the stratosphere. Additionally, because ozone chemistry depends on

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Florence Colleoni, Simona Masina, Annalisa Cherchi, and Doroteaciro Iovino

present-day level ( Fig. 1 ; Dutton et al. 2009 ). However, the atmospheric concentration of greenhouse gas (GHG) recorded in East Antarctica exhibits an overshoot at MIS 7 peak interglacial (≈245 kyr) before dropping abruptly to levels comparable to pre-MBE interglacials ( Fig. 1 ; below 260 ppm). In fact, after 245 kyr, CO 2 concentration drops below 240 ppm in less than 10 kyr, while for MIS 5, as an example of post-MBE interglacials, CO 2 remains above 260 ppm for almost 20 kyr ( Fig. 1

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Rolando R. Garcia and William J. Randel

1. Introduction A robust result of simulations of the atmospheric response to increases in greenhouse gases (GHG) is the acceleration of the stratospheric mean meridional, or Brewer–Dobson (BD), circulation (e.g., Butchart and Scaife 2001 ; Rind et al. 2001 ; Sigmond et al. 2004 ; Butchart et al. 2006 ; Eichelberger and Hartmann 2005 ; Olsen et al. 2007 ). A stronger BD circulation leads to changes in the transport of stratospheric tracers and in the stratospheric “age of air” (AOA

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Amanda C. Maycock, Christopher J. Smith, Alexandru Rap, and Owain Rutherford

perturbations applied locally in space. These have historically been used to explore the sensitivity of Earth’s energy balance to perturbations in composition at different altitudes ( Lacis et al. 1990 ; Shine 1991 ; Wang et al. 1980 ; Solomon et al. 2010 ), but more recently they have been used for other purposes, such as to examine responses to forcing in global climate models ( Soden and Held 2006 ; Smith et al. 2018 ). Such perturbations can include climate forcing agents (e.g., greenhouse gases) or

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Jennifer E. Kay, Marika M. Holland, Cecilia M. Bitz, Edward Blanchard-Wrigglesworth, Andrew Gettelman, Andrew Conley, and David Bailey

. Because the definition of Arctic amplification affects the identification of processes explaining Arctic amplification, we begin by defining Arctic amplification for the purposes of this study as the greater-than-global Arctic air or surface temperature warming in response to increased greenhouse gases. Both local feedbacks, such as the canonical positive surface albedo feedback (SAF), and heat flux convergence have been shown to affect Arctic warming and amplification in response to increased

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