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Clara Deser, Adam S. Phillips, Robert A. Tomas, Yuko M. Okumura, Michael A. Alexander, Antonietta Capotondi, James D. Scott, Young-Oh Kwon, and Masamichi Ohba

to answer from the short observational record alone. A recent example is the 2000-yr control simulation of the Geophysics Fluid Dynamics Laboratory (GFDL) model ( Wittenberg 2009 ), which exhibits large decadal-to-centennial modulation of ENSO behavior under constant boundary conditions (e.g., fixed greenhouse gas concentrations). Such internal variability implies that identifying externally forced changes in ENSO behavior from the relatively short observational record may have limited success

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Stephen Yeager, Alicia Karspeck, Gokhan Danabasoglu, Joe Tribbia, and Haiyan Teng

to a few decades by reducing the forecast uncertainty associated with intrinsic climate variability ( Hawkins and Sutton 2009 ). Initializing CGCM simulations with conditions that reflect the current observed state of the earth system could enhance predictive skill by synchronizing the slowly evolving internal variations in the model, in particular ocean variations, with those in nature. Initialization could also improve skill simply by reducing model bias in the early years of a prediction

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Gokhan Danabasoglu, Steve G. Yeager, Young-Oh Kwon, Joseph J. Tribbia, Adam S. Phillips, and James W. Hurrell

1. Introduction The Atlantic meridional overturning circulation (AMOC) is thought to play an important role in decadal and longer time-scale climate variability as well as in prediction of the earth’s future climate on these time scales. Through its associated heat and salt transports, AMOC significantly influences the climate of the North Atlantic and surrounding areas. In particular, changes in sea surface temperatures (SSTs) linked to AMOC variability can impact even global climate through

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Shih-Yu Wang, Michelle L'Heureux, and Jin-Ho Yoon

values of the WNP (DJF) and Niño-3.4 indices one year later (DJF+1yr). A considerable amount of decadal variability of the correlation between the WNP and ENSO is observed among the forcing experiments. While the Aerosol and Natural forcing experiments show significant correlations between the WNP and ENSO for brief periods between about 1940/50 through 1960/70, only the GHG forcing reveals strong increase in the correlations that begins during the period around 1950–70 and continues through the

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Gerald A. Meehl, Julie M. Arblaster, Julie M. Caron, H. Annamalai, Markus Jochum, Arindam Chakraborty, and Raghu Murtugudde

trends in SSTs, and the intraseasonal to interannual and decadal to multidecadal variability of the BoB SSTs and their role in modulating Indian monsoon variability can be effectively studied with CCSM4 for the processes involving natural variability and various anthropogenic effects. This is especially so given the improvements in the representation of phenomena that are critical for monsoon simulations such as the Madden Julian oscillation (MJO; see section 5 , and Subramanian et al. 2011 ; Zhou

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Laura Landrum, Bette L. Otto-Bliesner, Eugene R. Wahl, Andrew Conley, Peter J. Lawrence, Nan Rosenbloom, and Haiyan Teng

simulations confirm that volcanic forcing has induced a positive winter NAO response over northern Europe the first two winters after an eruption ( Fischer et al. 2007 ), decadal-length annual and late winter (February–March) cooling over western (especially interior) North America ( Wahl and Ammann 2010 ), and multidecadal variability of North Atlantic SSTs ( Otterå et al. 2010 ). The purpose of this paper is to provide an overview of the Last Millennium simulation of the Community Climate System Model

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Stephen J. Vavrus, Marika M. Holland, Alexandra Jahn, David A. Bailey, and Benjamin A. Blazey

is not monotonic throughout the century. There is no temperature increase during the entire decade of the 2040s and only a slight ensemble-mean warming in the 2090s. By contrast, the temperature rises relatively rapidly during the 2030s and 2080s. This highly uneven warming trend even under the most extreme RCP greenhouse-forcing scenario underscores the strong internal variability of Arctic climate and demonstrates that a decade-long warming hiatus in the future should not be surprising. Recent

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Laura Landrum, Marika M. Holland, David P. Schneider, and Elizabeth Hunke

and land cover change ( Lawrence et al. 2012 ). The Community Atmospheric Model version 3.5 was used offline with a fully interactive chemistry model to compute decadal-average monthly three-dimensional concentrations of ozone and aerosols ( Lamarque et al. 2011 ) from historical emissions ( Lamarque et al. 2010 ). Natural forcings for the twentieth century runs include volcanic aerosols ( Ammann et al. 2003 ) and solar variability ( Lean 2000 ; Wang et al. 2005 ). Further details of the

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Ernesto Muñoz, Wilbert Weijer, Semyon A. Grodsky, Susan C. Bates, and Ilana Wainer

. Yeager , 2012 : Mean biases, variability, and trends in air–sea fluxes and sea surface temperature in the CCSM4 . J. Climate , in press . Carton , J. A. , and B. Huang , 1994 : Warm events in the tropical Atlantic . J. Phys. Oceanogr. , 24 , 888 – 903 . Carton , J. A. , X. Cao , B. S. Giese , and A. M. da Silva , 1996 : Decadal and interannual SST variability in the tropical Atlantic Ocean . J. Phys. Oceanogr. , 26 , 1165 – 1175 . Chang , C.-Y. , J. A. Carton , S. A

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Matthew C. Long, Keith Lindsay, Synte Peacock, J. Keith Moore, and Scott C. Doney

observations, found that between 1981 and 2009 North Atlantic increased uniformly over the region at a rate just below that of . These authors demonstrate that climate variability can cause trends to deviate from trends on decadal time scales (see also Thomas et al. 2008 ), but over the long term (+25 yr), in the North Atlantic basically tracks . Metzl (2009) analyzed observations in the Indian Ocean sector of the Southern Ocean (south of 20°S), finding a rate of increase in surface ocean

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