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Guido Vettoretti, Marc d’Orgeville, William R. Peltier, and Marek Stastna

drives the mean surface easterlies and determines the resultant mean thermocline state, with warm waters building in the western Pacific and cool upwelling surface waters in the eastern Pacific. Changes in the mean Walker circulation, which are a vital component of ENSO dynamics, should therefore play a dominant role in determining mean changes of the Pacific equatorial thermocline and equatorial Pacific oceanic waveguide. In winter, the 200-mb streamfunction anomalies ( Fig. 5e ) indicate that the

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J. Paul Spence, Michael Eby, and Andrew J. Weaver

1. Introduction Variation in the poleward transport of heat by the Atlantic meridional overturning circulation (AMOC) is commonly evoked as a mechanism to explain large-scale climate events found in paleorecords ( Bond et al. 1993 ; Rahmstorf 2002 ). A wide range of modeling studies have demonstrated a weakened AMOC in response to surface freshwater forcing at North Atlantic Deep Water (NADW) formation sites (e.g., Stommel 1961 ; Weaver and Hughes 1992 ; Stouffer et al. 2006 ); specifically

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Garry K. C. Clarke, Andrew B. G. Bush, and John W. M. Bush

processors) and the run time is about 1 yr day −1 ; that is, the three simulations presented here each take nearly 50 days to execute. In some ocean modeling studies, diapycnal diffusivities are set artificially high in order to obtain a desired value for the meridional overturning circulation. Such high values are not justified from observations (e.g., Ledwell et al. 1998 ; Egbert and Ray 2000 ) and they tend to erode the tropical thermocline sufficiently that important global phenomena such as ENSO

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Michael S. Pritchard, Andrew B. G. Bush, and Shawn J. Marshall

solar and volcanic forcing: Idealized and realistic experiments. Climate Dyn. , 23 , 153 – 163 . Gordon , C. T. , and W. Stern , 1982 : A description of the GFDL global spectral model. Mon. Wea. Rev. , 110 , 625 – 644 . Hall , J. M. , and L. H. Chan , 2004 : Ba/Ca in benthic foraminifera: Thermocline and middepth circulation in the North Atlantic during the last deglaciation. Paleoceanography , 19 . PA4018, doi:10.1029/2004PA001028 . Hannachi , A. , 2001 : Toward a

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Marc d’Orgeville and W. Richard Peltier

western part of the basin, as shown, e.g., in the twentieth-century simulation presented herein; Fig. 1 ). This suggests that greenhouse gas–induced warming could interact with the PDO. Based on analysis of multicentury integrations performed using the Community Climate System Model, version 3 (CCSM3), atmosphere–ocean coupled general circulation model (AOGCM), the first goal of this paper is therefore to demonstrate that global warming may be expected to exert a strong influence on the natural North

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Stephen D. Griffiths and W. Richard Peltier

traced back over two billion years ( Williams 2000 ). Furthermore, the portion of energy converted to internal tides is thought to play an important role in vertical mixing of the deep ocean (e.g., Wunsch 2000 ). Changes to this mixing rate could lead to changes in ocean circulation and climate, as discussed by Wunsch (2005) , Munk and Bills (2007) , and Montenegro et al. (2007) . Tidal models, in which dissipation is parameterized, suggest that both internal tide dissipation and total

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