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Stephen D. Griffiths and W. Richard Peltier

that by evaluating U̇ in (2a) and (2c) at the intermediate variable Ũ m , rather than at U m , only one evaluation of U̇ is required per time step, the other being stored from the previous time step. (Equivalently, the Coriolis and drag terms can be treated using a second-order Adams–Bashforth scheme.) In addition to being second-order accurate in time, the major benefit of this scheme is that the maximum time step is twice the inverse of the maximum gravity wave frequency. Even so, the

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

associated with the propagation of atmospheric planetary waves excited during El Niño, which may give rise to the Pacific–North American (PNA) pattern ( Straus and Shukla 2002 ). Recent studies on the dynamics of ENSO teleconnections have investigated the effects of transient eddies, diabatic cooling processes ( DeWeaver and Nigam 2004 ; Trenberth et al. 2002 ), and sources of nonlinear ENSO variability (e.g., Hannachi 2001 ; Wu and Hsieh 2004 ), all of which likely contribute to the development of

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

scenarios ( Otto-Bliesner 1999 ; Federov and Philander 2000 ; Peltier and Solheim 2004 ) provides a means to assess the fidelity of these predictive models. Studies of the impact of freshwater forcing on tropical Pacific climate variability (e.g., Zhang and Delworth 2005 ; Dong and Sutton 2007 ) have discussed several of the mechanisms that could be involved in supporting an “atmospheric bridge” between the polar Atlantic and tropical Pacific Oceans. Timmermann et al. (2005) has suggested that an

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Marc d’Orgeville and W. Richard Peltier

correspond to more (less) sea ice coverage and a cooling (warming) of the atmosphere. Sea ice anomalies in the North Pacific–Arctic region, by modulating the albedo and therefore the surface atmospheric temperature, are able to generate a wave train that extends downstream along the coast of North America ( Honda et al. 1999 ; Alexander et al. 2004 ). It can be conjectured that the modulation of heat flux to the atmosphere from the Bering Sea would have the same effect [see, e.g., the study in the North

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

.4° are further integrated from the 3000-yr state of a 1.8° × 3.6° model. The two highest-resolution models have grid spacings that are finer than typical ocean eddy wavelengths (100–200 km) ( Stammer 1997 ), but coarser than the Rossby radius of deformation at high latitudes ( Smith et al. 2000 ; Oschlies 2002 ), which qualifies them as eddy permitting. All models are integrated under orbital, atmospheric (CO 2 = 280 ppm), and land surface conditions corresponding to the year 1850, rather than 8

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

than 5–10 yr. The climate system switch that has received the greatest attention and for which there is the strongest evidence is that associated with changing the operation of the North Atlantic meridional overturning circulation (MOC). The engine for this circulation is sinking of dense (cold and saline) seawater at high northern latitudes and the brake, by this hypothesis, is the freshening of surface waters by various means, including atmospheric precipitation and melt from glaciers and

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