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

(accumulation) and glacial demise (ablation) are ultimately determined by high frequency atmosphere–ocean processes. Initial progress has been made by forcing thermomechanical ice sheet models (ISMs) with seasonally varying climatological fields derived from general circulation models (GCMs). This method has proven to be instrumental in producing reconstructions of the Laurentide ice sheet at the Last Glacial Maximum (LGM) (e.g., Marshall et al. 2002 ; Huybrechts et al. 2004 ) and has been used to

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Shawn J. Marshall and Martin J. Sharp

within the context of a simplified sinusoidal annual temperature cycle: where T a is mean annual temperature, t is time in days, ϕ is a time lag, and τ is the length of the year (365.24 days). This assumes an annual temperature cycle with a half-amplitude A = T max − T a , where T max is the maximum summer temperature. Time t = 0 is taken as January 1, with ϕ introducing a time lag to describe heat capacity effects (seasonal thermal inertia). The latter is rarely introduced because

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Alex S. Gardner, Martin J. Sharp, Roy M. Koerner, Claude Labine, Sarah Boon, Shawn J. Marshall, David O. Burgess, and David Lewis

temperature fields and in the magnitude and spatial distribution of modeled glacier melt ( Greuell and Böhm 1998 ; Otto-Bliesner et al. 2006b ; Gardner and Sharp 2009 ). Near-surface temperature lapse rates vary on diurnal and seasonal time scales because of changes in the sensible heat flux between the free atmosphere and the underlying surface. This flux is influenced by temporal and spatial changes in free-atmosphere and surface temperatures, surface roughness, and wind speed. Because near

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

conditions that occurred at the onset of the Younger Dryas event. Hu et al. (2007) investigated the effects of keeping the Bering Strait opened or closed during a FWF event. In a set of FWF experiments using the NCAR CCSM2, Hu et al. (2007) noted that shutdown of the AMOC occurs in a similar manner in both the closed and open Bering Strait experiments but that the recovery was delayed by approximately 100 yr in the closed strait experiment because of reduced transports from the Atlantic into the

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Marc d’Orgeville and W. Richard Peltier

dependent in control simulations and also that it might be significantly reduced as a consequence of transient climate forcing because of continuous increase of the atmospheric CO 2 concentration ( Bryan et al. 2006 ). In the analysis presented in this paper, control simulations under both preindustrial and present-day perpetual seasonal cycle conditions will be compared to each other and also to the results of five simulations with increasing CO 2 concentration scenarios. The same model with the same

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Marc d’Orgeville and W. Richard Peltier

decadal variability ( Latif and Barnett 1996 ) superimposed on a lower-frequency modulation ( Minobe 1997 ; d’Orgeville and Peltier 2007 ), consisting of periods of stable sign separated by abrupt sign reversals as, for instance, in the case of the well-known 1976/77 climate shift ( Trenberth and Hurrell 1994 ; Latif and Barnett 1996 ). The PDO has been shown to have an impact on North American climate ( Zhang et al. 1997 ), and to modulate the effects of El Niño–Southern Oscillation (ENSO) over

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

Hudson Strait (B in Fig. 2b ), and the flood pulse from Lake Agassiz (C in Fig. 2c ). The forcing A + B + C represents the combined effects of rerouting and the release of stored water in the lake. The B + C forcing (no preflood flow to the Gulf of St. Lawrence) is appropriate to the case where the discharge to the St. Lawrence is highly turbid and possibly hyperpycnal, an idea that is explored in the next section. The A + B + C and B + C forcings can be viewed as end members of a continuum of

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