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We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bolling-Allerod (BA) warming. Our model reproduces several major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient C[O.sub.2] forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.

Byline: L. A. Gil-Alana (1) Abstract: This paper deals with the modelling of the global and northern and southern hemispheric anomaly temperature time series using a novel technique based on segmented trends and fractional integration. We use a procedure that permits us to estimate linear time trends and orders of integration at various subsamples, where the periods for the changing trends are endogenously determined by the model. Moreover, we use a non-parametric approach (Bloomfield P, Biometrika, 60:217--226, 1973) for modelling the I(0) deviation term. The results show that the three series (global, northern and southern temperatures) can be well described in terms of fractional integration with the orders of integration around 0.5 in the three cases. The coefficients associated to the time trends are statistically significant in all subsamples for the three series, especially during the final part of the sample, giving then some support to the global warming theories. Author Affiliation: (1) Faculty of Economics, University of Navarra, Edificio Biblioteca, Entrada Este, 31080, Pamplona, Spain Article History: Registration Date: 12/02/2008 Received Date: 05/08/2006 Accepted Date: 13/12/2007 Online Date: 30/04/2008

In complex spatial models, as used to predict the climate response to greenhouse gas emissions, parameter variation within plausible bounds has major effects on model behavior of interest. Here, we present an unprecedentedly large ensemble of >57,000 climate model runs in which 10 parameters, initial conditions, hardware, and software used to run the model all have been varied. We relate information about the model runs to large-scale model behavior (equilibrium sensitivity of global mean temperature to a doubling of carbon dioxide). We demonstrate that effects of parameter, hardware, and software variation are detectable, complex, and interacting. However, we find most of the effects of parameter variation are caused by a small subset of parameters. Notably, the entrainment coefficient in clouds is associated with 30% of the variation seen in climate sensitivity, although both low and high values can give high climate sensitivity. We demonstrate that the effect of hardware and software is small relative to the effect of parameter variation and, over the wide range of systems tested, may be treated as equivalent to that caused by changes in initial conditions. We discuss the significance of these results in relation to the design and interpretation of climate modeling experiments and large-scale modeling more generally. classification and regression trees | climate change | distributed computing | general circulation models | sensitivity analysis

A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positive-phase NAO favors the occurrence of European blocking events.

Through calculating the scatter diagrams of the streamfunction ([[psi].sub.P] or [[psi].sub.T]) versus potential vorticity (PV) ([q.sub.p] or [q.sub.T]), where tOp and tOt are the planetary-scale streamfunction and total streamfunction, respectively, and using a weakly nonlinear NAO model proposed in Part I of this paper, it is suggested that negative- and positive-phase NAO events may approximately correspond to free modes even though driven by synopticscale eddies. In a planetary-scale field, the [q.sub.P]([[psi].sub.P]) scatter diagram of an NAO event exhibits a linear multivalued functional relationship in a narrow region for the negative phase, but exhibits a linear single-valued functional relationship during the positive phase. It was also found that there is no steepening of the slope of the main straight line in the [q.sub.P]([[psi].sub.P]) scatter diagrams for two phases of the NAO event. Instead, the slope of the straight line in the scatterplots is time independent throughout the life cycle of the NAO event. However, when synoptic-scale eddies are included in the streamfunction field, the [q.sub.p]([[psi].sub.T] scatter diagram of the negative-phase NAO event shows a trend toward steepening during the intensification phase, and this tendency reverses during the decay phase. During the positive NAO phase the slope of the [q.sub.p]([[psi].sub.T]) scatter diagram shoals during the intensification phase and then steepens during the decay phase. Thus, it appears that the steepening and shoaling of the scatter diagrams of the streamfunction versus PV for the negative-and positive-phase NAO events are attributed to the effect of synoptic-scale eddies that force NAO events to form. Diagnostic studies using both composite and unfiltered fields of observed NAO events are presented to confirm these conclusions.

Byline: Christoph C. Raible (1), Carlo Casty (2), Jurg Luterbacher (2), Andreas Pauling (2), Jan Esper (3), David C. Frank (3), Ulf Buntgen (3), Andreas C. Roesch (4), Peter Tschuck (4), Martin Wild (4), Pier-Luigi Vidale (4), Christoph Schar (4), Heinz Wanner (2) Abstract: A detailed analysis is undertaken of the Atlantic-European climate using data from 500-year-long proxy-based climate reconstructions, a long climate simulation with perpetual 1990 forcing, as well as two global and one regional climate change scenarios. The observed and simulated interannual variability and teleconnectivity are compared and interpreted in order to improve the understanding of natural climate variability on interannual to decadal time scales for the late Holocene. The focus is set on the Atlantic-European and Alpine regions during the winter and summer seasons, using temperature, precipitation, and 500 hPa geopotential height fields. The climate reconstruction shows pronounced interdecadal variations that appear to 'lock' the atmospheric circulation in quasi-steady long-term patterns over multi-decadal periods controlling at least part of the temperature and precipitation variability. Different circulation patterns are persistent over several decades for the period 1500 to 1900. The 500-year-long simulation with perpetual 1990 forcing shows some substantial differences, with a more unsteady teleconnectivity behaviour. Two global scenario simulations indicate a transition towards more stable teleconnectivity for the next 100 years. Time series of reconstructed and simulated temperature and precipitation over the Alpine region show comparatively small changes in interannual variability within the time frame considered, with the exception of the summer season, where a substantial increase in interannual variability is simulated by regional climate models. Author Affiliation: (1) Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012, Bern, Switzerland (2) Institute of Geography, University of Bern, Hallerstrasse 12, CH-3012, Bern, Switzerland (3) Swiss Federal Research Institute WSL, Zurcherstrasse 111, CH-8903, Birmensdorf, Switzerland (4) Institute for Atmospheric and Climate Science ETH, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland Article History: Registration Date: 03/01/2006 Accepted Date: 09/11/2005 Online Date: 01/11/2006