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It is of fundamental importance to evaluate the ability of climate models to capture the large-scale atmospheric circulation patterns and, in the context of a rapidly increasing greenhouse forcing, the robustness of the changes simulated in these patterns over time. Here we approach this problem from an innovative point of view based on dynamical systems theory. We characterize the atmospheric circulation over the North Atlantic in the CMIP5 historical simulations (1851–2000) in terms of two instantaneous metrics: local dimension of the attractor and stability of phase-space trajectories. We then use these metrics to compare the models to the Twentieth Century Reanalysis version 2c (20CRv2c) over the same historical period. The comparison suggests that (i) most models capture to some degree the median attractor properties, and models with finer grids generally perform better; (ii) in most models the extremes in the dynamical systems metrics match large-scale patterns similar to those found in the reanalysis; (iii) changes in the attractor properties observed for the ensemble-mean 20CRv2c are artifacts resulting from inhomogeneities in the standard deviation of the ensemble over time; and (iv) the long-term trends in local dimension observed among the 56 members of the 20CR ensemble have the same sign as those observed in the CMIP5 multimodel mean, although the multimodel trend is much weaker.

Meridional transport of heat by transient atmospheric eddies is a key component of the energy budget of the middle- and high-latitude regions. The heat flux at relevant frequencies is also part of a dynamical interaction between eddies and mean flow. In this study we investigate how the poleward heat flux by high-frequency atmospheric transient eddies is modulated by North Atlantic weather regimes in reanalysis data. Circulation regimes are estimated via a clustering method, a jet-latitude index, and a blocking index. Heat transport is defined as advection of moist static energy. The focus of the analysis is on synoptic frequencies but results for slightly longer time scales are reported. Results show that the synoptic eddy heat flux is substantially modulated by midlatitude weather regimes on a regional scale in midlatitude and polar regions. In a zonal-mean sense, the phases of the North Atlantic Oscillation do not significantly change the high-latitude synoptic heat flux, whereas Scandinavian blocking and the Atlantic ridge are associated with an intensification. A close relationship between high-latitude (midlatitude) heat flux and Atlantic jet speed (latitude) is found. The relationship between extreme events of synoptic heat flux and circulation regimes is also assessed and reveals contrasting behaviors in the polar regions. The perspective that emerges complements the traditional view of the interaction between synoptic eddies and the extratropical flow and reveals relationships with the high-latitude climate.