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The resumption process of the North Atlantic meridional overturning circulation (MOC) is investigated in a series of freshwater hosing experiments using a comprehensive coupled climate model. Four different freshwater perturbations are applied to the North Atlantic between 50°N and 70°N leading to a substantial weakening of MOC and an expansion of winter sea ice cover over the Greenland-Iceland-Norwegian (GIN) Seas. Deactivating the freshwater forcing leads to a two-phase recovery of the MOC. The first phase is a slow basin-wide advection process whereas the second phase is an abrupt, decadal-scale transition, located in the North Atlantic. The slow basin-wide advection process to an intermediate state of the MOC is characterized by a restart of the deep water production in the area of the subpolar gyre and the Irminger Basin. However, the ice cover over the GIN Seas remains, whereas the surface density gradually increases in the GIN Seas on a century time scale. The length of this intermediate state depends on the strength of freshwater perturbation, which determines the magnitude of the meridional density gradient in the North Atlantic. The second transition phase is triggered by a model-specific density threshold reached in the GIN Seas. When passed, the MOC abruptly intensifies by ∼60% within 60-80 years. The analysis shows that this abrupt intensification only depends on surface salinity in the GIN Seas. The model results reveal that the simulated sea ice retreat in the GIN Seas and a rapid increase of the Greenland air temperature of the second phase of the resumption of the MOC are similar in amplitude to temperature proxies observed during Dansgaard-Oeschger events of the last ice age.

Extreme midlatitude cyclone characteristics, precipitation, wind speed events, their inter-relationships, and the connection to large-scale atmospheric patterns are investigated in simulations of a prolonged cold period, known as the Maunder Minimum from 1640 to 1715 and compared with today. An ensemble of six simulations for the Maunder Minimum as well as a control simulation for perpetual 1990 conditions are carried out with a coupled atmosphere-ocean general circulation model, i.e., the Climate Community System Model (CCSM). The comparison of the simulations shows that in a climate state colder than today the occurrence of cyclones, the extreme events of precipitation and wind speed shift southward in all seasons in the North Atlantic and the North Pacific. The extremes of cyclone intensity increases significantly in winter in almost all regions, which is related to a stronger meridional temperature gradient and an increase in lower tropospheric baroclinicity. Extremes of cyclone intensity in subregions of the North Atlantic are related to extremes in precipitation and in wind speed during winter. Moreover, extremes of cyclone intensity are also connected to distinct large-scale atmospheric patterns for the different subregions, but these relationships vanish during summer. Analyzing the mean 1,000hPa geopotential height change of the Maunder Minimum simulations compared with the control simulation, we find a similar pattern as the correlation pattern with the cyclone intensity index of the southern Europe cyclones. This illustrates that changes in the atmospheric high-frequency, i.e., the simulated southward shift of cyclones in the North Atlantic and the related increase of extreme precipitation and wind speed in particular in the Mediterranean in winter, are associated with large-scale atmospheric circulation changes

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Climate of the Past 9 (2013): 1111-1140, doi:10.5194/cp-9-1111-2013., Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs