Description: We use a global coupled atmosphere‐ocean sea‐ice model of intermediate complexity to demonstrate that wind‐forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind‐stress in our multi‐century‐long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2‐d climate models which typically do not capture wind‐driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed.

Description: We use a global coupled atmosphere-ocean sea-ice model of intermediate complexity to demonstrate that wind-forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind-stress in our multi-century-long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2-d climate models which typically do not capture wind-driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed.

Description: Global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the North Atlantic that disrupted deepwater production. These massive freshwater pulses known as Heinrich events arose, in part, from instabilities within the Laurentide ice sheet. Paleoevidence from the North Atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the Northern Hemisphere. In the tropical western Pacific sea, surface temperatures and salinity varied together with ocean and climate changes at high latitudes. Here we present results from coupled modeling experiments that shed light on a possible dynamical link between the North Atlantic Ocean and the western tropical Pacific. This link involves a global oceanic standing wave pattern brought about by millennial-scale glacial density variations in the North Atlantic, atmospheric teleconnections triggered by meridional sea surface temperature gradients, and local air-sea interactions. Furthermore, our modeling results are compared with hydrological records from the Cariaco basin, the Indian Ocean, the Sulu Sea, and northern Australia.

Description: Proxy records and results of a three dimensional climate model show that European summer temperatures roughly a millennium ago were comparable to those of the last 25 years of the 20th century, supporting the existence of a summer "Medieval Warm Period" in Europe. Those two relatively mild periods were separated by a rather cold era, often referred to as the "Little Ice Age". Our modelling results suggest that the warm summer conditions during the early second millennium compared to the climate background state of the 13th–18th century are due to a large extent to the long term cooling induced by changes in land-use in Europe. During the last 200 years, the effect of increasing greenhouse gas concentrations, which was partly levelled off by that of sulphate aerosols, has dominated the climate history over Europe in summer. This induces a clear warming during the last 200 years, allowing summer temperature during the last 25 years to reach back the values simulated for the early second millennium. Volcanic and solar forcing plays a weaker role in this comparison between the last 25 years of the 20th century and the early second millennium. Our hypothesis appears consistent with proxy records but modelling results have to be weighted against the existing uncertainties in the external forcing factors, in particular related to land-use changes, and against the uncertainty of the regional climate sensitivity. Evidence for winter is more equivocal than for summer. The forced response in the model displays a clear temperature maximum at the end of the 20th century. However, the uncertainties are too large to state that this period is the warmest of the past millennium in Europe during winter.

Description: Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections.

Description: Based on coupled modelling evidence we argue that topographically-induced modifications of the large-scale atmospheric circulation during the last glacial maximum may have led to a reduction of the westerlies, and a slowdown of the Pacific subtropical gyre as well as to an intensification of the Pacific subtropical cell. These oceanic circulation changes generate an eastern North Pacific warming, an associated cooling in the Kuroshio area, as well as a cooling of the tropical oceans, respectively. The tropical cooling pattern resembles a permanent La Niña state which in turn forces atmospheric teleconnection patterns that lead to an enhancement of the subtropical warming by reduced latent and sensible cooling of the ocean. In addition, the radiative cooling due to atmospheric CO2 and water vapor reductions imposes a cooling tendency in the tropics and subtropics, thereby intensifying the permanent La Niña conditions. The remote North Pacific response results in a warming tendency of the eastern North Pacific which may level off the effect of the local radiative cooling. Hence, a delicate balance between oceanic circulation changes, remotely induced atmospheric flux anomalies as well local radiative cooling is established which controls the tropical and North Pacific temperature anomalies during the last glacial maximum. Furthermore, we discuss how the aftermath of a Heinrich event may have affected glacial temperatures in the Pacific Ocean.

Description: A series of climate-change projections are conducted with LOVECLIM, a three-dimensional Earth system model of intermediate complexity that consists of a quasi-geostrophic atmospheric model (ECBILT), an ice-ocean general circulation model (CLIO), a dynamical model of the terrestrial biomass (VECODE), a thermomechanical model of the Greenland and Antarctic ice sheets (AGISM), and a model of the oceanic carbon cycle (LOCH). The global carbon cycle in the model is simulated by both VECODE and LOCH. ECBILT takes into account the topography and surface albedo changes computed by the ice-sheet model. CLIO accounts for the freshwater and latent heat fluxes resulting from the melt of the ice sheets and icebergs. The forcing of the AGISM by ECBILT-CLIO is considered in perturbation mode to avoid any systematic errors and to deal with the rather coarse-resolution of the climate model. We first perfom simulations with prescribed carbon-dioxyde concentrations in order to compare the model results with other studies.An ensemble of experiments over the last 500 years is first carried out aiming at validating the model against recontructions (Figures 1 and 2). The model is forced by two natural forcings: the changes in solar irradiance and volcanic eruptions. The anthropogenic forcings considered are the increase in greenhouse-gas concentrations, including troposheric ozone, as well as the evolution of the sulphate-aerosol load in the atmosphere. The model also accounts for the land-cover changes due to human activities.Climate changes during the 21st century are then studied through ensemble simulations performed with the model driven by various IPCCs SRES scenarios for greenhouse-gas and sulphate-aerosol concentrations. The model performance is assessed by comparing its results with similar results obtained by climate general circulation models.Idealised experiments are finally performed over the third millennium focusing mainly on the North Atlantic climate. In those simulations, the forcings are fixed to their 2100 values until the end of the millennium. A particular attention is paid to the change in freshwater flux from the Greenland ice sheet and its possible impact on the World Oceans thermohaline circulation. A sensitivity study over the third millenium is also carried out in order to assess the ice-sheet contribution to the modelled future climate change

Description: A three-dimensional global model of the Earth system suitable for studying the long-term evolution of climate (LOVECLIM) has been recently developed. This model is made up of a coarse-resolution three-dimensional atmospheresea-iceocean model (ECBILTCLIO), a dynamical model of the continental biosphere (VECODE), a comprehensive model of the oceanic carbon cycle (LOCH), and a high-resolution thermomechanical model of the Greenland and Antarctic ice sheets (AGISM). The atmospheric component has the big advantage that it has been simplified to a level that makes runs on a multi-century time-scale computationaly feasible, while at the time, producing results that, on the whole, are comparable to those of atmospheric general circulation models. The performance of the coupled model is evaluated by performing ensemble simulations over the period 15002000 and by comparing the model results to climate reconstructions available. In these simulations, the following forcings are taken into consideration : the variations in solar irradiance, the volcanic activity, the anthropogenic emissions of CO2, and the changes in concentration of other greenhouse gases and sulphate aerosols resulting from human activities. In the future, the model will be used to investigate the evolution of climate and sea level over the third millennium.