A multi-model comparison of Atlantic multidecadal variability     

Jin Ba  Noel S. Keenlyside  Mojib Latif  Wonsun Park  Hui Ding  Katja Lohmann  Juliette Mignot  Matthew Menary  Odd Helge Otterå  Bert Wouters  David Salas y Melia  Akira Oka  Alessio Bellucci  Evgeny Volodin 《Climate Dynamics》2014,43(9-10):2333-2348

A multi-model analysis of Atlantic multidecadal variability is performed with the following aims: to investigate the similarities to observations; to assess the strength and relative importance of the different elements of the mechanism proposed by Delworth et al. (J Clim 6:1993–2011, 1993) (hereafter D93) among coupled general circulation models (CGCMs); and to relate model differences to mean systematic error. The analysis is performed with long control simulations from ten CGCMs, with lengths ranging between 500 and 3600 years. In most models the variations of sea surface temperature (SST) averaged over North Atlantic show considerable power on multidecadal time scales, but with different periodicity. The SST variations are largest in the mid-latitude region, consistent with the short instrumental record. Despite large differences in model configurations, we find quite some consistency among the models in terms of processes. In eight of the ten models the mid-latitude SST variations are significantly correlated with fluctuations in the Atlantic meridional overturning circulation (AMOC), suggesting a link to northward heat transport changes. Consistent with this link, the three models with the weakest AMOC have the largest cold SST bias in the North Atlantic. There is no linear relationship on decadal timescales between AMOC and North Atlantic Oscillation in the models. Analysis of the key elements of the D93 mechanisms revealed the following: Most models present strong evidence that high-latitude winter mixing precede AMOC changes. However, the regions of wintertime convection differ among models. In most models salinity-induced density anomalies in the convective region tend to lead AMOC, while temperature-induced density anomalies lead AMOC only in one model. However, analysis shows that salinity may play an overly important role in most models, because of cold temperature biases in their relevant convective regions. In most models subpolar gyre variations tend to lead AMOC changes, and this relation is strong in more than half of the models.  相似文献   

Higher Laurentide and Greenland ice sheets strengthen the North Atlantic ocean circulation     

Xun Gong  Xiangdong Zhang  Gerrit Lohmann  Wei Wei  Xu Zhang  Madlene Pfeiffer 《Climate Dynamics》2015,45(1-2):139-150

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Conceptual model for millennial climate variability: a possible combined solar-thermohaline circulation origin for the ~1,500-year cycle   总被引：1，自引：0，他引：1  

Mihai Dima  Gerrit Lohmann 《Climate Dynamics》2009,32(2-3):301-311

Dansgaard-Oeschger and Heinrich events are the most pronounced climatic changes over the last 120,000 years. Although many of their properties were derived from climate reconstructions, the associated physical mechanisms are not yet fully understood. These events are paced by a ~1,500-year periodicity whose origin remains unclear. In a conceptual model approach, we show that this millennial variability can originate from rectification of an external (solar) forcing, and suggest that the thermohaline circulation, through a threshold response, could be the rectifier. We argue that internal threshold response of the thermohaline circulation (THC) to solar forcing is more likely to produce the observed DO cycles than amplification of weak direct ~1,500-year forcing of unknown origin, by THC. One consequence of our concept is that the millennial variability is viewed as a derived mode without physical processes on its characteristic time scale. Rather, the mode results from the linear representation in the Fourier space of nonlinearly transformed fundamental modes.  相似文献   

Sensitivity of the thermohaline circulation in coupled oceanic GCM — atmospheric EBM experiments     

Gerrit Lohmann  Rüdiger Gerdes  Deliang Chen 《Climate Dynamics》1996,12(6):403-416

We analyze the sensitivity of the oceanic thermohaline circulation (THC) regarding perturbations in fresh water flux for a range of coupled oceanic general circulation — atmospheric energy balance models. The energy balance model (EBM) predicts surface air temperature and fresh water flux and contains the feedbacks due to meridional transports of sensible and latent heat. In the coupled system we examine a negative perturbation in run-off into the southern ocean and analyze the role of changed atmospheric heat transports and fresh water flux. With mixed boundary conditions (fixed air temperature and fixed surface fresh water fluxes) the response is characterized by a completely different oceanic heat transport than in the reference case. On the other hand, the surface heat flux remains roughly constant when the air temperature can adjust in a model where no anomalous atmospheric transports are allowed. This gives an artificially stable system with nearly unchanged oceanic heat transport. However, if meridional heat transports in the atmosphere are included, the sensitivity of the system lies between the two extreme cases. We find that changes in fresh water flux are unimportant for the THC in the coupled system.  相似文献   

The atmospheric sulfur cycle in ECHAM-4 and its impact on the shortwave radiation   总被引：5，自引：0，他引：5  

J. Feichter  U. Lohmann  I. Schult 《Climate Dynamics》1997,13(4):235-246

 The atmospheric general circulation model ECHAM-4 is coupled to a chemistry model to calculate sulfate mass distribution and the radiative forcing due to sulfate aerosol particles. The model simulates the main components of the hydrological cycle and, hence, it allows an explicit treatment of cloud transformation processes and precipitation scavenging. Two experiments are performed, one with pre-industrial and one with present-day sulfur emissions. In the pre-industrial emission scenario SO₂ is oxidized faster to sulfate and the in-cloud oxidation via the reaction with ozone is more important than in the present-day scenario. The atmospheric sulfate mass due to anthropogenic emissions is estimated as 0.38 Tg sulfur. The radiative forcing due to anthropogenic sulfate aerosols is calculated diagnostically. The backscattering of shortwave radiation (direct effect) as well as the impact of sulfate aerosols on the cloud albedo (indirect effect) is estimated. The model predicts a direct forcing of −0.35 W m^-2 and an indirect forcing of −0.76 W m^-2. Over the continents of the Northern Hemisphere the direct forcing amounts to −0.64 W m^-2. The geographical distribution of the direct and indirect effect is very different. Whereas the direct forcing is strongest over highly polluted continental regions, the indirect forcing over sea exceeds that over land. It is shown that forcing estimates based on monthly averages rather than on instantaneous sulfate pattern overestimate the indirect effect but have little effect on the direct forcing. Received: 16 October 1996/Accepted: 24 October 1996  相似文献   

Reconstructing estuarine conditions: oyster shells as recorders of environmental change, Southwest Florida   总被引：1，自引：0，他引：1  

Donna M. Surge  Kyger C Lohmann  Glenn A. Goodfriend   《Estuarine, Coastal and Shelf Science》2003,57(5-6):737-756

Live-collected shells of the oyster, Crassostrea virginica, contain geochemical records of modern temperature and salinity, so records of prehistoric conditions may be obtained from subfossil shells. Restoration of channelized watersheds in Florida is receiving much attention, and plans for targeted watersheds require information about estuarine conditions before channelization. Lack of historical records necessitates alternative methods to understand pre-disturbance conditions. A ¹⁴C-calibrated, amino-acid geochronology based on racemization of glutamic acid yielded ages ranging from 190–1220 AD and from 1270–1860 AD for subfossil oysters from Blackwater River (near-natural watershed) and for Faka-Union Bay (channelized watershed), respectively. δ¹⁸O and δ¹³C values of subfossil shells from Blackwater River indicate salinity and summer temperatures similar to present. Winter temperatures recorded in shells from 190, 590, 720, and 1050 AD appear 1–5 °C colder than present winter temperatures, whereas the shell from 1220 AD records winter temperatures similar to modern winter temperatures. These temperature shifts may indicate change in climate or natural seasonal variation of winter temperature from year to year. Subfossils from Faka-Union Bay may reflect a complicated hydrology, which cannot be evaluated by isotopic compositions alone and demonstrates the need for development of independent elemental proxies for temperature and salinity. Decreases in δ¹³C from subfossil to modern shells may in part result from CO₂ added to the atmosphere from fossil fuel burning (the Suess effect). Subfossil δ¹³C that is >1‰ more positive than modern shells suggest a change in the dominant carbon sources from terrestrial C₄ or aquatic plants to C₃ plants (mangroves).  相似文献   

The Baltic Sea Experiment BALTEX: A brief overview and some selected results of the authors     

Ehrhard Raschke  Ute Karstens  Ralph Nolte-Holube  Rüdiger Brandt  Hans-Jörg Isemer  Dag Lohmann  Manfred Lobmeyr  Burkhardt Rockel  Rolf Stuhlmann 《Surveys in Geophysics》1998,19(1):1-22

The mechanisms responsible for the transfer of energy and water within the climate system are under worldwide investigation within the framework of the Global Energy and Water Cycle Experiment (GEWEX) to improve the predictability of natural and man-made climate changes at short and long ranges and their impact on water resources. Five continental-scale experiments have been established within GEWEX to enable a more complete coupling between atmospheric and hydrological models. One of them is the Baltic Sea Experiment (BALTEX).In this paper, the goals and structure of BALTEX are outlined. A short overview of measuring and modelling strategies is given. Atmospheric and hydrological model results of the authors only are presented. These include also the validation of precipitation using station measurements as well as validation of modelled cloud cover with cloud estimates from satellite data. Furthermore, results of a large-scale grid based hydrological model to be coupled to atmospheric models are presented.This research has never been possible without the contribution of research groups and operational institutions from all 10 member countries. We concentrate here on results obtained at the GKSS research center.  相似文献   

Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin     

Charles N. Alpers  David L. Dettman  Kyger C. Lohmann  Dragan Brabec 《Journal of Geochemical Exploration》1990,38(1-2)

Stable isotope ratios of oxygen and carbon were determined for CO₂ in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO₂ concentrations detected previously by McCarthy et al. (1986). Values of δ¹³C in soil gas CO₂ from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ¹³C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C₃ (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C₃ and C₄ (Hatch-Slack) plants, together with atmospheric CO₂ are the likely sources of carbon in soil gas CO₂. Values of δ¹⁸O in soil-gas CO₂ range from 32 to 38‰ (SMOW). These δ¹⁸O values are intermediate between that calculated for CO₂ gas in isotopic equilibrium with local groundwaters and that for atmospheric CO₂. The δ¹⁸O data indicate that atmospheric CO₂ has been incorporated by mixing or diffusion. Any CO₂ generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO₂ taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ¹³C value of soil-gas CO₂ was observed, contrary to what would be expected if the anomalous CO₂ were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C₃ plant material were responsible for most CO₂ generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO₂ anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO₂ concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.  相似文献   

Stability of the glacial thermohaline circulation and its dependence on the background hydrological cycle     

V.?RomanovaEmail author  M.?Prange  G.?Lohmann 《Climate Dynamics》2004,22(5):527-538

Different reconstructions of glacial sea-surface temperatures (SST) are used to force a hybrid coupled atmosphere–ocean model. The resulting glacial states differ in global salinity and temperature distributions, and consequently in the strength of the thermohaline circulation. Stability analysis of the Atlantic Ocean circulation, by means of freshwater-flux hysteresis maps, reveals mono-stability for each glacial background state, which appears to be a robust feature of the glacial ocean. We show that this behaviour is directly linked to the hydrological cycle. A monotonic relation between the freshwater input necessary for reaching the off-mode and the hydrological budget in the Atlantic catchment area, accounts for the sensitivity of the oceans circulation. The most sensitive part of the hydrological balance appears to be in the tropical and subtropical regions suggesting that the Achilles heel of the global conveyor belt circulation is not restricted to the northern North Atlantic where convection occurs.  相似文献   

Die verteilung der Lichtschwachen Sterne im Offenen Sternhaufen NGC 2477     

Werner Lohmann 《Astrophysics and Space Science》1973,24(2):531-533

The relative frequency of the fainter stars increases from the centre of the cluster to the boundary.  相似文献