CGD's Dr. Bette Otto-Bliesner

Abstract

This special issue grew out of three sessions at the 2003 American Geophysical Union Fall Meeting: "The Last Interglacial I", chaired by Bette L. Otto-Bliesner and Gifford H. Miller; "The Last Interglacial II", chaired by Meixun Zhao; and "Climate of the Last Glacial-Interglacial Cycle: New Insights From Models and Data", chaired by Noah S. Diffenbaugh and C. Mark Eakin. These sessions were comprised of oral and poster presentations reporting on recent study of the dynamics shaping glacial and interglacial climates of the past 160,000 years. Specifically, as the titles suggest, the first two sessions focused on the climate of the last interglacial, while the last focused on the climate of the last glacial and the current interglacial.

Support: National Science Foundation.

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Ammann, C.M., F. Joos, D.S. Schimel, B.L. Otto-Bliesner, and R.A. Tomas, 2007: Solar influence on climate during the past millennium: results from transient simulations with the NCAR Climate System Model, Proc. National Academy Sci., 104, 3713-3718.

Abstract

The potential role of solar variations in modulating recent climate has been debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Because solar variability before the satellite period must be scaled from proxy data, large uncertainty exists about phase and magnitude of the forcing. We used a coupled climate system model to determine whether proxy-based irradiance series are capable of inducing climatic variations that resemble variations found in climate reconstructions, and if part of the previously estimated large range of past solar irradiance changes could be excluded. Transient simulations, covering the published range of solar irradiance estimates, were integrated from 850 AD to the present. Solar forcing as well as volcanic and anthropogenic forcing are detectable in the model results despite internal variability. The resulting climates are generally consistent with temperature reconstructions. Smaller, rather than larger, long-term trends in solar irradiance appear more plausible and produced modeled climates in better agreement with the range of Northern Hemisphere temperature proxy records both with respect to phase and magnitude. Despite the direct response of the model to solar forcing, even large solar irradiance change combined with realistic volcanic forcing over past centuries could not explain the late 20th century warming without inclusion of greenhouse gas forcing. Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century

Figure caption: Comparison of NCAR CSM simulations with proxy reconstructions and instrumental data. (a) Reconstructed NH average surface temperature anomalies over the past millennium. All series are as published originally and no additional scaling has been performed, but annual records have been smoothed with a 50-year-long Gaussian filter. All series are relative to 1901-1960 averages computed from original data. (b) Northern hemisphere surface temperature from the low- (green), medium- (red), and high-scaled (blue) solar forcing simulations compared with the range spanned by the annual proxy-based reconstructions. This range does not include a systematic error analysis, it only illustrates the current debate regarding the amplitude of hemispheric multidecadal to century-scale temperature variations of the past. (c) Simulated versus the instrumental (gray) record of global average surface temperature (gray, thick solid line). The time series of the low (green), medium (red), and high (blue) solar forcing experiments were smoothed by using an 11-year Gaussian filter. Anthropogenic forcings were included in the primary experiments (solid lines) but held at 1870 AD conditions in 1870-2000 AD branch experiments (dashed lines). See publication for references.

Support: National Science Foundation.

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Weber, S.L., S. S. Drijfhout, A. Abe-Ouchi, M. Crucifix, M. Eby, A. Ganopolski, S. Murakami, B. Otto-Bliesner, and W. R. Peltier, 2007: The modern and glacial overturning circulation in the Atlantic ocean in PMIP coupled model simulations. Climate of the Past, 3, 51-64.

Abstract

This study analyses the response of the Atlantic meridional overturning circulation (AMOC) to LGM forcings and boundary conditions in nine PMIP coupled model simulations, including both GCMs and Earth system Models of Intermediate Complexity. Model results differ widely. The AMOC slows down considerably (by 20-40%) during the LGM as compared to the modern climate in four models, there is a slight reduction in one model and four models show a substantial increase in AMOC strength (by 10-40%). It is found that a major controlling factor for the AMOC response is the density contrast between Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) at their source regions. Changes in the density contrast are determined by the opposing effects of changes in temperature and salinity, with more saline AABW as compared to NADW consistently found in all models and less cooling of AABW in all models but one. In only two models is the AMOC response during the LGM directly related to the response in net evaporation over the Atlantic basin. Most models show large changes in the ocean freshwater transports into the basin, but this does not seem to affect the AMOC response. Finally, there is some dependence on the accuracy of the control state.

Figure caption: Analysis of processes within the Atlantic basin and those originating in the Southern Ocean for the PMIP2 (circles) and PMIP1.5 (squares) simulations: CCSM (green), Hadl2 (black), MIROC (red), ECBilt (dark blue), UVic (purple), Hadl1.5 (black), MRI (light blue), UTor (dark blue) and ClimC (purple). (Top left) The response in AMOC strength versus that in the Atlantic density contrast at the southern end of the Atlantic basin versus that at the northern end. (Bottom left) The response in AMOC strength versus that in the density contrast between AABW at its source region and NADW at its source region. (Top right) The response in AMOC strength versus that in net evaporation over the Atlantic basin. (Bottom right) The response in AMOC strength versus the strength of the deep reversed cell. Densities are given in kg/m3 (relative to a reference value of 1000), circulation strengths and evaporation in Sv.

Support: National Science Foundation.

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Otto-Bliesner, B.L., C.D. Hewitt, T.M. Marchitto, E. Brady, A. Abe-Ouchi, M. Crucifix, S. Murakami, and S.L. Weber, 2007: Last Glacial Maximum ocean thermohaline circulation: PMIP2model intercomparisons and data constraints. Geophysical Research Letters, 34, L12706, doi:10.1029/2007GL029475.

Abstract

The ocean thermohaline circulation is important for transports of heat and the carbon cycle. We present results from PMIP2 coupled atmosphere-ocean simulations with four climate models that are also being used for future assessments. These models give very different glacial thermohaline circulations even with comparable circulations for present. An integrated approach using results from these simulations for Last Glacial Maximum (LGM) with proxies of the state of the glacial surface and deep Atlantic supports the interpretation from nutrient tracers that the boundary between North Atlantic Deep Water and Antarctic Bottom Water was much shallower during this period. There is less constraint from this integrated reconstruction regarding the strength of the LGM North Atlantic overturning circulation, although together they suggest that it was neither appreciably stronger nor weaker than modern. Two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing.

Figure caption: Atlantic Ocean meridional overturning circulations (Sv) simulated by the PMIP2 coupled atmosphere-ocean models for (top) modern and (bottom) Last Glacial Maximum.

Support: National Science Foundation.

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Braconnot, P., B. Otto-Bliesner, S. Harrison, Abe-Ouchi, M. Crucifix, C. D. Hewitt, M. Kageyama, A. Kitoh, O. Marti, U. Merkel, T. Motoi, R. Ohgaito , W. R. Peltier, J. Valdes, L. Weber, Y. Zhao, 2007: Results of the PMIP2 coupled simulations of the mid-Holocene and Last Glacial Maximum. Part 1: experiments and large scale features. Climate of the Past, 3, 261-277.

Abstract

A set of coupled ocean-atmosphere simulations using state of the art climate models is now available for the Last Glacial Maximum and the Mid-Holocene through the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP2). This study presents the large-scale features of the simulated climates and compares the new model results to those of the atmospheric models from the first phase of the PMIP, for which sea surface temperature was prescribed or computed using simple slab ocean formulations. We consider the large-scale features of the climate change, pointing out some of the major differences between the different sets of experiments. We show in particular that systematic differences between PMIP1 and PMIP2 simulations are due to the interactive ocean, such as the amplification of the African monsoon at the Mid-Holocene or the change in precipitation in mid-latitudes at the LGM. Also the PMIP2 simulations are in general in better agreement with data than PMIP1 simulations.

Figure caption: Annual mean tropical cooling (°C) at the last glacial maximum: comparison between model results and palaeo-data. (Centre panel) simulated surface air temperature changes over land are displayed as a function of surface temperature changes over the oceans, both averaged in the 30°S to 30°N latitudinal band, for all the PMIP 2 OA simulations (color) and the PMIP1 simulation (grey) The comparison with palaeo-data uses two reconstructions: (upper panel) over land the distribution of temperature change is estimates from various pollen data from (Farrera et al., 1999); (right panel) over the ocean the distribution of SST is estimated from alkenones in the tropics (Rosell-Melé et al., 1998) Caution: in this figure, model results are averaged over the whole tropical domain and not over proxy-data locations, which may bias the comparison (e.g. Broccoli and Marciniak, 1996). See publication for references

Support: National Science Foundation.

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Braconnot, P., B. Otto-Bliesner, S. Harrison, Abe-Ouchi, M. Crucifix, C. D. Hewitt, M. Kageyama, A. Kitoh, O. Marti, U. Merkel, T. Motoi, R. Ohgaito , W. R. Peltier, J. Valdes, L. Weber, Y. Zhao, 2007: Results of the PMIP2 coupled simulations of the mid-Holocene and Last Glacial Maximum. Part 2: feedbacks with emphasis on the location of the ITCZ and mid- and high-latitude heat budgets. Climate of the Past, 3, 279-296.

Abstract

A set of coupled ocean-atmosphere(-vegetation) simulations using state of the art climate models is now available for the Last Glacial Maximum (LGM) and the Mid-Holocene (MH) through the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP2). Here we quantify the latitudinal shift of the location of the Intertropical Convergence Zone (ITCZ) in the tropical regions during boreal summer and the change in precipitation in the northern part of the ITCZ. For both periods the shift is more pronounced over the continents and East Asia. The maritime continent is the region where the largest spread is found between models. We also clearly establish that the larger the increase in the meridional temperature gradient in the tropical Atlantic during summer at the MH, the larger the change in precipitation over West Africa. The vegetation feedback is however not as large as found in previous studies, probably due to model differences in the control simulation. Finally, we show that the feedback from snow and sea-ice at mid and high latitudes contributes for half of the cooling in the Northern Hemisphere for the LGM, with the remaining being achieved by the reduced CO2 and water vapour in the atmosphere. For the MH the snow and albedo feedbacks strengthen the spring cooling and enhance the boreal summer warming, whereas water vapour reinforces the late summer warming. These feedbacks are modest in the Southern Hemisphere. For the LGM most of the surface cooling is due to CO2 and water vapour.

Figure caption: (a) Surface albedo (ratio) of the control simulation averaged from 20°W to 30°E and plotted as a function of latitude. The different color lines correspond to the PMIP2 OA simulations. The dotted lines correspond to the set of PMIP1 SSTf simulations. (b) Relationship between the mean surface albedo (%) of the region extending from 18°N to 26°N and 20°W to 30°E and the location of the mean core of the ITCZ (degrees of latitude) over West Africa averaged from 20°W to 30°E and diagnosed following the definition of Sect. 2.1 from the pre-industrial simulations.

Support: National Science Foundation.

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National Research Council, 2006: Surface temperature reconstructions for the last 2,000 years. The National Academies Press, Washington, D.C., 141 pp.

Because widespread, reliable instrumental records are available only for the last 150 years or so, scientists estimate climatic conditions in the more distant past by analyzing proxy evidence from sources such as tree rings, corals, ocean and lake sediments, cave deposits, ice cores, boreholes, glaciers, and documentary evidence. For example, records of Alpine glacier length, some of which are derived from paintings and other documentary sources, have been used to reconstruct the time series of surface temperature variations in south-central Europe for the last several centuries. Studying past climates can help us put the 20th century warming into a broader context, better understand the climate system, and improve projections of future climate.

Support: National Science Foundation.

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Jansen, E., J. Overpeck, K.R. Briffa, J.-C. Duplessy, F. Joos, V. Masson-Delmotte, D.O. Olago, B. Otto-Bliesner, W.R. Peltier, S. Rahmstorf, R. Ramesh, D. Raynaud, D.H. Rind, O. Solomina, R. Villalba, D. Zhang, 2007: Palaeoclimate. In: Climate Change 2007: The Physical ScienceBasis. Contribution of Working Group I to the Fourth Assessment Report of the IntergovernmentalPpanel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (eds.)]. Cambridge University press, Cambridge, United Kingdom and New York, NY, USA, 433-497.

This chapter assesses palaeoclimatic data and knowledge of how the climate system changes over interannual to millennial time scales, and how well these variations can be simulated with climate models. Additional palaeoclimatic perspectives are included in other chapters.

Palaeoclimate science has made significant advances since the 1970s, when a primary focus was on the origin of the ice ages, the possibility of an imminent future ice age, and the first explorations of the so-called Little Ice Age and Medieval Warm Period. Even in the first IPCC assessment (IPCC, 1990), many climatic variations prior to the instrumental record were not that well known or understood. Fifteen years later, understanding is much improved, more quantitative and better integrated with respect to observations and modelling.

Figure caption: The Last Glacial Maximum climate (approximately 21 ka) relative to the pre-industrial (1750) climate. (Top left) Global annual mean radiative influences (W m^-2) of LGM climate change agents, generally feedbacks in glacial-interglacial cycles, but also specified in most Atmosphere-Ocean General Circulation Model (AOGCM) simulations for the LGM. The heights of the rectangular bars denote best estimate values guided by published values of the climate change agents and conversion to radiative perturbations using simplified expressions for the greenhouse gas concentrations and model calculations for the ice sheets, vegetation and mineral dust. References are included in the text. A judgment of each estimate's reliability is given as a level of scientific understanding based on uncertainties in the climate change agents and physical understanding of their radiative effects. Paleoclimate Modelling Intercomparison Project 2 (PMIP-2) simulations shown in bottom left and right panels do not include the radiative influences of LGM changes in mineral dust or vegetation. (Bottom left) Multi-model average SST change for LGM PMIP-2 simulations by five AOGCMs (Community Climate System Model (CCSM), Flexible Global Ocean-Atmosphere-Land System (FGOALS), Hadley Centre Coupled Model (HadCM), Institut Pierre Simon Laplace Climate System Model (IPSL-CM), Model for Interdisciplinary Research on Climate (MIROC)). Ice extent over continents is shown in white. (Right) LGM regional cooling compared to LGM global cooling as simulated in PMIP-2, with AOGCM results shown as red circles and EMIC (ECBilt-CLIO) results shown as blue circles. Regional averages are defined as: Antarctica, annual for inland ice cores; tropical Indian Ocean, annual for 15°S to 15°N, 50°E to 100°E; and North Atlantic Ocean, July to September for 42°N to 57°N, 35°W to 20°E. Grey shading indicates the range of observed proxy estimates of regional cooling: Antarctica (Stenni et al., 2001; Masson-Delmotte et al., 2006), tropical Indian Ocean (Rosell-Mele et al., 2004; Barrows and Juggins, 2005), and North Atlantic Ocean (Rosell-Mele et al., 2004; Kucera et al., 2005; de Vernal et al., 2006; Kageyama et al., 2006). See IPCC report for references.

Support: National Science Foundation.

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Title: A Numerical Study of the South Atlantic circulation at the Last Glacial Maximum. Authors: Gabriel Clauzet, I. Wainer, A. Lazar, E. Brady, B Otto-Bliesner. Palaeogeography, Palaeoclimatology, Palaeoecology, 253 (2007) 509-528.

In this study,we examine the simulation results from the paleoclimate version of the National Center of Atmospheric Research coupled Climate System Model (CSM 1.4) for the Last Glacial Maximum (LGM) in order to understand changes in the South Atlantic (SA) circulation relative to the Present Day (PD). The LGM simulation is validated with the available proxy data in the region. The results show good agreement, except in the eastern equatorial and eastern SA region, where the model is not able to reproduce the correct cloud cover and the associated air-sea interactions. Ocean transport in the PD simulation is in good agreement with observational estimates. Results show that at subsurface levels there are two distinct patterns: (i) strengthening of the transport for the LGM in the southern SA (35°S to 25°S); and (ii) weakening of the mass transport in the northern SA(25°S to the Equator). In intermediate layers, there is an intensification of the subtropical gyre and a northward shift of the South Equatorial Current (SEC) bifurcation for the LGM. This leads to the intensification of the southward transport by the Brazil Current (BC) and the associated BC recirculation cell in the southern basin for the LGM. This shift in the position of the SEC bifurcation leads to a weakening in the northward transport and the western recirculation of the central SEC in the northern basin. This northward shift of the SEC (upper limit of the subtropical gyre) is consistent with the northward shift observed in the subtropical convergence zone and suggests a displacement of the sub-tropical gyre 3°-5° towards the Equator. In deeper layers, a shallower and weaker North Atlantic Deep Water (NADW) circulation in the LGM contributes to the reduction of the southward transport in the northern part of the basin and is associated with a greater northward intrusion of Antarctic Bottom Water. This intrusion plus the increase of the Indian Water inflow is responsible for the northward transport intensification in the southern basin.

Figure caption: (Top) The Atlantic Ocean cross section of the zonally-averaged of salinity (psu) (left panels) and temperature (°C) (right panels). The black line delimits the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW): (a)-(c) Present Day; (b)-(d) LGM.

Support: NCAR and in part by grants FAPESP-00/02958-7, FAPESP 01/04920-0, CNPq 300223/93-5 and CNPq 300040/94-6.