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- 1. Exploring atmospheric, Earth system, and solar processes, variability, and change
- 2. Investigating the interaction of the atmosphere, the broader Earth system, and human society
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- Research Catalog
CGD 2008 Profiles in Science: Dr. Bette Otto-Bliesner
Summary of achievements
Bette Otto-Bliesner
Building on successful "snapshot" simulations with CCSM, Bette Otto-Bliesner's glacial-interglacial simulations are now focusing on transient climate changes. In 2008, Bette and Esther Brady (CGD/CCR), Liu, Carlson, and He (University of Wisconsin), Clark (Oregon State), Jacobs (Argonne), and Erickson (Oak Ridge) continued using a large computing grant from DOE INCITE to run a set of synchronously coupled transient ocean-atmosphere-dynamic vegetation CCSM3 simulations of the past 21,000 years (TraCE-21). This simulation, which is now at 14.5 ky BP, provides a strong test on CCSM for its climate sensitivity to various forcings, especially, the greenhouse forcing, as well as its capability for the simulation of abrupt climate changes. It marks a new era in paleoclimate model-data comparison by allowing for a direct comparison of time series between model and data. A series of CCSM3 simulations to provide a detailed look at the 8.2ka event, an abrupt climate change associated with anomalous freshwater flow into the North Atlantic, is also in progress with Morrill and Wagner (CIRES). Two workshops, SynTraCE-21000, are being convened by Otto-Bliesner and her collaborators to prepare a three-dimensional synthesis and database of the transient evolution of the Earth system over the last 21,000 years.
Bette Otto-Bliesner is also working on simulations that fit closely with current and future themes of the Paleoclimate Modeling Intercomparison Project (PMIP). With Nan Rosenbloom (CGD/CCR), a CAM/CLM simulation has been run for the mid-Pliocene (~3 myrs ago), possibly the closest paleo analog for the equilibrium climate with current CO2 levels. In addition, working with SOARS student Zi Zi Searles of San Francisco State University, sensitivity simulations have shown the importance of warm sea surface temperatures off the coast of California for correctly simulating proxy indicators of a wetter western US during the mid-Pliocene. A coupled carbon-climate simulation of the Last Glacial Maximum with ocean and land biogeochemistry, as a first step towards determining what controls glacial-interglacial variations in the greenhouse gases, is currently in the spinup phase. This is a joint project with Mahowald (Cornell), Doney (WHOI), Moore (UC-Irvine), Joos (Bern), and Lindsay (CGD/OCE).
Bette Otto-Bliesner is also organizing the PMIP2 Workshop, to be held 14-19 September in Estes Park, where over 70 international participants will discuss future model-data intercomparison projects of PMIP and develop a White Paper of proposed paleoclimate simulations for AR5.
Publications
Otto-Bliesner, B.L., and E.C. Brady, 2008: PMIP2 Climate Model-Proxy Data Intercomparisons for LGM. PAGES Newsletter, 16, 18-20.
Figure 1.
High resolution figure
Abstract: PMIP initially focused on two periods, the Last Glacial Maximum (LGM; ca. 21 cal kyr BP) and the mid-Holocene (MH; ca. 6 cal kyr BP). The experiments were designed to examine the climate response to Milankovitch orbital forcings for the MH and the presence of large ice sheets and low greenhouse gas (GHG) concentrations for the LGM. Seventeen modeling groups participated in simulations of these time periods with atmosphere-only models (PMIP1), and twelve groups in the second phase of the project (PMIP2) using ocean-atmosphere or ocean-atmosphere-vegetation models. With the incorporation of coupled atmosphere-ocean-sea ice models into PMIP2, new comparisons to proxy data can now be used in evaluating the capabilities of current climate models to simulate climate conditions different than present. Here, we describe two such com-parisons of the PMIP2 LGM simulations to glacial proxy data: deep-ocean tempera tures and salinities in the Atlantic Ocean, and sea ice extent around Antarctica.
Figure caption: Theta (potential temperature) and salinity for modern (open symbols) and LGM (filled symbols) estimated from data (black symbols with error bars) at ODP sites (Adkins et al., Science, 2002) and predicted by PMIP2 models. Site 981 (Δ) is located in the North Atlantic (Feni Drift, 55°N, 15°W, 2184 m). Site 1093 ( ) is located in the South Atlantic (Shona Rise, 50°S, 6°E, 3626 m). Only CCSM included a 1 psu adjustment of ocean salinity at initialization to account for fresh water frozen into LGM ice sheets; predicted salinities for the other models have been adjusted to allow comparison.
Hu, A., B.L. Otto-Bliesner, G.A. Meehl, W. Han, C. Morrill, E.C. Brady, and B. Briegleb, 2008: Response of Thermohaline Circulation to Freshwater Forcing under Present-Day and LGM Conditions. J. Climate, 21, 2239-2258.
Figure 2.
High resolution figure
Abstract: Responses of the thermohaline circulation (THC) to freshwater forcing (hosing) in the subpolar North Atlantic Ocean under present-day and the last glacial maximum (LGM) conditions are investigated using the National Center for Atmospheric Research Community Climate System Model versions 2 and 3. Three sets of simulations are analyzed, with each set including a control run and a freshwater hosing run. The first two sets are under present-day conditions with an open and closed Bering Strait. The third one is under LGM conditions, which has a closed Bering Strait. Results show that the THC nearly collapses in all three hosing runs when the freshwater forcing is turned on. The full recovery of the THC, however, is at least a century earlier in the open Bering Strait run than the closed Bering Strait and LGM runs. This is because the excessive freshwater is diverged almost equally toward north and south from the subpolar North Atlantic when the Bering Strait is open. A significant portion of the freshwater flowing northward into the Arctic exits into the North Pacific via a reversed Bering Strait Throughflow, which accelerates the THC recovery. When the Bering Strait is closed, this Arctic to Pacific transport is absent and freshwater can only be removed through the southern end of the North Atlantic. Together with the surface freshwater excess due to precipitation, evaporation, river runoff, and melting ice in the closed Bering Strait experiments after the hosing, the removal of the excessive freshwater takes longer, and this slows the recovery of the THC. Although the background conditions are quite different between the present-day closed Bering Strait run and the LGM run, the THC responds to the freshwater forcing added in the North Atlantic in a very similar manner.
Lee, J. E., I. Fung, D. J. DePaolo, and B. Otto-Bliesner, 2008: Water isotopes during the last glacial maximum: New GCM calculations. Geochimica et Cosmochimica Acta, 72, A525-A525.