Climate Modelling Group
School of Earth and Ocean Sciences

PI PROJECT DESCRIPTION



TITLE: Paleoclimatic experiments using coupled atmosphere ocean models.

PI: Andrew J. Weaver -- School of Earth and Ocean Sciences, University of Victoria

CO-I: Andrew Bush -- Department of Earth and Atmospheric Sciences, University of Alberta

Summary: To date there have been virtually no paleoclimatic experiments conducted with fully coupled atmosphere-ocean models. In this project it is proposed to initiate research in this direction through the use of a locally developed energy/moisture balance atmosphere model which has been coupled to thermodynamic ice and global ocean models, and the Geophysical Fluid Dynamics Laboratory (GFDL) coupled atmosphere ocean general circulation model will be used for a suite of parallel experiments. A first step of this research will be to use these coupled models to investigate the climate of the Last Glacial Maximum (LGM -- 21 KBP), the mid Holocene warm period (6 KBP) and the competing effects of CO2 and orbital forcing. In addition, during phase II of the CSHD project, a continental ice sheet model will be incorporated into the UVic model in collaboration with the Clarke group at UBC and the Peltier group at the University of Toronto. This will be done in order to try and capture the growth and decay of continental ice sheets within a model of the Earth's climate system. In collaboration with the Fung group at the University of Victoria, a carbon cycle model will also be included in the UVic model as will appropriate vegetation albedos for the LGM and 6 KBP periods. The ultimate technical goal of this project is to develop a fully coupled model of the climate system that is sufficiently simple to allow for the running of numerous sensitivity experiments, yet sufficiently realistic that the results from the coupled model will give a quantitative understanding of the various feedbacks and processes at work in recent Earth's history. The ultimate scientific goal of this project is to use the UVic and GFDL coupled models to understand the climate and climate feedbacks of the LGM, the Holocene and the transition between the two.

Project Description

The last glacial maximum (LGM) around 21 KBP has been one of the most studied periods in Earth history. Early CLIMAP (1976, 1981) attempts to reconstruct sea surface temperature (SST) have suggested that relative to the present, LGM global SSTs were on average 1.7oC cooler in August and 1.4oC cooler in February. These reconstructions further suggested that tropical SSTs in the LGM were similar to those of the present climate whereas in the North Atlantic, SSTs were up to 10o - 12oC colder in places. While more recent evidence (Sikes and Keigwin, 1994) corroborate the CLIMAP findings that tropical SSTs were only slightly cooler at the LGM compared to the present, additional evidence is contradictory. For example, recent evidence from coral records from Barbados (Guilderson et al., 1994) and the southwest Pacific (Beck et al., 1997), ice core records from Peru (Thompson et al., 1995), noble gas measurements in Brazil (Stute et al., 1995), ocean core records from the western equatorial Atlantic (Curry and Oppo, 1197) and the Indian Ocean (Bard et al., 1997) suggest LGM Atlantic tropical temperatures were significantly below the present.

Previous modelling studies of the LGM have fallen into two categories. In the first, ocean-only models have been used in which surface forcing consists of restoring temperature and salinity to LGM reconstructions with specified AGCM LGM surface wind stress fields (Fichefet et al., 1994; Seidov et al., 1996; Winguth et al., 1996). The second class of modelling studies involves the integration of atmospheric general circulation models (AGCMs) with either fixed sea surface temperature or mixed layer ocean models at the lower boundary (Gates, 1976; Hansen et al., 1984; Manabe and Broccoli, 1985; Kutzbach et al., 1986; Rind, 1987; Lautenschlager and Herterich, 1990; Hall et al., 1996; Crowley and Baum, 1997) . Most of these modelling studies have found tropical air/sea surface temperatures along the lines of those of CLIMAP, although this simply reflects the fact that CLIMAP data was used as a boundary condition. Webb et al. (1997) on the other hand were able to obtain relatively cool subtropical SST in their LGM simulation using a version of the NASA-GISS model in which present day oceanic heat transports were maintained. They argued that reduced evaporation in the subtropics with present day ocean heat transport lead to reduced water vapour, and hence outgoing longwave radiation absorption in the atmosphere. This, cooling mechanism, they argued, was amplified by other processes in the coupled system. Based on our understanding of ocean circulation, however it is unclear how such a present day oceanic heat transport could have been maintained in the LGM when the radiative forcing was dramatically different from the present day.

To date it has not been possible to run fully coupled atmosphere-ocean GCM experiments for LGM scenarios due to the necessity of applying flux adjustments to keep the present-day climate stable. The use of flux adjustments is only valid for small perturbations away from the present climate, whereas the LGM clearly represents a large perturbation. In the present project we will examine the climate of the LGM using two parallel approaches. In the first approach, a fully coupled model will be used which does not need to employ flux adjustments (Fanning and Weaver, 1997). The atmospheric component of this coupled model is relatively simple in that it consists of energy and moisture balance equations and a parameterization for wind feedbacks. It is to this coupled model that we will couple a continental ice sheet model in collaboration with the Clarke group at UBC and the Peltier group at the University of Toronto. Similarly, a carbon model will be incorporated into this model in collaboration with the Fung group at the University of Victoria. This fully coupled numerical laboratory will then be used to investigate the role of orbital versus CO2 forcing of the LGM climate. In addition, we will be able to examine various feedbacks (e.g., water vapour, albedo, ocean heat transport) on the LGM response. We also hope to shed light on the debate regarding the ocean surface temperatures in the LGM.

In a parallel approach, the GFDL coupled model will be used to investigate similar problems and feedbacks in the LGM climate with an emphasis on the atmospheric response rather than the oceanic response. The GFDL model will be integrated in a fully coupled mode without flux adjustments for several hundred years. This time scale for spin up will allow the surface of the ocean to adjust, although the deep ocean will not have equilibrated. It is not possible to integrate this model to a full equilibrium due to both computational constraints and the necessity of using flux adjustments. Once more, results from this model will be compared to observational data and with the results from the UVic coupled model. The atmospheric component of the model will also be run with SST anomalies specified from the results of the UVic coupled model. This will allow us to explore the atmospheric response in more detail and with greater realism.

At 6000 years before present (BP), the continental ice volume was approximately equivalent to that of today. It is therefore thought that the only major climatic forcing factor which was different from present was the insolation via different orbital parameters. Numerical simulations of this period have been performed by a number of different atmospheric GCMs. While the models correctly predict a stronger south Asian monsoon and increased precipitation along the southern margin of the Sahara Desert, none of the simulations capture the extent of Saharan rainfall that has been inferred from lake level data (Wright et al., 1993). The fact that all models fail in this regard -- not only for 6 KBP but also for 9 KBP when the insolation differences were magnified -- suggests that we are neglecting a significant forcing factor by changing only the orbital parameters.

Atmospheric simulations of early Holocene climate have traditionally been forced with present day SSTs. Geoarcheological evidence, however, suggest the possibility that during the warm period of the early Holocene (11K-5K BP) eastern tropical Pacific SST were significantly warmer than they are today (Sandweiss et al., 1996). The data indicate that at that time El Niño conditions were permanent, in contrast to the interannual occurrence of the modern El Niño. While it is unclear just how great the spatial extent of the SST warming was (DeVries et al., 1997), additional evidence from Peruvian ice cores (Thompson et al., 1995) indicate a moist and tropical climate where desert conditions exist today. The desertification of coastal Peru is a consequence of the La Niña cycle of the Southern Oscillation, suggesting that the ice core evidence supports the notion of a permanent El Niño state. However, the south Asian monsoon is known to weaken during modern El Niño events. Yet convincing evidence from early Holocene upwelling indices off the Somali coast indicate a much stronger monsoon circulation (Prell, 1984). Through the use of the GFDL coupled model we will address the question as to what is the relative impact of SST forcing versus orbital forcing in determining the strength of the Asian monsoon and whether a simulation of the early Holocene which prescribes El Niño SST delivers a climate that is more consistent with all the available proxy data. Given that the El Niño phase of the Southern Oscillation impacts the global climate in a profound way through flooding, drought, and severe weather, the existence of a permanent El Niño would have important implications for the climate of the early Holocene. Two approaches will be used. In the first, the GFDL AGCM will be used with prescribed SSTs. In the second, SST anomalies will be derived from the equilibrium climate of the UVic coupled model, driven by 6 KBP orbital forcing, and used as a lower boundary condition for the GFDL AGCM.

Studies of tropical SST variations have thus far focused on the El Niño/Southern Oscillation phenomenon. The atmosphere-ocean interactions that are at the heart of this phenomenon involve an essentially adiabatic, horizontal redistribution of warm surface waters within the tropics (Philander, 1985). This redistribution leaves the mean depth of the tropical thermocline unchanged. To address the questions as to what processes determine that mean depth and under what conditions this mean depth can be increased (a state of affairs that favours a permanent El Niño) or decreased (something that probably happened during the LGM), we will explore the processes that maintain the sharp, shallow tropical thermocline.

In summary, a hierarchy of numerical experiments will be conducted in order to address the above issues. Atmosphere-only simulations using the GFDL AGCM will be undertaken in an attempt to determine the relative roles of SST forcing versus orbital forcing on early Holocene climate. These simulations will be configured for 6 KBP and 21 KBP. In this way we will determine whether permanent El Niño SST produces an early Holocene climate that is more in accord with the proxy data than a climate forced by modern SST. The second stage of experiments will involve an investigation into the impact of changing orbital parameters and atmospheric CO2 on the coupled atmosphere-ocean system -- an impact which is entirely unknown at this point. This second stage will involve the use of a fully coupled atmosphere-ocean GCMs as detailed above. Emphasis will be placed on the dynamics which maintain the depth of the tropical thermocline and how orbital parameters and radiative forcing associated with CO2 changes affect those dynamics. Given the controversy surrounding tropical SST during glacial periods, we intend to explore not only early Holocene climates with the coupled model but also the climate of the Last Glacial Maximum. The answers to these questions will provide an important contribution to our knowledge of the evolution of our atmosphere-ocean system.

Two final components of this research project are to: 1)-- examine the transition from the LGM to the Holocene using the UVic coupled model into which continental ice sheet and carbon models have been incorporated. This involves the integration of the entire coupled system from 21KBP to 6KBP (a task within the capabilities of the PIs computing resources) under changing orbital forcing. 2)-- examine (using the UVic coupled model into which a continental ice sheet model has been incorporated) whether or not the onset of northern hemisphere glaciation can be attributed to the closure of the Isthmus of Panama ~ 3.2 million years ago. These two projects must be viewed as extremely ambitious and high risk. If successful, the knowledge gained from these two projects would be extremely fundamental and of utmost importance.

Percentage of time for CSHD:

Weaver 50%
Bush 50%

Note: Weaver will be on a Steacie fellowship for the 1998-1999 academic year and on sabbatical (in Victoria) during the 1999-2000 academic year. As such, he will be able to spend 100% of his time during these first two years on research.

Graduate student training: Weaver is currently supervising 4 PhD, 4 MSc students and 5 Research Associates. Two PhD students will work within this project (Masakazu Yoshimori and Olaf Dravnieks) on the ice sheet/carbon cycle model implementation and sensitivity analysis, respectively. In addition 1 MSc student (Daithi Stone) and one Research Associate (Simon Evans) will work in this project. The 3 students mentioned are in their first year of graduate school and so are taking courses. They will start research in May 1998. The research associate will join the group in early 1998. Bush will take on two new graduate students as part of this project. We envision that two of the five students listed above (one based at UVic and one at U of A) will spend some of their research time at both institutions.

Relationship of existing funds, especially NSERC operating grants, to this project: The research proposed here has not been proposed in any other proposal written by the PI. The PIs main research activities have been in the area of climate/climate variability and he has only recently moved into the field of paleoclimate. His NSERC Strategic, Operating and Steacie Grants are supporting research in the area of understanding the present-day climate or the paleoclimatic eras of the Cretaceous and the Ordovician. these later two periods are not of direct interest to the CSHD project. Some of the work describing attempts at understanding the processes that maintain the sharp shallow thermocline will be supported by the CO-I's NSERC Operating Grant.

Level of interest in other global change projects: The PI is extensively involved in, has sat on a number national and international committees and advisory boards of, and invited to give numerous plenary/scientific lectures to WOCE, CLIVAR, ACSYS and IPCC projects.

References

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Crowley, T.J. & Baum, S.K. J. Geophys. Res. 102, 16463-16480 (1997).
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DeVries, T.J., et al., Science 276, 965 (1997).
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