Development of a reduced-complexity climate model and applications on glacial-interglacial timescales

Ritz, Stefan (2011). Development of a reduced-complexity climate model and applications on glacial-interglacial timescales (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Global climate models are very valuable tools in climate sciences and are therefore used in many areas. The applications differ depending on the complexity of the components represented in the model. Three-dimensional physical models of the ocean and the atmosphere permit the analysis of the present-day and past dynamics within each component and to project future anthropogenically induced changes. Model results can be compared to observations of temperature, salinity, and various tracers such as chlorofluorocarbons and radiocarbon. For the analysis of past states and transient changes of the climate system where observations are not available, information must be reconstructed from proxy data from climate archives such as tree-rings, marine and lake sediments, ice sheets and glaciers, corals, and speleothems. With climate models, the gained information can be interpreted by simulating these proxies.
In contrast to comprehensive climate models, climate models of reduced complexity have the benefit that simulations of a certain time interval require much less CPU time. This permits a larger number of simulations as well as simulations over longer time intervals. The high computational efficiency of this category of climate models is very favorable for studies where novel tracers are implemented, sensitivities are quantified, or new hypotheses are tested, because these studies involve many simulations. Reduced complexity climate models are however generally of greatly reduced complexity compared to comprehensive climate models, processes are simplified or ignored, and many important forcings are prescribed.
The aim of this thesis is to extend the cost-efficient Bern3D three-dimensional reduced-complexity ocean model by a two-dimensional energy and moisture balance atmosphere in order to make the model suitable for transient simulations in the past and into the future. Strong emphasis is put on the cost-efficiency of the coupled ocean-atmosphere model, as the model should remain fit for the types of studies mentioned above. This is documented in several paleoclimatic applications performed with the coupled model.
Chapter 1 discusses the energy balance of the atmosphere and gives an overview on the current state of knowledge concerning the occurrence of glacial-interglacial cycles and the processes involved in the last deglaciation. In a third part, Earth System Models of Intermediate Complexity (EMICs) are introduced and positioned within the suite of coupled ocean-atmosphere models. Then, the current state of the Bern3D EMIC is summarized.
Chapter 2 describes in detail the energy and moisture balance model that is coupled to the Bern3D ocean model, and the modern and last glacial maximum (LGM) state of the coupled ocean-atmosphere model is presented. Two first applications of the coupled model are discussed. In the first, several simulations of the past 800,000 years are performed and the contributions of the different forcing factors to the glacial-interglacial atmospheric temperature change are analyzed. From these simulations, the sensitivity of ocean temperature to atmospheric temperature, Atlantic meridional overturning circulation (AMOC), and Antarctic Bottom Water strength is analyzed at 23 locations. In a second application the equilibrium climate sensitivities of the modern and of the LGM state are compared. The temperature rise for a doubling of the CO2 concentration from LGM conditions is with 4.3◦C notably larger than in the modern case (3◦C). The relaxation time scale of atmospheric temperature is strongly dependent on the response of AABW to the CO2 change.
Chapter 3 presents a new method to qualitatively and quantitatively reconstruct the evolution of the AMOC strength on glacial-interglacial timescales by the combination of paleoclimate records and climate model simulations. Two reconstructions of the past 330 kyr are performed, one based on proxies of deep ocean temperature, the other on time series of sea-surface temperature proxies. Both AMOC reconstructions suggest an increase at glacial inceptions followed by a decrease throughout glacial intervals. Because of the age-scale uncertainties of the temperature reconstructions it is concluded that the method is not suitable to detect abrupt ocean circulation changes. However, the method may be useful to quantitatively estimate the magnitude and long-term trends in the AMOC strength on glacial-interglacial timescales.
In Chapter 4, a proposition is tested where past global mean ocean temperatures can be reconstructed by measuring noble gas concentrations in ice core bubbles. For this, krypton, xenon, argon, and molecular nitrogen are implemented into the Bern3D model and the characteristics of these novel paleoclimatic proxies are explored. It is concluded that atmospheric noble gas concentrations are suitable proxies of global mean ocean temperature. Changes in ocean volume need to be considered when reconstructing ocean temperatures from noble gases. Calibration curves are provided to translate ice-core measurements of krypton, xenon, and argon into a global mean ocean temperature change.
Chapter 5 analyzes the decline of atmospheric ∆14C during the last deglaciation that has not yet been successfully explained. The contributions of changes in the radiocarbon production rate, rapid changes of the AMOC, and of the difference between the LGM and modern state of the ocean are quantified. Depending on the carbon distribution within the ocean during the LGM, the LGM-to-modern atmospheric ∆14C difference ranges between 40 h and 219 h. Modeled LGM benthic-planktonic radiocarbon age differences are compared to reconstructions from marine sediments.
Chapter 6 discusses the effect of abrupt ocean circulation changes on marine radiocarbon reservoir age. Model results are compared to observations of the Younger Dryas, a cold interval during the last deglaciation. It is found that reservoir ages decrease by about 100 yr in the Atlantic as a consequence of a shutdown of the AMOC. The effect of changes in sea-ice cover also play a major role. Therefore, changes in marine reservoir age need to be considered when dating marine organisms during the deglaciation. In this study the ocean-only model is used as the study was carried out prior to the development of the atmospheric component of the model.
Finally, an outlook is given in Chapter 7.

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