Simulations using the CCSM3 comprehensive AOGCM: Mechanisms of abrupt climate change and natural variability during the last centuries

Renold, Manuel (2007). Simulations using the CCSM3 comprehensive AOGCM: Mechanisms of abrupt climate change and natural variability during the last centuries (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Proxy evidence from many tracers such as ice cores in Greenland and Antarctica reveal that the earth’s climate has changed several times in the past. The last 700,000 years are punctuated by transitions between glacial and interglacial periods. Different periods have been detected and linked to changes in the earth’s orbit, eccentricity, axial tilt, and precession. The last of such a cycle ended approximately ten thousand years ago. The global climate moved from a glacial to a much warmer climate. Since then, the climate has been in a remarkably stable state. Throughout the entire Holocene only small fluctuations of the climate such as the Little Ice Age, the Medieval Warm Period or the 8.2 ka event are known.
To assess the predictability of future climate changes it is of crucial importance to have a profound knowledge of past climate history and the mechanisms controlling these changes naturally. The climate changes of the past leave their traces in many proxies, such as marine and lake sediments, corals, ice cores, and glaciers. Climate records reflect local as well as large-scale variability of the climate. Climate models represent an excellent instrument to combine these proxies of different time scales and spatial distributions to investigate past climate history and the related mechanisms.
The power of today’s computers permits the use of complex climate models simulating numerous physical and chemical processes. In conjunction with supercomputers and PC-clusters, simulations of hundred to thousand model years are possible to calculate, even with complex Atmosphere-Ocean-General-Circulation models (AOGCM). In contrast to climate models with reduced complexity, the AOGCM simulate many modes of internal variability. This is a great advantage, because it allows the investigation of atmospheric modes.
In this thesis the AOGCM Community Climate System Model (CCSM) was used to carry out two types of experiments. One addresses Dansgaard-Oeschger/Heinrich events, and a second is focused on simulations of the past 500 years. The former experiments have been motivated by the limited knowledge about stadial to interstadial fluctuations during the past ice age. The mechanisms, which initiate such an event, are still being debated. Several “freshwater hosing” experiments have been done to investigate the transition from an interstadial to a stadial climate state. The experiments are also performed to investigate the influence of the collapsed Meridional Overturning Circulation (MOC) on El Niño-Southern Oscillation (ENSO).
The later experiments are focused on past climate variablity during the last 500 years. The impact of solar irradiance and volcanic forcing on climate is investigated during the Maunder Minimum, the period of AD 1640 to 1715. Additional studies have been done to investigate the climate of the Indian ocean in the 19th and 20th century, in particular the precipitation in East Africa. Comparisons with lake sediments from Lake Victoria and glacial proxies from Mt. Kilimanjaro show that the model captures climate changes during this period. This permitted the investigation of possible mechanisms that are responsible for droughts and floods in East Africa.
The freshwater hosing experiment further served as a base for investigation about experimental designs. There are two types of methods to carry out freshwater hosing experiments. One is to add freshwater directly into the forcing area, another is to remove salt out of the areas. To satisfy the global salt budget, the surplus of salt is then redistributed to the rest of the ocean. Both methods are routinely used in freshwater experiments. It was found that the redistribution of salt was responsible for an artificial warming in high latitudes of the ocean. The globally uniform salt flux caused heat transport anomalies to high latitudes, due to a change in the related bolus velocity field.
The thesis is organized as follows: The first chapter presents an introduction of large-scale ocean circulation with a focus on the last ice age. This is done with the goal of embedding artifical freshwater hosing experiments into a more climatological context. The second part of the introduction discusses the second phase of the last millennium and gives a brief overview of the Maunder Minium period and an introduction into a recently detected atmospheric-ocean mode operating in the Indian Ocean. The second chapter explains the used experimental design for all studies presented in this thesis. Two different versions of the CCSM model are used, therefore a discussion of the differences and improvements is presented, as well as a short model description. The subsequent chapters presents published and submitted papers about the earlier mentioned topics.

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