Renner, Stefanie (2005) Simulation der letzten Eiszeit mit ECHAM5: Sensitivität gegenüber Randbedingungen und Modellauflösung. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 102 pp.

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Abstract

Simulations of the last glacial maximum (LGM) were carried out using the atmospheric general circulation model ECHAM5. Glacial boundary conditions are prescribed, such as lower atmospheric greenhouse gas concentrations and changes in orbital parameters as well as the increased topography and albedo of the ice sheets over North America and Europe. The boundary conditions are set to glacial values first separately and then in combination. The atmospheric response is analysed for model simulations at three different horizontal resolutions. The greatest influence originates from the ice sheets. In the Northern Hemisphere winter season, the glacial topography is the main cause for the decrease in the global mean air temperature. In summer, the largest part of the cooling is due to the higher albedo. In particular in the boreal summer season, topography and albedo are producing opposite anomalies. The meridional temperature gradient in the LGM is much stronger than in the control simulation. Temperature anomalies are nevertheless generally lower than in former studies because present sea surface ternperatures have been prescribed. The anornalies due to LGM greenhouse gas concentrations and orbital parameters are negligible. If all four boundary conditions are set to ice age values, the winter response largely reflects the response to the ice sheet topography, whereas in summer the glacial albedo is most important. The total response is approximately a linear combination of the responses to the individual forcings. The atmospheric circulation also changes in response to the different boundary conditions, in particular to the presence of the continental ice sheets. Over the North American (Laurentide) ice sheet a glacial anticyclone develops, as well as a stronger wave pattern in the 500 hPa geopotential height field. The North Atlantic jet stream is strengthened due to a higher gradient in the isohypse field over the North Atlantic. The model's horizontal resolution has a strong effect on the topography response. Especially above the ice sheets, the temperature response is stronger at higher horizontal resolution. The boreal winter transient eddies are most sensitive to the model resolution. The T21 response pattern is significantly different from those simulated in higher resolution experiments. At T42, the Atlantic storm track is further extends further eastward, causing positive precipitation anomalies over western Europe. The higher importance of the stationary waves compared to the transient eddies during the LGM found in earlier model simulations has also been simulated. Temperature, wind and precipitation do not exhibit a significant response in the Indian Summer Monsoon region and thus do not suggest a weakening of the Indian Summer Monsoon.

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