The influence of glacial boundary conditions on the climate system during the last glacial maximum.
The aim of this thesis is to explore and understand some major climate mechanisms that were responsible for atmospheric and oceanic changes during the LGM (21,000 years ago). A coupled global atmosphere ocean model of intermediate complexity is used to study the influence of glacial boundary conditions on the climate system during the LGM in a systematical manner. A web of atmospheric interactions is disentangled which involves changes of the meridional temperature gradient and an associated modulation of the atmospheric baroclinicity. This in turn drives anomalous transient eddy momentum flux which feedback onto the zonal mean circulation. Moreover, the modified transient activity, weakened (strengthened) in the North Pacific (Atlantic), leads to a meridional re-organization of the atmospheric heat-transport, thereby feeding back to the meridional temperature structure. Furthermore, it is argued that modifications of the large-scale atmospheric circulation during the LGM may have led to a slowdown of the Pacific subtropical gyre as well as to an intensification of the Pacific subtropical cell. These oceanic circulation changes generate an eastern North Pacific warming, an associated cooling in the Kuroshio area, as well as a cooling of the tropical oceans, respectively. The tropical cooling pattern resembles a permanent La Nina state which in turn forces atmospheric teleconnection patterns that lead to an enhancement of the subtropical warming by reduced latent and sensible cooling of the ocean. In addition, the radiative cooling due to atmospheric CO2 and water vapour reductions imposes a cooling tendency in the tropics and subtropics, thereby intensifying the permanent La Nina conditions. Hence, a delicate balance between oceanic circulation changes, remotely induced atmospheric flux anomalies as well as local radiative cooling is established which controls the tropical and the North Pacific temperature anomalies during the LGM. The LGM simulation exhibits an intensified Atlantic overturning cell, associated with an enhanced formation of North Atlantic Deep Water. This enhancement can be attributed to the strong surface cooling in high latitudes and brine release in areas of seasonally varying sea-ice extent. In turn, the intensified meridional overturning circulation leads to an enhanced poleward heat transport that is required to equilibrate the strong tropical-extratropical temperature contrast during the LGM. The modeling results compare well with some recent paleoreconstructions.