ALBERT

Description: A coupled global atmosphere–ocean model of intermediate complexity is used to study the influence of glacial boundary conditions on the atmospheric circulation during the Last Glacial Maximum in a systematical manner. A web of atmospheric interactions is disentangled, which involves changes in the meridional temperature gradient and an associated modulation of the atmospheric baroclinicity. This in turn drives anomalous transient eddy momentum fluxes that feed back onto the zonal mean circulation. Moreover, the modified transient activity (weakened in the North Pacific and strengthened in the North Atlantic) leads to a meridional reorganization of the atmospheric heat transport, thereby feeding back onto the meridional temperature structure. Furthermore, positive barotropic conversion and baroclinic production rates over the Laurentide ice sheets and the far eastern North Pacific have the tendency to decelerate the westerlies, thereby feeding back to the stationary wave changes triggered by orographic forcing.

Description: We present a numerical eigenmode analysis of an intermediate El Nin˜o–Southern Oscillation (ENSO) model which is driven by present-day observed background conditions as well as by simulated background conditions for the Last Glacial Maximum (LGM) about 21,000 years ago. The background conditions are obtained from two LGM simulations which were performed with the National Center for Atmospheric Research climate system model (CSM1.4) and an Earth system model of intermediate complexity (ECBilt-CLIO). Our analysis clearly shows that the leading present-day unstable recharge-discharge mode changes its stability as well as its frequency during LGM conditions. Simulated LGM background conditions were favorable to support large-amplitude self-sustained interannual ENSO variations in the tropical Pacific. Our analysis indicates that off-equatorial climate conditions as well as a shoaling of the thermocline play a crucial role in amplifying the LGM ENSO mode.

Description: Based on coupled modelling evidence we argue that topographically-induced modifications of the large-scale atmospheric circulation during the last glacial maximum may have led to a reduction of the westerlies, and 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 Niña 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 vapor reductions imposes a cooling tendency in the tropics and subtropics, thereby intensifying the permanent La Niña conditions. The remote North Pacific response results in a warming tendency of the eastern North Pacific which may level off the effect of the local radiative cooling. Hence, a delicate balance between oceanic circulation changes, remotely induced atmospheric flux anomalies as well local radiative cooling is established which controls the tropical and North Pacific temperature anomalies during the last glacial maximum. Furthermore, we discuss how the aftermath of a Heinrich event may have affected glacial temperatures in the Pacific Ocean.

Description: 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.