Uta Krebs-Kanzow

The Atlantic meridional overturning circulation (AMOC) exerts a strong influence on global climate by providing up to one quarter of the maximum meridional heat transport of the coupled climate system (Wunsch, 2005, Trenberth and Caron, 2001). Climate models consistently predict a slow-down of the AMOC in response to rising atmospheric CO2 concentrations for the coming decades (Schneider et al, 2007). The rate of the slow-down is however poorly constrained and adds considerable uncertainties to model projections of the future climate. Moreover, the natural variability of the AMOC beyond inter annual time scales is not known as direct continuous measurements only exist for the past 6 years (Kanzow et al, 2010). Paleo reconstructions reveal that the AMOC was subject to strong variation during both the last glacial period and the transition to the present Holocene warm period. The deglaciation is of particular interest as it represents the most recent period of global warming associated with significantly increasing atmospheric CO2 concentration. Since paleo data do not allow to quantify the amplitude of past AMOC variations and since the climate response might depend on the background climate, it is a major problem to separate past AMOC induced climate variations from superimposing climate signals related to changes in ice-volume, greenhouse gas concentrations or orbital parameters. However, during the last glacial period events of extremely weakened AMOC exhibit common characteristics such as low northern hemispheric temperatures and shifts in low latitudinal precipitation (as review see Hemming, 2004 and references therein) which might represent a fingerprint of AMOC variations. The proposed work will focus on the identification of fingerprints of the AMOC which are valid over an as wide as possible range of climate-controlling parameters using a suite of climate model experiments which differ in continental ice caps, greenhouse gas concentration, orbital parameters and geometry of major ocean passages. Ultimately, this shall be used to decipher the role of AMOC variation during the deglaciation and to define and evaluate proxies, which potentially reconstruct AMOC variations indirectly from observed climate patterns on centennial to millennial time scales.

Today the Earth's climate is rapidly warming. There is ample evidence for other, dramatic transitions to warmer climates in the Earth's history. Understanding climate transitions of the past is key to provide better predictions for the coming centuries. Compared to equilibrium simulations, however, the transient simulation of past climates is still in its infancy. The aim of this project is to provide a transient climate simulation of the Last Glacial Termination with a state-of the-art coupled atmosphere ocean general circulation model (AOGCM) to investigate changes in the Atlantic meridional overturning circulation (AMOC) and the atmospheric hydrological cycle in a globally warming climate. The simulation will cover the period between the end of the Last Glacial Maximum (LGM, 19 kyr B.P1) and the onset of the warm Bølling-Allerød (about 15 kyr B.P.) and will be forced by changes in atmospheric CO2 and methane concentrations, insolation, and deglacial meltwater discharge. Together with idealized climate simulations and vegetation model experiments this simulation will be used to • quantify the individual contributions of the simultaneously changing forcings to the observed climate evolution • analyze the relationship between Atlantic meridional heat transport and the atmospheric hydrological cycle • develop techniques to constrain reconstructions of precipitation and AMOC strength