Hochschulschrift:
Dissertation, Universität Hamburg, 2017
Anmerkungen:
Literaturverzeichnis: Seite 111-123
Zusammenfassung: Deutsch
Text englisch, Zusammenfassung in deutscher und englischer Sprache
Beschreibung:
State-of-the-art coupled climate models produce very di erent states of the Atlanti c Meridional Overturning Circulation (AMOC) in simulations of the La st Glacia l Maximum (LGM). In particular, many of them fail to capture the shoaling of the North Atlantic Deep Water (NADW) cell, which is indicated by paleo records. The cause for these differences is not yet well understood. Simulations with the Max Planck Earth System Model (MPI-ESM) are used to improve this understanding by studying the sensitivity of the AMOC and the deep Atlantic water masses to different sets of forcings. Analysing the individual contributions of the glacial forcings reveals that the glacial ice sheets cause an increase in the overturning strength and a deepening of the NADW cell, while the low greenhouse gas (GHG) concentrations cause a decrease in overturning strength and a shoaling of the NADW cell. The effect of the orbital conguration is negligible. The effects of the ice sheets and the GHG reduction balance each other in the deep ocean so that no shoaling of the NADW cell occurs in the full glacial state. The mechanism behind the shoaling of the NADW cell is analysed by simulating the AMOC response to different GHG concentrations with linearly decreasing radiative forcing. In order to capture a possible non-linear response, the different GHG concentrations are applied to a setup with glacial ice sheets and to a setup with preindustrial ice sheets. In the simulations with glacial ice sheets, the AMOC decreases linearly with the radiative forcing once the atmospheric pCO2 is below 284 ppm. To simulate a shoaling relative to the preindustrial AMOC state, GHG concentrations below the glacial level are necessary. Antarctic Bottom Water (AABW) needs to become more saline than NADW to achieve the necessary shoaling. Brine release and shelf convection in the Southern Ocean are key processes for the salinity increase of AABW. In the simulations with preindustrial ice sheets, the AMOC strength responds nonlinearly to the decreasing radiative forcing. There are two distinct AMOC modes: A strong and deep mode at high GHG concentrations, and a weak and shallow mode at low GHG concentrations. The strong AMOC mode becomes unstable at a pCO2 between 230 ppm and 206 ppm. The weak AMOCmode becomes stable at a pCO2 between 206 ppm and 185 ppm. In the weak AMOC mode, AABW is as salty as or saltier than NADW, and the Nordic Seas do not contribute to the formation of NADW. In a simulation with 206 ppm, both AMOC modes are unstable and the AMOC oscillates between the two unstable states. These self sustained oscillations are caused by salinity changes in the tropical and subpolar Atlantic in combination with interactions between the subpolar gyre and deep convection in theNordic Seas. The AMOC does not switch into the weak mode in the simulations with glacial ice sheets, because the glacial ice sheets increase the AMOC strength by enhancing the density gain in the North Atlantic.