Mechanisms of Centennial AMOC Variability in CESM1

By acting as a global heat buffer and water supply, the ocean plays a critical role in influencing climate variability. For instance, the Atlantic Meridional Overturning Circulation (AMOC) has often been connected to climate variability on timescales ranging from decades to millennia. However, AMOC variability on centennial timescales has often been overlooked due to the limited availability of long climate model simulations as well as the scarcity of suitable paleoclimate proxies. Models that do simulate centennial variability all show a salinity anomaly being transported to the deepwater formation regions in the North Atlantic Ocean; the underlying mechanism differs between models however, further complicating the understanding of centennial variability. These mechanisms can be broadly categorized into two groups: either the salinity anomaly originates in the subtropics following changes in local precipitation, or it derives from the Arctic Ocean as a result of anomalies in sea ice concentration. In the case of CESM1, which is one of the few CMIP5 models that show clear centennial variability, regression analysis suggests that the former process dominates. To provide more detail on the origin of centennial AMOC variability in CESM1, we will present its complex spatial-temporal patterns using Multi-channel Singular Spectrum Analysis (MSSA), which is a technique enabling the identification of propagating patterns of variability in time series of spatial fields. MSSA will be applied to multi-millennial simulations and focus of the analysis will be on the identification of the propagation mechanisms, e.g. associated phase differences between tracer fields, responsible for centennial variability.