The objective of this award is to test the hypothesis that solar variability forced tropical climate change during the last glaciation. A strong correlation between millennial scale Holocene solar variability and North Atlantic sea-ice drift proxies has been found. Previous modeling studies indicate that the North Atlantic region is unlikely to force rapid tropical climate change. In contrast, a preliminary climate simulation of the last glaciation indicates that tropical climate is sensitive to changes in solar irradiance. It is hypothesized that solar-driven tropical climate change may have driven rapid extra-tropical climate oscillations. The strength of this hypothesis is that it explains both tropical and extra-tropical glacial marine proxy data with a known forcing.
This award will support a series of late Pleistocene (Marine Isotope Stage 3) coupled ocean-atmosphere-sea-ice experiments using NCAR 's paleo-climate system model to test this hypothesis. After establishing an equilibrium control experiment, a series of branch runs forced by plausible changes in solar irradiance will be completed. The experiments will be compared and time series of important climate fields will be analyzed to identify the response of the climate to solar forcing. Because of the potentially important role for tropical water vapor transports to influence glacial climate, the passive tracer capabilities of Community Atmospheric Model (CAM) 2.0, will be implemented to determine the pathways and fluxes of water vapor from the tropical oceans. In order to identify climate linkages between tropical and extra-tropical regions, a second series of experiments will be conducted with the imposed solar forcing restricted to specific regions (e.g., the tropics). In this way, it will be possible to distinguish the direct climate response to solar variability from the indirect response resulting from dynamical changes. Finally, model results will be compared with existing records of climate change. This research should provide helpful insight into tropical - extra-tropical linkages in climate.
