Abstract ATM-9523787 Ghil, Michael University of California, Los Angeles Title: Internal and External Causes of Climatic Changes The PI will study three general areas of climate: 1. He will continue research on the physical mechanisms of Quaternary glaciations. Simple and intermediate models of these mechanisms will be developed further, including zero-, one-, two- and three dimensional models of the paleoceanic thermohaline circulation (THC), in addition to the previously-modeled radiation balance, ice-sheet mass balance, and hydrologic cycle. Model results will be compared in the spectral and temporal domain with marine, ice-core and continental records, emphasizing rapid variations associated with marine Heinrich and ice-core Dansgaard- Oeschger events, and their spatial patterns. 2. Research on the low-frequency variability of the atmosphere, emphasizing intraseasonal oscillations in the midlatitude atmosphere, has proceeded through theoretical studies with a hierarchy of simple and intermediate models, observational studies of the longest upper-air datasets, and studies using the UCLA general circulation model. The PI shall complete this research with a direct comparison of intermediate-model, GCM, and observational-analysis results. A laboratory study of barotropic flow over topography should also shed further light on the oscillations' physical mechanisms. 3. Natural climate variability on the interdecadal time scale is the third, and new area of research. Preliminary, univariate studies of instrumental records will be extended to explore this variability's spatial patterns. Two- and three-dimensional THC models will be coupled with active and passive atmospheric and sea- ice models. These models will be studied first for constant radiative forcing and then for different scenarios of changes in this forcing, due to solar variability, volcanic eruptions and greenhouse-gas increases. The interaction of these long-periodic, intermittent or g radual changes with those caused by the short- periodic annual cycle will be examined carefully. All three thrusts of the investigation are aimed at clarifying the interaction between the climatic system's natural variability - intraseasonal, interdecadal or millennial - and the variable forcing: external to the atmosphere, in the intraseasonal case, and to the climate system as a whole, in the other two. The tools to be used are those of dynamical systems theory, for the model studies, and of nonparametric statistics, for the model diagnostics and data analysis. Ultimately, the proposed research should improve our understanding of the factors that contribute to or limit the system's predictability on each one of the three time scales under consideration.
