Climate variability involves strong interactions among climate systems, and certain phenomena, such as El Niño, that arise from interactions and that could not exist in the individual systems alone. The Climate Systems Interactions (CSI) group, led by Prof. J. David Neelin, develops theory and modeling aimed at understanding these interactions.   The group specializes in the application of hierarchical climate modeling: building a hierarchy of models of successively less complexity, until the phenomenon has been distilled down to its essential elements. The more complex models aim to simulate the phenomena, while the simpler models allow theoretical understanding. Many climate research groups make some use of hierarchical modeling; a particular concern of this group is to practice it systematically and attempt to make the derivation of the simpler and intermediate complex members of the hierarchy as clean as possible.

The Theoretical Climate Dynamics (TCD) group, led by Prof. Michael Ghil, studies climate dynamics on all time scales -- from intraseasonal, through interannual and interdecadal, to millennial -- using the methods of dynamical systems theory.  They apply these methods to observations, numerical models, and experiments concerning the climate system -- the atmosphere, ocean, bio- and cryosphere -- through collaboration with researchers in North America and on other continents.