As the record of past climate becomes clearer, the existence of regimes has emerged as a primary characteristic of the climate system. Present climate is now known to represent one regime among others including glacial climates such as characterized recent intervals of the Pleistocene and the much warmer equable climates of the Eocene and Cretaceous. An important test of climate theory is the ability to explain the record of climate change in terms of atmosphere-ocean dynamics, and the response of the dynamics to internal and external changes.

The hypothesis is advanced that variation in the strength and extent of the symmetric circulation is an important mechanism meditating climate change. Using a simplified model we explore dynamical variables that influence the symmetric circulation and the physical processes that may have been modified these dynamical variables to maintain the warm equable climates.