An idealized general circulation model is constructed of the ocean's deep circulation and CO2 system that explains some of the more puzzling features of glacial-interglacial CO2 cycles, including the tight correlation between atmospheric CO2 and Antarctic temperatures, the lead of Antarctic temperatures over CO2 at terminations, and the shift of the ocean's δ13C minimum from the North Pacific to the Atlantic sector of the Southern Ocean. These changes occur in the model during transitions between on and off states of the southern overturning circulation. We hypothesize that these transitions occur in nature through a positive feedback that involves the midlatitude westerly winds, the mean temperature of the atmosphere, and the overturning of southern deep water. Cold glacial climates seem to have equatorward shifted westerlies, which allow more respired CO2 to accumulate in the deep ocean. Warm climates like the present have poleward shifted westerlies that flush respired CO2 out of the deep ocean.