MULTIPLE EQUILIBRIA, NATURAL VARIABILITY, AND CLIMATE TRANSITIONS IN AN IDEALIZED OCEAN-ATMOSPHERE MODEL

An idealized coupled ocean-atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-sc ale fluid motions in the climate system. The atmospheric component is an eddy-resolving two-level global primitive equation model with simpl ified physical parameterizations. The oceanic component is a zonally a veraged sector model of the thermohaline circulation. The two componen ts exchange heat and freshwater fluxes synchronously. Coupled integrat ions are carried out over periods of several centuries to identify the equilibrium states of the ocean-atmosphere system. It is shown that t here exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. E ach of the coupled equilibria has a close analog in the ocean-only mod el with mixed boundary conditions. The oceanic circulation in the coup led model exhibits natural variability on interdecadal and longer time scales. The dominant interdecadal mode of variability is associated wi th the advection of oceanic temperature anomalies in the sinking regio ns. The sensitivity of the coupled model to climatic perturbations is studied. A rapid increase in the greenhouse gas concentrations leads t o a collapse of the meridional overturning in the ocean. Introduction of a large positive surface freshwater anomaly in the high latitudes l eads to a temporary suppression of the sinking motion, followed by a r apid recovery, due primarily to the high latitude cooling associated w ith the reduction of oceanic heat transport. In this evolution, the se condary roles played by the atmospheric heat transport and moisture tr ansport in destabilizing the thermohaline circulation are compared, an d the former is found to be dominant.