Multiple equilibria, natural variability, and climate transitions in an idealized ocean-atmosphere model

Abstract An idealized coupled ocean–atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-scale fluid motions in the climate system. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The two components exchange heat and freshwater fluxes synchronously. Coupled integrations are carried out over periods of several centuries to identify the equilibrium states of the ocean–atmosphere system. It is shown that there exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. Each of the coupled equilibria has a close analog in the ocean-only model with mixed boundary conditions. The oceanic circulation in the coupled model exhibits natural variability on interdecadal and longer timescales. The dominant interdecad...