Modeling long-term climate changes with equilibrium asynchronous coupling

The use of the equilibrium asynchronous coupling (EAC) scheme is proposed as a strategy to better understand long-term climate changes in a fully coupled ocean-atmosphere general circulation model. The EAC scheme requires each component model to be integrated to its equilibrium before being coupled to the other component. Use of this scheme has the distinct advantage of being able to clarify the nature of the coupling between the ocean and atmosphere, because each asynchronous iteration takes the form of a sensitivity experiment. Basic features of the EAC scheme are first studied in an energy balance model. It is found that the convergence rate of the EAC scheme is proportional to the damping rate in the atmosphere or surface ocean, but is inversely proportional to the coupling strength between the ocean and atmosphere. Furthermore, the seasonal cycle response converges much faster than the annual mean response. Using realistic parameters, the seasonal cycle response should converge in a few iterations. The EAC scheme is further applied to a coupled ocean-atmosphere general circulation model to study the tropical monsoon climate of the early Holocene. The convergence behavior of the sea surface temperature is found to agree with the theory derived from the energy balance model study. The EAC scheme is further used to investigate the role of ocean-atmosphere feedback in modifying the response of monsoons to orbital forcings in the early Holocene. It is found that the ocean exerts a positive feedback on the North African monsoon, but a negative feedback on the Indian monsoon.