Modeling long-term climate changes with equilibrium asynchronous coupling

The use of the equilibrium asynchronous coupling (EAC) scheme is proposed a s a strategy to better understand long-term climate changes in a fully coup led ocean-atmosphere general circulation model. The EAC scheme requires eac h component model to be integrated to its equilibrium before being coupled to the other component. Use of this scheme has the distinct advantage of be ing able to clarify the nature of the coupling between the ocean and atmosp here, because each asynchronous iteration takes the form of a sensitivity e xperiment. 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 p roportional to the damping rate in the atmosphere or surface ocean, but is inversely proportional to the coupling strength between the ocean and atmos phere. Furthermore, the seasonal cycle response converges much faster than the annual mean response. Using realistic parameters, the seasonal cycle re sponse should converge in a few iterations. The EAC scheme is further appli ed to a coupled ocean-atmosphere general circulation model to study the tro pical monsoon climate of the early Holocene. The convergence behavior of th e sea surface temperature is found to agree with the theory derived from th e energy balance model study. The EAC scheme is further used to investigate the role of ocean-atmosphere feedback in modifying the response of monsoon s to orbital forcings in the early Holocene. It is found that the ocean exe rts a positive feedback on the North African monsoon, but a negative feedba ck on the Indian monsoon.