The impact of orbital forcing on the climate of an intermediate-complexity coupled model

In a transient simulation of the Holocene climate with ECBilt orbital forcing induces a quasi-linear response in the Northern Hemisphere mean climate state. Summer warming over land induces a stronger monsoon circulation in the subtropics and associated adjustment in precipitation patterns. This results in a freshening of the tropical Atlantic and Indian oceans, whereas the Pacific is more saline in the early Holocene. Although the response simulated by ECBilt is too weak, it is qualitatively correct compared with proxy data and with earlier PMIP time-slice simulations with more complex atmospheric models. The present model version exhibits rapid transitions between a 'weak overturning' and a 'strong overturning' state in the Southern ocean. It is found that the small response in the global ocean fields to the orbital forcing modifies the frequency and duration of these events significantly.